Multi-scale modeling of the fatigue behavior for short fiber reinforced thermoplastics

This PhD thesis deals with the investigation of damage initiation in a short glass fiber reinforced polyamide under fatigue loading. This material belongs to Short Fiber Reinforced Plastics (SFRPs) and is widely used in load-bearing applications in the automotive sector. Lifetime prediction models represent a powerful tool for optimizing structural parts in the early stage of a project reducing the number of prototypes needed before the series production. Multi scale predictive models aim to integrate the relevant damage mechanisms in order to obtain accurate estimation of the lifetime to failure reducing empirical parameters and assumptions. The aim of this PhD thesis is to gain insight into the conditions for damage initiation in a short glass fiber reinforced polyamide under fatigue loading in order to prepare the basis for the development of a multi-scale, mechanism-based lifetime prediction model. This objective was addressed through three main activities: 1) The quantification of the lifetime to crack initiation during fatigue tests (Chapter 4); 2) The investigation of the fatigue damage mechanisms (Chapter 5 - 6); 3) The study of the local stress concentrations at crack initiation (Chapter 7). Chapter 4 describes the development of an optical method for the quantification of the lifetime to crack initiation during fatigue tests. Using the proposed experimental method, it was possible to generate a set of fatigue data to crack initiation for the calibration of a lifetime prediction model. The investigation of the damage mechanisms constitutes a major part of the PhD. In Chapter 5 an extensive damage investigation on PA66-GF35 plain and notched specimens is presented. In Chapter 6 the influence of the fiber volume fraction on the damage mechanisms is investigated. Fatigue damage mechanisms were studied at multiple scales by means of Field Emission Scanning Electron Microscopy (FESEM). Damage investigation was carried out analyzing either the fracture surface of failed specimens or the polished side surface of specimens subjected to interrupted fatigue tests. Specific fractographic features were analyzed and compared with the results from the literature. These are: ductile / brittle matrix fracture behavior, fiber failure / pull out, degree of fiber-matrix interfacial adhesion. Particular attention was devoted to the analysis of the fiber-matrix interface. Mirror-clean fibers on the fracture surface indicate fiber-matrix debonding. By contrast, fibers covered by a resin layer suggest the damage occurs in form of matrix-cracking in the resin, at a certain distance from the interface. In Chapter 7, the influence of the fiber distribution on the local stress concentrations at crack initiation is studied. Stress concentrations are potential locations for damage initiation. In SFRPs, fiber-fiber and fiber-matrix interactions lead to stress concentrations at micro-scale. X Ray Computed Tomography (X-Ray CT) was used for the quantitative description of the fiber distribution around a molded notch, at crack initiation. For this purpose, a fatigue test of a notched specimen was interrupted before failure. A sample surrounding the notch and including a crack propagated during the fatigue test was scanned by means of X-Ray CT. A manual procedure for reconstructing the real fiber orientation distribution around the notch is proposed. The reconstructed volume was simulated with the FEM code ABAQUS with the aim to study the stress concentrations at crack initiation. Finally, the findings of the experimental and modeling activities were used for the development of a preliminary multi-scale approach for the prediction of the crack initiation in a short glass fiber reinforced polyamide under fatigue loading. This activity is presented in Chapter 8