Applicability of memory rules during cyclic stress-strain response of polymers PA6 and PA66 GF30

Polymers are rapidly emerging materials that are taking over the place in structural applications formerly reserved for metals. Structural materials require reliable load-bearing capability during the entire lifetime of products. Available simulation tools already offer constitutive models that are well applicable to metals but have not yet been approved for polymers. Typically, models with kinematic hardening incorporate Masing rule complemented by three memory rules to simulate the cyclic response of metals during variable loading. Conversely, the research presented here has been focused on polyamides as a load-bearing-capable substitute for metals in low-cycle fatigue applications. In particular, it has been studied whether natural PA6 and glass-fibre-reinforced PA66 GF30 follow the memory rules during variable amplitude loading. Incremental step tests were performed initially to determine the material parameters of the stabilised response. A high variation for the stabilised stress-strain response could reach up to 42.5% for PA6 and 24.5% for PA66 GF30 during incremental step tests. Moreover, the natural PA6 exhibited a higher degree of symmetry of the stress- strain result with less variation in stress than the reinforced PA66 GF30. Next, variable amplitude tests were performed which contained the three memory rules in the load history. The stress scatter during the variable amplitude loading in repeated tests has been observed between approximately & PLUSMN;15% and & PLUSMN;10% for natural PA6 and PA66 GF30, respectively. Finally, the Armstrong-Frederick model was used to carry out the simulations of the stress-strain response during the same variable load history. Comparison of the simulated and experimental results revealed that for both polymers, applicability of memory rules can be recognised.

Automated summary

Polymers are rapidly replacing metals in structural applications. This study focused on polyamides as a substitute for metals in low-cycle fatigue applications. The research aimed to determine whether natural PA6 and glass-fibre-reinforced PA66 GF30 follow the memory rules during variable amplitude loading. The results showed that both polymers exhibited applicability of memory rules.