Meaning
Structural analysis of polymer parts subjected to high stress must account for a regime where the relationship between stress and strain depends on both time and the magnitude of the applied load. This behavior is called non linear viscoelasticity and it occurs when the deformation exceeds the linear threshold of the material. In this region, the creep rate increases disproportionately with higher stress levels.
The behavior does not apply to very low stress levels where the material stiffness remains independent of the load.
Stress Dependency
Polymeric chains slip and disentangle more rapidly when they are subjected to higher loads. Under non linear viscoelasticity, doubling the stress results in more than double the strain over a given time. This behavior cannot be calculated using simple linear models.
Moulding Implication
Molded parts often contain high levels of internal stress and variable molecular orientation due to the injection moulding process. These localized variations mean that different areas of the part will experience different levels of non linear viscoelasticity when a load is applied. Regions with high molded-in stress will deform much faster than expected, which can lead to premature failure or warping under load.
Structural Analysis
Designing load-bearing polymer parts requires using non-linear finite element models that can capture this complex behavior. Engineers must input stress-strain data from isochronous curves at multiple stress levels to ensure the software calculates the correct deformation over time. Ignoring this behavior leads to underestimating the deflection of the part, which can cause structural failure in applications such as snap-fits or load-bearing brackets where the material is pushed beyond its linear limit.
