Fatigue accumulated damage in notched quasi-isotropic composites under high-temperature conditions: A discussion on the influence of matrix nature on the stress energy release rate

Abstract

In order to investigate the contribution of matrix nature to the fatigue behaviour, the purpose of the present work is to establish the correlation between material toughness and macroscopic damage accumulation during tensile cyclic loading in the brittle (C/epoxy) and ductile (C/Polyphenylene sulfide) matrix systems. More specifically, this article presents a fracture mechanics-based approach to compute the strain energy release rate during fracture along with the macroscopic transverse crack growth in fatigue. The knowledge of energy-absorbing processes is important as they are responsible for the toughness of the composite. Woven-ply laminates are characterised by matrix-rich regions which may stop or slow down the growth of fatigue cracks by absorbing the mechanical energy through local plastic deformations at the cracks tip depending on matrix nature. With respect to C/epoxy laminates, the local plastic deformations at the cracks tip are prominent in highly ductile composite systems (e.g. C/Polyphenylene sulfide), and ultimately results in fatigue behaviour virtually independent of the applied stress level under high temperatures T > Tg. To evaluate the influence of matrix ductility and toughness on fatigue damage, a damage variable d based on the measurement of longitudinal stiffness at each cycle was used. A model derived from a Paris law and a fracture mechanics criterion were combined to: (i) evaluate the fatigue crack growth – (ii) compare the changes in the strain energy release rate G and the macroscopic damage d during cyclic loading. Macroscopic damage appears to be well correlated with the strain energy released during fracture.

The fatigue of carbon fibre reinforced plastics - A review

“…Indeed, open hole CFRP are more prone to cracking through delamination and as such, improving the throughthickness performance of CFRP is recognisably important. Undamaged 3D CFRP, though weaker than undamaged UD CFRP, has a better resistance to macro-delamination (as described above) and this results in the improved fatigue performance of 3D composites over UD composites when the samples contain macro-defects such as open holes (3D woven [167] or stitched [168]) and notches [169,170], with normalised S-N curves. One notable exception can be found in Tsai et al [171] who showed that the superposition of normalised S-N curves for notched and unnotched samples of 3D woven CFRP were almost identical.…”

About the high temperature fatigue performance of thermoplastic-based composite laminates for aeronautical applications

“…Thus, in laminated composites consisting of a thermoplastic (TP) matrix, the ductility and toughness of the TP matrix play a significant role in the fatigue performance of woven-ply laminates by modifying the damage mechanisms and their chronology. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Vieille et al have proposed a detailed literature review on the fatigue behaviour of thermoplasticbased laminated composites, more specifically under high temperature conditions. 14 New materials often lack the necessary database for a fast application in cyclic loaded PMCs parts due to time-consuming testing series.…”

“…Under the working hypothesis that cracks are the main contributors for heat emission, TP-based composites are characterized by a different behavior in comparison to thermosets under fatigue loading. [24][25][26][27][28] At low stress levels, instead of crack formation, it appears that the TP matrix relaxes under fatigue loading. This relaxation could potentially lead to reduce crack growth but might also result in modifying the damage chronology.…”

“…PEEK, PEKK, PAEK) and their interesting mechanical properties make it possible to consider their integration in structures in hot zones, which can reach 200°C, currently reserved for conventional materials, such as metals and ceramics. 20,21 Temperature also plays an important role on the fatigue properties of TP-based composites, [22][23][24][25][26][27][28][29][30][31][32][33][34] as well as the ductility of the matrix. 35,36 These two factors are correlated since an increase in temperature will make the behavior of the matrix more ductile and promote the activation of the viscous mechanisms of the matrix.…”

“…Indeed, depending on the ductility of the matrix, these zones can play a role of barrier to cracks increasing the intrinsic toughness of the laminate. 28,37 It can therefore become interesting to associate a UD reinforcement with a ductile TP matrix, with respect to the fatigue behavior. 29,38 In addition, this effect can be amplified by the increase in temperature, especially at T>T g .…”