Abstract
The paper concerns the mechanical performance of continuous fibre/thermosetting polymer matrix composites reinforced in the through-thickness direction with fibrous or metallic rods or threads in order to mitigate against low delamination resistance. Specific illustrations of the effects of microfasteners in reducing delamination crack growth are made for Z-pinned and tufted composites. Response to loading in such ‘structured materials’ is subject to multiple parameters defining their in-plane and out-of-plane properties. Single microfastener mechanical tests are well suited to establish the crack bridging laws under a range of loading modes, from simple delamination crack opening to shear, and provide the basis for predicting the corresponding response of microfastener arrays, within a given material environment. The fundamental experiments on microfasteners can be used to derive analytical expressions to describe the crack bridging behaviour in a general sense, to cover all possible loadings. These expressions can be built into cohesive element constitutive laws in a finite-element framework for modelling the effects of microfastener arrays on the out-of-plane mechanical response of reinforced structural elements, including the effects of known manufacturing imperfections. Such predictive behaviour can then be used to assess structural integrity under complex loading, as part of the component design process.This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.
Highlights
The class of composite materials made by combining high-performance continuous carbon fibres with a stiff thermosetting resin matrix has long been the material of choice for advanced composite structures in the aerospace industry, because of its high specific stiffness
EHT = 20 kV WD = 9.5 mm compared with experimental data across a range of Z-pin array tests at different mode mixities; double cantilever beam (DCB), end loaded split (ELS) and mixed mode bending (MMB). Success of such models means that for the first time a capability exists for the design of any composite structural element with microfastener through-thickness reinforcement, allowing assessment of the interaction between the microscale bridging behaviour of individual microfasteners and the global load-carrying capacity of the laminate coming from external boundary conditions
The tufting technology adoption is likely to benefit from lessons learnt previously from two-sided stitching and Z-pinning
Summary
The class of composite materials made by combining high-performance continuous carbon fibres with a stiff thermosetting resin matrix has long been the material of choice for advanced composite structures in the aerospace industry, because of its high specific stiffness. This traditional low-delamination resistance can be tackled successfully by the insertion of rigid microfasteners in the through-thickness direction, at least as far as resistance to crack growth is concerned This much has been established for the case of thin rigid composite rods (Z-pins) inserted into a prepreg stack prior to cure [1,2,3] and for thin threads stitched or tufted into dry fibre preforms prior to resin infusion and cure [4,5]. This paper presents the testing and characterization methodology and indicates how our understanding of the effects of the different structural parameters on the micromechanisms of failure under different loading conditions informs the model development strategy
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