The objective of this work is to develop a plane strain apparatus for "soft" (unconfined compressive strengths below 35 MPa) rock and concrete, based upon a stiff-frame design. The unique feature of the device is that no kinematical constraints are imposed on the formation of a failure plane. By placing the upper platen on a linear bearing, the specimen has the freedom to form a failure mechanism consisting of two rigid bodies separated by a localized zone of deformation. The stresses are statically determinate throughout the load history, even at large displacements. The failure process is mapped by the locations of acoustic emissions. The problem of frictional constraint is addressed and an instrumentation plan is suggested. The research projects for which the plane strain apparatus will be used, involve the verification of bifurcation phenomena and the determination of material parameters for constitutive models.
