: An overview of the virtual crack closure technique is presented. The approach used is discussed, the history summarized, and insight into its applications provided. Equations for two-dimensional quadrilateral elements with linear and quadratic shape functions are given. Formula for applying the technique in conjuction with three-dimensional solid elements as well as plate/shell elements are also provided. Necessary modifications for the use of the method with geometrically nonlinear finite element analysis and corrections required for elements at the crack tip with different lengths and widths are discussed. The problems associated with cracks or delaminations propagating between different materials are mentioned briefly, as well as a strategy to minimize these problems. Due to an increased interest in using a fracture mechanics based approach to assess the damage tolerance of composite structures in the design phase and during certification, the engineering problems selected as examples and given as references focus on the application of the technique to components made of composite materials.

/pdf/virtual-crack-closure-technique-history-approach-and-53akn4ewzt.pdf

Virtual crack closure technique: History, approach, and applications

The finite element analysis of delamination in laminated composites is addressed using interface elements and an interface damage law. The principles of linear elastic fracture mechanics are indirectly used by equating, in the case of single-mode delamination, the area underneath the traction/relative displacement curve to the critical energy release rate of the mode under examination. For mixed-mode delamination an interaction model is used which can fulfil various fracture criteria proposed in the literature. It is then shown that the model can be recast in the framework of a more general damage mechanics theory. Numerical results are presented for the analyses of a double cantilever beam specimen and for a problem involving multiple delamination for which comparisons are made with experimental results. Issues related with the numerical solution of the non-linear problem of the delamination are discussed, such as the influence of the interface strength on the convergence properties and the final results, the optimal choice of the iterative matrix in the predictor and the number of integration points in the interface elements. Copyright © 2001 John Wiley & Sons, Ltd.

Finite element interface models for the delamination analysis of laminated composites: mechanical and computational issues

This paper is a review of low-velocity impact responses of composite materials. First the term ‘low-velocity impact’ is defined and major impact-induced damage modes are described from onset of damage through to final failure. Then, the effects of the composite's constituents on impact properties are discussed and post-impact performance is assessed in terms of residual strength.

Review of low-velocity impact properties of composite materials

https://research.birmingham.ac.uk/portal/files/20583243/Oliveux_Dandy_Leeke_Current_status_recycling_fibre_Progress_Materials_Science_2015.pdf

Current status of recycling of fibre reinforced polymers: Review of technologies, reuse and resulting properties

A continuum damage model for composite laminates: Part I - Constitutive model

Investigation of Factors Influencing Dynamic Response of a Tensile Split Hopkinson Pressure Bar

From plants tracking the sun to the aerodynamics of bird wings, shape change is key to the performance of natural structures. After years of reliance on mechanical joints, human engineering now focuses on improving aerodynamic efficiency through smooth, full form changes in material geometry, achieved using technologies such as morphing composites. Promising improved power generation and efficiency in wind turbines and safer more sustainable aircraft and cars, these materials can achieve both large geometric changes with low energy requirements by cycling between several stable physical states and more gradual changes in geometry by exploiting coefficient of thermal expansion mismatch and structural anisotropy, shape memory polymers and 4D printing. The merits and limitations of these various shape change systems are the subject of extensive and ongoing academic research and both commercial and defence industry trials to improve the viability of these technologies for widespread adoption. Shape change capabilities are often associated with problems in material cost, mass, mechanical properties, manufacturability, and energy requirements. Nonetheless, the considerable and rapid advances in this technology, already resulting in successful trials in advanced civilian and military aircraft and high-performance cars, indicate that future research and development of this materials platform could revolutionise many of our most critical power generation, defence and transport systems. 

Functional flexibility: The potential of morphing composites

We propose novel zone-based hybrid laminate concepts for improving the high-velocity impact (HVI) response of baseline carbon fibre-reinforced polymer (CFRP) composites while maintaining similar areal weights and retaining substantial in-plane mechanical properties by requiring that about 80% (by mass) of the baseline CFRP is kept in the hybrid concepts. We identify three zones along the thickness of the laminate according to their role during HVI and implemented tailored materials in these zones to improve the HVI response. We studied a wide range of materials, including: fibre reinforcements of carbon (thin- and thick-plies), glass, aramid and ultra-high molecular weight polyethylene (UHMWPE); a shape memory alloy/carbon weave; and ceramic, alumina and titanium sheets. All laminate concepts have similar areal weights for a meaningful comparison. After defining the various concepts, we manufactured and measured their specific dissipated energy under HVI, and finally carried out post-mortem analysis (including C-Scan and microscopy). The results show up to 95% improvement in energy dissipation over the baseline quasi-isotropic (QI) CFRP configuration. 

Novel zone-based hybrid laminate structures for high-velocity impact (HVI) in carbon fibre-reinforced polymer (CFRP) composites

Structural composites with smart functionalities of self-healing and self-sensing are of particular interest in the fields of aerospace, automotive, and renewable energy. However, most of the current self-healing methodologies either require a relatively complex design of the healing network, or sacrifice the initial mechanical or thermal performance of the carbon fibre composite system after introducing the healing agents. Herein, an extremely simple methodology based on commonly used thermoplastic interleaves has been demonstrated to achieve repeatable easy-repairing and self-sensing functionalities, alongside enhanced mechanical performance in comparison with unmodified carbon fibre/epoxy system. Moreover, due to the high glass transition temperature of the thermoplastic, the repairable composites are shown to have an unchanged storage modulus up to 80 °C, solving the previous limitation of repairable epoxy matrix systems with thermoplastics. High retention of peak load (99%) and a decent recovery of interlaminar fracture toughness (34%) was achieved. Most importantly, the mechanical properties remained greater than the unmodified system after four consecutive cycles of damage and healing. Repeatable in-situ damage sensing was achieved based on the piezoresistive method. This “new” discovery based on an “old” approach, which is fully compatible with current composite manufacturing, may overcome existing conflicts between mechanical performance and healing functions, providing a new solution to extend components’ service life towards a more sustainable development of the composite sector. 

https://doi.org/10.1016/j.compositesa.2022.107337

Smart and repeatable easy-repairing and self-sensing composites with enhanced mechanical performance for extended components life

/pdf/exploring-the-tensile-response-in-small-carbon-fibre-52cit2v075.pdf

Paul Robinson

Papers

Investigation of Factors Influencing Dynamic Response of a Tensile Split Hopkinson Pressure Bar

Functional flexibility: The potential of morphing composites

Novel zone-based hybrid laminate structures for high-velocity impact (HVI) in carbon fibre-reinforced polymer (CFRP) composites

Smart and repeatable easy-repairing and self-sensing composites with enhanced mechanical performance for extended components life

Exploring the tensile response in small carbon fibre composite bundles