Abstract
This paper is the first detailed investigation of the fatigue behavior of pseudo-ductile unidirectional (UD) thin-ply interlayer hybrids made of thin-ply carbon/epoxy plies sandwiched between standard thickness glass/epoxy plies under two scenarios: without any initial damage (pristine hybrids) and after the introduction of damage in the laminates by loading past the pseudo-yield point (overloaded hybrids). The laminates were subjected to different percentages of the critical stress level at which multiple fragmentation of the carbon plies was established (knee-point stress). The stress levels for fatigue delamination initiation and growth were evaluated experimentally. Based on the experimental work, it was observed that (1) when pristine hybrid composites were fatigued well below the carbon failure strain, at a stress level of 80% of the knee-point stress, there is no stiffness reduction after a significant number of cycles (105 cycles) (2) gradual stiffness reduction and very slow delamination growth was observed for pristine hybrid composites when fatigued at 90% of the knee-point stress, (3) when overloaded hybrid composites were fatigued at 90%, 80% and 70% of the knee-point stress, they did not fail immediately but delaminated slowly (4) the slow growth was due to the low energy release rate of the thin-ply hybrid composites (5) the strain energy release rate approach related to delamination rates provides a good way to characterize the fatigue damage accumulation of overloaded hybrid composites and as a basis to predict the fatigue life.
Highlights
Fibre reinforced composites offer outstanding combinations of strength and stiffness together with low density [1], making them an attractive choice of material for lightweight structural applications such as wind turbine blades [2], sports equipment [3], spacecraft and aerostructures [4], motorsport [5], where weight saving, and durability are major concerns
This paper has presented the mechanical properties of unidirectional thin-ply hybrid composites subjected to static and cyclic tensile loads
The following was concluded: 1. When pristine hybrid specimens were fatigued at 80% of σk no stiffness reduction up until 105 cycles was observed because the specimens were loaded well below the first carbon layer fracture strain
Summary
Fibre reinforced composites offer outstanding combinations of strength and stiffness together with low density [1], making them an attractive choice of material for lightweight structural applications such as wind turbine blades [2], sports equipment [3], spacecraft and aerostructures [4], motorsport [5], where weight saving, and durability are major concerns. Including new technology such as thin carbon prepregs has been shown to improve the mechanical properties of quasi-isotropic composites under various loading conditions [20,21,22,23] towards higher failure strains due to suppression of matrix cracking and delamination The reason for this behaviour is that thinner plies have lower energy release rates, thereby delaying the propagation of inter- and intralaminar cracks. The tensile fatigue behaviour of thin-ply quasi-isotropic (QI) carbon laminates reported by Sihn et al [20] has shown superior mechanical properties (higher residual tensile strength) compared to the quasiisotropic thick ply laminates up to 5.104 cycles at 60% of the ultimate tensile strength of the QI thin specimens The reason behind this is the absence of microcracks and delamination for the thin-ply specimens due to the low energy release rate. The comparison between the fatigue behaviour of UD thin-ply carbon/ glass hybrid composites with its constituent parent materials is outside the scope of this paper
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