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Unraveling the complex behavior of wet granular media has been challenging, because the effective stresses transmitted through the particles have remained unobservable. The authors use their recently developed extension of photoelasticity for coupled fluid-granular systems to image particle stresses during fracture by air injection in an oil-saturated granular pack. They discover two states of the granular medium evolving with the fractures, plus an ``effective-stress shadow'' behind the propagating fracture tips. This study points out the power of photoporomechanics for studying coupled fluid-solid processes in granular media, including energy recovery, gas venting, and geohazards.
Meng et al. (Tue,) studied this question.