Hinged Rigid Beam Fracture Specimen for Characterization of Lattice and Thin-Sheet Materials

Abstract Background Measuring the mode-I toughness of two-dimensional (2D) lattice materials and other thin-sheet materials poses a significant challenge with existing testing techniques. For example, material compression ahead of the crack or unstable crack growth frequently arise during testing of these materials and can complicate toughness measurements. Objective This study investigates a new experimental method, the hinged rigid beam (HRB), to evaluate the mode-I toughness of elastic-brittle 2D lattice and thin-sheet materials. Methods The HRB uses stiff beams and a hinged boundary to create a monotonically decreasing tensile stress along the length of the test material in the direction of the crack path. An analytical model, corrected using finite element studies, allows the critical strain energy release rate to be extracted from experimental data collected in HRB testing. Tests on homogeneous poly(methyl methacrylate) (PMMA) are performed to validate the technique and model. Then, 2D triangular and hexagonal lattices with varying relative densities are characterized using the HRB. Results Compliance measurements from experiments on homogeneous PMMA closely match the corrected model, and toughness measurements are consistent with previously reported values. Stable crack growth was observed in the tested lattice specimens, and toughness values were readily calculated. Toughnesses are compared to models for lattice fracture that use simple scaling laws. Good agreement is observed between experiments and model, especially at lower relative densities. As the relative density of the triangular lattices increased, the failure mode transitioned from strut-based to node-based, and the measured toughness values diverge from the models. Conclusions The HRB method allows for stable crack growth and prevents alternative modes of failure, like buckling, in thin-sheet materials and 2D lattices. This new experimental approach can be used for the fracture testing of a wide range of thin or highly compliant materials.