State Of The Art In Crack Propagation

C. Timbrell, R. Chandwani, G. Cook

Journée Scientifique 2004: Les méthodes de dimensionnement en fatigue, Centre de Compétences Matériaux & Conception (CCM&C), Fribourg, Switzerland, Oct 27 2004

This paper discusses the issues involved in numerical crack growth prediction for general 3D cracks and describes the state of the art methods that are available to practising engineers. This is a wide ranging subject in which no single theoretical method is appropriate for all cases. Different approaches are adopted, for example, for crack propagation under static load, sustained load, fatigue load and impact load. The historical pedigree of the various approaches dictates the extent to which commercial software can provide practical solutions on a day-to-day basis. A brief overview is given of the relevant fracture mechanics parameters and their use in crack growth prediction under various load conditions.
The difficulties imposed by real-life problems are further compounded by the complex 3D geometries that are involved. These complexities may arise from general component shape such as turbine disk-to-blade connections or from individual geometric discontinuities such as chamfers or stiffeners. Further complications may be introduced from a variety of sources including residual stress effects, propagation along dissimilar material interfaces and propagation in non-metallic materials or metals which are non-homogenous, large grained or anisotropic.
A number of numerical approaches are discussed and the advantages and disadvantages of each are noted. Difficulties associated with growth of a general 3D crack front are considered in general and with respect to each method.
Of the various load types that may cause crack propagation, fatigue is the most advanced in terms of useable prediction capabilities. The current state of the art for fatigue crack propagation allows for growth of multiple non-planar defects through a 3D structure under general mixed mode loading. Stress ratio and load interaction effects may be included within the analysis. The most general integration schemes allow for proprietary crack growth models, including stress ratio and temperature dependency. In addition, the effect of a static load component such as residual stress may be included within the analysis. Sustained load damage (e.g. Creep) may be combined with instantaneous damage due to rainflow counted fatigue cycles.