crack tip opening displacement ctod - a fracture mechanics test offered by element materials...

3/7/14 11:05 PMCrack Tip Opening Displacement CTOD - A Fracture Mechanics Test Offered by Element Materials Technology

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Crack Tip Opening Displacement CTOD - A FractureMechanics Test Offered by Element MaterialsTechnology

Topics CoveredBackground

Increasing Material DemandsCrack Tip Opening Displacement CTODThe CTOD Test Process

Sample Machining for the CTOD TestCommonly Used Test SpecimensTesting Nomenclature for the CTOD TestThe Importance of the Pre-Fatigue Crack

Pre-CrackingFatiguing the SampleInitiation and Propagation of a Fatigue CrackInitiation and Growth of a CrackTracking Crack LengthStress DistributionsCrack Front Examination

FractureMaintaining Constant TemperatureStress Intensity Factor

Data AnalysisCTOD Failure TypesPop-in FailuresCalculating CTOD Values

The Final Result

Background

Even the strongest and hardest-working materials on earth - pipes, drills, munitions, girders -crack. A crack is the beginning of failure, but when and how dramatically will the failure occur?

Increasing Material Demands

The world demands more-greater quantities of energy, taller buildings, longer pipelines, fasterand higher-flying aircraft. Material scientists have responded with stronger and more durablemetals, but every one, given the right set of circumstances, will crack.

We know more and demand more of materials than ever before. Rather than reacting toproblems as they occur, more and more industries are choosing to be proactive in failureprevention by testing materials properties beyond the normal testing requirements. One suchtest that goes beyond traditional material property analysis is the CTOD test, which is gainingpopularity in the oil and gas industry.

Crack Tip Opening Displacement CTOD

Crack Tip Opening Displacement test or CTOD is one of a family of fracture mechanics teststhat measures the resistance of a material to growing a crack. Similar tests (i.e., da/DN, K1C,KEE, and J1C) can determine fracture resistance of a material, but CTODis particularly suited

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KEE, and J1C) can determine fracture resistance of a material, but CTODis particularly suitedto pipeline and drilling equipment. The CTOD test is used to determine the fracture mechanicsproperties of ductile materials and can be thought of as the simulated opening of a pre-existing fatigue crack prior to fracture. The data that result from this opening can be used forcritical defect assessment, in which the critical defect size can be determined.

The CTOD Test Process

Please note that the following is a simplified version of the CTOD test process and does notcover all aspects of the test, such as personalized testing specifications.

A CTOD test can be broken into 4 main steps:

1. Machining of the test specimen (Sample Machining);

2. Fatiguing of the specimen within specified limits (Pre-Cracking);

3. Breaking of the specimen under controlled conditions (Fracture);

4. Post analysis of the specimen and resultant data to obtain the CTOD value (Data Analysis).

Sample Machining for the CTOD Test

Unlike other destructive material tests, the CTOD test has multiple factors that can affect theresultant value. Not only is the test temperature a variable, but the specimen size can alsoaffect the results, as well as the conditions in which the result can be used. It is important touse the maximum thickness of specimen possible when performing the test. As a general rule,if a material meets the TOD test requirements at a given test size, then the results can beextrapolated to apply to thinner sections, but not thicker.

Figure 1. Machining test samples for CTOD testing.

Commonly Used Test Specimens

For structural and pipe materials used in the oil and gas industries, the most commonly usedspecimens are a rectangular three-point bend or a square three-point bend. The rectangularthree-point bend is preferable, except where there is limited material or a surface notch needsto be evaluated.

Testing Nomenclature for the CTOD Test

As with other destructive material tests, the CTOD value can vary, depending on the directionof the test. The various testing specifications have their own nomenclature to describe thesample and notch direction in respect to the grain flow or weld direction. This nomenclature istypically the same as that of a charpy test.

The Importance of the Pre-Fatigue Crack

The calculation of the final CTOD value is dependent on the depth of a pre-fatigue crack fromthe surface of the specimen. As it is impractical to fatigue a crack from the actual specimensurface, the specimen is machined to include a notch, which will act as the initiation point ofthe fatigue crack and be included in the overall length of the fatigue crack used for thecalculation of the CTOD value.

National standards are used for the actual testing criteria.

Pre-Cracking

On completion of machining of the specimen, an actual fatigue crack is induced at the base ofthe starter notch. This crack must be of sufficient length to bypass any area of plasticdeformation that may have been occurred during the machining process. The crack length istypically based on the size of the sample, the method of notch manufacture, the width of the

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typically based on the size of the sample, the method of notch manufacture, the width of thenotch, the shape of the notch, and practical time restraints. The overall length from thesurface of the sample to the crack tip must fall within given parameters. Other factors thatalso must be considered include the angle of the crack in respect to the specimen and thedifference in length of the crack as seen on the exposed surfaces. The operation is typicallyperformed in air at room temperature.

Fatiguing the Sample

Fatiguing the sample requires a minimum and maximum fatigue load. If the loads selected toinduce the fatigue crack are too low, then the fatigue time may become restrictive-or, at theextreme, a crack may not develop. If the loads are too high, then a plastic zone may resultwhich would affect the CTOD result-or, again at the extreme, the sample may fractureprematurely. The national standards specify criteria to ensure a valid test sample, including: aminimum to maximum load ratio of less than 0.1, a change in stress intensity relating to themodulus of the material, and a maximum load based on the material tensile properties,specimen size and span used.

Figure 2. Fatiguing the CTOD test sample.

Initiation and Propagation of a Fatigue Crack

Initiation and propagation of a fatigue crack is dependent on the configuration of the sample,the crack length and the loading conditions. This relationship determines the stress intensityfactor (K) at the fatigue crack tip, and can be determined for a three-point bend by thefollowing formula:

[Where K is the stress intensity factor, F is the load, S the span, B the specimen thickness, Wthe specimen width and a the crack length].

Figure 3. Fatigue loading of the CTOD test sample.



During the fatigue operation, W, B and S remain constant. The equation demonstrates adefinitive relationship between the crack length, load and stress intensity.

Initiation and Growth of a Crack

To initiate and grow a fatigue crack for a CTOD test, various methods can be used.

a. Constant Load - A constant load amplitude (load) is the most common situation.However, for the purposes of growing a fatigue crack for a CTOD it is not the mostpractical. In order to meet the validity requirements imposed by the specifications, the loadratio selected would have to be determined based on the final crack length. As a result, thecrack growth rate would increase as the length increased; however, it would be slow in theinitial stages of crack length. This method can result in an extended time to grow the crackof the required length.

b. Decreasing Load - By calculating the load restrictions for a given crack length, it ispossible to start the fatigue operation with a high load and decrease it to the limitsrequired as the crack grows. Done carefully, this can save time in obtaining valid fatiguecrack front, but it should be noted that reducing the load by too great an amount canresult in the crack propagation slowing or even stopping. In this case, a given number offatigue cycles would be needed to initiate the crack again.

c. Constant K - During the fatiguing of a CTOD sample, S, W and B will remain constant. Assuch, the relationship between the crack length, load and stress intensity can be utilized inthe growth of a fatigue crack. From the equation it can be seen that by keeping the changein stress intensity constant, the load will drop proportionally as the crack length increases.

This method will result in an even load drop as the crack grows and will prevent the crackarrest that can occur when method b is used.

It is possible to combine aspects of the three methods to further increase to efficiencies of thecrack propagation. By starting with a high K and reducing it as the crack extends, one canreduce the time necessary to grow a crack while keeping within the specification requirements.

Tracking Crack Length

Tracking the actual crack length can be done in a number of ways, such as:

a. Visual measurement can be made on the sample. Using this method, only the crack lengthat the outer surface can be determined. To enhance the crack, non-destructive testingtechniques such as dye penetrant or magnetic permeability work well.

b. The compliance technique depends on a 5th order polynomial in which the coefficients arebased on the specimen geometry and material properties. Typically, a clip gage is attachedto the sample at the machined opening and electronically records the opening that is thenrelated to the crack length. The recorded length can then be used to automatically adjustthe load, based on the method decided for the crack growth, resulting in a smooth loaddrop.

c. The potential drop across the crack depends on ohms law: as a crack grows the potentialwill increase. As with the compliance technique, this method can be directly associated withthe load control and hence give a smooth load transition.



Figure 4. Tracking crack length of CTOD test samples.

While performing the fatigue operation, it is important to remember that only the outer surfacecan be measured and confirmed. The fatigue is propagating across a plane inside the sample,and as such the length cannot be visually confirmed until the test is complete and the samplefractured open. The compliance and potential drop techniques can provide information aboutthe internal situation of the fatigue crack.

Stress Distributions

Variance in length across the fatigue crack front increases in materials in which an even stressdistribution is not present, i.e. in a weldment. In these cases, various operations may benecessary to produce a linear crack front. Precompression of the sides of the sample andreverse bending are two of the most common techniques employed.

Crack Front Examination

On completion of the fatigue operation, the visible crack front must be visually examined toensure compliance to the specification, e.g. within length tolerances from the surface andbetween sides, straightness and the absence of any obvious surface bifurcations.

Figure 5. Examination of fracture surfaces can provide information about the type of failurethat has taken place.

Fracture

The actual breaking of the specimen is performed under monotonic conditions, which meansthat the sample is under increasing load until fracture, and at a static temperature.

Maintaining Constant Temperature

Fractures can be affected by temperature, therefore it is important to control the temperaturethroughout the test. Testing in a liquid alcohol bath with CO2 as a cooling medium is one of themost common methods to achieve this.

Stress Intensity Factor

The rate of testing is determined by the change in the stress intensity factor during the initialapplication of load. As was seen in the equation, the stress intensity is dependent on the loadand crack length. Since the crack length is not measurable until the sample is fractured, it isnot possible to confirm the actual testing rate until completion of the test. An estimated cracklength must be used to determine the testing rate-with the actual test rate confirmed to bewith in the validation limits.

During the application of the load, a clip gage is used to measure the opening at the mouth ofthe fatigue crack. This opening is plotted against the load applied.



Figure 6. Use of a clip gage allows crack opening width to be measured.

Data Analysis

After the sample has been fractured, scientists perform various operations to determine theCTOD type and value.

The fracture face must be examined in conjunction with the plot of the load vs. the crackmouth opening. From this, the type of fracture can be determined.

CTOD Fracture Types

Three main categories of fracture exist:

a. m in which the fracture face exhibits tearing and the final fracture occurs underdecreasing load

b. u in which the fracture face exhibits tearing and the final fracture occurs underincreasing load

c. c in which the fracture face does not exhibit tearing and the final fracture occurs underincreasing load

Figure 7. The various modes of failure during CTOD testing.

Pop-in Failures

A 4th type of failure can occur which is known as a pop-in. In this situation, either a load drop,a displacement increase, or both is observed, and the load then recovers to exceed the initialcondition. When a pop-in occurs, the material has partially fractured; however, the remainingligand is sufficient to withstand the increase in load. It is often possible to see the cause of thepop-in on the fracture face. The validity of the pop-in is evaluated based on the changes inload and/or displacement. If deemed valid, the final calculation of the CTOD value is based onthe load and displacement at the pop-in occurrence.

The length of the fatigue fracture and any tearing (in the case of a u type fracture only) shouldbe measured. The fatigue crack length is used in the CTOD calculation.

Calculating CTOD Values

From the plot, the maximum load and the plastic component (Vp) of the crack opening isdetermined for use in the CTOD calculation.

The CTOD value is calculated from the following formula:

where is the CTOD, F is the load, S the span, B the specimen thickness, W the specimenwidth, a the crack length, v the poisons ratio, Vp the plastic component corresponding to theload at the critical event, z is the clip gage height and YS is the yield at test temperature.

The Final Result

When the graphical data has been analyzed, the sample measured and examined, and the



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When the graphical data has been analyzed, the sample measured and examined, and theCTOD value calculated, the validity of the result must also be evaluated.

As discussed above, some of the validity requirements of the CTOD test cannot be determineduntil the test is completed. A value may be obtained, there may be a minimum value of CTODand/or type of fracture restrained, but, the test must also be valid. It is possible to have aresult with a sufficient value to meet the specification requirement, but still have an invalidtest. Similarly, your result may be lower than required with an invalid test. In these cases, theresult obtained should not be used and the test should be repeated.

Source: Element Materials Technology.

For more information on this source please visit Element Materials Technology.

Date Added: Apr 18, 2006 | Updated: Jun 11, 2013

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