Meaning
Localization of plastic strain in a narrow region due to heat generated during rapid deformation represents a failure mechanism where thermal diffusion is slower than the rate of mechanical energy input. Plastic deformation under high strain rates triggers adiabatic shear banding in solid polymers. The process occurs when the rate of local heating exceeds the rate of thermal dissipation, leading to localized softening.
This phenomenon is particularly relevant in high-speed machining and ballistic testing of polycarbonate and polymethyl methacrylate.
Thermal Consequence
Thermal energy accumulation raises the local temperature of the material during high-speed shear deformation. This temperature increase lowers the yield strength of the polymer in the sheared zone. A runaway thermal instability develops because the softened material undergoes further concentrated strain.
Shear Localization
Structural instability occurs within the deformed zone as the deformation rate increases, forcing the strain to concentrate in a thin layer. The shear band becomes narrower as strain accumulates, creating a distinct path of highly oriented chains. Microstructural studies of failed injection molded parts demonstrate that the polymer molecules align parallel to the shear direction.
This localized alignment reduces the resistance of the material to crack initiation and propagation, ensuring that any subsequent mechanical impact leads to rapid crack propagation and brittle failure.
Polymer Failure
Rapid fracture along the path of the shear band completes the failure process. Components subjected to impact loads split along these bands of localized deformation, demonstrating a low energy absorption capacity. The presence of these bands indicates that the design limits of the polymer have been exceeded during dynamic loading.
