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Fracture Toughness and the Grey Area Between Pass and Fail

Tue, 20 August, 2019

It’s straightforward for hardness limits, or strength requirements - the material test results either meet the required values or they don’t. But, when it comes to fracture toughness, the complexity of the test can leave room for confusion when trying to meet a given crack tip opening displacement (CTOD) requirement.

An Industrial Member recently got in touch with TWI to discuss just such a concern. A project specification required a CTOD of 0.35mm to be met at -10°C in a thick section steel weld. While two of the three fracture toughness tests comfortably met this requirement, the third result was a little below the required value. What advice could TWI offer this engineer to go back to their client and help accept the welds? Or else, what could be done to improve the fracture toughness?

Fortunately, TWI not only has significant experience in performing these tests within our extensive test labs, but also carries out research and development of fracture test methods, often in support of test standards, on which TWI staff are represented on both the UK and International committees. We could think of several things to help!

Fracture toughness testing involved loading a bar of metal, containing a notch that has been sharpened by growing a fatigue crack to the required depth, until the point of failure. The fatigue crack shape can be affected by residual stresses in the weld and, if the shape is sufficiently irregular, it can make the result outside the limits of the test standard. Published papers by TWI engineers can help quantify how the toughness might change for specimens with excessive pre-crack curvature, or where the pre-cracking forces are higher than the standard permits, or when testing a weld mismatch higher than within the range of the standard. This research, usually supported by TWI’s Core Research Programme, is done so that we can help our Members with just such issues, backed up with quantitative evidence.

In the case of the current enquiry, the tests had met the requirements of the standard. The next approach was to look at the load versus displacement traces from the tests and match them to the appearance of the fracture surfaces. The three results had all exceeded maximum load and were fully ductile, indicating the welds were on the upper shelf of the ductile to brittle transition curve at -10°C. By outlining the significance of this to the enquirer, it provided a good basis for them to go back to their client. The CTOD requirement in a specification is often to avoid the risk of brittle fracture of the structure in service, but the actual CTOD value required can be rather arbitrary. The upper shelf toughness is ultimately limited by the steel’s tensile properties when it tears and yields, rather than the initiation of a brittle crack. This weld was not going to fracture, but it also wasn’t going to achieve much higher values of CTOD, even at warmer test temperatures. The assurance that the client was seeking in the weld’s performance might still have been achieved despite the actual value of CTOD being a little too low. A thirty minute discussion with TWI might have prevented the engineer from having to scrap the entire fabrication and start again.

But what if their client still wasn’t convinced? With fracture toughness data, the next stage could be to perform a fitness-for-service calculation, using procedures such as those in BS 7910 in TWI’s CrackWISE5 software. The worst-case combination of maximum service stresses, the upper flaw acceptance limit, and the lowest value of CTOD could all be combined to determine whether the structure would still nonetheless be safe from failure. Sometimes the CTOD ‘requirement’ in a specification can be rather arbitrary, instead of being based on fracture mechanics calculations to determine value of fracture toughness that is actually necessary for the safety of the structure.

TWI’s experts in fracture, from providing practical help with performing the tests, through to guidance with data interpretation and support with fracture mechanics calculations, help our Members navigate their way when fracture toughness results fall into these ‘grey areas.’

If you want to know more about fracture toughness testing to help with your own work in specifying, performing or interpreting fracture toughness tests, why not come along to TWI’s Industrial Members event on fracture toughness testing on 30-31st October?

As well as TWI experts offering classroom sessions covering Standards, specimen preparation, post-test metallography and R-curve analysis, delegates will be shown the range of testing facilities at TWI’s Granta Park facility.

For more information please email:


contactus@twi.co.uk