Multiscale modeling of the damage and failure of homogeneous and short-fiber reinforced thermoplastics under monotonic and fatigue loadings

Reinforced and unreinforced thermoplastic polymers (TPs) are widely used in a range of industrial sectors such as in automotive, aerospace, sporting goods, consumer electronics, and many other domains, because of their interesting properties and their relatively ease of production. Their behavior is rather complex as it is time, strain rate and temperature dependent and couples both viscoelastic and viscoplastic modes of deformation. As a result of their important expansion, produced parts are more likely to be subjected to extreme operating conditions such as fatigue, or exposed to aggressive environments. The effect of these demanding conditions is reflected in a degradation of the material’s mechanical properties throughout its lifetime, which is usually the cause of material failure. Hence numerical prediction tools for damage and failure are required, in order to obtain reliable estimates of the material response. The aim of this thesis is to numerically predict the damage and failure of both thermoplastic polymers (TPs) and misaligned short fiber reinforced thermoplastic polymers (SFRTPs), considered as heterogeneous materials mainly under mechanical fatigue loading with large number of cycles: high cycle fatigue (HCF). The modeling approaches take into account multiaxial stresses and the microstructure of the studied materials. To predict the effective response of the studied heterogeneous materials from their microstructure, multiscale approaches were employed. These approaches represent the modern way of modeling materials because they enable to understand, quantify and optimize the relationship between the final properties of a material or a part at the macroscopic scale and the microstructure. In the first part of this thesis, the focus is on the modeling of the behavior of unreinforced TPs, motivated by experimental observations showing the important role of the TP matrix in the behavior of the SFRTPs. In the second part some of the developed tools are applied to predict the fatigue behavior of the SFRTPs. A damage model, which takes into account the viscoelasticity coupled with viscoplasticity, was developed for homogenous TP materials. A key concept in the proposed approaches is that the damage and defects, causing TPs and SFRTPs failure under HCF, are localized at limited zones within the material, in agreement with experimental findings. Mean field homogenization techniques were developed and employed in order to take into account localized fatigue damage zones, the latter are modeled by so-called "weak spots", which are assumed to have a viscoelastic viscoplastic damaged behavior. The results of the different proposed modeling approaches were validated against available experimental results. For unreinforced TPs, the experimental data were collected from the open literature. However, for the SFRTPs, our partners in the EUREKA/DURAFIP project which aims at the lifetime investigation of short-glass fiber reinforced Polyamide 66, provided the experimental results of monotonic and fatigue tests.