Mechanical behaviour of fully densified silica glass under Vickers indentation

When subjected to constrained deformation loadings, such as during an indentation test, silica glass experiences complex deformation mechanisms including densification and volume-conservative shear plasticity. The densification mechanism may increase the density up to more than 21%. The question of the mechanical behaviour of an already fully densified glass sample naturally arises. This issue is one of the key points to address when one tries to propose a constitutive model of pristine silica glass. Indeed, during an indentation test, which is a popular test for this task, beneath the indenter tip, some regions might be fully densified. What is their behaviour, after saturation in densification and during loading, is therefore an issue to address. Moreover, this is a crucial point for exploring the transition from plasticity to cracking, which is of paramount importance and a long-term objective for predicting the lifetime of glass products subjected to contact loadings such as impact or scratching.In this paper, a quantitative identification of fully densified silica mechanical constitutive behaviour is made by using instrumented indentation testing and finite element analyses. The use of such an indirect method to assess the mechanical behaviour of this material comes from its brittle behaviour in unconstrained deformation modes usually employed in metals plasticity. It is shown here that fully densified silica behaves as a von Mises material (rate-independent shear plasticity without strain-hardening) like some crystalline metals. The yield strength and yield strains are, at the contrary, much higher than for alloys: respectively 6.5 GPa and 6.1%. This mechanical modelling, as well as the plastic parameters values found, are in excellent agreement with very recent experimental and numerical simulations on silica glass and silicate glasses.The high value of the yield strain is found to explain unusual indentation features such as a unusual long range residual piling-up while sinking-in is predicted during loading, as well as low values of the ratio hardness-to-yield strength.

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