Fracture Toughness

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In a recent case of component failure, the importance of the assumed material properties was highlighted. Material property in this case refers to the fracture toughness of a metal, and its dependence on various parameters. Unfortunately fracture toughness of a metal is not a rigorous unchanging ‘property‘ of the material, such as its melting point, coefficient of expansion, or density, but is dependent on other physical factors, not normally associated as related to material properties, such as size, temperature or loading rate.<\strong>

In the case in question, the effect of loading rate (or ‘strain rate’) on the materials fracture toughness was not considered fully, as it was assumed that the fracture toughness was universal and unchanging and not dependent on other physical factors such as loading rate. This is not uncommon and many material properties, such as yield stress, are dependent on factors such as composition, forming history and heat treatment, but independent of other variables.

In the case of ‘fracture toughness’ (or potential to fracture), there is also dependence on temperature, yield stress, size, and strain rate. While many physical properties are independent of dimension or stress rate, fracture toughness is unique in that the measured toughness IS also dependent on size (or section thickness), temperature and loading rate. This dependence is aptly illustrated in the well-known Charpy impact tests, where the material (Charpy) toughness is clearly a function of test temperature. For example, many steels are brittle at low temperatures but exhibit higher toughness and ductility at higher temperatures (or the so called ‘upper shelf”). Similarly, toughness is also inversely proportional to yield strength. Toughness also depends (unusually perhaps) on thickness, where thin sections are more ductile than thick sections which can fail in a brittle manner.

Of special interest in the case in question, which involved the failure of a structure during a full-scale impact test, were the effects of high strain rates associated with the impact loading. The effect is irrelevant in most conventional (slow) loading rates as the effect is small, and there is typically only about a 10% drop in fracture toughness over a whole decade of increased strain rate, which is seldom considered in practise. However, this effect does become significant if the strain rate change is high. In such cases, which include impact or explosive loading, the fracture toughness is reduced significantly. For example, a strain rate change of three to four orders of magnitude results in a consequent reduction in fracture toughness of 40 to 50%, and effectively brittle behaviour can occur. In such cases, the assumption that the fracture toughness is essentially constant (or about the same as more normal slow loading rates) breaks down, with significant and obviously deleterious consequences for the component.

It is important in considering new designs or high strain rate applications, that the appropriate values of fracture toughness be considered and incorporated, if the engineering system is to be considered fully functional. In the case in question, this phenomenon was overlooked, with catastrophic consequences for the whole engineering structure and costly test program.

The message is clear – be sure that your assumptions regarding material properties are indeed correct and are applicable at the strain rates relevant to the system, or bear the cost of sudden catastrophic failure!

This is the last Tech Tip of a very busy year where our tech tips have not been consistent – we trust you have found the tips informative and they have been useful in helping improve integrity/prevent failure. Please note we will be closed from 14 December 2023 and will be back on 4 January 2024