Influence of large deformation on plastic yield inception and associated mechanism during spherical indentation of soft materials

Abstract

Soft materials typically experience large deformations before the onset of plastic yielding during indentation. The inception of plastic yield and associated mechanisms are significantly different from those of hard materials under small deformation, where the influences of large deformation are investigated in this paper. A parametric finite element model is developed to solve the indentation of elastic-plastic soft half-spaces by rigid spherical indenters, encompassing a wide range of soft materials. The stress and deformation fields at the onset of plastic yielding, along with the critical contact parameters for initial yield, are accurately determined. A soft material range, referred to as the FYD range, in which finite deformations are required for initial yield, is identified. Within the FYD range, the characteristics of the stress field exhibit alteration, the individual stress components reduce unevenly, the Mises stress substantially changes, and the load and indentation needed for plastic yielding inception significantly reduce. The interaction between large deformation and frictional traction results in significantly different yield behaviors compared to those observed in hard materials at small deformation. Three novel yield deformation mechanisms are found to be universal and driven by large deformations. These include the significant compression of the stress concentration region, significant flattening of the stress concentration region, and the occurrence of a partial tension zone on the contact surface. Large deformations strongly influence the mechanisms associated with the inception of plastic yield, revealing unique plastic yield behaviors in soft material indentation. These findings provide new insights into the physics of soft material indentation under large deformations.