Dynamic flow and failure of confined polymethylmethacrylate

The static and dynamic mechanical behavior of confined commercial polymethylmethacrylate (PMMA) is investigated. Cylindrical specimens are confined by means of a tightly fit metal sleeve and subjected to compression tests. The rate and pressure sensitivity of PMMA are characterized over a range of strain rates of ε˙=10-3–104s-1. In the quasi-static regime, the material is quite ductile, exhibiting noticeable barreling. The dynamic failure mode consists of axial splitting and fragmentation at the lower strain rates (ε˙<2000s-1) and confining pressures. But, at the higher strain rates and confining pressures, the failure mode changes to adiabatic shear formation of a conical plug. This failure mode transition, identified by SEM fractographic analysis, can be explained by the fact that the confinement delays the operation of damage micromechanisms, as evidenced from the post-peak slope. The response of this polymer is described using a fitted rate-dependent Drucker–Prager pressure-sensitive model, according to τflow=66.78γ˙0.06933+0.223p, for 10-3⩽γ˙⩽104s-1. This work shows that confining PMMA turns this brittle material into a ductile one, exactly like other brittle materials such as ceramics. The present results thus fit into a general description of the pressure induced brittle–ductile transition, including rate sensitivity effects for this material.