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Research project: Teardrop cracking: mechanism and design criteria

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The phenomenon of teardrop cracking has been observed to occur in high strength Nickel-base superalloys such as those used in high pressure turbine discs. Teardrop cracking results in the growth of cracks with a characteristic non-planar morphology under nominally mode I loading.

3D profile teardrop
3D profile teardrop

In particular, large 'shear' regions are observed, typically at 45° to the loading direction, extending in from the specimen free surface and enclosing a central planar 'teardrop' region nominally perpendicular to the loading direction. Such a crack morphology is difficult to explain in terms of conventional fatigue mechanics and leads to concerns about the accuracy of crack growth rate data and predictions of crack trajectories in components. Past work has established certain dependencies between the degree of 'teardrop formation' and the material and loading parameters, e.g. grain size, test temperature, specimen geometry etc, but no mechanism has been determined.

Teardrop crack reconstruction
Teardrop crack reconstruction

Although historically the effect has been observed largely during specimen testing a number of cracks of complex morphology have now been observed on component testing. There is therefore a pressing need to understand the occurence of this phenomena and to develop criteria allowing the assessment of likelihood of teardrop cracking and its effect on component fatigue life during the design process. The ultimate goal is to develop an understanding of the mechanism of teardrop cracking and deliver assessment methodologies that can be incorporated into the component design and lifing process.

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