Acceleration wave speeds in a hypoplastic constitutive model

The paper presents a theoretical investigation of acceleration waves in a plastic material described by an incrementally non-linear hypoplastic constitutive equation. Speeds of plane acceleration waves and their dependence on the stress state are calculated. The spectrum of possible wave speeds is found to be continuous, which is in contrast to discrete wave speed spectra in incrementally linear models. Two types of ill-posedness are revealed, known as flutter ill-posedness and stationary discontinuity. The wave speed analysis is also performed by the characteristic method, leading to different equations compared to the acceleration wave approach. It is proved that for the considered hypoplastic constitutive equation both approaches give identical wave speed spectra.