Material Property Report

Introduction

There are three types of properties involved when selecting a material; which are chemical property, physical property, and mechanical property. Here, we will be focussing on mechanical property which deals with the force applied to the material. Property of material is very important when selecting a material for any applications. Because it is crucial to select the safest and most effective material for the application. We will be testing sample materials for its property through tensile testing, hardness testing, and impact testing.

Objectives.

1. Relate properties and applications

2. Effect of % Carbon on properties of steel

3. Study about the fracture mechanism of each materials

4. Relation between HR and TS of steel and their use

5. Application about tensile and impact result

6. Effect of strain rate on material

Pitichai Rajatawipat • ADME Section 6 • Chulalongkorn University

Theory

1. Strength

The strength of a metal is the ability to withstand the action of external forces without breaking. This property is inherent in the material itself and must be determined by experiment. Tensile strength, also called ultimate strength, is the maximum strength developed in a metal in a tension test. The tension test is a method for determining the behavior of a material under an actual stretch loading. This test provides the elastic limit, elongation, yield point, yield strength, tensile strength, and the reduction in area.

1.1 Tension test

To perform the tension test a specimen of the material is made into a standard shape and size. Before testing, two small punch marks are identified along the specimen’s length. These marks are located away from both ends of the specimen because the stress distribution at the ends is somewhat complex due to gripping at the connections. Example is shown in Fig. 1(a).In order to apply an axial load with no bending of the specimen. A testing machine like the one shown in Fig. 2 is then used to stretch the specimen at a very slow, constant rate until it reaches the breaking point. The Result is load required to maintain this uniform stretching.

Fig. 1 Typical steel specimen

A frequency interval during the test, data is recorded of the applied load P, as read on the dial of the machine or taken from a digital readout. Also, the elongation δ = L − L0 between the punch marks on the specimen may be measured using an extensometer. This value of δ(delta) is then used to calculate the average normal strain in the specimen. It is also possible to read the strain directly by using an electrical-resistance strain gauge, which looks like the one shown in Fig. 3. The operation of this gauge is based on the change in electrical resistance of a very thin wire or piece of metal foil under strain.

Fig. 2 Tensile testing machine


Fig. 3 Strain gauge

1.2 The stress-strain diagram There are two ways in which it is normally described.

1.2.1 Conventional stress-strain diagram The nominal or engineering stress is determined by dividing the applied load P by the specimen’s original cross-sectional area A0. This calculation assumes that the stress is constant over the cross section and throughout the region between the gauge points.

Nominal or engineering strain is found directly from the strain gauge reading, or by dividing the change in the specimen’s gauge length, δ, by the specimen’s original gauge length L0. Here the strain is assumed to be constant throughout the region between the gauge points. Thus,

1.2.2 True stress-strain diagram The stress and strain computed by using actual cross-sectional area and specimen length at the instant the load is measured are called true stress and true strain, and a plot of true stress and true strain is called the true stress-strain diagram. The plot is shown by the light line in Fig. 4. Both the conventional and true stress-strain diagram are practically coincident when the strain is small. The differences between the diagrams begin to appear in the strain-hardening range, where the magnitude of strain becomes more significant. In particular, there is a large divergence within the necking region. Here it can be seen that the specimen actually supports a decreasing load, since A0 is constant when calculating engineering stress, σ=P/A0. However, from the true stress-strain diagram, the actual area A within the necking region is always decreasing until fracture, σf’. Therefore, the material actually sustains increasing stress, since σ=P/A. The tensile test specimen also provides another property of metal known as its modulus of elasticity, also called Young`s modulus, E. This is the ratio of the stress to the elastic strain. It relates to the slope of the curve to the yield point. The modulus of elasticity is important to the designers and is incorporated in many design formulas.


1.2.3 Ductility (Ductile materials) The ductility of a metal is the property that allows it to be stretched or otherwise changed in shape without breaking and to retain the changed shape after the load has been removed. The ductility of a metal can be determined from the tensile test. This is done by determining the percent of elongation and percent reduction in area.

Percent elongation

Gauge marks are made across the point where fracture will occur. The increase in gauge length related to the original length times 100 is the percentage of elongation.

Percent reduction in area

Reduction of area is another measure of ductility and is obtained from the tensile test by measuring the original cross-sectional area of the specimen and relating it to the cross-sectional area after failure.

1.2.4 Brittle materials Materials that exhibit little or no yielding before failure are referred to as brittle materials. Gray cast iron is an example, having a stress-strain diagram in tension as shown in portion AB of the curve in Fig. 4.

Fig.4Stress‐Staindiagramforgraycastiron


1.2.5 Strain energy The energy related to the strains in the material is called strain energy. It is convenient to formulate the strain energy per unit volume of material. This is called the strain energy density.

Modulus of resilience When the stress reaches the proportional limit, the strain energy density is referred to as the Modulus of resilience (Fig. 8(a)). Physically a material’s resilience represents the ability of the material to absorb energy without any permanent damage to the material.

2. Hardness The hardness of a metal is defined as the resistance of a metal to local penetration by harder substance. The hardness of metals is measured by forcing a hardened steel ball or diamond into the surface of the specimen, under a definite weight, in a hardness testing machine. The popular machine is the Rockwell hardness tester, which utilizes a diamond that is forced into the surface of the specimen. Different loads are used to provide different scaled. Smaller loads are used for softer materials

Basics of Rockwell Hardness Testing

Fig.5


Basics of Brinell Hardness Testing

Relation of hardness to other material properties

3. Impact Resistance

Resistance of a metal to impacts is evaluated in terms of impact strength. A metal may possess satisfactory ductility under static loads but may fail under dynamic loads or impact.Impact strength is most often determined by the Charpy test. It is sometimes measured by the Izod test. Both types of tests use the same type of pendulum-testing machine. The Charpy test specimen is a beam supported at both ends and contains a notch in the center. The specimen is placed on supports and struck with a pendulum on the side opposite the notch. The accuracy and location of the notch is of extreme importance.

Impact Test

The impact test is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's toughness and acts as a tool to study brittle-ductile transition.


Fig.6

Charpy Impact Test

The test specimen is machined to a 10mm x 10mm (full size) cross-section, with either a "V" or "U" notch. Sub-size specimens are used where the material thickness is restricted. Specimens can be tested down to cryogenic temperatures.


Fig.7Impacttest

Experiment

Apparatus

Tensile test

The universal testing machine, Shimadzu Autograph AG-IS—100 kN, isused to perform the tensile test. The forces are obtained directly from load cell of thetesting machine. An extensometer with gauge length 50 mm is used to detect thedisplacement during the tests. The tests are controlled and the raw data of load anddisplacement are collected by using the computer programming.

Fig.9specimen


Fig.8The universal testing machine

Hardness test

The Avery Type 6402, Hardness Tester are used. The tests areperformed following the standard and the results are directly read from the machine.Check the calibration of the Rockwell Machines with Standard Calibration Test Blocksfor the scale selected.

Impact test

The AVERY type 6703 impact tester are used to determine the impacttoughness of steel and aluminum. The pendulum is used to strike the standard specimen. The impact toughness is directly read from the machine.


Fig.10Avery6402

Fig.11Avery6703

Description of Measurands and Measurement Instruments

Micrometer used to measure the sizes of specimensExtensometer for measuring displacement for tensile testNotch checked equipment and specimen setting equipment for impact test

Procedure

Tensile test

This experiment we used the computer to correct the data. The extensometer will measure the elongation data of material and the computer program will use this data to plot the graph. This lab we should be careful about the grip because this equipment is expensive and easily to broken.

1. Preparation of the test machine.

2. Measure the dimension and find the average dimension of each specimens.

3. Mark 8 points of 70 mm gauge lengths on the specimen (The failure should occur sway to the smaller diameter side.)

4. Set the computer by chose the information that wanted from the graph; example, Break force, stoke, yield point, and etc.

5. The capacity set to 100 kN, install data collected from each specimens.

6. Set the system to focus on the position of gauge marks to protect the damage happen to the extensometer.

7. Insert the specimen into the tensile testing machine by tight the top end to the load cell.

8. Connect an extensometer at the 70 mm marks of the specimen. (For the first use of the opening machine, the tensile testing machine must be calibrated.

9. Computer could set up the system.

10. Set the stoke value to be zero, the elongate value could be found correctly


11. Extension value also set to be zero, because the initial length is not extended.

12. Tight the other end of the specimen to the load cell.

13. Collect the value of the initial force.

14. Set the removal extensometer warning that suit to the material.

15. Start the test.

16. Immediately remove the extensometer after warning showed at the desktop of computers. This process you should be carefully because this equipment is very expensive.

17. The specimen fail then removed from the extensometer. This process will occur the noise sound when material.

18. Measure the final length of the marked points and measure the necking point.

19. Replacement of specimen.

*Note that only brass that necking will occur at the center of work pieces so you should connect the extensometer at center of work piece. Other work piece we connect extensometer at lower the center.

Hardness test

The hardness testing can be done by many methods for example Rockwell, Brinell,

Vickor which will give the different result. All of the results need to be comparing to the table of

each test to see the result. Our tests use the Rockwell to do the tests which have the different scale

due to the material that we use.

1.Clean the surface of the testing material by grinding

2.Use the sand paper to clean up the surface.

3.Calibrate the testing machine before the testing.

4.Measure the thickness of the testing piece.

5.Find the spot that want to press it.

6.Press the testing piece by applies the minor load which is 10kgf.

7.Apply the major load to the work piece and hold about 10-15 second due to the material and release the major load and read the value.


8. The pressing use the cell load to press, in each scale use different weight of cell load. The scale C is press by the total weight of 150kgf and scale B is use the total weight of 100kgf. We should use the suitable scale for each material. Check the scale should we used by the value of the hardness if it not in range 20-70 we change the scale.

9.We check 2 times for sure that the material in the suitable scale.

10.If the hardness over the range we change the scale but if it in range we continue the test.

11.The scale that we use is control by the indenter and the material properties and the specimen of the testing piece.


Impact test

The impact testing can be done by using the work piece which have the specimen that

require from the testing machine.

1. Check the specimen of the work piece.

2. Calibrate the machine.

3. Swing the hammer without any work piece then read the error of the machine and mark out so we can adjust with the result.

4. Put the hammer and the specimen into position.

5. Release the hammer to the strike specimen and reading the result and done forget to adjust it with the error of the machine.



Results

Tensile test

From the data acquired from the experiment, we can draw a stress-strain curve like the one shown in Figure 12. From the graph we can see that shaft steel has the highest yield strength. The material with the highest toughness is stainless steel. We can see the behavior of each material before it breaks too, the most exciting one is Brass which necking does not occur. The construction steel has the highest difference between yield and tensile strength and has the longest yield.

The Modulus of Elasticity can be computed by the slope of the elastic period. The yield point can be defined with the 0.2% offset. Calculations are available in Appendix E, values are shown in Table 1.

Table 1

Material Modulus of Elasticity (MPa) Yield Strength (MPa) Tensile Strength (MPa)

Aluminum 66.35 219.53 243.73

Brass 97.29 318.77 424.39

Construction Steel 210.01 486.04 678.22

Shaft Steel 172.29 669.93 858.55

Stainless Steel 178.78 617.12 722.63


Fig. 12 Stress-Strain Curve

Fig. 13 Stress-Strain Curve of Steel

*Note: Discussion of this graph is available in the discussion part


X

XX

X

X

X

X

Hardness test

The result of hardness test can be viewed in Table 2 below. For this experiment, we will be using two main scales; HRB and HRC for determining the hardness. We used scale HRH for Aluminum. We need to test first if the material exceed 20 on the scale, if yes you must use scale HRC if not we can use HRB.

And from the Appendix C we can convert HRB to HRC and even to HRN. The hardness of material can be used to convert to tensile strength from this table. We can compare the converted tensile strength to the ones tested by tensile testing in Figure 12. The % error of tensile strength is also calculated and shown in Table 2.

Table 2

Impact test

The result of Charpy impact testing is shown in Table 3. We have already minus 2 for

friction of the hammer. Temperature while testing is 29 degree Celsius.

Table 3


Material HRC HRB HRHConverted

tensile strength (MPa)

%error

Aluminum 99.50

Brass 68.00

Construction Steel 13.00 94.00676

**tensile test 678.22

0.30%

Shaft Steel 23.00 100.00814

**tensile test858.88

5.20%

Material Trial 1 Trail 2 Trail 3 Average Energy Lost (ft.lb)

Aluminum 18 22 19 21

Brass 12 13 14 13

Shaft Steel 26 24 28 26

Discussion

1. Materials properties and application

Aluminum: Aluminum is the most abundant metal in the Earth’s crust. It has high strength-per-weight ratio. Thus it is extensively used in application that requires low weight and good strength. Applications such as automobile parts, packaging (cans, foils, etc.), constructions (windows, doors, etc.), and CPU heat sinks are composed of aluminum.

Brass: Brass is an alloy of copper and zinc. It has high hardness and is water resistance. Applications using brass are water pipes, and musical instrument. The on the downside, brass is quiet expensive and allows less elastic movement.

Stainless Steel: Stainless steel has high oxidation-resistance in air at ambient temperature. It can be used in constructing structure but is not extensively used due to its high price. Applications where stainless steel excel are cookware, cutlery, surgical instrument.

Shaft Steel: As the name says, it is often used in producing shaft for high load power transmitting. Due to its hardness, it can operate under high load before plastic deformation begins.

Construction Steel: The construction steel has the greatest ability to absorb energy. Because in construction for safety, we can’t allow the concrete to crack. The steel also has high ductility for absorption.

2. Effect of % Carbon on properties of steel

The % carbon in steel has the ability to alter the steel’s properties. As we can see from the graph for instance, construction steel has lower carbon than shaft steel. As a result, the construction steel is more ductile than that of shaft steel. Also it is more harder than shaft steel. Thus, the more carbon presented in the steel, the steel becomes more brittle and more harder.

3. Study about the fracture mechanism of each materials

The study of fracture mechanism would allow us to determine the difference in fracture surface between brittle and ductile materials. By doing the tensile test, we saw that ductile material induces the “cup and cone” behavior. The ductile material failed by maximum shear direction of 45 degree. On the other hand, brittle material have a flat surface is seen in impact testing of brass. Brass, as a brittle material does not have necking while fracture, it breaks instantly. While ductile materials have necking and air gaps.


4. Relation between hardness value and tensile strength

The tensile strength of steel can be achieved by testing the hardness first to obtain the value of hardness then convert it to tensile strength through the conversion table. But for some material, there are no hardness value in the table, so we can not determine the tensile strength of it. The tensile strength obtained from the table if compared to the tested tensile strength achieved by tensile testing is appropriate and safe. Although there are some minor errors but the wide range of the curves are acceptable.

5. Application about tensile and impact result

From tensile test we can achieve the toughness of each material by determining the area under each curve. For example, the area of Brass is three times more than that of Aluminum. But in impact test, we achieve the toughness directly from the data. In impact testing Aluminum is two times more than that of Brass. The ratio change is caused by strain rate. Tensile test have low strain rate, on the other hand impact test has very high strain rate.

6. Effect of strain rate on material

Strain rate is the rate at which the material deforms. Material having high strain rate tends to be more brittle than those having lesser strain rate. Thus, brittle material have less toughness than ductile material. Each types of tests also induce the strain rate of material. Impact test causes the high strain rate of the material while tensile test causes low strain rate on the material. Although the decreasing in toughness caused by the strain rate is not the same in each material.


Conclusion

From the experiment, we are able to select the most appropriate material for a specified application upon knowing the material’s properties. We also know that the carbon content presented in the steel affects its ductility, the lower the carbon the higher the ductile. More over, we learned about the fracture of each type of material; ductile material tends to break in cup&cone shape while brittle material tends to break flat. We can also predict the tensile strength of a material from hardness test and the value is more safe than the ones gathered from the stress-strain curve. We can find the material’s toughness by using area under curve for tensile test and compare directly for impact test, strain rate is what determine the change in strain rate. And last, we learned that the strain rate are different in each type of tests; tensile test will induce low strain rate to the material while impact test will induce very high strain rate.


Appendix

Appendix A

Typical Application of Rockwell Hardness Scales

HRA....Cementedcarbides,thinsteelandshallowcasehardenedsteelHRB....Copperalloys,softsteels,aluminiumalloys,malleableirons,etc.HRC....Steel,hardcastirons,casehardenedsteelandothermaterialsharderthan100HRBHRD....ThinsteelandmediumcasehardenedsteelandpearliticmalleableironHRE....Castiron,aluminiumandmagnesiumalloys,bearingmetalsHRF....Annealedcopperalloys,thinsoftsheetmetalsHRG....Phosphorbronze,berylliumcopper,malleableironsHRH....Aluminium,zinc,lead.

HRK, HRL, HRM, HRP, HRR, HRS, HRV(Soft bearing metals, plastics and other very softmaterials.

Appendix B

Charpy impact test

A standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a given material's toughness and acts as a tool to study temperature-dependent brittle-ductile transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. But a major disadvantage is that all results are only comparative.

The apparatus consists of a pendulum axe swinging at a notched sample of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after a big fracture.

The notch in the sample affects the results of the impact test,[3] thus it is necessary for the notch to be of a regular dimensions and geometry. The size of the sample can also affect results, since the dimensions determine whether or not the material is in plane strain. This difference can greatly affect conclusions made.

The quantitative result of the impact test—the energy needed to fracture a material—can be used to measure the toughness of the material and the yield strength. Also, the strain rate may be studied and analyzed for its effect on fracture. The results can be used to determine the ductility of a material. If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile. Usually a material does not break in just one way or the other, and thus comparing the jagged to flat surface areas of the fracture will give an estimate of the percentage of ductile and brittle fracture.


http://en.wiktionary.org/wiki/standardized

http://en.wiktionary.org/wiki/standardized

http://en.wikipedia.org/wiki/Strain_(materials_science)

http://en.wikipedia.org/wiki/Strain_(materials_science)

http://en.wikipedia.org/wiki/Energy

http://en.wikipedia.org/wiki/Energy

http://en.wikipedia.org/wiki/Fracture

http://en.wikipedia.org/wiki/Fracture

http://en.wikipedia.org/wiki/Toughness

http://en.wikipedia.org/wiki/Toughness

http://en.wikipedia.org/wiki/Pendulum

http://en.wikipedia.org/wiki/Pendulum

http://en.wikipedia.org/wiki/Axe

http://en.wikipedia.org/wiki/Axe

http://en.wikipedia.org/wiki/Infer

http://en.wikipedia.org/wiki/Infer

http://livepage.apple.com/

http://livepage.apple.com/

http://en.wiktionary.org/wiki/quantitative

http://en.wiktionary.org/wiki/quantitative

http://en.wikipedia.org/wiki/Yield_strength

http://en.wikipedia.org/wiki/Yield_strength

http://en.wikipedia.org/wiki/Ductility

http://en.wikipedia.org/wiki/Ductility

Appendix C

Hardness Comparison Table


Appendix D

Rockwell Hardness Scale


Appendix E

0.2% offset graph

Appendix F

Elastic Modulus


Reference

[1] ASSC. Prof. Asi Bunyajitradulya, Engineering Mechanical Laboratory, 2009, p.p. 15-31

[2]http://www.auto-met.com/Rockwell_hardness_tester/images/ROCKWELL_HARDNESS_SCALES_chart.jpg

[3]http://www.azom.com/Details.asp?ArticleID=965

[4]http://en.wikipedia.org/wiki/Modulus_of_elasticity


http://www.auto-met.com/Rockwell_hardness_tester/images/ROCKWELL_HARDNESS_SCALES_chart.jpg




http://www.azom.com/Details.asp?ArticleID=965

http://www.azom.com/Details.asp?ArticleID=965

http://en.wikipedia.org/wiki/Modulus_of_elasticity

http://en.wikipedia.org/wiki/Modulus_of_elasticity

Contents

Introduction page 1

Objectives page 1

Theory page 2-7

Experiment page 8-12

Results page 13-15

Discussion page 16-17

Conclusion page 18

Appendix page 19-22

Reference page 23


Abstract

When designing a structure, an engineer should concern most about the safety of the structure. By knowing the property of each materials, engineers would have an advantage when designing a safe structure. Because the property of materials would allow engineers to predict the maximum load that would be allowed to the structure. Property of materials can be achieved by doing numerous test, for example tensile test, hardness testing, and impact testing. From these tests, we would be able to gather numerical data and to see how each materials react to certain tests.


Documents

Material Property Report