Materials Testing Guide

Materials Testing eBookA Compilation of Technical Tips, Questions and Answers, and Customer Features from the TechNotes Newsletter

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Tensile Testing 4 Green Plastic: The Garbage Dump Killer? 5

Study Shows Differences in Mesh Materials for Hernia Repair 7

Why Alignment is Important in Tensile Testing 8

Testing of High Strength Rebar 9

Hidden Sensors Provide Extra Safety 10

The Bridge to Safety 11

What's Inside Your Arteries? Testing Could Reveal Your Risk of Stroke 12

Q: We take great care to ensure our test setup is consistent and our test equipment is as good as it can

be, but our Poissons ratio values still show too much variability. Is there anything else we can do? 13

Multi-Purpose Grip Shields 14

Q: Can I trust my strain figures when they are derived from crosshead position rather than from an

extensometer? 15

Q: When testing some specimens, the strain values appear to go backwards when the specimen is

yielding. Could extensometer slippage be causing this effect? 16

Q: Which grips are best for testing thin metal specimens? 17

Can clip-on extensometers affect my strain results when testing thermoplastics? 18

Do Your Test Results Change When Your Operators Change? 19

Study Reveals Benefits of Video Extensometers 20

Protect Your Investment with Proper Use of Grip Accessories 21

Tightening Your Wedge Grips 22

Q: How can I get better r-value results when using clip-on extensometers? 23

Q: What style of extensometer do I need? 24

Faster, More Consistent Testing With Pneumatic Grips 25

Q: How do I select an extensometer when determining a yield stress? 26

Worn Grip Faces? 27

Choosing the Right Grips 28

Why Am I Not Seeing Upper Yield? 29

What to Consider When Measuring Plastics 30

How can I improve the accuracy and repeatability of my Poisson's Ratio results? 31

The Best Solution for Gripping Low-Force Specimens 32

Why do I see a negative load after clamping my tensile specimen? 33

Indicating the Correct Gauge Length for Your Specimen 34

Grip Attachment Techniques 35

Q: Why does the speed of tensile testing after yield vary from material specification to material

specification? In your opinion, is there a significant difference in results? 36

Q: Why do I see a negative load value when I grip my specimen? 37

Q: Why am I getting low modulus values from my test machine? 38

Protecting Our Environment: Reducing Waste in Landfills 39

The Invisible Rebar: Microscopic Nanotubes Dramatically Increase Material Strength 40

Compression Testing 41 The Tower of Babel: Testing the Possibilities 42

A New Hip Material 43

Recent Testing Uncovers Titanics Mystery 44

From CO2 to Solid Rock 45

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Hardness Testing 46 Increasing Efficiency in Knoop and Vickers Testing 47

Best Practices: Which Rockwell Scale to Use 48

Q: How far apart should I space each Rockwell hardness test 49

Hardness Testing on Cylindrical Specimens 50

The Difference between a Knoop and a Vickers Test 52

Q: What is the difference between a Knoop and a Vickers test? 52

How GR&R Helps Your Rockwell Testing Process 53

Q: What is a Jominy test? 54

How Can Testing Strengthen Your Smile? 55

ASTM E18-07: New Changes will Affect Your Rockwell Hardness Indenters 56

Hardness Testers: Closed-Loop or Deadweight? 57

Different Rubber Hardness Scales for Your Testing Needs 58

Select Jaw Faces Based on the Hardness of Your Specimens 59

Q: How do I know when my hardness test block is no longer useful? 60

Impact Testing 61 Why Should I Instrument My Impact Tests? 62

Why Instrumented Impact Testing is Becoming More Popular 63

Q: What Causes the First Peak in the Load Curve of My Impact Test Data? 65

Q: How Much Energy Should I Use for My Impact Test? 66

Damaged Tups Change Results 67

Fatigue Testing 68 Q: I want to perform cyclic testing on my static testing machine. How fast can I go? 69

3M Ensures Quality under Different Test Conditions 70

Lab-grown Tissue 71

MacGyver-style Leg Brace May Reduce Amputations 72

Volvo Meets the Challenges of High Strain Rate Testing 73

Characterizing Spinal Range of Motion for Development of Improved Devices 74

Patients Own Tissue Repairs Torn Ligaments 75

Simulating Physiological Conditions of Implants 76

Bend Testing 77 Q: What is the difference between a single-point and a 4-point flexure test? 78

The Impenetrable Ship 79

Torsion Testing 80 Q: How can I measure the torsional properties of a pipe or cylinder? 81

Environmental Testing 82 Testing at High or Low Temperatures 83

Using Grips in a Low Temperature Chamber 84

Component Testing 85


Challenges in Testing Biomedical Components 85

Q: How does side loading and specimen/component misalignment of varying geometries of medical

devices and implants affect my test results? How should I best address these challenges? 87

How does the mechanical testing of solar cells contribute to the "green energy" initiative? 88

Software Tips 89 Q: When writing a procedure in Bluehill Software, how do I deal with "Toe Compensation" (as described

in ASTM D882) when testing the secant modulus (1%) of a thin film (1-5 mils)? Should I add a preload?

And how much is appropriate? 90

Capturing Testing in Action 91

Q: The way we currently test for N-value is cumbersome. We are looking for a way to improve productivity.

Is there a way that we can get the program to automatically assign the uniform elongation at the end of

the calculation, instead of having to do it manually? 92

Q: What happens if power is suddenly lost during a test? Will I lose all my data in Bluehill? 93

Q: How can I be certain my extensometer is ready to use? 94

Correcting for Compliance 95

Benefits of the Preload Feature in Bluehill Software 96

Q: Do I Need to Enter Dimensions for Each Specimen? 97

Service and Calibration Tips 98 Are You Always "Investigation-Ready?" 99

Q: Can you give me a letter certifying that my test is in accordance with a specific ASTM or ISO standard?

101

Q: What does accreditation mean and how does it affect testing standards? 102

How do you move a 250,000 pound deadweight stack, while maintaining its integrity and accuracy? 103

Errors in Testing 104 Are You Receiving the Highest Quality Test Results? 105

What is Data Rate? 106

Q: What is the Relationship between Accuracy and Resolution? 107

When You Shouldn't Balance the Load Cell 108

Test Specimen Cutting and Stamping 109

Testing Standards 110 Q: What types of international testing standards are used in the medical device industry? 111

Q: What is 21 CFR Part 11 and how does it affect me? 112

Q: I've been testing to ASTM test standards and now I've been asked to do the ISO equivalent. What is the

difference between ASTM and ISO? Can I use my existing test fixtures? 113

Q: What testing standards serve as guidelines and requirements for the development and manufacture of

hip implants? 114

Customer Stories 115 Research Institute Partners with Private Steel Company 116

Materials Science for Young Minds 118

The Sound of Quality 119

The Science Behind Superhuman Strength 120

Formula 1 Racer Gears Up With Carbon Fiber 121

Materials Testing Explored in High School 122

Tensile Testing


A tensile test, also known as tension test, is probably the most fundamental type of mechanical test you can

perform on material. Tensile tests are simple, relatively inexpensive, and fully standardized. By pulling on

something, you will very quickly determine how the material will react to forces being applied in tension. As the

material is being pulled, you will find its strength along with how much it will elongate.

Tensile Testing

Tensile Testing


Green Plastic: The Garbage Dump Killer?

The Great Pacific Garbage Patch is a colossal floating garbage dump in the northern Pacific Ocean. Roughly the

size of Texas, it lies between Hawaii and San Francisco. It contains about 3.5 million tons of trash, much of it

plastic--shoes, toys, bags, pacifiers, wrappers, toothbrushes, and bottles are only part of what can be found in

this dump. A similar dump exists in the Atlantic Ocean.

The global buildup of plastic, both in the sea and along every shoreline, is an environmental nightmare. Most

commercial plastics are produced from petroleum. These plastics degrade into small pieces so plastic waste

builds up and can exist for many years. A great deal of research has taken place to develop biodegradable

plastics that break down with exposure to sunlight, water or dampness, bacteria, enzymes, and so on. Instron

customer Metabolix, Inc. has been researching for two decades to develop a commercially viable

biodegradable plastic from corn sugar and has recently made the leap from research to commercial production

with their product Mirel.

Plastics produced from plant material are not new; they have been around for more than 150 years. First

produced in 1845, polylactic acid (PLA), a thermoplastic polyester, was made by fermenting various

agricultural products such as cornstarch. Dow Chemical revived PLA production in the 1950s, but high

production costs precluded its widespread use. In the 1980s, the British chemical company Imperial Chemical

Industries (ICI) developed Biopol, a bioplastic produced through bacterial action. Polyhydroxyalkanoate (PHA)

polymers are produced by most species of bacteria from food sources such as plant sugars and oils. One of

these PHAs, known as polyhydroxybutyrate (PHB), has properties similar to those of polypropylene. But once

more, ICI was unable to produce Biopol cheaply enough to compete with conventional plastics. Monsanto

purchased Biopol from ICI in 1996. In 1998, Monsanto discontinued its bioplastics operations due to high

costs and limited commercial opportunities. It sold its interests to the U.S. bioscience company Metabolix that

began researching and developing a cost-effective process for manufacturing PHB-based plastics. In 2006,

Metabolix formed a joint venture called Telles with the agricultural giant Archer Daniels Midland to

commercialize a bioplastic under the name Mirel.

Mirel is designed as a suite of products, each of which can

withstand heat and cold, is capable of containing food products,

and biodegrades in natural soil and marine environments, home

composting and industrial composting facilities, where these

facilities are available. However, like nearly all bioplastics and

organic matter, Mirel is not designed to biodegrade in

conventional landfills. The rate and extent of Mirels biodegradability depends on the size and shape of the articles

made from it. As with any new material, its testing requirements

have been extensive. Its product data sheet gives mechanical

test specifications for tensile strength?, elongation at break,

flexural modulus, flexural strength, notched IZOD impact values,

and melt flow figures, using ASTM and ISO standards.

Photo courtesy of Mirel

Tensile Testing


After initially producing Mirel bioplastics in a pilot plant, Telles opened a new production plant in Clinton, Iowa,

USA, in December 2009 with a 50,000 ton/year capacity.

One of the first Mirel products is the injection molding grade used to make 60% of the pen components for the

$1.25 Biodegradable Paper Mate pen made by Newell Rubbermaid. The pen costs more to manufacture, but

Paper Mate forecasts a strong demand. Other potential applications are cups, food containers, beverage

cartons, razor handles, brushes, applicators, cell phones, erosion control netting, plant pots, and plant clips.

The success of the venture is partially linked to consumers continued and increasing demand for green products, though businesses also use the material as a cost savings measure, in applications where

biodegradation saves time and labor. The market appears confident that the demand is there, with Metabolix

more than doubling its share price since February despite a $38 million loss last year. It remains to be seen if

the current enthusiasm to take care of the environment can eventually have the effect of shrinking or even

eliminating the ocean garbage dumps.

What is Tensile Strength?

Ultimate strength of a material subjected to tensile loading. It is the maximum stress developed

in a material in a tensile test.

Tensile Testing


Study Shows Differences in Mesh Materials for Hernia Repair

Twenty years ago a patient undergoing hernia surgery would be marked by a noticeable scar, endure a long

recovery time, and according to a medical study, up to 20% of these patients would experience a recurrent

hernia. Due to medical advancements, hernia surgery is now less invasive, has a quicker recovery time, and

decreased risk of recurrence (less than 1%). What is this magical medical advancement? Laparoscopic

surgery.

According to Dr. Corey Deeken,

Director of the Biomedical

Engineering and Biomaterials

Laboratory at Washington

Universitys School of Medicine, it is important for surgeons to choose an

appropriate prosthetic mesh material

when performing laparoscopic hernia

repair.

In the world of hernia repair, there are so many materials and pre-

formed sizes available for surgeons

to choose from, Deeken said. The mesh that is right for a particular

patient and type of repair may not be

the best choice for the next patient.

Deeken, a biomedical engineer,

wants to give surgeons more

standardized information to compare

when choosing what is best for their

patients. This includes a recent project to characterize the properties of a variety of mesh materials available

for hernia repair applications. During this project, Deeken and her team used a tensile testing system to

measure the biomechanical properties of more than 25 different hernia repair materials using techniques such

as suture retention and tear testing, as well as standard uniaxial and mesh strength testing. Deeken hopes to

present the data from this study at an upcoming surgical conference to make surgeons aware of differences in

the biomechanical properties of hernia repair materials.

Tensile Testing


Why Alignment is Important in Tensile Testing

Laboratories performing low-cycle fatigue tests know how important it is to have good alignment of the test

specimen relative to the principle stress axis. There is an increasing awareness of the role alignment can play

in the accuracy of tensile testing results.

Organizations, such as NADCAP and ASTM, are addressing this in the form of laboratory accreditation and

methodology for measuring alignment. For example, a NADCAP audit checklist for a composite materials

testing lab will now include an alignment check of the testing instrument and refer to ASTM E1012 - Standard

Practice for Verification of Test Frame and Specimen Alignment under Tensile and Compressive Axial Force

Application as the method of checking alignment. This process ensures the testing instrument is capable of performing tensile tests that produce less than 10% bending for non-brittle materials and less than 5%

bending for brittle materials.

To meet the bending requirements noted above, the testing instrument must be designed and built to a high

standard and the alignment of the loading frame, load cell and grips must be measured to determine the

percent bending. This is typically done using an alignment specimen having a total of 12 strain gauges; four at

the upper gauge length area, four in the center, and four at the lower gauge length area. The outputs of the 12

strain gauges are used to calculate concentricity error and angularity error. Our AlignPro Software is available

to perform the calculations for percent bending and provide a guide to the adjustments needed to correct for

bending that exceeds acceptable limits.

Tensile Testing


Testing of High Strength Rebar

Many standards govern rebar testing including: ASTM A 370, ASTM A 615, ASTM A 996, BS 4449 and EN

10002-1. The mechanical tests these standards outline can be demanding on operators and testing

equipment. So, when testing large rebar samples #14 (all grades) we suggest using a single test space load frame in lieu of traditional dual test space styles. For this test we used a 1500 KN model, which has a

capacity of 1500 kN (337,500 lbf) and accommodates rebar specimens ranging in length from 400 mm to

700 mm.

This load frame features a top-mounted hydraulic actuator which places the

loading area at ground level. This significantly reduced our lifting requirements

for loading the heavy rebar specimens. Additionally, we were able to perform

both tension and bend tests on the rebar sample by adding compression

adapters to the tension grips. This saved change-over time because we didnt need to use the overhead crane to remove the large, heavy tension grips. Adding

the compression adapter and bend fixture took only a few minutes and involved

tightening a few screws.

For the tension test we used hydraulic wedge grips because the initial clamping

force reduced grip slippage on the uneven surface of the rebar. These hydraulic

wedge grips accept rebar specimens from 10 mm (0.39 in) to 70 mm (2.75 in) in diameter. The grip jaws are

vee-shaped with a custom-cut groove to accept the ribs found on rebar.

Finally, we used an automatic extensometer to capture strain. The model we selected, an M300B, has an

adjustable gauge length from 10 mm to 300 mm (required for most rebar applications). It automatically

clamps to the ribs of the rebar surface when a test is started and unclamps at a specified point during the test.

The strain data can be used for required modulus? and yield? calculations.

What is Modulus?

Rate of change of strain as a function of stress. The slope of the straight line portion of a stress-

strain diagram. Tangent modulus of elasticity is the slope of the stress-strain diagram at any point.

Secant modulus of elasticity is stress divided by strain at any given value of stress or strain. It also is

called stress-strain ratio. learn more >>

What is Yield?

Indication of maximum stress that can be developed in a material without causing plastic

deformation. It is the stress at which a material exhibits a specified permanent deformation and is a

practical approximation of elastic limit. learn more >>

Tensile Testing


Hidden Sensors Provide Extra Safety

When someone says "fiber optics", you most likely think of telecommunications and not aircrafts or stadiums.

However, fiber optics can be embedded in structures to continuously monitor mechanical strain and

temperature changes, a technological breakthrough in the sensing industry.

Historically, such measurements were captured using electrical-type sensing devices, but in extreme

environments, such technology can be vulnerable. On the other hand, optical fiber sensors are rugged,

efficient, and extremely light, making them particularly interesting for the aerospace industry.

Fiber Optic Sensors & Sensing Systems (FOS&S), a

Belgium-based company, turns optic fiber into a sensor

by exploiting a physics law known as the Bragg condition.

Simply put, through exposing the core of a fiber to intense

ultraviolet light, the reflective properties can be used to

measure temperature and strain.

FOS&S is currently working on two notable projects. The

first is monitoring the structural health of the Athens

Olympic Velodrome roof structure (designed by the

famous architect Santiago Calatrava). The second project

is the in-flight structural health monitoring of aircraft

structures for Airbus. FOS&S uses an Instron testing

system for the calibration of its strain sensors and

performs tensile tests on composite samples embedded

with Fiber Optic Sensors.

"Placing fiber optic sensors in structural elements of an

airplane enables continuous monitoring of the actual

distribution of mechanical strain and temperature data

within these structures," says Mark Voet, CEO of FOS&S.

"This way, it immediately alerts operators of abnormal load situations like excessive vibration and internal

damage, allowing them to take the appropriate remedial action."

Photo courtesy FOS&S

Photo courtesy of FOS&S

Tensile Testing


The Bridge to Safety

Eighty percent of all earthquakes occur along the edge of the Pacific Coast. So far in 2007, there have been

nearly 15,000 detected earthquakes worldwide. Depending on its force, some buildings, roadways or bridges

could collapse.

When sitting in stopped traffic on a bridge, do you

wonder how it holds hundreds of tons without

collapsing? California Department of

Transportation (Caltrans) ensures California is

getting the highest quality materials for bridge

and highway projects throughout the state by

testing materials, from concrete to reinforcing

bars to structural steel components and couplers.

With a daily volume of nearly 300,000 vehicles,

one of the busiest bridges in the USA is the 71

year old west-coast San Francisco-Oakland Bay

Bridge (SFOBB). This 4.5 mile (7.2 km) long

bridge consists of two major spans. Once

deemed impossible to build, Caltrans designated

the SFOBB as the emergency lifeline route to use

in disaster response activities. This requires the

bridge to be secure, fully functional, and

earthquake-resistant. In 1989, the bridge closed

for more than a month due to repairs needed

after the Loma Prieta earthquake. In response,

the eastern span between Oakland and Yerba

Buena Island is now being replaced by an entirely

new crossing making the bridge less susceptible to damage during an earthquake. This is known as the East Span Seismic Safety Project.

"We are using Instron's testing system to tensile test large diameter steel

bars (#14 and #18) to ASTM A 615, ASTM A 706 and ASTM A 722

specifications," said Rosme Aguilar, the Structural Materials Testing Lab

Branch Chief. "This custom built 2 million pound (8,896 kN) capacity

system has replaced our existing testing system because its 1 million

pound (4,448 kN) capacity could no longer handle materials of larger

diameter and strength that require a higher capacity."

The system, which stands more than 26 feet (8 meters) high, is located at

the Structural Materials Testing Lab in Sacramento, CA. As California's

only state transportation testing lab accredited by the American

Association for Laboratory Accreditation (A2LA), it quickly responded to a

recent bridge collapse due to a tanker truck explosion. The lab had the

responsibilities of assisting with the damage assessment to determine if

the material properties of the steel girders and bent caps had been

compromised due to the heat from the tanker truck fire. Remarkably, the

damaged bridge was fully functional in 18 days.

Photo courtesy of CalTrans

Tensile Testing


What's Inside Your Arteries? Testing Could

Reveal Your Risk of Stroke

Strokes are the second most commonly feared condition; in

fact 2 out of 3 people know someone who has suffered a

stroke. In order to better understand prevention and

treatment, many researchers are studying the causes of

strokes, including the Department of Engineering at The

University of Cambridge under the direction of Dr. Michael

Sutcliffe.

Together with his colleagues, Dr. Sutcliffe is studying plaque

(a material that is deposited on the walls of the arteries)

and the hardening of the carotid artery, which can lead to a

stroke. The aim of their research is to develop better

methods for estimating a persons risk of having a stroke and to improve therapy selection.

Dr. Sutcliffe is testing plaque-filled arteries using an Instron

3367 30kN Universal Testing Machine to understand the

modulus and strength changes associated with plaque

growth. These results will be used in models of the carotid

artery to predict rupture of the artery and fluid flow

patterns.

"In the future, we will be working with others to link the way

cells change their shape under stress with tissue-level

testing and clinical experimental work. Our aim is to

understand how the stresses these plaques experience in

the arteries affect the way they grow," said Dr. Sutcliffe.

Tensile Testing


Q: We take great care to ensure our test setup is consistent and our test

equipment is as good as it can be, but our Poissons ratio values still show too

much variability. Is there anything else we can do?

A. Poissons Ratio is defined by the division of transverse strain by axial

strain. Instron has carried out extensive

reproducibility studies to investigate

inconsistent results between labs, as

well as within individual labs. Difficulty

in calculating the ratio relates directly to

the measurement of transverse and

axial strain at very small strain ranges.

As you indicate, a consistent setup with

accurate equipment is vital. For most

plastics, the recommended

extensometer is a high-resolution

biaxial extensometer. It is equally

important to use the appropriate grips.

Pneumatic side acting grips are

preferred since they are self-aligning

and offer adjustable clamping

pressures, which allows for consistent

clamping forces on the specimen from

one to the next.

You should try setting up a small preload value on all your future test methods for plastics.

When specimens are initially placed into grips, they can be subjected to small compressive forces. These

forces can cause specimens to bend imperceptibly, causing inaccurate and inconsistent results. We have

shown that establishing a small preload as a part of the test method eliminates those compressive forces on

specimens and improves the repeatability of results.

What is Preload?

A test segment where the crosshead moves to load the specimen to a specified value before a test

starts. Data is not captured during the preload segment.

Tensile Testing


Multi-Purpose Grip Shields

The new Instron pneumatic side-acting grips are supplied with adjustable

jaw face shields on either side of the grip. You can adjust the position of

the shields so that you can insert a specimen between the jaw faces, but

the shields help to prevent you from inadvertently placing a finger between

the jaws.

Many people dont realize that the shields also provide useful guidance for specimen centering. There are two centering guides on the shields, one for

round specimens and the other for flat specimens.

A notch in the shield arms is aligned with the center of the grip jaws. This

notch is useful when mounting a round or a thin specimen such as wire or

thread. When the shields are correctly installed and aligned for the

specimen size, you insert the specimen between the shields and hold it

against the notch while you close the grip jaws. You are then assured that

the specimen is centered.

For flat specimens, there are marks engraved at intervals on the shield

arms equidistant from the center. When inserting a flat specimen, you use

the marks as a guide to accurately locating the specimen in the center of

the grip jaws.

Tensile Testing


Q: Can I trust my strain figures when they are derived from crosshead position

rather than from an extensometer?

A. Crosshead movement is measured using a high-resolution encoder. When you move the crosshead with no

specimen installed, the reported measurement of that movement is often more accurate than for many

extensometers.

However, when you install a specimen and apply a tensile or compressive

load, the accuracy of the measurement of crosshead movement becomes

dependent upon the system compliance.

Compliance refers to the tendency of the various components of a test

system to deflect under load. Consider every component in a test system as

equivalent to a very stiff spring. When you apply a load to that component,

even a major item such as a crosshead, it will deflect, either bending,

stretching, or compressing. If it is a very stiff spring the deflection is tiny,

but still measurable. Compliance is the inverse of stiffness; the stiffer, the

less compliant.

There are three sources of compliance in a system: the load frame, the load string components, and the

specimen itself.

The load frame is designed with a very high stiffness. Instron measures the stiffness at a particular load and

publishes that figure as part of the specifications of the load frame.

Load string compliance is usually not known. There may be few or many components in a load string; grips or

fixtures, couplings, one or more load cells, and so on. Many components do not have published stiffness

values.

The specimen compliance is usually what you are trying to measure.

As a rule of thumb, if the compliance of your specimen is around 100 times greater than the compliance of the

load frame and the load string components, you can assume that the reported crosshead movement is

equivalent to the deflection experienced by the specimen. However, if you are testing a very stiff specimen, you

should always use an extensometer to measure specimen deflection.

If using an extensometer is not possible, then you should evaluate the system compliance before the test.

Either install an extremely stiff specimen and apply a tensile force, or install compression platens and apply a

compressive force with the platens touching each other. The resulting deflection measurement gives a close

indication of the system compliance. When you test the specimen, you can remove this value from the result.

Tensile Testing


Q: When testing some specimens, the strain values appear to go backwards

when the specimen is yielding. Could extensometer slippage be causing this

effect?

A. Yes it could be, but if you are aware of it and you have mounted

the extensometer correctly its unlikely. Its more probable that the strain is really going backwards.

Many metal alloys have a non-homogenous structure with grains of

different sizes and orientation, and they also contain various

impurities. Under loads that are sufficient to cause the material to

yield, bands of localized plastic deformation, known as Luders bands, can form in the otherwise unyielding portion of the material.

These bands of dislocations are the main contributor to the

discontinuous yielding portion of the stress/strain curve. They can

occur both inside and outside of the gauge length of the specimen, moving along the length of the specimen as

the load increases.

Your extensometer is probably reacting to yields that are occurring both inside and outside of the gauge length,

which can create this phenomenon of backwards strain.

Tensile Testing


Q: Which grips are best for testing thin metal

specimens?

A: Screw side-action grips open the door for specimen slippage,

high standard deviation, and low throughput. We recommend self-

tightening wedge grips for metal applications. They offer

improvement in all of these areas, do not require any tools, and are

easy to use.

Once the specimen is inserted between the jaw faces, manually

turn the lever to close the wedged faces and apply only a slight

amount of clamping force. This is sufficient enough for the jaw

faces to pull on the specimen once the test is started. The clamping

force increases as the specimen is pulled, eliminating jaw breaks

that are normally caused by high initial clamping force. The exact

model of grips and faces often requires a discussion about the

material youre testing.

Tensile Testing


Can clip-on extensometers affect my strain results when testing thermoplastics?

There are a variety of attributes used to describe

thermoplastics since properties are dependent on the

polymer, as well as additives. In some instances,

thermoplastics are relatively soft, so knife edges on

traditional clip-on style extensometers may cause premature

failures. This occurs when high stress points are created

where the knife edges contact the specimen. In other

instances, thermoplastics may be rigid, if glass or talc is

added. For these materials, significant energy releases may

occur at failure, possibly damaging the clip-on extensometer

since they are in direct contact with the specimen.

Non-contacting video extensometers overcome both issues by

providing a means to measure specimen strain without

having direct physical contact with the specimen. A high

resolution digital camera and real-time image processing

allows the device to acquire accurate strain data without

interfering with the specimen.

Tensile Testing


Do Your Test Results Change When Your Operators Change?

During a tensile test the specimen is subjected to a force purely in the tensile direction along a single axis. If

the specimen is not aligned properly, it will be pulled along multiple axes, which can cause premature

specimen failure and adversely affect any measurements captured during testing. Also, operators could load

the specimen differently or inaccurately causing erroneous end results.

To ensure proper alignment and accurate results amongst multiple operators, we suggest using a specimen

alignment device. These specimen centering devices (which are shaped like an "L") are attached to the grip

and help center the specimen within the grip faces. The operator can adjust the centering device making sure the specimen is centered and not over-inserted. Once the operator has the centering device set properly,

each subsequent specimen will be in the same location in the grips, allowing for accurate, repeatable results.

Tensile Testing


Study Reveals Benefits of Video Extensometers

We conducted a study to better understand the extensometry needs

of our customers. We found that many of our customers

experienced common issues while testing strain with clip-on

extensometers. Included are a few solutions from that study, along

with information about an alternative to clip-on extensometers.

From those surveyed, our study revealed:

Problem: 77% of those testing fragile, expensive, or delicate

specimens (including tendons and sutures) struggled to capture

strain without damaging their sample. These customers reported

that the weight of a clip-on extensometer influenced the sample's

behavior under test.

Solution: Since video extensometers do not come in contact with the

specimen, it makes them less damaging to the samples. They can

also be used with in vivo testing of biomedical samples.

Problem: Customers testing specimens that break violently were

unable to use a clip-on extensometer through failure. They also

reported problems with broken extensometers and felt uneasy

about lab operators removing a clip-on device while the specimen

was under load.

Solution: Video extensometers offer lab operators the convenience of capturing strain through failure since

they do not need to be removed.

Problem: Many of our customers testing at high and low temperatures struggled to find traditional

extensometry solutions that worked well with chambers.

Solution: 86% of those who used chambers preferred video extensometers over traditional clip-on styles.

In conclusion: we discovered that, of our customers who have used both traditional-style and video

extensometers, 77% preferred the video technology. Learn more about non-contact video extensometry, or

contact an applications specialist.

Tensile Testing


Protect Your Investment with Proper Use of Grip Accessories

Do you notice your internal crosshead grip jaws extending outside of the system's crosshead while running

tests? If you do, this may cause extensive damage to the machine. This style of grip is most commonly used in

our SATEC Series, a product line designed to deliver high-capacity tensile forces up to 3,000 kN. Since this force is so high, it can deform the machine's crosshead if the proper accessories are not used. This damage

may be irreparable and require replacement of several costly components, not to mention cause downtime.

To prevent this, we suggest using grip spacers (also called filler plates) to accommodate different sizes of

specimens while keeping the jaws inside the crosshead.

View an animation on In-head Grip Parts & Grip Accessories

Tensile Testing


Tightening Your Wedge Grips

A common mistake many customers make to troubleshoot specimen slippage

when using their mechanical wedge style tensile grips is over tightening them.

Over tightening a wedge grip can damage the grip and exert unwanted load on

the specimen. The mechanical design of a wedge grip works in the following

way:

1. A tension force is applied to the specimen 2. This tension force causes the specimen to pull downward on the jaw

faces (provided there is good bite between the jaw faces and the

specimen)

3. The faces slide through the grip body along the wedge path 4. The faces then squeeze the specimen

This entire process is self-tightening the higher the tensile load, the harder the jaw faces squeeze in on the specimen. While over-tightening isn't an

effective way to improve slippage, customers can minimize specimen slippage

by improving the bite through the use of proper jaw faces and ensuring the

specimen contacts at least 2/3 of the grip faces.

Tensile Testing


Q: How can I get better r-value results when using clip-on extensometers?

A: Determining r-value for ASTM E 517 requires precise measurement of axial and transverse strain. When

using clip-on extensometers, make sure you are practicing

the following techniques:

1. Set gauge lengths

2. Align instruments on the specimen

3. Zero instruments with no load on the specimen

4. Check that the knife edges do not deform the specimen

5. Be certain that the specimen is not bent

6. Ensure specimen markings havent deformed the specimen

7. Be certain specimens have smooth edges and meet

ASTM E 517

Tensile Testing


Q: What style of extensometer do I need?

A: There are two main styles of extensometers contacting and non-contacting. Contacting extensometers are widely used and provide accurate strain measurement. However, some applications (like biological tissue or

thin film) demand a device that won't damage the specimen or affect test results. Non-contacting

extensometers provide an ideal solution for delicate specimens, for specimens that break violently, for tests

conducted in a chamber, and for specimens of varying lengths and elongations. An Instron Applications

Engineer can recommend the correct instrument for your testing.

Tensile Testing


Faster, More Consistent Testing With Pneumatic Grips

While screw side action grips are appropriate for certain applications, you may experience long setup times,

premature specimen breaking at the jaws (due to over-tightening) or specimen slippage (due to under-

tightening). In addition, you will always need one hand to tighten the grip while the other hand holds the

specimen, which is not always convenient and may result in a misaligned specimen.

Upgrading to pneumatic side action grips could make your testing easier and faster.

Tensile Testing


Q: How do I select an extensometer when

determining a yield stress?

A: Extensometers are available from 1% to 3000%+ full

scale travel, but using the longer travel is not always the

best solution. When testing stiff specimens, such as steel,

an extensometer with 10% or less travel is recommended to

ensure adequate resolution for the determination of yield.

On the other hand, materials such as plastics commonly

yield at greater strain values, and therefore an instrument

with 50% travel is recommended. Long travel instruments

(100% or more) should be

reserved for high-elongation

specimens, such as rubber. An

Instron Application Engineer can

recommend the correct

instrument for your specimen

type.

Tensile Testing


Worn Grip Faces?

Efficient gripping of your test specimen is important for reliable, trouble-free testing. Like any tool, you need to

keep your jaw faces in good condition for optimum performance.

Chipped, worn or clogged teeth on jaw faces can produce slippage and with

it, the temptation to use excessive force, increasing the likelihood of jaw

breaks.

Unevenly worn faces can also produce undesirable bending effects.

Rubber-coated faces can gradually degrade over time in your shop

environment, particularly in higher temperature conditions.

Cord and yarn grips rely on a smooth, polished surface for optimum

resistance to jaw breaks. The original surfaces can wear with heavy use.

The best way to restore lost gripping efficiency is with a new set of jaw

faces.

Tensile Testing


Choosing the Right Grips

Successful gripping solutions require the specimen to be held in a way that

prevents slippage and jaw breaks and ensures axiality of the applied force. In

some cases the gripping requirements are very specific and a purpose-

designed grip or fixture is necessary to meet a particular testing standard.

However, in most cases, you can use general purpose accessories. General

purpose grips and fixtures have the advantage of being able to grip a wide

variety of specimen types and materials using a range of options such as

different jaw faces, alignment fixtures, etc.

The most important step in successful gripping is to choose the best set of

grips for your specimen type. To learn more about different grips and fixtures,

browse our online Accessories Catalog.

Tensile Testing


Why Am I Not Seeing Upper Yield?

Are you testing for upper yield strength, but not seeing a "dip" in your stress/strain curve? This is often the

result of using improper test control parameters. During yielding, the strain rate needs to be as constant as

possible. This is best achieved by using crosshead position or strain control*.

For example, if you run a test in stress control at the onset of yielding, the testing machine will accelerate to

maintain the desired stress rate. Incorrectly running in load control causes unwanted acceleration. This

prevents the stress from dropping relative to the increase in strain. As a result, the upper yield strength

calculation will fail because it cant find the dip in the stress-strain curve (a zero or negative slope).

To correct this situation, set up the test to use stress control during the first half of the elastic portion. Prior to

the onset of yielding, switch to either position or strain control. We have software packages that are designed

to allow for control transition.

* Refer to test standards such as ASTM E8, ISO 6892 or EN10002 for the allowable test rates

during yielding.

Tensile Testing


What to Consider When Measuring Plastics

When injection molding plastics, the outer surface of the specimen may collapse inward, causing it to form a

concave shape. Commonly referred to as "sink", it may cause variations in the specimen's thickness as a result

of the concave surface.

When a specimen exhibits sink, it is important to understand the affect it may

have on stress-based calculations, such as yield stress or modulus. Sink will

often cause the measured cross-sectional area to appear larger than what it

actually is and this can result in lower modulus, yield stress, and other stress-

based calculations.

Methods to measure cross-sectional area may vary depending on the standard

you are testing to.

Standards may specify different procedural requirements, as well as different

requirements for the measuring device itself. In some cases, it may not be possible to account for sink. For

potential solutions, view our animation.

Tensile Testing


How can I improve the accuracy and repeatability of my Poisson's Ratio results?

Poisson's Ratio? is the ratio of transverse strain divided by axial strain in

the elastic region of a uniaxial tensile test. It is a measure of how much of

a material contracts under tensile conditions, and is typically on the order

of 1/3 (0.3). Since the displacement associated with transverse strain can

be 10 to 12 times smaller than the displacement for axial strain (~4 times

smaller gauge length multiplied by ~3 times smaller displacement), the

accuracy of Poisson's Ratio is often limited by the accuracy of the

transverse extensometer.

Improving the accuracy and repeatability is best achieved by using a high-

resolution biaxial extensometer designed specifically for this purpose. View

our complete solution for testing Poisson's Ratio.

What is Poissons Ratio?

Ratio of lateral strain to axial strain in an axial loaded specimen. It is the constant that relates

modulus of rigidity to Young's Modulus in the equation:

E = 2G(r + 1)

Where E is Young's Modulus; G, modulus of rigidity; and r, Poisson's ratio. The formula is valid only

within the elastic limit of a material. A method for determining Poisson's ratio is given in ASTM E-

132.

Tensile Testing


The Best Solution for Gripping Low-Force Specimens

Low-force biomedical testing applications vary widely, and include specimens

such as native tissue, bio-engineered tissues, hydrogels, and contact lenses. In

most cases, these specimens are tested in a heated, fluid environment that

simulates physiological conditions; in other cases, the specimens are hydrated

for several hours before testing. Generally, most customers assume that rubber-

coated or serrated faces provide the ideal gripping

solution. But do they?

Rubber-coated faces tend to cause specimen

slippage, while serrated faces cause premature

failure.

A study conducted by the Instron Application Lab proved the best gripping

solution to be sandpaper or a grip surface called SurfAlloy, a surface that

resembles sandpaper. This slightly roughened surface provides enough friction to

prevent slipping, and not too much grit that could cause premature failure.

Tensile Testing


Why do I see a negative load after clamping my tensile specimen?

This is due to the fact that material is being forced out of the grip as a result of the squeezing, which can cause

a compressive load on the specimen, even with the best grip in particular for softer materials such as elastomers.

When the sample is clamped in one grip, there is no apparent load on the sample since it still has a free end.

However, when it is squeezed by the second grip, the material flows out of the grip, causing the specimen to be

in compression. This will show up as a

negative load on the readout before the test has begun.

If this is the case, you should NOT balance

out the load because the load you see is

real; balancing it would introduce error into

your test results. If you are experiencing

this, you need to move the machine's

crosshead to remove the compressive load.

There are two ways to do this:

1. Manually adjust the crosshead, for

example with a thumbwheel; or

2. Through software features, like the

preload function

Alternatively, we suggest using the load

protect feature, which limits the maximum

force applied to your specimen by

automatically ensuring the force on your

specimen remains within the pre-set

bounds. It removes the possibility of the

crosshead going into compression in real

time.

Tensile Testing


Indicating the Correct Gauge Length for Your Specimen

Understanding how specimen dimensions differ is important when setting up your calculations for a tensile

test. Most calculations are based off stress and strain, and since both are dimension dependent, it is

important to specify the correct values.

For specimens that have the same cross-sectional area from end to end (tubes, rods, rectangles and fibers),

the gauge length is determined by simply measuring the distance between the grip faces (refer to image).

However, the most common shape is the "dog bone" specimen (refer to image). Unlike the specimens

mentioned above, its non-uniform shape often causes mistakes in identifying the gauge length. When a "dog

bone" specimen is tested, most of the stretching occurs within the narrow region and not in the tabs because

they have a larger cross-sectional area. Since most of the stretching occurs within the narrow region, that

length should be used as the gauge length.*

*Note: There is a small amount of stretch within the tabs of the specimen. In order to get the most accurate

strain results, we suggest using an extensometer.

Tensile Testing


Grip Attachment Techniques

Do you have a loose connection between your

grips and the testing frame? If so, you could have

slack in your load train that can cause blips in

your curves, skew modulus data or alter

extension results. There are two solutions to

make sure you have a rigid connection. If your

grips are equipped with a check nut, make sure

it's tightened against the adapter, away from the

grip. We use the check nut consistently in our lab

because it's simple and

convenient. If you do not have

a check nut, an effective and

inexpensive solution is to place

a spring below the lower grip

inside the clevis adapter.

These techniques will remove

the slack in your load train and

ensure you're measuring true

extension and not the

movement of your grips.

Tensile Testing


Q: Why does the speed of tensile testing after yield vary from material

specification to material specification? In your opinion, is there a significant

difference in results?

A: In tensile testing, most materials are sensitive to the rate at which they are stretched, meaning some of their

properties are dependent on the rate of straining during the test. This effect is most noticeable after plastic

flow occurs, although some properties can be affected while in the elastic region. There is no intrinsically

correct strain rate for a given material, but to allow comparison of test results it is important for all tests to be

done within a range of rates. Test

standards define the range over

which results will be consistent

and therefore, comparable.

Tensile Testing


Q: Why do I see a negative load value

when I grip my specimen?

A: The closing action of wedge action grip jaws often

applies a compressive load to the specimen. If your

indicator is set to auto-zero at the start of the test, you

may see lower load values. Remove the auto-zero

function for the load channel to correct the low reading.

Another way to reduce negative load caused by wedge

action grips is to use the specimen protect feature

available on most newer Instron control systems.

Tensile Testing


Q: Why am I getting low modulus values from my test machine?

A: Elastic modulus values are affected

mainly by how strain is measured. Small

errors in strain measurement can result in

large errors in modulus values. If

crosshead extension is used for strain, it

will include small deflections of the

testing machine and the grips under load,

which are added to the specimen

elongation. This results in artificially large

strain values and lower-than-expected

modulus values. The most accurate

approach for modulus measurement is to

measure strain by applying an

extensometer directly to the gage length

of the sample, thereby eliminating errors

from other sources of deformation.

Tensile Testing


Protecting Our Environment: Reducing Waste in Landfills

More than 60% of the refuse going to local landfills is business/industrial waste. Much of the plastic from this

waste could actually be recycled. However, in order to recycle plastics, the materials must be recovered from

the items they are part of; and the many plastic types must be separated from non-plastic materials and from

each other.

MBA Polymers, Inc. is leading the

way with research, development

and even large-scale commercial

efforts in plastic recycling. MBA

Polymers, and its newly opened

manufacturing plants in China and

Austria, recover high-value plastics

from popular electronics such as

computers, televisions and even

automobiles. Using a proprietary

separation process developed over

the past 12 years by R&D Manager

Brian Riise and several colleagues,

MBA is able to remove non-plastic materials from complex durable

goods and recover purified streams of ABS and high-impact polystyrene

flakes. These flakes are then extruded into pellets; a process that

requires less than 10% of the energy needed to manufacture virgin

plastics. MBA then measures several common mechanical properties,

including tensile properties, using an Instron universal testing system.

"After the MBA separation process, we are able to create products with

mechanical properties similar to what you would find in a virgin plastic.

We tend to sell these pellets to customers who normally use virgin

plastics, including some very demanding electronic equipment

manufacturers," says Riise.

Photo courtesy of MBA Polymers, Inc.

Tensile Testing


The Invisible Rebar: Microscopic Nanotubes Dramatically Increase Material

Strength

One of the exciting new building blocks for very small systems is carbon nanotubes (CNTs). These single- or

multi-walled cylinders, made up of carbon atoms, are about 1/100th of the diameter of one piece of human

hair.

What makes CNTs attractive is that they are light (about 1/6 the weight of steel), strong (about 100 times

stronger than steel), electrically conductive (more conductive than Copper), thermally conductive and UV

absorbing.

A promising application for CNTs is nanocomposites, where

tubes are combined with another material (either an epoxy or

polymer). The CNTs behave much like fibers in wood or rebar

in concrete. The fibers are strong and make up most of the

strength, whereas the matrix holds the fibers in place and

makes it a usable material.

In 2004, Nanocomposites, Inc. licensed the Rice University

patented process for functionalizing CNTs, a process which

affects the surface of the nanotubes and makes them more

suitable for mixing with polymers.

The process dramatically reduces the CNTs tendency to stick

together, thereby allowing them to mix and bond with the

matrix, significantly improving mechanical properties. For

example, by adding treated CNTs to a rubber compound, Nanocomposites,

Inc. measured a 35% increase in ultimate tensile strength.

Additionally, 90% of the material's strength is retained at temperatures up to 400F (204C).

Nanocomposites, Inc. used an Instron electromechanical testing machine equipped with a video extensometer

to measure the mechanical properties of their materials at ambient and elevated temperatures.

Photo courtesy of Michael Strck

Compression Testing


A compression test determines behavior of materials under crushing loads. The specimen is compressed and

deformation at various loads is recorded. Compressive stress and strain are calculated and plotted as a stress-

strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and, for

some materials, compressive strength.

Compression Testing

Compression Testing


The Tower of Babel: Testing the Possibilities

The story of the Tower of Babel has fascinated scholars for centuries. The goal of the builders was to reach the

heavens. An ancient document called the Book of Jubilees mentions the tower's height as being 5433 cubits

and 2 palms, which is almost 2.5 kilometers (about 1.55 miles). That is certainly higher than any man-made

structure today, but is that possible? The building materials of the time

were simply bricks of mud and straw. So just how tall could the tower

have been?

Instron had the opportunity recently to meet with Professor Linn Hobbs

of the Department of Materials Science and Engineering at

Massachusetts Institute of Technology (MIT). Dr. Hobbs along with two

colleagues teaches the course Materials in the Human Environment that investigates the development of materials and technologies

through human history. As evidenced by the name, the range of class

teachings and projects is wide and our discussions included research

into brick and mortar construction, natural-fiber rope bridges in the

Andes, and whether the Egyptian pyramid blocks were cast in place

rather than quarried and then lifted into place. The well-appointed MIT

laboratories have several Instron test instruments that enable the

students to evaluate the capabilities of the materials that they produce

during the class.

To estimate the possible theoretical height of a brick-built tower, Dr.

Hobbs has his students manufacture bricks using clay, sand, and straw

folded together. Some bricks are sun-dried while others are fired in a

furnace, as they would have been as building technology advanced. A

series of empirical compressive tests on the bricks using an Instron

electromechanical testing system evaluates their individual strength

and from these values they can calculate the possibilities.

The sun-dried bricks withstand compressive loads up to 4000 lb/sq in.

A pyramidal structure built with these bricks and with a wide base to

spread the weight of the structure could reach around 1500 ft., or

around a quarter of a mile. However, a new technology had developed

that imparted much greater strength to the bricks; they were baked in

wood-fired furnaces. When baked bricks are compressed, they can

withstand 20,000 lb/sq in., which equates to a possible height for the

tower of 10,500 ft or almost two miles high. Thats around four times as high as the worlds tallest building, the Burj Khalifa in Dubai. Its also high enough to have given altitude sickness to any Mesopotamians

strong enough to reach the top!

The aim of this fascinating inter-disciplinary course is to teach

innovative thinking to our future materials scientists, civil and

construction engineers, archeologists, architects, and so on, through an

understanding of how materials and their uses and physical properties

have developed over time. It doesnt hurt that building walls, pyramids, and plant-fiber bridges is great fun as well.

Photo courtesy of MIT

Compression Testing


A New Hip Material

The first surgery to replace a damaged hip joint with an artificial joint was performed just 50 years ago. Today

more than 190,000 hip replacement surgeries are performed in the USA

alone.

During this time, there have been many improvements to the surgical

techniques and to the technologies and materials of the replacement

joints but inherent problems remain. One of these is the slow

deterioration of bone tissue around the prosthetic material due in part to

uneven load distribution between the prosthetic and the bone itself.

Dr. Afsaneh Rabiei, a professor of mechanical, aerospace, and biomedical

engineering at North Carolina State University has recently developed a

new composite metal foam material that offers, among many other

possibilities, the development of new hip joint prostheses that may

overcome this problem.

Artificial hip joints are usually manufactured using solid titanium, which is

many times stiffer than the bone into which it is secured. The implant

therefore assumes the majority of the loads exerted by walking and

running. Regular load-bearing exercise is an important factor in good

bone health. The bone around the implant, being now deprived of much

of the load, loses density and strength, a phenomenon known as stress

shielding. In time this deterioration, together with other changes due to

biological reactions with the cement used to secure the implants to the

bone, can cause the implant to loosen, resulting in the need for further

surgery to reseat or replace the joint.

Metal foams have been around since the late 1940s. Most are developed by introducing gases into molten

metal, which cools to form a matrix of thin-walled metal. However, the cellular structure is difficult to control,

leading to variations in cell wall thickness and random cell shapes and sizes. The resulting mechanical

properties of the material are unpredictable and inconsistent.

Dr. Rabieis composite metal foam material uses preformed hollow metal spheres. These are packed together randomly, and the spaces between

the spheres are filled with metal powder. The whole is then sintered to

form a sturdy composite structure. The foam displays superior

compressive strength and energy absorption capabilities as compared to

existing metal foams, while exceeding strength to density ratios.

The ability to control the size, the wall thickness, and the percentage of

spheres added to the matrix allows close control of the stiffness and

durability of the metal foam. The foam can therefore be manufactured to

closely match the stiffness of bone, thus eliminating stress shielding.

Other benefits of the new material are energy absorption, so they

cushion the shock of each step. The composites pores also provide places where natural bone can grow and anchor the implant in place.

The combination and predictability of these properties offers promise for use in other applications where light

weight, high stiffness and energy absorbsion capabilities are important, such as automobile crumple zones,

and structural members in air, naval, and space craft.

Photo courtesy of Arthritis Research & Therapy

Photo courtesy of Dr. Afsaneh Rabiei

Compression Testing


Recent Testing Uncovers Titanics Mystery

The Unsinkable Ship sank in less than 3 hours back in 1912. Did the Titanic sink simply due to the impact of

an iceberg and the speed of the ship or was it a malfunction in the mechanical property of a key material

holding the ship together?

A recent study, conducted by Tim Foecke of the National Institute of Standards and Technology (NIST), and his

colleagues, tested the rivets of the ship's hull; rivets that were made of wrought iron, not steel like the rest of

the ship's rivets. The one big difference: wrought iron tends to soften at lower temperatures.

Using a SATEC Series universal testing system, Foecke and colleagues simulated the ship's design with 2

pieces of 1-inch thick steel plates held together with wrought-iron rivets. Through a compression test, they

were able to simulate the force on the rivets and found that the rivet heads broke off, proving their

substandard quality. As the rivet heads popped, the steel plates separated, allowing water to pour into the

ship's hull at a very fast rate.

"If the wrought iron rivets were up to standards, they would have been fine," says Foecke. "But since there was

no method for quality checking, the rivets used on the Titanic were not up to standards, which caused them to

fail prematurely."

Compression Testing


From CO2 to Solid Rock

Did you know that each day we pump 70 million tons of CO2 into

the Earth's atmosphere?

Suzanne Hangx, M.Sc. of Utrecht University and her Dutch

colleagues at CATO are using Instron equipment in their research

to remove this greenhouse gas from the atmosphere. They are

studying CO2 Capture and Storage (CCS), a technology that may

provide 100 years of CO2 storage beneath the

Earth's surface. So how does CCS work? Below

the Earth's surface there is a vast volume of

storage space available through unminable coal

beds, depleted oil and gas reservoirs and aquifers.

CO2 is captured at power plants and pumped

underground into these storage spaces. As the

CO2 spreads through the reservoir or aquifer it will

partially dissolve into the present pore water,

which results in the formation of an acid fluid. This

fluid interacts with the porous rocks and causes the carbon to settle out through mineralization, resulting in a

stable, solid rock.

In addition, there are several organizations around the world performing CCS research, ensuring it doesn't lead

to undesirable sinking of the Earth's surface. In order to understand and quantify the effects of CCS, Hangx

performed constrained compression tests on granular CO2-injected rock samples using an Instron static

testing system and a special compaction vessel

"Our results show that geomechanical processes, like grain cracking, are significantly inhibited in CO2-injected

samples and geochemical effects are negligible on short time scales. Our testing is proving that CCS is a viable

and safe way to reduce greenhouse gas emissions," says Hangx.

Currently, there are a handful of CCS test sites around the world. With expanded implementation, CCS may

allow precious time to work on improving energy efficiency and using renewable energy sources.

Photo courtesy of Michael Strck

Hardness Testing


Simply stated, hardness is the resistance of a material to permanent indentation. It is important to recognize

that hardness is an empirical test and therefore hardness is not a material property. This is because there are

several different hardness tests that will each determine a different hardness value for the same piece of

material. Therefore, hardness is test method dependent and every test result has to have a label identifying

the test method used.

Hardness Testing

Hardness Testing


Increasing Efficiency in Knoop and Vickers Testing

Two of the most common hardness tests

are Knoop and Vickers that are used in

micro and macro testing. These tests

determine the material hardness based

on measuring the size of a diamond-

shaped impression from an application of

a force. The nature of the test process

typically dictates a relatively light force

application, resulting in extremely small

impressions that must be manually

measured. Traditional techniques involve

the use of microscopes with objective

lenses to manually measure through an

eyepiece. This is a time-consuming,

subjective, and potentially error-filled

process. Its not uncommon for a technician to manually produce and

measure hundreds of indentations during

a day which means that operator fatigue could compromise the measurements.

During the past several years, automated processes have become a more popular technique. What used to

take 25 minutes to test manually now takes 5 minutes to test with an automatic tester. Newer technology

eliminates much of the hardware that created operational challenges and cluttered workspaces. It typically

consists of:

Automatic rotating turret

Actuation in the Z axis for applying the indentation and for automatic focusing of the specimen

Automatic XY traversing motorized stage and USB video camera integrated to the test frame

Stage movement through a virtual joystick or stage controllers

Together, these produce a fully-automated hardness testing system. When loaded with samples and a stored

program, it can be left alone to automatically make, measure, and report on an almost a limitless amount of

indentation traverses.

"Using an automated tester is very useful. The biggest benefit to our lab is the amount of time it

saves us. What used to take us 1.5 weeks to test now takes us 2.5 days. Automated testing allows

for less human error and frees up time for the operator to do other jobs. Plus, it saves us money."

Dipak Patel, Prudential Steel

Hardness Testing


Best Practices: Which Rockwell Scale to Use

Rockwell hardness values are a combination of a hardness number and a scale symbol representing the

indenter and the minor and major loads. The symbol HR and the scale designation represent the hardness

number. The combination of indenter and test force make up the Rockwell scale. These various combinations

make up 30 different scales and are expressed as the actual hardness number followed by the letters HR and

then the respective scale. A recorded hardness number of HRC 63 signifies a hardness of 63 on the Rockwell

C Scale. Higher values and

Rockwell scales indicate harder

materials, such as hardened steel

or tungsten carbide.

The majority of applications are

covered by the Rockwell B and C

Scales for testing steel, brass, and

other metals. However, the

increasing use of other materials

requires a basic knowledge of the

factors that must be considered in

choosing the correct scale. The

choice is between the regular and

superficial hardness tests (a

lighter, 3 kg minor load test), and

the diamond and various carbide

ball indenters

The operator relies on the engineering specifications that are established at the material design. If no

specification exists or there is doubt about the suitability of a predetermined scale, refer to our Best Practices

article published in Industrial Heating Magazine.

Request a Hardness Wall Chart

Hardness Testing


Q: How far apart should I space each

Rockwell hardness test

A: Indent spacing is a common concern during

specimen testing or coupon block verification. The

purpose for these distances is to ensure that any

new indentation is not influenced by work hardening

of the materials edge or material around a previous indentation. The accepted criteria are that the

distance from the center of any indentation should

be at least three times the diameter of the

indentation. The distance from material edge to the

center of any indentation should be at least two and

one-half times the diameter of the indentation. Also,

the edge distance requirement ensures that the

indentation's area of contact permits proper

support.

Hardness Testing


Hardness Testing on Cylindrical Specimens

When performing hardness tests on cylindrical, convex, or concave surfaces, the

operator should understand that the actual results may be inaccurate due to the

curvature of the material. In most cases, these inaccurate results should be

accounted and adjusted for when reporting actual material hardness. Due to the

material curvature, several important factors may contribute to the invalid reading

including the actual material hardness, the applied force, the size and

shape of the indentation, and the diameter or radius of the test

piece.

However, there are many techniques operators should consider to minimize

errors.

Correction Factors

If the curved sample is used for material control purposes only, there may

be sufficient information and comparative data generated that allows operators to benchmark values and

processes. To make correction factors necessary, as indicated by ASTM, it is advisable to compare the

hardness of the rounded material with the hardness value of a flat piece. In a convex (curvature that extends

outward) or cylindrical piece, the reduction in lateral support will result in the indenter penetrating further into

the material which translates to apparent lower hardness readings. In this case a correction factor must be

added to the generated result. In

contrast to convex surfaces,

concave surfaces will provide

higher material support due to the

curvature towards the indenter

and result in apparently harder

material due to production of a

shallower indent. In this case, a

correction factor must be

subtracted. If the diameter of the

material is greater than 25 mm

the surface will provide suitable

surface structure for testing and

corrections are not required.

Lower diameter materials will

need the correction factor added

to the test result.

Proper Test Type

If the diameter of the material is smaller than 3.175 mm, Rockwell testing is not recommended. Instead,

operators should use a Knoop or Vickers test, which can accommodate lower diameters down to thin gage

wires. Most digital Rockwell testers provide the means to input the diameter of the curve. This input

automatically adds or subtracts a correct factor from the test results. In manual dial gage testers, ASTM

correction tables should be referenced to determine the correction factor. All corrections produce approximate

results and should not be expected to meet exact specification.

Hardness Testing


Hardness Accessories

Another important factor in testing curved material is

proper specimen support. The supporting anvil should

be selected to match the specimen geometry and to

ensure exact alignment of the indenter to the radius. It

should be rigid and provide full support to prevent

deformation.

ASTM E18 is a good reference for anvil selection. The

anvil must position the test specimen perpendicular to

the indenter. A V style anvil is ideal for supporting cylindrical parts. A cylindron anvil is suitable for larger

diameter parts. Elongated parts that extend beyond the

frame should be supported with a Vari-rest type fixture

to prevent part tilt or movement. Specialized anvils can

accommodate varying geometries and radiuses.

Hardness Testing


The Difference between a Knoop and a Vickers Test

Knoop and Vickers tests are used in micro and macro hardness testing to determine material hardness. It is

based on measuring the impression from an application of a force.

The Knoop test uses a diamond indenter ground to pyramidal form that produces a diamond shaped

indentation with an approximate ratio between long and short diagonals of 7:1. The depth of indentation is

about 1/30 of its length. When measuring the Knoop hardness, only the longest diagonal of the indentation is

measured. Originally the Knoop Hardness Number (KHN) was calculated by using this length and load in a

formula. Then, look-up tables became a popular source to find the KHN. Currently, most KHN results are

generated by digital measurement that automatically calculates the hardness number.

The Vickers test uses a ground squared pyramid. The depth of the indentation is about 1/7 of the diagonal

length. Unlike the Knoop test, when calculating the Vickers Diamond Pyramid hardness number, both

diagonals of the indentation are measured. The mean of these values used in a formula with the load

determines the Hardness Vickers value (HV). Similar to the Knoop test, tables of these values are available,

and the most current techniques utilize electronic or imaging measurements.

When choosing a test type you need to review the material, surface finish, geometry, thickness, uniformity and

other characteristics.

Q: What is the difference between a Knoop and a Vickers test?

A: Knoop and Vickers hardness scales are used for determining the hardness of a range of samples, including

thin materials or wires, coatings and small precision parts. In both cases, the hardness value is determined by

measuring the size of the indent and the test forces range from 1 g 100 kgf. They're defined by ASTM test methods E 384 and E 92. The Vickers indenter is a pointed, square-based diamond and the Knoop indenter is

a rhombic-based diamond.

Hardness Testing


How GR&R Helps Your Rockwell Testing Process

A GR&R study determines how much of the tolerance in your testing process comes from the variation in the

equipment and the operators. When operator error or equipment error becomes a significant portion of the

tolerance, it's hard to determine if the results are accurately measured.

Performing GR&R reveals a lot about how well your system is reading Rockwell hardness; provides insight to

potential problems; and determines if you need

additional testing, such as direct verification.

A study conducted on 30 testers used daily

showed that 90% failed a direct verification

even though they passed an indirect verification

using test blocks. These testers consume most

of the allowable tolerances.

Adjustments using test block verification do not

accurately characterize an instrument's

performance.

A full GR&R study involves multiple operators

performing 90 tests using Rockwell test blocks. The calculated results reveal the inaccuracy of the tester.

Acceptable GR&R values vary depending on the tester type (analog, digital, closed loop), as well as the quality,

condition, and calibration status of the tester.

Hardness Testing


Q: What is a Jominy test?

A: A Jominy test is a method for determining the hardenability of

steel. A test piece that typically measures 25 mm x 100 mm is

heated to a pre-determined temperature and quenched by a jet of

water sprayed onto one end. When the specimen is cold,

hardness measurements using the Rockwell HRC scale (10 kg

minor and 150 Kg major forces) are made at specific intervals

along the test piece from the quenched end. Test results are then

plotted on a standard chart. Hardness values are the highest at

the quenched end of the specimen. You should find that the

values decrease proportionally as you move to the other end.

We have found that using holding fixtures improves the accuracy

of our results. We recommend using automatic software and

stages to increase throughput. This setup accommodates from

one to several bars at once, and performs the tests at pre-

programmed intervals, while automatically plotting the data.

Hardness Testing


How Can Testing Strengthen Your Smile?

There are an abundance of commercials luring us to buy different toothpastes each promising a different outcome: teeth whitening, cavity fighting, or breath freshening. Although having that bright white smile is

appealing to most, without the strength of enamel even the whitest of teeth will still decay.

Established in the 1940s, the Indiana University School of Dentistry has been researching enamel strength for

nearly 70 years. One of the school's many groundbreaking findings includes the first successful stannous

fluoride formula, the active decay-preventing agent in Crest toothpaste. Since 1999, Dr. Domenick Zero,

Professor and Department Chair and Director, Oral Health Research

Institute, and his staff have been studying various oral treatments and

preventative methods to understand their effect on the hardness of tooth

enamel. Without strong enamel, teeth become soft and prone to decay.

"Conducting hardness tests on tooth enamel allows us to measure how

much demineralization, or breakdown, of the tooth enamel has occurred,

based on changes in the size of the indentation," says Dr. Zero. "The

larger the indent the more demineralization of the enamel."

Reversing this damage isn't an easy task. However, after more than

10,000 hardness tests, the group has proven that the breakdown of

minerals in enamel can be repaired by remineralization a process which is enhanced by fluoride and helps to

harden the enamel. Zero's

studies show that when the

tooth is remineralized, the

indents get smaller.

For his research, Dr. Zero

conducts baseline

microhardness tests on the

tooth enamel. Then, he places

the tooth specimens on

dentures (or a similar

appliance), which is placed in

the mouth, and worn throughout

the study. The specimen is

eventually removed, tested again

for microhardness, and compared

to the baseline test results.

"We're able to get answers much quicker and with much less expense than if

we've had to run a full clinical trial measuring tooth decay," says Zero. "It's a way of getting clinically-relevant

info without taking years to do the study."

Hardness Testing


ASTM E18-07: New Changes will Affect Your Rockwell Hardness Indenters

The latest changes to the ASTM E18 standard require suppliers to verify the geometry of indenters to meet

E18-07 compliance. This new requirement ensures that every Rockwell diamond indenter tip is verified for

correct cone angle and radius.

Why is this important?

The new standard ensures improved

performance of the indenters

throughout the testing range of

applicable Rockwell scales.

Old indenters verified to previous

revisions are not compliant to the

new standard and cannot be used

when testing to ASTM E18-07

unless they are verified and re-

certified to the new standard.

It's easy to verify if your indenters

are compliant to the revised

standard just view a copy of your indenter calibration certificate.

According to E18- 07, it is required

for the manufacturer to be ISO/IEC

17025 accredited by an

accreditation agency recognized by

the ILAC agreement. Examples of

such approved accrediting bodies

are NVLAP, UKAS and A2LA. The

compliance of your indenters can be

verifi

Documents

Materials Testing Guide