Viscoelastic Response of Silicone Additively Manufactured Direct Ink Write (DIW) Foams under Repetitive Compression

Additively manufactured (AM) foam was compressively strained into the plateau region through a reduced design of experiments. Its dynamic stiffness, or complex modulus, declined across 10,000 cycles of small deformation in the plateau region. This rate of change of stiffness, when fit to a simple power law, is correlated with the degree of nonlinearity of the material’s deformation. Nonlinearity is defined as the nonlinear viscoelastic parameter, calculated as total harmonic distortion. Experiments with low nonlinearity tend to maintain their complex modulus across cycles, while materials undergoing highly nonlinear deformations decreased their modulus. The nonlinearity of the experiment is dependent upon the stress that the material experiences as well as its strain rate. Strain rate correlates with the odd resonance nonlinearity while the complex modulus relaxation rate is correlated to the even resonances. Two different material structures were tested, face centered tetragonal (FCT) and simple cubic (SC). As SC has more overlapping strand areas, and thus a shorter load path length through the sample, the initial stress is higher at the same relative porosities. The SC material therefore experiences greater nonlinearity, 5% total harmonic distortion, than the FCT material with 2% total harmonic distortion. Thus, the SC structure shows much lower stiffness retention than FCT. These findings indicate that when selecting material for repetitive cyclic compressions, a more stable low stress material will maintain its dynamic performance more uniformly than an instable high stress material.