Investigation of Short Fiber Reinforced Polymers fatigue behaviour using specimens of highly coherent fibres orientation
Topic(s) : Special Sessions
Co-authors :
Andrea CANEGRATI (ITALY), Martulli LUCA MICHELE (ITALY), Andrea BERNASCONI (ITALY)Abstract :
Short Fibre Reinforced Polymers (SFRP) display a highly variable, process induced Fibre Orientation (FO). Local anisotropic properties are commonly evaluated through advanced multiscale material models adopting Pseudo-Grains (PGs) discretization and homogenization schemes, coupled with the FO. Each PG is considered as a fictitious unidirectional (UD) composite, whose fatigue life may be predicted by micro-mechanical continuum damage evolution models [1] or otherwise estimated from master S-N curve [2]. These models are calibrated on the evolution of elastic and viscous properties and on the fatigue limits of UD SFRP, respectively. Experimental methods, consisting of testing specimens either injected to shape or extracted at nominal orientations from plates, show limitations to evaluate those quantities due to the heterogeneous FO [3]. In [3] it was suggested that the specimen’s core may hinder the accurate evaluation of the fatigue strength of SFRP with fibres perpendicular to the load. Alternatively, the UD fatigue properties of SFRP could be estimated with a reverse engineering approach, provided that the microstructural details are available. In this framework, the uncertainties in the evaluation of the fatigue limits of UD SFRP usually leads to part overdesign.
The goal of the present study is to analyse specific information delivered from SFRP displaying highly coherent and heterogeneous FO, so to improve established fatigue models. 0° and 90° integer and shell-only SFRP specimens were fatigue tested. The evolution of their dynamic modulus and their area of the hysteresis cycle over fatigue was evaluated (see Fig)1 as well as their cycles dependent fatigue limits.
Specimens with highly coherent FO deliver more representative information about the fatigue behaviour of an almost UD SFRP than integer ones. The experimental data of shell-only specimens will be applied to the calibration refinement of the PGs based fatigue models. Similarly, master S-N curve will be derived from their S-N curves, then implemented in fatigue modelling approaches as [2]. The lifetime prediction performance of these fatigue models could benefit from enhanced calibration stage and from empirical fatigue limits closely approximating those of UD SFRP.
It is eventually proposed a strain measurement improvement to be applied to cyclic mean strain unifying fatigue models [4]. Their mean strain parameter is defined as the average slope of the mean strain over the specimen’s midlife, times the testing frequency. A set of images of the speckled specimen was captured in the cycle range of interest and analysed by digital image correlation. The model’s parameter computed by local strains measurements at failure location (Fig2) and by average gauge length strain were compared and their performance analysed.
The goal of the present study is to analyse specific information delivered from SFRP displaying highly coherent and heterogeneous FO, so to improve established fatigue models. 0° and 90° integer and shell-only SFRP specimens were fatigue tested. The evolution of their dynamic modulus and their area of the hysteresis cycle over fatigue was evaluated (see Fig)1 as well as their cycles dependent fatigue limits.
Specimens with highly coherent FO deliver more representative information about the fatigue behaviour of an almost UD SFRP than integer ones. The experimental data of shell-only specimens will be applied to the calibration refinement of the PGs based fatigue models. Similarly, master S-N curve will be derived from their S-N curves, then implemented in fatigue modelling approaches as [2]. The lifetime prediction performance of these fatigue models could benefit from enhanced calibration stage and from empirical fatigue limits closely approximating those of UD SFRP.
It is eventually proposed a strain measurement improvement to be applied to cyclic mean strain unifying fatigue models [4]. Their mean strain parameter is defined as the average slope of the mean strain over the specimen’s midlife, times the testing frequency. A set of images of the speckled specimen was captured in the cycle range of interest and analysed by digital image correlation. The model’s parameter computed by local strains measurements at failure location (Fig2) and by average gauge length strain were compared and their performance analysed.