Abstract
Z-pin reinforcements in composite materials significantly enhance the through-thickness strength by providing strong bridging forces on the delaminated surfaces of a damaged laminate. The bridging forces have been shown to be more effective in mode I dominated delamination cracks than in mode II shear dominated cracks. One solution to help improve the Mode II delamination resistance of Z-pins is to incline the pins such that the angle between the longitudinal axis of the pin and the shear force load vector is reduced. In this study, the Mode II resistance of laminated composite reinforced through the thickness with inclined Z-pins is characterised. There is a notable increase in the apparent fracture toughness of composites when Z-pins are better aligned with the shear load vector (inclined) compared to the conventional, orthogonally inserted Z-pins. Brittle, catastrophic failure however occurs when the inclined Z-pins are angled against the shear load vector. For many structures the direction of the local direction of shear load cannot always be predicted, therefore a general approach for inclined Z-pins using a±θ configuration was also investigated. With this setup, a modest improvement in the apparent fracture toughness was obtained.
Highlights
Laminated composite materials lack through-thickness reinforcements (TTR), which often leads to their failure through delamination
Aligning a Z-pin with the load vector ensures that the dominate forces acting on the Z-pin are frictional forces as opposed to shear forces which cause Z-pins to fail catastrophically resulting in low energy absorption
The Z-pin insertion angle is directly aligned with the load vector
Summary
Laminated composite materials lack through-thickness reinforcements (TTR), which often leads to their failure through delamination. In spite of the performance disparities of Z-pins in Mode I and Mode II being raised, there is little research work presented in the open literature that offers solutions to these issues This is important for the case of out-of-plane impact, where the resulting delamination cracks are driven by mode II shear. Cartié et al [6] in a study to understand crack bridging mechanisms of Z-pins, characterized the behaviour of T300/BMI Zpins in uni-directional (UD) carbon fibre/epoxy laminates for insertion angles from −10° to 15° under shear loading. No study to date has looked at the resistance of an inclined Z-pin to Mode II delamination propagation, where the concept is expected to enhance performance relative to orthogonally inserted pins In this investigation the bridging effect of T300/BMI Z-pins in Mode II has been studied by changing their orientation in a laminated composite. The fundamental findings from this study are compared on a more structural level by analysing the Mode II resistance of inclined pins using the End-Loaded Split (ELS) specimens
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