Press forming a 0/90 cross-ply advanced thermoplastic composite using the double-dome benchmark geometry

A pre-consolidated thermoplastic advanced composite cross-ply sheet comprised of two uniaxial plies orientated at 0/90° has been thermoformed using tooling based on the double-dome bench-mark geometry. Mitigation of wrinkling was achieved using springs to apply tension to the forming sheet rather than using a friction-based blank-holder. The shear angle across the surface of the formed geometry has been measured and compared with data collected previously from experiments on woven engineering fabrics. The shear behaviour of the material has been characterised as a function of rate and temperature using the picture frame shear test technique. Multi-scale modelling predictions of the material’s shear behaviour have been incorporated in finite element forming predictions; the latter are compared against the experimental results

Prediction of the Failure Locus of C/PEEK Composites under Transverse Compression and Longitudinal Shear Through Computational Micromechanics

The potential of computational micromechanics to predict the failure locus of a unidirectional C/PEEK composite subjected to transverse compression and longitudinal shear was established. Numerical simulations were compared with the experimental results of Vogler and Kyriakides [Vogler TJ, Kyriakides S. Inelastic behavior of an AS4/PEEK composite under combined transverse compression and shear. Part I: Experiments. Int J Plasticity 1999;15:783–806], which contain detailed information of the matrix and fiber properties as well as the failure micromechanisms during multiaxial loading. Analyses were based in the finite element analysis of a three-dimensional representative volume element of the lamina microstructure and included the main deformation and failure mechanisms observed experimentally, namely matrix shear yielding and interface decohesion. In addition, the numerical predictions of the failure locus for composites with strong and weak interfaces were compared with those obtained by current phenomenological failure models to establish the accuracy and range of validity of these criteria

A New Approach to Fibrous Composite Laminate Strength Prediction

A method of predicting the strength of cross-plied fibrous composite laminates is based on expressing the classical maximum-shear-stress failure criterion for ductile metals in terms of strains. Starting with such a formulation for classical isotropic materials, the derivation is extended to orthotropic materials having a longitudinal axis of symmetry, to represent the fibers in a unidirectional composite lamina. The only modification needed to represent those same fibers with properties normalized to the lamina rather than fiber is a change in axial modulus. A mirror image is added to the strain-based lamina failure criterion for fiber-dominated failures to reflect the cutoffs due to the presence of orthogonal fibers. It is found that the combined failure envelope is now identical with the well-known maximum-strain failure model in the tension-tension and compression-compression quadrants but is truncated in the shear quadrants. The successive application of this simple failure model for fibers in the 0/90 degree and +/- 45 degree orientations, in turn, is shown to be the necessary and sufficient characterization of the fiber-dominated failures of laminates made from fibers having the same tensile and compressive strengths. When one such strength is greater than the other, the failure envelope is appropriately truncated for the lesser direct strain. The shear-failure cutoffs are now based on the higher axial strain to failure since they occur at lower strains than and are usually not affected by such mechanisms as microbuckling. Premature matrix failures can also be covered by appropriately truncating the fiber failure envelope. Matrix failures are excluded from consideration for conventional fiber/polymer composites but the additional features needed for a more rigorous analysis of exotic materials are covered. The new failure envelope is compared with published biaxial test data. The theory is developed for unnotched laminates but is easily shrunk to incorporate reductions to allow for bolt holes, cutouts, reduced compressive strength after impact, and the like

Meso-scale Finite Element (FE) modelling of biaxial carbon fibre non-crimp-fabric (NCF) based composites under uniaxial tension and in-plane shear

Non-crimp-fabrics (NCF) are promising materials in aerospace applications. The complex internal structure of NCF composites could influence the in-plane performances, which needs to be comprehensively studied. The novel three-dimensional (3D) meso-scale repeated unit cell (RUC) models were proposed for biaxial NCF composites based on the Finite Element (FE) method to conduct a systematic parameter study, including layup sequence, out-of-plane tow waviness, resin-rich areas, transverse tow placements and delamination. The meso RUC model could effectively predict the homogenised uniaxial tensile and in-plane shear properties of biaxial NCF composites based on their meso-scale constituent and material properties. A multiscale framework was also developed for biaxial NCF composites. A micromechanical representative volume element (RVE) model provided homogenised mechanical properties for tows, and a macroscopical FE model validated the test results using the homogenised results obtained from meso RUC models. The numerical results were in good agreement with the experiment results. Therefore, the multiscale framework provides an insight into the critical parameters influencing the in-plane properties of NCF composites and an analysis tool for NCF material design

Composite structural materials

Overall emphasis is on basic long-term research in the following categories: constituent materials, composite materials, generic structural elements, processing science technology; and maintaining long-term structural integrity. Research in basic composition, characteristics, and processing science of composite materials and their constituents is balanced against the mechanics, conceptual design, fabrication, and testing of generic structural elements typical of aerospace vehicles so as to encourage the discovery of unusual solutions to present and future problems. Detailed descriptions of the progress achieved in the various component parts of this comprehensive program are presented

Computational micromechanics of the transverse and shear behavior of unidirectional fiber reinforced polymers including environmental effects

Qualification of Fiber Reinforced Polymer materials (FRP’s) for manufacturing of structural components in the aerospace industry is usually associated with extensive and costly experimental campaigns. The burden of testing is immense and materials should be characterized under different loading states (tension, compression, shear) and environmental conditions (temperature, humidity) to probe their structural integrity during service life. Recent developments in multiscale simulation, together with increased computational power and improvements in modeling tools, can be used to alleviate this scenario. In this work, high-fidelity simulations of the material behavior at the micro level are used to predict ply properties and ascertain the effect of ply constituents and microstructure on the homogenized ply behavior. This approach relies on the numerical analysis of representative volume elements equipped with physical models of the ply constituents. Its main feature is the ability to provide fast predictions of ply stiffness and strength properties for different environmental conditions of temperature and humidity, in agreement with the experimental results, showing the potential to reduce the time and costs required for material screening and characterization.The authors would like to acknowledge the support provided by AIRBUS SAS through the project SIMSCREEN (Simulation for Screening Composite Materials Properties). Additionally, C.S. Lopes acknowledges the support of the Spanish Ministry of Economy and Competitiveness through the Ramón y Cajal program. The help of Dr. Miguel Monclús and Dr. Jon Molina in the experimental work is also gratefully acknowledged

Experimental and numerical studies on forming and failure of woven thermoplastic composites

Fuel efficiency, weight reduction, and sustainability are major global challenges fuelling research into advanced material systems. There is an urgent necessity to manufacture light weight products from highly recyclable, lightweight materials. Woven thermoplastic composites are attractive light weight candidates for the replacement of metallic parts in a wide range of industries from automotive to aerospace. They offer attractive benefits such as high specific strength, balanced thermomechanical properties, improved fatigue and wear resistance, and recyclability. However, there are two major concerns needed to be addressed properly before they can be adopted into mainstream manufacturing industries: forming and failure. This thesis investigates formability and failure behaviour of a woven self-reinforced polypropylene composite (SRPP) using a custom-built press, an open die configuration, and a real time 3D photogrammetry measurement system (ARAMIS). Specimens with novel geometries having different aspect ratios and fibre orientations were formed until catastrophic failure. Deformations and strains were measured to construct a strain-based path dependant failure envelope in a principal strain space. Optical microscopy investigations were conducted to reveal the relation between incorporated failure mechanisms and deformation modes of the composite. Then, experimental forming and failure behaviours of SRPP were benchmarked against a woven glass-fibre reinforced polypropylene composite (GRPP). Characterisation experiments were conducted on SRPP composite using a universal testing machine and a real time strain measurement system to elucidate mechanical behaviours of the composite at room temperature. The highly nonlinear behaviour of SRPP necessitated adopting an incremental deformation theory to develop constitutive stress-strain relations and construct an orthotropic material model. Material and failure models were coded in FORTRAN and implemented into a finite element analysis using the Abaqus-Implicit solver. A finite element model with a nonlinear contact condition was developed to predict formability and failure behaviours of the SRPP during stamp forming process. Comparison between experimental and finite element analysis results proved high accuracy and reliability of the developed numerical model in predicting formability and failure of a woven self-reinforced polypropylene composite. The finite element analysis predictions demonstrated the potential of the developed numerical model to accurately predict strain path, evolution of surface strains, and failure initiation in woven composites. Finally, wrinkling behaviour of the SRPP composite was investigated through a novel Modified Yoshida Buckling Test (MYBT). The inadequacy of the current wrinkling measures to predict compressive instability in woven composites was shown. A more reliable, wrinkling-sensitive criterion, based on gradient of principal strains, was proposed. An important conclusion drawn from this study indicates that proper selection of forming path, fibre orientation and specimen dimensions facilitates manufacturing complex parts from woven thermoplastic composites. The developed numerical model showed the potential to predict failure of the thermoplastic composites experiencing complicated loading conditions. This process eliminates the need to conduct expensive, time consuming trial and error manufacturing processes to achieve flawless products made of woven thermoplastic composites

Production and mechanical characterization of graphene micro-ribbons

Patterning of graphene into micro- and nano-ribbons allows for the tunability in emerging fields such as flexible electronic and optoelectronic devices, and is gaining interest for the production of more efficient reinforcement for composite materials. In this work we fabricate micro-ribbons from CVD graphene by combining UV photolithography and dry etching oxygen plasma treatments. Raman spectral imaging confirms the effectiveness of the patterning procedure, which is suitable for large-area patterning of graphene on wafer-scale, and confirms that the quality of graphene remains unaltered. The produced micro-ribbons were finally transferred and embedded into a polymeric matrix and the mechanical response was investigated by in-situ mechanical investigation combining Raman spectroscopy and tensile/compressive tests

Multiscale analysis and damage tolerance of carbon fibre biaxial non-crimp-fabric composites

Composite materials are attractive in aerospace and automotive applications due to their high stiffness-to-weight ratios. Non-crimp-fabric (NCF) reinforced composites have been receiving attention in the composite market due to their cost-effectiveness and excellent mechanical performance. However, compared to traditional unidirectional laminates (UD), NCF composites have different mechanical behaviours due to their heterogeneous internal structures. The ability to predict the mechanical performance of NCF composites is necessary for a robust and reliable design. The objective of this PhD research was to develop numerical methods to predict the in-plane mechanical behaviour and damage tolerance of the carbon fibre reinforced biaxial NCF composites. The in-plane mechanical behaviours of NCF composites were investigated at different scales by conducting multiscale analyses in the LS-DYNA finite element (FE) software. The macroscopical FE modelling results were validated by a series of in-plane characterisation tests of biaxial NCF composites. The compression-after-impact (CAI) test was adopted to assess the damage tolerance of the composite laminates. The complex failure mechanisms of NCF composites involved in a CAI failure process were comprehensively studied by experimental methods. The experimental results contributed to the validation of FE models to predict the low-velocity impact (LVI) and CAI behaviours of NCF composites in LS-DYNA. Furthermore, different laminate designs were employed to change the CAI behaviour of NCF composites by altering layup sequence and ply-level hybridisation. An optimised scheme was proposed to enhance the CAI behaviour of NCF composites, providing a practical guide to damage tolerance design.Open Acces