Crack-front shapes analysis

However, for PSA layers we need to introduce two modifications which complicate the analysis the adhesives are both viscoelastic and strained in the nonlinear elastic regime. In other words the term Gc will include a dissipative term and the term E should be replaced with a high-strain equivalent controlled by the nonlinear elastic properties as shown in Fig. 22.14 and 22.15. As a result, the crack front will not have the same shape as the classical interfacial crack and the exact nature of the stress distribution at the crack tip will be unknown. 

The geometry of a crack is very important to fracture mechanics analysis and will be considered carefully. Based on the experimental study, the surface crack is assumed to have a cone shaped crack face with a curved line as the crack front. The position of an element on the crack surface in figure 1 may be written as... 

In general, the geometry and loading encountered in three-dimensional crack problems is too complex for the stress intensity factors (SIF) to be solved analytically. The SIF calculation is further complicated because it is a function of the position along the crack front, the crack size and shape, type of loading and geometry of the structure. Therefore, in this study the current available numerical analysis tools are used to perform a fracture mechanics analysis. 

Energy is reflected by boundaries like conjunctions between materials, joints and open cracks or by the free surfaces. Their shape can be irregular and causes perturbations of the wave fronts. Aggregates and reinforcement bars have also a strong scattering effect on waves. The complexity of the recorded waveforms allows only the analysis of distinct parts of the signal, generally the interpretation of the first arrival of a wave. 

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