Fracture Toughness IPresented by Carl Ziegler Stork 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

Griffiths Crack TheoryFracture 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 soTest the slide by bending and record the load to break itLeave a slide for a few days and then test it the same wayTest 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

ExplanationThe 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.

Griffiths Crack Theory (ii)Griffiths 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 relationIrwin, 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 its used

How its usedFracture 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 3rdDefect sizeLoadingFracture Toughness

How its 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 ConditionsThe 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 SpecificationsUS SpecificationsASTM E399ASTM E1290ASTM E1820ASTM B646ASTM E1152ASTM E813ASTM E1737ASTM 561Plus 117 more in ASTMNon US SpecificationsBS 7448 pts 1 -4ISO 15853EN ISO 12737Plus over 200 other referenced specificationsThere 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 specificationTesting uses the CTS sample. The specialist machining requirements of this specification means that standard size samples are typically used., typically in or 12.5mm gradations.

Specifications Used ASTM E1290This 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 m 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 E1820This 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 StressTwo 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 StrainA 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 StressLoads 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

The Effect of ThicknessExamination 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

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 ControlServo 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 MechanicsAn 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 CorrelationsA 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 testingUsing 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 TensileThis 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

The whole basis of fracture mechanics is the need to provide the energy required to turn a solid into two separate pieces. The energy needed to produce two new surfaces depends on the material and other factors. This energy in these cases is supplied by straining the material putting potential energy into the system which is converted during the new surface formation.

The theoretical strength of a material assumes no defects , cracks and an infinitely long material with no edges to provide problems with a simplistic analysis. Materials have none of these.Griffith tried to adjust fracture mechanics calculations to materials with some ductility. Note however that these equations till have limitations on sample sizes etc. some of which are transferred into restrictions during FM testing very simple test showing how cracks markedly affect the ability of a material to withstand loadModified theory with addition of ductility to the material, for materials much closer to real life engineering materialsFracture toughness values are used to predict failure in structures based on loads, defects and the material constant TOUGHNESS. As costs increase the old method of making things bigger to last longer is not financially viable, it can also have the opposite effect, heavier materials may actually cause failures from other methods than overload. Like an equilateral triangle, knowing any two sides allows the third to be calculated. In this instance the relationships are between the fracture toughness of the material (actual or calculated), the size of a defect and the loading seen by that defect.A company wanted to increase the oil/gas export from A to B, by using fracture toughness determinations they were able to increase the pressure in an existing pipeline giving high flow without having to build a complete new pipeline, saving large amounts of money and timeThis assumes that the toughness of the material is known either from testing at qualification, or from coupons removed from the structure. The job previously mentioned had toughness measurement determined from left over qualification material kept in the yard several years after the qualification work was carried out.From the actual design stresses and estimates of minimum detectable crack sizes a minimum required toughness can be specified for the procedure. This is using critical assessments to drive qualification requirements, rather than a value grasped from NDE has limitations on the size of defects it can find, based on the geometry of the joint, the material types etc. If you know the minimum size of defect you can detect and size accurately, with a known loading condition this can be used to determine the minimum fracture toughness that the structure must be made from so as to not fail.

There are a multitude of toughness specifications out there, in oil field work they are typically BS7448 pts 1 and 2 and ASTM 1290. The BS standard is currently the only one that covers the results typical of oil industry materials, in that 1290 has no provision for Delta M values and is reliant on the sample failing before plastic collapse.

We primarily carry out CTOD testing, with only occasional J1C and K1C testing,

The K1C specification only, the same requirements in this are in all of the other specifications listed below, however 399 does not allow for any other than fully brittle fracture testing.

Any brittle fracture showing little ductility should be assessed against the requirements of K1C validity, however the higher fatigue loads allowed in the CTOD specifications means that most brittle fractures could never be a valid K1C anyway.The requirement to carry out R curves for samples failing by plastic collapse M values makes this potentially very expensive and slow for our clients.

We do test to the e02 specification if required as it allows M values.

Most of or client specifications stipulate a M value or a minimum toughness which cannot be determined form this specification

Recent changes to this makes cracking very slow, with rates 8-10 times slower. It does mean that for low toughness materials that K1Cs can be determined form the same test, however since most Offshore engineering materials do not show brittle behavior then this is redundant.

Most of or client specifications stipulate a M value or a minimum toughness which cannot be determined form this specification BS 7448 is the UK standard on which all other CTOD standards is based, it includes specific tests for welded joints Part 2. The metallurgical examinations are based on work carried out by labs other than the Welding Institute on materials for the North Sea after the introduction of Controlled Rolled steels and similar The two terms which cover the possible conditions of strain at a crack tip, plain strain gives the lowest toughness as all strain is perpendicular to the crack and there is no lateral displacements to absorb additional energy. The strain is effectively perpendicular to the crack front and the bonds are in simple tensionPlane stress has a component of strain along the crack front meaning that some energy is used to move material and not just break the bonds so the measured toughness is higher. Only very thick materials or very wide materials show anything other than predominantly Plane stress conditionsAs thickness increases the ability to hold load is higher but the chances of brittle failure at the same stress is also higher, leading to a lower toughnessNearly all fractures show shear lips at the edge, even if they are very small. The extent of shear shows how brittle a material is, low shear - low toughness.A high yield does not mean a high toughness, neither does a low one, the ratio of the yield to the ultimate is more import. A low ratio is more likely to be tough than a high one, although like all rules of thumb, they can be wrong. A long elongation on a tensile is typically a good indicator of toughnessEffectively as the temperature lowers the yield to ultimate ratio increaseAs the strain rate increases you get a similar increase in yield to ultimate ratio, but it is not enough on its own to give the variations noted. High strain rates give less time for strains to be redistributed enforcing a larger effective plane strain component and lower toughnessEnvironment can have a major effect on how fast a sample fails due to the effects of hydrogen, corrosion etc. In few cases does the environment itself effect toughness directly. Even liquid metal embrittlement takes a definable amount of time to become apparent. These affects must be considered in other aspects of designAs in many other cases, with two control methods, people did research programs into the differences and it was found that there was a difference in the fastest load control against the slowest displacement control speeds. Other researchers indicated differences in Pop in behavior. Little if any notice was taken regarding the response rate of the machine and the stiffness.

Pop ins are going to be more prevalent in displacement control as the sudden crack extension relieves loads dropping the driving force, a slow load controlling machine will also indicate pop ins more readily than one with a potentially higher piston travel speed. It doesnt really take a research program to see this. Displacement testing was chosen as the way for CTOD testing, this effectively reduced the amount of testing that could be carried out in a day. Under load control it could be possible to test 60 samples in a day, under load control 10+ is difficult especially for very high toughness materials.

Control would possibly be best related to service, most Structural items have displacement constraints, while pipelines and pressure vessels are more controlled by load. These variations have never been implemented.

Sample sizes are always stated as minimums. Often this is the critical high stress section dimension and is related to the design or fabrication.

Other areas may be significantly thicker and have potentially lower toughness's but are less susceptible to crack formation.

Fracture design depends on stress, defect, and toughness, if there is no defect or its effect is minimal than a lower toughness is not a problem.Material tested at room temperature does not give the same results as material tested at sub zero or elevated temperatures. The effect of temperature can be dramatic:

A thick gas manifold gave perfectly acceptable results at the normal minimum operating temperature of -20C, equivalent to a a cold winters day. However this was a gas line and it was decided to test the material at the temperature that would occur if the gas line was blown down, re pressurized and then blown down again -55C!

The 5 thick CTOD samples failed by completely brittle fracture throwing the 5x5x12 samples out of the frame completely. The operating procedures for the plant had to be changed to prevent the system dropping below -40C a substantial cost penalty if there was a problem.No matter which test is being carried out the principles are the same:Prepare the sample to the dimensions and tolerances of the specificationsA pre-machined notch is extended to a final length by fatigue cracking using loads determined by the relevant specification, this produces a crack with a tip radius approaching 0 which is one of the basis of the theories.Under a controlled loading rate the load verses opening of the sample is plotted typically this is the opening of the top of the sample, but may be any one of several other measurements possible

Using values determined from the plot a value of toughness is derived. Typically there are validity requirements on various aspects of the tests that need to be met.

These are mentioned just to allow you to know they existThere have been large numbers of correlations published over the last 30 years, some work for limited ranges but others are less useful. The rough correlation that good Charpy values give good Fracture Toughness values is typically true, however like all such rules there are instances when it is not true or even inversed. During an investigation at a research lab I worked in we compared a recently published (30 years ago) paper giving a correlation, we compared over 200 sets of K1C values and didnt get a single match!A very expensive test, requiring large equipment and large test samples, good for research possibly but of little use for qualification and routine testingSENT has been a growing test in the oil business, however some of the largest exponents of the test have recently gone back to CTOD and R curve tests due to the lack of a consistent standard to work too. There are two test methods and it is obvious from their geometries that they would give differences in values.

The DENT came out 30 years ago and the problems existed then of giving acceptable cracks. It may be good in uniform high quality base materials but as soon as you add material variability form welds etc, it becomes much more difficult.The test provided a fully plain strain test sample as the circular crack effectively provided the infinitely long crack with no edge effects to provide plain stress areas.