During the manufacturing of fibre-reinforced laminates, undesired phenomena are produced, among which the residual stresses need investigation and analysis. The authors have previously presented the research done on the predictive modelling and measurement of the distortions happening during the manufacturing of fibre metal laminates (FML). In this paper, primary measurements of residual stresses are presented. First, the mechanisms governing the development of residual stresses in polymeric laminates are described. Then, the measurement techniques applicable to composite and hybrid materials are reviewed. The hole-drilling method is used to estimate the stress levels in the aluminium sheet of the FML. The results give a new understanding of the stress state in the FML. Comparisons made with the model predictions show the consistency of the results and indeed the need for further improvements for the model validations in terms of the residual stress values in the aluminium sheets.

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This paper reviews predictive models developed for the development of residual stresses and shape distortions during the manufacturing of thermoset polymeric/composite materials. Different sources that produce residual stresses and shape changes in the laminated panels are described and reviewed. An overview is presented on the characterisation and predictions of the phenomena resulting in residual stresses. The focus will be on the models accounting for the parameters during the cure cycle of the thermoset composite materials published in the literature from 2005 until 2018. The material types covered here range from thermoset adhesives, full composites, and fibre metal laminates. Furthermore, selected works are reviewed on the reduction of the shape changes and residual stresses of composites and fibre metal laminates consisting of thermoset polymers.

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The quality control of Glare panels manufacturing is an important and complex process including the evaluation of the quality of the basic constituents, namely the aluminium sheets, and the glass fibre reinforced pre-preg. In particular, the handling of aluminium sheets is one of the most critical steps for the manufacturing of Glare laminates. The unintentionally induced deformations, referred to as kinks, can significantly affect the geometry and the mechanical properties of the final laminate. This paper considers the effects of kinks in Glare laminates by developing a comprehensive investigation based on controlled kink manufacturing in aluminium sheets, non-destructive and destructive evaluation of laminates with kinks, and the impact on the compressive ultimate strength. The results contribute to the understanding of the kink induced defects and to define thresholds for improving future automated laminate manufacturing.

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Distortions and residual stresses are predicted in Fibre Metal Laminates (FMLs) under large deformations. A new modelling procedure is presented for small and large deformation analysis of thermo-viscoelastic problems of orthotropic materials. The material model is implemented in a finite element package which can be used for cure and/or temperature dependent response of composites undergoing large rotations but with small strains.Temperature-dependent and viscoelastic responses are characterised for GLARE, as the mostly used type of FMLs. The geometrically nonlinear thermo-viscoelastic model is used to predict the manufacturing-induced warpage of panels. The curing stresses are calculated from a previously developed model accounting for chemical shrinkage and stiffness evolution of the prepreg layers during cure. The shape deviation of some non-symmetric GLARE panels are predicted and compared to the real measurement of fabricated laminates. The accuracy of the model is verified which can be used in further studies to improve the precision of manufacturing and assembly and also to have better prediction of the fatigue life and residual strength.

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Gaps and overlaps between pre-preg plies represent common flaws in composite materials that can be introduced easily in an automated fibre placement manufacturing process and are potentially detrimental for the mechanical performances of the final laminates. Whereas gaps and overlaps have been addressed for full composite material, the topic has not been extended to a hybrid composite material such as Glare, a member of the family of Fibre Metal Laminates (FMLs). In this paper/research, the manufacturing, the detection, and the optical evaluation of intraply gaps and overlaps in Glare laminates are investigated. As part of an initial assessment study on the effect of gaps and overlaps on Glare, only the most critical lay-up has been considered. The experimental investigation started with the manufacturing of specimens having gaps and overlaps with different widths, followed by a non-destructive ultrasonic-inspection. An optical evaluation of the gaps and overlaps was performed by means of microscope image analysis of the cross sections of the specimens. The results from the non-destructive evaluations show the effectiveness of the ultrasonic detection of gaps and overlaps both in position, shape, width, and severity. The optical inspections confirm the accuracy of the non-destructive evaluation also adding useful insights about the geometrical features due to the presence of gaps and overlaps in the final Glare laminates. All the results justify the need for a further investigation on the effect of gaps and overlaps on the mechanical properties.

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This paper presents results of material characterisation experiments on the hygrothermal viscoelastic behaviour of unidirectional laminates of continuous carbon-fibre reinforced polyamide 6. The material behaviour when subjected to the automotive painting process is of interest. Coefficients of thermal- and -moisture expansion were determined from dilatometer experiments and micrometer measurements together with weighing, respectively. Diffusion coefficients were generated from thermogravimetric analysis and fitted with the Arrhenius equation. Dynamic mechanical analysis and digital image correlation of quasi-static tensile tests were performed to obtain a relaxation curve and a major Poisson's ratio, respectively. The Williams-Landel-Ferry equation was fitted to the time shift factors.

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During the automated manufacturing of fibre reinforced laminates, defects can be produced. Gaps and overlaps between adjacent prepreg layers can be produced in composites during the tape-layup process. However, the topic is not yet studied for hybrid materials, in which metal sheets and thin prepreg layers lead to different effects due to the defects than in full composites. Here, the effect of gaps and overlaps on the mechanical properties of the Fibre metal laminates (FML) is evaluated. Specimens are manufactured with a specified width of gaps/overlaps and the mechanical performance of the panels is evaluated by some selected mechanical tests. Gaps show to have a considerable effect on the mechanical performance of FML. Compression strength of samples with overlaps was rather increased. Discussions are presented on the influence on each mechanical property according to the severity of the defect (gaps/overlap) and the failure mode(s) under consideration.

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Gaps in pre-preg plies, in a case when adjacent plies do not align perfectly, may represent a significant issue in an automated fibre placement manufacturing process and can be detrimental to the properties of the final laminates. In order to tackle the consequences of gaps in the composite material, a comprehensive research has to be performed. Starting from existing manufacturing processes, the effects of gaps were investigated in different ways, including the first indication of their impact on the mechanical properties. The focus of this paper is to extend the state of the art in gaps in composite materials to hybrid composite materials such as Glare, a member of Fibre Metal Laminates (FMLs). The investigation started with the manufacturing of Glare specimens having gaps with different widths, followed by a non-destructive ultrasonic inspection. Also, an optical evaluation of the gaps was performed by microscope image analysis of the cross sections of the specimens. Results from the ultrasonic inspections revealed the presence of areas corresponding to the gaps areas. The optical evaluations supported the ultrasonic results and showed the presence of fibre waviness and delaminations. Finally, the ultimate tensile strength of comparable specimens was determined, proving the detrimental effect of gaps on the strength of the final laminate.@en

A new method is presented to model the interactions between aluminum/composite and composite/composite layers in the Finite Element Method (FEM) simulation of forming processes of Fiber Metal Laminates (FMLs) and inhomogeneous composite laminates. Finite Element simulation is based on a thermo-viscoelastic material model. The method divides the composite laminates and FMLs into continuum and homogeneous aluminum and composite layers and polymeric layers between them. The polymeric layer has a rigid contact to the aluminum and composite layers and its shear strains result in relative displacements of the laminates and creates the required deformations. The method is applied to a press forming simulation of composite laminates (PVC/woven glass fabric) and related FMLs at 140 °C and 160 °C. The simulation is carried out using ABAQUS explicit FE package and the results of mechanical and thermal simulations are compared with experimental results. The results demonstrate the accuracy of this method that models the interactions in the forming simulations of composite laminates and FMLs.

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In this paper, the study of residual stresses in Fibre Metal Laminates (FMLs) is improved using a thermo-viscoelastic model developed for composites which have an orthotropic viscoelastic response. Prepreg layers of GLARE consist of epoxy adhesive that has temperature dependent and viscoelastic response in thermal environments such as the curing cycle. Thermal and viscoelastic response of the adhesive is already characterised using thermo-mechanical methods. In order to get the response for unidirectional fibre-epoxy composite (prepreg) layers, the Correspondence principle is utilised with self-consistent micromechanics equations in the Laplace domain. By reverting, the thermo-viscoelastic response of the composite layer is obtained in the time domain. A user defined material model is added to ANSYS able to analyse the orthotropic thermo-viscoelastic behaviour. Using the developed model, residual stresses of GLARE are calculated. Comparison to the curvature measurements of manufactured non-symmetric laminates, shows the accuracy of the modelling and the improvements with respect to the thermo-elastic analyses.

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