Fracture and failure of shear-jammed dense suspensions under impact

Impacted with sufficiently large stress, a dense, initially liquidlike suspension can be forced into a solidlike state through a process known as shear jamming. While the onset of shear jamming has been investigated extensively, much less is known about the resulting solidlike state in the high-stress limit and, in particular, about its failure mode. We report on experiments that produce such high-stress failure by impacting dense suspensions with a cylindrical rod moving at controlled speed. Using suspensions of cornstarch particles we vary the impact speed over several orders of magnitude and change the fluid viscosity as well as the surface tension in order to identify the conditions for failure. The results are compared with similarly dense suspensions of potato starch or polydisperse silica particles. In all cases where the shear-jammed suspension fails by fracturing, similar to brittle solids, we observe two types of cracks: a primary circular crack around the impacting rod followed by secondary radial cracks. Mapping out the onset of radial fracturing for different particle volume fractions ϕ and impact speeds, we identify the requirements for failure via crack formation to occur with at least 50% likelihood and record the corresponding normal stress σ_{N} on the impactor. We find that this likelihood is not particularly sensitive to changes in particle diameter, but increases when the solvent's viscosity or its surface tension are reduced. In the state diagram for dense suspensions we use these data to delineate the upper limit of shear-jammed rigidity and the crossover into a fracture regime at large ϕ and large σ_{N}, several orders of magnitude above the stress required for the onset of shear jamming. We find that the onset of fracturing in many cases is correlated with signatures of internal ductile deformation of the shear-jammed material underneath the impactor, observable in σ_{N} as a function of axial strain. However, for smaller suspension volumes and larger impact speeds, we find strain hardening up to the point of fracturing. This more brittle behavior results in a modulus that, just before crack formation, is roughly an order of magnitude larger than what we observe in shear-jammed suspensions undergoing internal ductile deformation.