Multiaxial Strength and Failure Criteria for Printed Photopolymers: Combined Loading, Anisotropy and Model Validation

The strength of a printed photopolymer under combined loading cannot be reconstructed safely from one tensile strength and one elastic modulus. Tension, compression and shear can activate different deformation and fracture processes, while print orientation, cure, conditioning, temperature, rate, specimen geometry and local stress gradients alter the observed failure state. Familiar criteria such as maximum stress, von Mises, Drucker-Prager, Mohr-Coulomb, Tsai-Hill and Tsai-Wu encode different assumptions. Selecting one by convenience does not establish its applicability, and fitting an interaction coefficient to the same data used for evaluation does not validate prediction.

This review defines a non-compensatory qualification framework for multiaxial strength of printed photopolymers. It separates the material coordinate system from the specimen and loading axes, requires proportional and non-proportional paths to be declared, and distinguishes material failure from grip, fixture, free-edge, notch and instability failures. Uniaxial data may anchor directional limits but cannot identify combined-load interaction terms. Calibration must therefore use independently measured loading paths, preserve tension-compression asymmetry where observed, report parameter identifiability and uncertainty, and reserve additional paths for prospective validation. The direct printed-photopolymer corpus supports orientation- and state-dependent strength and distinct failure morphologies, but direct multiaxial evidence remains limited. Accordingly, this article supplies a bounded test and validation architecture rather than universal coefficients or a preferred failure criterion.

Keywords: printed photopolymer; multiaxial strength; combined loading; anisotropy; failure criterion; interaction parameter; calibration; validation