Rate-Dependent and Anisotropic Mechanical Behavior of Elastic-50a and Flexible-80a Photopolymer Resins

Rapid adoption of Stereolithography (SLA) has enabled the fabrication of complex elastomeric components for applications ranging from soft robotics to protective gear. Despite the commercial prevalence of resins such as Formlabs Elastic 50 A (E50A) and Flexible 80 A (F80A), a comprehensive characterization of their non-linear, time-dependent mechanical behaviour remains absent from the literature. In this study, we characterize the mechanical response of these two elastomers across a broad spectrum of strain rates under uniaxial tension, compression, stress relaxation, and cyclic loading, while evaluating the influence of build orientation. Both materials exhibit pronounced strain-rate sensitivity and non-linearity: E50A shows a largely isotropic response, while F80A displays measurable anisotropy that grows with strain rate. Standard tensile testing geometries were affected by manufacturing artifacts producing deformation beyond the gauge region, necessitating an optical strain-tracking methodology, and occasional failure initiation outside the gauge section, indicating a need for AM-specific testing standards. A visco-hyperelastic constitutive model was developed, coupling a piecewise four-term Ogden potential with a four-term Prony series, calibrated against the monotonic and relaxation data and verified against the Baker-Ericksen inequalities. Across the seven nominal strain rates tested, the model reproduced the experimental response with a maximum NRMSE of 9.5% for both materials. An additional set of 1000 s relaxation experiments tested the strain-time separability assumption: separability holds within ~3-8% across each material's clustering strain range, with a worst-case deviation of ~13%. Direct LS-DYNA implementation using the standard MAT_077_O material card reproduced the experimental response, confirming that the calibrated model is readily implementable for simulation-driven design of SLA-printed elastomeric components.