A crystal plasticity phenomenological model to capture the non-linear shear response of carbon fibre reinforced composites

A hardening response is often observed for shear-dominated large deformation of Carbon Fibre Reinforced Plastics (CFRP). This non-linear response is often modelled by fitting a strain hardening law against experimental stress-strain curves. Inspired by a crystal plasticity framework, a phenomenological model is developed to capture matrix shearing and fibre rotation of CFRP under finite strain. This phenomenological model is first verified by simple shear and transverse compression tests, followed by comprehensive validations against measured stress-strain responses of unidirectional (UD) and cross-ply composite laminates subjected to quasi-static loading. The analytical and finite element predictions of CFRP lamina under simple shear loading confirm that the initial yielding is governed by the shear yield strength of the matrix, while the hardening behaviour is dependent on the modulus and rotation of the carbon fibres. This model accurately predicts the non-linear behaviour of CFRP under off-axis loading without the need of an empirical curve-fitted strain hardening law

Estimating the mode I through-thickness intralaminar R-curve of unidirectional carbon fibre-reinforced polymers using a micromechanics framework combined with the size effect method

A three-dimensional micromechanics framework is developed to estimate the mode I through-thickness intralaminar crack resistance curve of unidirectional carbon fibre-reinforced polymers. Finite element models of geometrically-scaled single edge notch tension specimens were generated. These were modelled following a combined micro-/meso-scale approach, where the region at the vicinity of the crack tip describes the microstructure of the material, while the regions far from the crack tip represent the mesoscopic linear-elastic behaviour of the composite. This work presents a novel methodology to estimate fracture properties of composite materials by combining computational micromechanics with the size effect method. The size effect law of the material, and consequently the crack resistance curve, are estimated through the numerically calculated peak stresses. In-depth parametric analyses, which are hard to conduct empirically, are undertaken, allowing for quantitative and qualitative comparisons to be successfully made with experimental and numerical observations taken from literature

Transmission laser welding of thermoplastics by using carbon nanotube web

Laser welding of transparent and semi-transparent thermoplastics using layers of carbon nanotube (CNT) web as absorbant is reported. Single lap shear specimens were manufactured placing the layers of CNT-web between two polyethylene terephthalate glycol-modified (PETG) sheets, that were successively irradiated with laser power at a wavelength of 1064 nm. Optical analyses were performed to assess the transmittance of the joint under different configurations; for the single layer of CNT web a transmittance of 83 %, in the visible range, was obtained after welding. Single-lap shear tests were performed and a shear strength of 23 MPa was obtained when using one layer of CNT-web. The investigated technology allows using a solid film as laser absorbing material, replacing conventional liquid or dye that need to be processed and applied on the surface before welding, thus speeding up the manufacturing process

Atmospheric Pressure Plasma-Synthesized Gold Nanoparticle/Carbon Nanotube Hybrids for Photothermal Conversion

In this work, a room-temperature atmospheric pressure direct-current plasma has been deployed for the one-step synthesis of gold nanoparticle/carboxyl group-functionalized carbon nanotube (AuNP/CNT-COOH) nanohybrids in aqueous solution for the first time. Uniformly distributed AuNPs are formed on the surface of CNT-COOH, without the use of reducing agents or surfactants. The size of the AuNP can be tuned by changing the gold salt precursor concentration. UV–vis, ζ-potential, and X-ray photoelectron spectroscopy suggest that carboxyl surface functional groups on CNTs served as nucleation and growth sites for AuNPs and the multiple potential reaction pathways induced by the plasma chemistry have been elucidated in detail. The nanohybrids exhibit significantly enhanced Raman scattering and photothermal conversion efficiency that are essential for potential multimodal cancer treatment applications

Comparison of different quasi-static loading conditions of additively manufactured composite hexagonal and auxetic cellular structures

Auxetic cellular structures have the potential to revolutionise sandwich panel cores due to their potential superior energy absorption capability. Because of their negative Poisson's ratio, auxetics behave counterintuitively and contract orthogonally under an applied compressive force, resulting in a densification of material in the vicinity of the applied load. This study investigates three cellular structures and compares their compressive energy absorbing characteristics under in-plane and axial loading conditions. Three unit cell topologies are considered; a conventional hexagonal, re-entrant and double arrowhead auxetic structures. The samples were additively manufactured using two different materials, a conventional Nylon and a carbon fibre reinforced composite alternative (Onyx). Finite element simulations are experimentally validated under out of and in-plane loading conditions and the double arrowhead (auxetic) structure is shown to exhibit comparatively superior energy absorption. For the carbon fibre reinforced material, Onyx, the specific energy absorbed by the double arrowhead geometry was 125% and 244% greater than the hexagonal (non-auxetic) and re-entrant (auxetic) structures respectively

The role of interfacial properties on the intralaminar and interlaminar damage behaviour of unidirectional composite laminates: Experimental characterization and multiscale modelling

The development of the latest generation of wide-body passenger aircraft has heralded a new era in the utilisation of carbon-fibre composite materials. One of the primary challenges facing future development programmes is the desire to reduce the extent of physical testing, required as part of the certification process, by adopting a ‘certification by simulation’ approach. A hierarchical bottom-up multiscale simulation scheme can be an efficient approach that takes advantage of the natural separation of length scales between different entities (fibre/matrix, ply, laminate and component) in composite structures. In this work, composites with various fibre/matrix and interlaminar interfacial properties were fabricated using an autoclave under curing pressures ranging from 0 to 0.8 MPa. The microstructure (mainly void content and spatial distribution) and the mechanical properties of the matrix and fibre/matrix interface were measured, the latter by means of nanoindentation tests in matrix pockets, and fibre push-in tests. In addition, the macroscopic interlaminar shear strength was determined by means of three-points bend tests on short beams. To understand the influence of interfacial properties on the intralaminar failure behaviour, a high-fidelity microscale computational model is presented to predict homogenized ply properties under shear loading. Predicted ply material parameters are then transferred to a mesoscale composite damage model to reveal the interaction between intralaminar and interlaminar damage behaviour of composite laminates

Blast resilience of composite sandwich panels with hybrid glass-fibre and carbon-fibre skins

The development of composite materials through hybridisation is receiving a lot of interest; due to the multiple benefits, this may bring to many industries. These benefits include decreased brittle behaviour, which is an inherent weakness for composite materials, and the enhancement of mechanical properties due to the hybrid effect, such as tensile and flexural strength. The effect of implementing hybrid composites as skins on composite sandwich panels is not well understood under high strain rate loading, including blast loading. This paper investigates the blast resilience of two types of hybrid composite sandwich panel against a full-scale explosive charge. Two hybrid composite sandwich panels were mounted at a 15 m stand-off distance from a 100 kg nitromethane charge. The samples were designed to reveal whether the fabric layup order of the skins influences blast response. Deflection of the sandwich panels was recorded using high-speed 3D digital image correlation (DIC) during the blast. It was concluded that the combination of glass-fibre reinforced polymer (GFRP) and carbon-fibre reinforced polymer (CFRP) layers in hybrid laminate skins of sandwich panels decreases the normalised deflection compared to both GFRP and CFRP panels by up to 41 and 23%, respectively. The position of the glass-fibre and carbon-fibre layers does not appear to affect the sandwich panel deflection and strain. A finite element model has successfully been developed to predict the elastic response of a hybrid panel under air blast loading. The difference between the maximum central displacement of the experimental data and numerical simulation was ca. 5% for the hybrid panel evaluated

Modelling the nonlinear behaviour and fracture process of AS4/PEKK thermoplastic composite under shear loading

The accurate determination of non-linear shear behaviour and fracture toughness of continuous carbon-fibre/polymer composites remains a considerable challenge. These measurements are often necessary to generate material parameters for advanced computational damage models. In particular, there is a dearth of detailed shear fracture toughness characterisation for thermoplastic composites which are increasingly generating renewed interest within the aerospace and automotive sectors. In this work, carbon fibre (AS4)/thermoplastic Polyetherketoneketone (PEKK) composite V-notched cross-ply specimens were manufactured to investigate their non-linear response under pure shear loading. Both monotonic and cyclic loading were applied to study the shear modulus degradation and progressive failure. For the first time in the reported literature, we use the essential work of fracture approach to measure the shear fracture toughness of continuous fibre reinforced composite laminates. Excellent geometric similarity in the load-displacement curves was observed for ligament-scaled specimens. The laminate fracture toughness was determined by linear regression, of the specific work of fracture values, to zero ligament thickness, and verified with computational models. The matrix intralaminar fracture toughness (ply level fracture toughness), associated with shear loading was determined by the area method. This paper also details the numerical implementation of a new three-dimensional phenomenological model for carbon fibre thermoplastic composites using the measured values, which is able to accurately represent the full non-linear mechanical response and fracture process. The constitutive model includes a new non-linear shear profile, shear modulus degradation and load reversal. It is combined with a smeared crack model for representing ply-level damage initiation and propagation. The model is shown to accurately predict the constitutive response in terms of permanent plastic strain, degraded modulus as well as load reversal. Predictions are also shown to compare favourably with the evolution of damage leading to final fracture

Modelling the crush behaviour of thermoplastic composites

Thermoplastic composites are likely to emerge as the preferred solution for meeting the high-volume production demands of passenger road vehicles. Substantial effort is currently being directed towards the development of new modelling techniques to reduce the extent of costly and time consuming physical testing. Developing a high-fidelity numerical model to predict the crush behaviour of composite laminates is dependent on the accurate measurement of material properties as well as a thorough understanding of damage mechanisms associated with crush events. This paper details the manufacture, testing and modelling of self-supporting corrugated-shaped thermoplastic composite specimens for crashworthiness assessment. These specimens demonstrated a 57.3% higher specific energy absorption compared to identical specimen made from thermoset composites. The corresponding damage mechanisms were investigated in-situ using digital microscopy and post analysed using Scanning Electron Microscopy (SEM). Splaying and fragmentation modes were the primary failure modes involving fibre breakage, matrix cracking and delamination. A mesoscale composite damage model, with new non-linear shear constitutive laws, which combines a range of novel techniques to accurately capture the material response under crushing, is presented. The force-displacement curves, damage parameter maps and dissipated energy, obtained from the numerical analysis, are shown to be in a good qualitative and quantitative agreement with experimental results. The proposed approach could significantly reduce the extent of physical testing required in the development of crashworthy structures

Virtual testing of composite structures: Progress and challenges in predicting damage, residual strength and crashworthiness

The entry into service of the Boeing 787 and the Airbus A350 XWB heralded a new era in the utilisation of carbon fibre composite material in the primary structure of passenger aircraft. With an estimated 20?% airframe weight reduction in comparison to equivalent conventional aluminium aircraft, commensurate savings in fuel consumption per revenue passenger kilometre, superior fatigue and corrosion resistance and the promise of reduced maintenance schedules for the operators, it is likely that these materials will continue to feature prominently in future aircraft development programmes. Nonetheless, these ‘all-composite’ aircraft have incurred high development costs which is not a sustainable business model if composites are to be exploited across the product range of airframe manufacturers, especially towards the smaller single-aisle passenger aircraft. The high costs of materials and tooling are exacerbated by slow production rates and the extensive level of physical testing required as part of the development and certification process.The increased use of simulation at all levels of the development cycle provides tremendous opportunities for reducing costs and improving production efficiencies. While the aerospace industry has been at the forefront of incorporating computational tools in the design and optimisation of aircraft, the use of composites has brought with it a new set of challenges in developing reliable and robust simulation tools. This chapter addresses the development and use of numerical modelling aimed at reducing the extent of physical testing. The ultimate objective is to enable certification by simulation which, in essence, requires the ability to reliably predict damage. This chapter will therefore focus on predicting damage initiation and propagation, the residual strength of damaged structures, and assessing the energy-absorbing capacity of composite structures for crashworthiness assessments. While the emphasis is primarily on aerostructures, the automotive and railway industries are exploring similar lightweighting strategies where issues such as crashworthiness are of paramount importance and where simulation will likewise play a prominent role