Implementation of 3d nonlinear material model in finite element code

Abstract

In certain applications, there is a growing need to use a nonlinear material model that could account for viscoplasticity, viscoelasticity and damage. Bio-based composites, composites used in demanding environments, and manufacturing process optimization are just a few examples of such fields. On top of that, composite structures are becoming more complex. Thus, using widely used simplistic 1D material models is not sufficient anymore for these applications. Even for relatively simple conditions, 3D material models show significantly higher stresses than typically used 1D models. Figure 1 shows thermal stresses during the cooling process within a mold. The plate is cooled down in two steps – first, the “fast” cooling process, where the plate is cooled down from 120°C to 70°C in 600s, followed by the “slow” cooling process until 20°C in 1200s. Often, the 1D models are used with the assumption that the plate in the lateral direction can move freely. However, in reality, the mold will constrain the movement in the lateral direction. Thus, the 1D models will significantly underestimate the stresses within the material. Simulations using experimentally determined master curves are performed for two different cases using two different assumptions: a) the time-temperature shift factor is temperature independent (notation a=1) and b) the shift factor is temperature dependent (notation a(T)) and it changes following the temperature change. Although only a part of the nonlinearity expected within the material is included in the shift factor, there is a significant difference in the modeling curves. With temperature-dependent shift factors, stresses are lower than with the other assumption