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Softening Induced Instability of a Stretched Cohesive Granular Layer
First, we report on a cellular pattern which spontaneously forms at the surface of a thin layer of a cohesive granular material submitted to in-plane stretching. We then present a simple model in which the mechanism responsible for the instability is the ‘‘strain softening’’ exhibited by humid granular materials above a typical strain. This analysis indicates that such an instability should be observed in any system exhibiting a weakening in response to strain.
Second, we perform measurements of the strain field associated with such structures and, in addition, we characterize the mechanical properties, cohesion and shear modulus, of the samples. We show that for high cohesion, the layer is fragile and the surface deformation is highly nonlinear, whereas for low cohesion, a smooth and linearly growing structure is observed when the external stretching is increased. Analysis of the wavelength as a function of cohesion along with independent measurement of the shear modulus indicate that a simple model of strain softening is acceptable provided that the cohesion lead to the formation of clusters in the granular material.
At last, we consider the mechanical response of a layer of foam subjected to the same type of deformation.
Softening Induced Instability of a Stretched Cohesive Granular Layer,
H. Alarcón, O. Ramos, L. Vanel, F. Vittoz, F. Melo and J.-C. Géminard, Phys. Rev. Lett. 105, (2010) 208001.
Flexural fracturing of a cohesive granular layer,
J.-C. Géminard, L. Champougny, P. Lidon and F. Melo, Phys. Rev. E 85, (2012) 012301.
Effect of cohesion and shear modulus on the stability of a stretched granular layer,
H. Alarcón, J.-C. Géminard and F. Melo, Phys. Rev. E 86 (2012) 061303.