Job Knowledge 76 Ctod

7/29/2019 Job Knowledge 76 Ctod

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Mechanical TestingJob knowledge76:

CTODTestingThe concept of fracture toughness was introduced in an earlier Connectarticle,

Job knowledge 71 , which discussed the Charpy-V test, a simple qualitative testthat gives only an indication of the toughness of a metal.The next few articles will look at the tests that enable fracture toughness to be

accurately measured in a quantitative manner by using a full size specimen

containing a crack with loading that is representative of service conditions.This allows a fitness-for-purpose analysis to be carried out which enables a critical

defect size to be calculated. Thus, prior to fabrication, realistic acceptancestandards can be set and decisions on appropriate NDE techniques and detection

sensitivities can be made.For equipment already in service, it is possible to justify the continued use of

cracked or otherwise flawed components until such time as repair or replacement

can be effected. Such engineering critical assessments can save an operator largeamounts of time and money, running into perhaps hundreds of millions of pounds

in the case of an oil rig for example. Whilst the Crack Tip Opening Displacement(CTOD) test was developed for the characterisation of metals it has also been

used to determine the toughness of non-metallics such as weldable plastics.The CTOD test is one such fracture toughness test that is used when some plastic

deformation can occur prior to failure - this allows the tip of a crack to stretch and

open, hence 'tip opening displacement'.Unlike the inexpensive 10mm by 10mm square Charpy-V test piece with a blunt

machined notch, the CTOD specimen may be the full thickness of the material,will contain a genuine crack and will be loaded at a rate more representative of

service conditions. Conventionally three tests are carried out at the relevanttemperature to ensure consistency of results.

The test piece itself is 'proportional' - the length, depth and thickness of each

specimen are inter-related so that, irrespective of material thickness, eachspecimen has the same proportions.

There are two basic forms - a square or a rectangular cross section specimen. Ifthe specimen thickness is defined as 'W', the depth will be either W or 2W with a

standard length of 4.6W. A notch is machined at the centre and then extended bygenerating a fatigue crack so that the total 'defect' length is half the depth of the

test piece- see Fig.1. A test on a 100mm thick weld will therefore require a

specimen measuring 100mm wide, 200mm deep and 460mm long - an expensiveoperation, the validity of which can only be determined once the test has been

completed.

Fig.1.

Proportiona

lrectangularcross

section

CTODspecimen

The test is performed by placing the specimen into three point bending andmeasuring the amount of crack opening. This is done by means of a strain gauge


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attached to a clip placed between two accurately positioned knife edges at the

mouth of the machined notch ( Fig.2)

Fig.2. Typical test

arrangement. Thespecimen can be

easily immersed ina cooling bath

As bending proceeds, the crack tip plastically deforms until a critical point isreached when the crack has opened sufficiently to initiate a cleavage crack. This

may lead to either partial or complete failure of the specimen. The test may be

performed at some minimum temperature eg the minimum design temperature or,more rarely, at a range of temperatures.As a rule of thumb, a CTOD value of between 0.1mm and 0.2mm at the minimum

service temperature is regarded as demonstrating adequate toughness.

The values that are required for the calculation of toughness are firstly the load atwhich fracture occurs and secondly the amount by which the crack has opened at

the point of crack propagation ( Fig.3).

Fig.3. Position of CTOD specimen

immediately prior to crack propagation

Since the length of the crack and the opening at the mouth of the notch are

known it is a simple matter to calculate the crack tip opening by simple geometry.Whilst the test is in progress the results are recorded automatically on a

load/displacement chart that is similar in some respects to the tensile test curveillustrated in Fig.3 inJob knowledge 69.


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The CTOD curve is a plot of stress versus strain ( Fig.4). This illustrates the

various shapes of curve that may be produced - (a) is a test where the test piece

has fractured in a brittle manner with little or no plastic deformation. (b) exhibitsa 'pop-in' where the brittle crack initiates but only propagates a short distance

before it is arrested in tougher material - this may occur several times giving thecurve a saw tooth appearance or after this one pop-in deformation may continue

in a ductile manner as in (c) which shows completely plastic behaviour.

Fig.4. Load vs crack opening

displacement curves showing threetypes of fracture behaviour

The location of the notch in the weld HAZ or parent metal is important as anincorrectly positioned fatigue crack will not sample the required area, making the

test invalid. To be certain that the crack tip is in the correct region, polishing andetching followed by a metallurgical examination are often carried out prior to

machining the notch and fatigue cracking. This enables the notch to be positioned

very accurately. This examination may be carried out after testing as furtherconfirmation of the validity of the test results.

Once the sample is broken open the crack surface is examined to ensure that thefatigue crack has a reasonably straight front. The residual stresses present in a

welded joint may cause the fatigue crack front to be irregular - if this is excessivethe test may be invalid. To overcome this problem the test piece may be locally

compressed at the machined notch tip to redistribute the residual stress.Two depressions each side of the sample can often be seen where this

compression has been carried out. The fatigue cracking itself should be carried

out using a low stress range. The use of high stresses to speed up the fatiguecracking process can result in a large plastically deformed area ahead of the

fatigue crack and this will invalidate the results of the test.


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Other causes of test failure can unfortunately only bedetermined once the test has been completed and thecrack surface examined. The precise length of thefatigue crack is measured - this is required for theanalysis - but if the length of the crack is not withinthe limits required by the specification the test isinvalid. If the fatigue crack is not in a single plane, ifthe crack is at an angle to the machined notch or if thecrack is not in the correct region the test will need tobe repeated.

Related specifications

BS 7448 Parts 1- 4 Fracture Mechanics Toughness Tests

BS 6729 Determination of the Dynamic Fracture

Toughness of Metallic Materials.

BS 7910 Guide on Methods for Assessing the Acceptability

of Flaws in Metallic Structures.

ASTM E1820 Standard Test Method for Measurement of

Fracture Toughness.

This article was written by Gene Mathers.

Copyright 2005 TWI LtdInformation and advice from TWI and its partners are provided in good faith and based, where appropriate,on the best engineering knowledge available at the time and incorporated into TWI's website inaccordance with TWI's ISO 9001:2000 accredited status. No warranty expressed or implied is givenregarding the results or effects of applying information or advice obtained from the website, nor is anyresponsibility accepted for any consequential loss or damage.

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Job Knowledge 76 Ctod