Échelon cracks in soft solids (Vol. 45 No.3)

Stepped crack surface developing from a straight notch (dashed line).

While under pure tension loading, crack surfaces are usually planar, whereas under superimposed shear they generally exhibit steps. Explaining the emergence of this ubiquitous instability remains a challenge in fracture mechanics. We study it here for a highly deformable solid (a hydrogel) and show that:
- échelon steps appear beyond a finite shear/tension threshold;
- contrary to linear elastic fracture mechanics predictions, they do not emerge homogeneously along the crack front via a direct bifurcation, but nucleate on local toughness/stiffness fluctuations. As such, the échelon instability continues the cross
-hatching one, observed on soft solids under pure tension, here biased by shear loading.

We argue that this behavior results from the controlling role of elastic non-linearity.

These results point to the importance of studying whether they remain relevant for stiffer materials, in order to assess the validity limit of the linear elastic approximation.

O. Ronsin et al., "Crack front echelon instability in mixed mode fracture of a strongly nonlinear elastic solid", EPL, 105, 34001 (2014)