Fracture Toughness I

Presented by Carl ZieglerStork Testing and Metallurgical Consulting

Houston, TX

Fracture Toughness Part I

EnergyFracture is all about energy, the energy

needed to break the atomic bonds of the material and produce a new surface.

Fracture Mechanics is about using that to provide a method of calculating whether something will fracture!

Energy (ii)Early work showed that the theoretical

strength of a material was far superior to that actually achieved, and research was carried out as to why there was such a huge difference:

Calculated strength ~10,000 MPaActual Strength ~100MPa

Griffith’s Crack Theory Fracture Mechanics theory was invented during

World War I by the English aeronautical engineer, A.A. Griffith, to explain the failure of brittle materials. Griffith's work was motivated by two contradictory facts: The theoretical stress needed for breaking atomic

bonds is approximately 10,000 MPa. The stress needed to fracture bulk glass is around

100 MPa. He discovered that the reduction in strength

could be related to the size of defects in the material

TestingA simple experiment to show this: Take glass microscope slides and heat

them to just below melting and hold for an hour or so

Test the slide by bending and record the load to break it

Leave a slide for a few days and then test it the same way

Test a slide that has not been heated

ResultsThe loads required to break the slides

show significant variation with the freshly heated slide showing significantly higher load bearing capacity than the other slides. Although still nowhere near the theoretical strength, the heated slide can be 5-10 times stronger.

The results of the unheated slide and the one left for a period of time are very similar

Explanation The heated slide has flowed and filled small

surface cracks, presenting a material much closer to a uniform uncracked material, and needing more energy to break and create two new surfaces.

The untreated slide has a surface covered with fine micro cracks, which reduce the energy needed to produce the new surfaces.

The treated slide gradually gains surface cracks due to strains in the material.

Griffith’s Crack Theory (ii) Griffith’s theory was based around Brittle

Fracture: failure being sudden, no crack growth prior to the failure, and little-to-no ductility.

It also assumes an infinitely wide plate so that edge effects do not apply (edges cause ductility), and assumed an infinitely sharp notch.

This was not really appropriate to most engineering materials, which typically exhibit some ductility before failure.

Irwin's modification of Griffith's energy relation

Irwin, working out of U.S. Navy Research laboratories in WWII, expanded Griffith's theory to allow for the natural ductility of most engineering materials. This work assumed that the plastic deformation at the crack tip was small in relation to the crack length.

Later work by Irwin and others expanded the applicability of the theories to engineering materials.

Fracture Toughness Part II:

How it’s used

How it’s used Fracture mechanics testing derives a value that can be

used in design work to ensure that the fabrication does not fail by brittle fracture. Tied in with fatigue work and corrosion rates, it can allow a life (or a remaining life) to be assigned to a fabrication.

The nature of that value is dependent on the type of fracture toughness testing undertaken, but all values can be used for the same end result.

Note that Charpy testing is a form of fracture toughness testing, but its small size and high impact rate make its values more useful as acceptance tests rather than as design values.

TheoryKnow any 2, calculate the 3rd

Defect size Loading

Fracture Toughness

How it’s used (ii)

Known fracture toughness properties have allowed pipelines to be upgraded for higher production without resorting to costly new lines.

Structures found to contain cracks can be analyzed to predict remaining life spans, potentially saving on redundant repairs.

Pressure vessels can be designed on a “leak before break” principle, saving on costs and increasing safety.

The fracture toughness value calculated by any method can be used by engineers to provide data on the safety of a design:

From a Known ToughnessUsing a known toughness material – from

qualification tests possibly:The maximum stress condition a structure can

maintain with a known defect size can be calculated. Can be used in design or upgrading work.

The maximum size of defect a loaded structure can have in it without failure can be calculated. This can be used to set NDE acceptance requirements .

For Known Stress Conditions

The minimum fracture toughness required to prevent failure with a known defect size – an existing defect, or one extrapolated to a certain life.

The maximum size of defect that can be sustained for a known toughness

Using the calculated stresses from engineering calculations or measurements allows you to calculate:

From a Known Defect SizeUsing a known pre-existing defect or the

minimum detectable by NDE, allows you to calculate:The maximum load sustainable without

failure.The minimum fracture toughness value

required to prevent failure.

Fracture Toughness Part III

Specifications

Fracture Mechanics Specifications

US SpecificationsASTM E399ASTM E1290ASTM E1820ASTM B646ASTM E1152ASTM E813ASTM E1737ASTM 561Plus 117 more in ASTM

Non US SpecificationsBS 7448 pts 1 -4ISO 15853EN ISO 12737Plus over 200 other

referenced specifications

There are a multitude of specifications in use for Fracture Toughness testing covering structural steels, aluminum alloys, ceramics etc.

Specifications Used In the oil business, we deal mainly with ferrous materials,

but fracture toughness tests are used for pressure vessels (chemical and nuclear), aviation, structural etc.

Oil has primarily used the CTOD tests using SENB (single edged notched beam) samples.

R curve tests are carried out more often to give a value of toughness at the initiation of tearing, and is becoming more common.

K1C is not typically carried out, as few materials for offshore need that form of testing. It is, however, very common in pressure vessel and aviation fields with high strength low ductility materials

Specifications Used ASTM E399

For K1C tests, the earliest formal toughness specification

Testing uses the CTS sample. The specialist machining requirements of this

specification means that standard size samples are typically used., typically in ½” or 12.5mm gradations.

Specifications Used ASTM E1290

This specification has been changed and is now less relevant to the Oil industry with its highly ductile materials.

Specified in API1104 originally, but is limited now in Oil industry work due to the inability to derive δm values.

Current specification requires R curve determinations when a material does not fail by “C” or “U,” causing additional costs and time.

Specifications Used ASTM E1820

This specification, like E1290, seems to have been revised primarily to the aerospace and pressure vessel industries.

Current version has revised fatigue specifications which extend preparation to several days with increased expense. Used for R curves.

Specification no longer has a “M” value, but has a new “EOT” – end of test – value instead, which is the value when you stop the test and are no longer at maximum load, so it is dependent on when “you” stop.

Specifications Used BS 7448-1/2

The most commonly used specification for CTOD tests.

Part 1 covers base materials. Part 2 covers welded materials.Part 2 has requirements for notch placement

and validations, these have been seldom used in USA but are very common in European qualification work; current specifications from several oil companies are now requiring these requirements.

Fracture Toughness Part IV

What affects toughness

Plane Strain and Plain Stress Two terms that help explain some of the aspects

of Fracture Toughness that are intrinsic to the testing of material and defining their toughness values.

It should be noted that the effect of straining rate is not covered in detail here. Some materials show a strain rate dependence which can serve to effectively increase the yield point of a material. So, for the following discussions bear in mind that sudden impacts can make a difference to toughness properties.

Plane Strain A material in a plane strain

condition shows strains only perpendicular to the crack direction, with no strains along the crack direction.

This is most nearly attained in large sections with material either side of the crack preventing movement of the material.

Plane Strain conditions give the lowest Fracture Toughness values and typically produce brittle fractures

Plane Stress Loads across the crack produce a

displacement along the crack; this becomes more prevalent the closer to the surface and the lower the yield of the material (and is hence affected by temperature and material thickness).

Under Plane Stress conditions materials fail by a ductile mode.

This condition is most prevalent in oil industry engineering materials due to thickness and yields.

The Effect of ThicknessAs materials get

thinner, the amount of material under plane stress decreases, increasing the likelihood of a ductile failure mode

Fracture Toughness

Kc

Thickness

K1C

Plane Stress

Plane Strain

Mixed Mode

The Effect of Thickness Examination of a fracture

surface of a fracture mechanics test can show the extent of the plane strain and plane stress seen by the sample. The more flat, featureless area there is, typically the lower the toughness values, as more of the material is in the Plane Strain condition.

The Effect of YieldThe higher the yield of the material, the

closer to the surface you can be and still have a Plane Strain condition. Since the toughness of the sample is dependent on the amount of Plane Strain material, the more there is, the lower the toughness. This partially explains why materials get more brittle as they get colder.

The Effect of TemperatureAs temperature decreases, the toughness

of a material decreases. The extent of that change, and the temperature over which it occurs, varies from material to material. Some materials exhibit a sharp transition others a gentle change, while others show no distinct change at all.

The Effect of Temperature

Fracture

Toughness

Transition

Temperature

Brittle Fracture

Ductile Failure

Temperature

The Effect of Loading RateAs strain rates increase the toughness at

any temperature tends to decrease, the amount this happens is dependent on the materials.

The Effect of Loading Rate

The Effect of EnvironmentThe effect of environment on toughness is

seldom directly tested, although it can have a significant effect. For design work, corrosion, etc. is considered in the life calculations, and it is unlikely that a material susceptible to something like stress corrosion cracking or hydrogen embrittlement would be used in a structural application.

Testing Control Servo hydraulic machines were uncommon

when FM testing first started, so 2 different control methods were needed to allow both types of equipment to be used: Load – for hydraulic machines which could only

control a load change Displacement – for screw driven machines which

were optimal for displacement control.

Both control modes were used, however most tests now are in displacement control

Fracture Toughness Part V

Testing

Testing: Sample Size! The effect of Plane Strain on the toughness of a

material is the reason behind test requirements: To test a minimum thickness of material . . .

ensures that the value quoted is the minimum for that configuration. The value of thickness is usually related to the material in the most highly stressed areas, or those most likely to contain defects (e.g. weld necks). The thicker the material, the lower the value typically, although there is a minimum value for a material it may be at thicknesses above those that are used, or that can be tested, or at temperatures below those ever seen in service.

Testing: Sample Size! (ii)To test at a service temperature . . .

Again, ensures that the maximum Plane Strain content is tested to give the lowest results.

Often a requirement may have a temperature below the service temperature, this is used to cover thicker materials, strain rates, or safety factors.

Fracture Mechanics An appropriate sample is prepared:

for CTOD, the maximum size of sample possible is usually made.

for K1C, the size is typically standardized to a series of fixed dimension, ideally larger than the minimum thickness required to be tested.

By fatigue, the machined notch is extended to provide the sharpest possible notch tip as required by the theory.

Controlled loading of the sample is carried out, measuring the load and the mouth opening/load point displacement.

The relevant Fracture toughness value and its validity is calculated.

Fracture Toughness Part VI

Other tests

Other Fracture Mechanics TestsSeveral other fracture toughness tests

have been put forward for a variety of reasons. Many have not lasted, others have found niche areas, whilst others needed machine and measurement advances before they could be effectively used.

Other Tests (i)Charpy Correlations

A CTOD test is large and expensive, both as a test and in preparation of material, especially in the heavy section materials used in the 70s-90s. Correlations to impact toughnesses were published on a regular basis with great promises of the cost savings. None of these would do more than correlate a particular metal at a particular temperature range – normally upper or lower shelves. It never replaced CTOD and other full size tests

Other Tests (ii)Wide Plate testing

Using large section plates loaded in tension, a fast fracture was started at one edge of the plate and a running crack developed.

The test was too expensive for the offshore oil business at the time as it was only really available only at the R&D center that put the test up as a viable alternate to CTOD testing

Other Tests (iii)Single Edged Notch Testing (SENT)

Becoming more common, this test uses a tensile sample notched on one side, and tested in tension to provide a fracture toughness value.

Double Edged Notched Tensile (DENT)Like the SENT but with both edges notched,

the problems of cracking these really prevents their use.

Other Tests (iv)Circular Cracked Tensile

This uses a round tensile sample which is notched and has a circular fatigue crack introduced into the sample before testing. Problems with getting consistently valid cracks stopped this from becoming a cheap alternative to CTOD testing. A method was developed, but by that time CTOD had become the accepted test in the offshore oil business

End of Fracture Toughness I

Questions?

Part II to be continuedPart II of this webinar will cover specifics of

the main methods of testing fracture toughness samples.

Additionally:Validation will be described The metallurgical examinations to BS7448 pt 2LimitationsOther tests