Extrinsic plastic hardening of polymer thin films in flat punch indentation

The effect of contact aspect ratio and film to substrate elastic modulus ratio on stress vs. strain up to the point of yield during flat punch thin film indentation of an elastic-plastic film

“…A radially symmetric simulation was used to investigate indentation under load control to about 25% the film thickness. Our results indicate that a well aligned punch contacting a stiffly supported film at high contact aspect ratio exhibits features characteristic of uniaxial strain, demonstrated previously by our experiments with polymer films on silicon (Brazil et al, 2020;Brazil et al, 2021). With sufficient indentation depth and high enough aspect ratio, a complete mechanical parameterization of an elastic-plastic constitutive condition characterized by confined elastic modulus, bulk modulus, and yield stress can be extracted from a single loading curve (Brazil et al, 2020).…”

“…Unlike the Oliver-Pharr technique that relies on an assumption of the way a deforming sample contacts a presumed tip shape function with depth, a simple mean stress can be directly calculated from flat punch load data at any point during the deformation. Work investigating flat punch, supported thin film contact beyond the zero strain limit exists (Li, 2002;McManamon et al, 2020;Luo et al, 2022), but generally does not consider the high contact aspect ratio limit which renders the deformation highly uniform and allows the formation of a representative common strain exploited by the layer compression test (Brazil et al, 2020;Brazil et al, 2021).…”

Densification of a polymer glass under high-pressure shear flow

“…We have recently shown that this confined compression testing geometry, shown as an insert in Fig. 1(a), results in uniform longitudinal uniaxial strain well past the point of first plastic yield [31,32]. Figure 1(a) plots indentation stress versus strain curves for four individual indents to different peak stresses into the atactic polystyrene (aPS) film.…”

“…The flat punch indentation of a thin polymer film closely approximates confined uniaxial strain compression, which enables residual stresses and the resultant elastic densification to be determined analytically [31,32]. For a linear elasticperfectly plastic material loaded to a peak stress σ p > Y c , the residual strain on unload is…”

“…Finite element simulations of the confined compression of an elastic-plastic material (see Supplemental Material [29]) confirm the validity of this equation, therefore enabling volume changes resulting from elastic pinning through residual stresses to be subtracted from the total densification. Despite the somewhat idealized nature of this linear elasticplastic model when applied to glassy polymers, we have shown it to be extremely effective in describing the complex stresses and strains these materials are subjected to in confined compression-in particular, the residual stress state upon unloading [32]. For the density measurement problem, we employ a unique approach based on a combination of AFM film thickness measurements and STXM to measure the relative mass density of the indented region through the bulk of the film without reference to the pileup surrounding the residual impression.…”