Large deformation mechanical behavior and constitutive modeling of oriented PMMA

Compared with the unoriented polymethyl methacrylate (PMMA), oriented PMMA has much better mechanical properties, hence widely used in engineering structures. To establish a new constitutive model that is suitable for oriented PMMA at different strain rates and investigate the large deformation mechanical behavior of oriented PMMA, compression experiments of oriented PMMA were conducted at low (0.001 s⁻¹, 0.01 s⁻¹) and high (2000 s⁻¹, 3000 s⁻¹) strain rates. The effects of orientation degree (ϕ) on the mechanical behavior were clarified by making a quantitative comparison of unoriented PMMA (ϕ=0) with the PMMA of various orientation degrees (ϕ=65%, 72%, and 85%). The experimental results demonstrated that orientation degree has different effects on the mechanical behavior of PMMA at different strain rates. Particularly, it was found that the strain hardening behavior after yielding can be significantly enhanced with the increase of orientation degree. According to the experimental results, the influence mechanism of orientation degree on the microscopic deformation of PMMA was revealed, and the theoretical relationship between orientation degree and the entropic resistance was established by introducing an initial deformation gradient. On this basis, a modified physically-based constitutive model of oriented PMMA was proposed. Numerical simulations and analysis of PMMA with different orientation degrees were conducted using the proposed model. The stress–strain curves predicted by the present model were in good agreement with the experimental results, which indicates the proposed constitutive model can well describe the large deformation mechanical behavior of oriented PMMA under different strain rates.