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Echelon cracks in soft solids

Under pure tension loading, crack surfaces are usually planar. However, when superimposing shear parallel to the crack front, steps are systematically observed on the cracked surfaces, the segments between steps rotating toward pure tension loading while the front propagates. Understanding the emergence of this ubiquitous instability remains a challenge in fracture mechanics that researchers of the « Multiscale Mechanics of Soft Solids » team from INSP tried to assess.

Researchers studied this instability in a highly deformable solid (a gelatin hydrogel) in which the crack propagation velocity can be controlled. They have thus been able to follow the evolution of initially flat crack submitted to a mixed traction+shear loading (figure 1). They have shown that the steps only appear above a finite threshold of the ratio of shear and traction stresses. Below the threshold the crack front remains straight but rotates towards a pure tension configuration after propagating over a length imposed by the sample thickness. Profilometric analysis of the fracture surfaces (figure 1) showed that, contrary to linear elastic fracture mechanics predictions, the echelon cracks do not emerge homogeneously along the crack front via a direct bifurcation, but nucleate on local toughness/stiffness fluctuations acting as local stress concentrators.

JPEG Figure 1
Fracture surface Crack surface (8x10 mm2) developing steps (vertical bar 500μm) from a straight notch (dashed line) submitted to mixed loading (traction + shear).

They showed the effect of shear is to favor a crack front instability observed under pure tension in soft solids (gels, elastomers) showing strong non linear elasticity : stress inhomogeneities due to the statistical quenched structural disorder of the material induce the folding of the crack front, leaving steps on the crack surface. The echelon instability continues the « cross-hatching », here biased by shear loading.

These results show the essential role of elastic non-linearities and points to the importance of studying whether they remain relevant for stiffer materials, in order to assess the validity limit of the linear elastic approximation.

« Crack front échelon instability in mixed mode fracture of a strongly nonlinear elastic solid »
O. Ronsin, C. Caroli et T. Baumberger
EPL, 105, 34001 (2014)

Olivier Ronsin