Distinguishing between irreversible damage and time-dependent effects in polymers

In-depth knowledge of property deterioration over time is vital for lifetime assessment in many applications. Such a deterioration may be caused by microscopical damage phenomena such as voids or crazes. These can accumulate and form cracks via coalescence, which eventually leads to catastrophic failure. However, the determination of permanent damage in polymers is challenging due to their viscoelastic behavior. The decay of mechanical properties such as the Young’s modulus may be caused by irreversible damage, but can also be attributed to time-dependent effects.
High resolution imaging techniques, for example computer tomography (CT), are usually required to determine the extent of microstructural damage. Such highend equipment tends to be expensive and not readily available. In this contribution, a combination of easy-to-use methods is offered as alternative in order to distinguish between permanent damage and time-dependent effects. A critical strain can also be determined as an estimate for the onset of damage. On the one hand, load-unload tests are used in order to depict the decay of the modulus with each cycle. This approach, however, is shown to suffer heavily from the viscoelastic behavior of polymers. Thus, no definite information on structural health can be given. On the other hand, the influence of testing time is eliminated by pre-straining the material and performing dynamic-mechanical analysis (DMA) after a relaxation period. The modulus in these measurements remains constant until a critical value of pre-strain, at which the microstructure is affected for the first time. These assumptions are validated with μ-CT measurements, where a good aggreement between the pre-strained DMA results and the actual microstructure is observed.