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This paper presents a three-dimensional Progressive Failure Model based on the chain of bundles able to represent the stiffness loss in unidirectional composite materials loaded in the fibre direction. A representative volume element with a random distribution of fibres with their own radius is considered. Complete stress distributions around fibre breaks are obtained by associating a damage variable to the loss of stress transfer capability along the ineffective length and applying local stress concentrations. The model has been validated by comparing it against the literature results and exhibits good agreement with hybrids and non-hybrid composites. The aim of this model is to simulate the tensile response of unidirectional composite systems dominated by fibre fragmentation mechanisms using a very reduced computational effort, even for larger representative volume elements, compared to micro-mechanical finite element models.

The quest for impact damage tolerant laminates by tailoring stacking sequences has led to nonconventional laminates whose ply sequences are not limited to 0, ±45 and 90°. Departing from the hypothesis that compression after impact (CAI) strength is impaired by the presence of delaminations, a ply sequence was defined by selecting the mismatch angles between plies so as to maintain a central sublaminate with no, or small, delaminations. An experimental test campaign was devoted to validate this hypothesis. To that purpose, baseline and blocked-ply laminates were included in the study. Specimens were tested under low velocity impact followed by compression according to ASTM standards. Delaminations were identified with Ultrasonic C-Scan. The results show delamination locations being successfully predetermined by controlling the mismatch angle, as well as the ensuing improvement in compressive strength retention after impact The authors acknowledge the financial support of the Spanish Ministerio de Ciencia e Innovación under the project MAT2015-69491-C3-1-R (MINECO/FEDER, UE)

The introduction of Variable-Stiffness Laminates requires not only the use of advanced numerical simulation tools but also better understanding of manufacturing induced effects. These computational tools can give beneficial insight that will lead to an effective reduction of test campaigns as well as to increase design possibilities. This paper presents a virtual testing approach to study the influence of tow-drop effects, generated in the manufacturing of steered-fibre composites, by means of nonlinear finite element analyses within the framework of fracture and damage mechanics. X-ray computed tomography was used to characterise the embedded resin-rich areas. Both the progressive failure mechanisms and the ultimate failure loads can be predicted with high realism when compared with experimental observations. In addition, the differences of the tow-drop effects on plain and notched laminates can also be predicted accurately The authors acknowledge the financial support of the Ministerio de Economía y Competitividad of Spain under project MAT2009-07918. The first author would also like to thank the Generalitat de Catalunya for the FI pre-doctorate Grant 2010FI_B00658

The introduction of tow-steered laminates in structural applications demands reliable design methodologies to predict and simulate their mechanical response. The full details about the mechanisms that lead to damage and failure of these novel panels are not yet known. In this work, a numerical analysis tool has been developed for structural simulations in a three-dimensional domain. This paper presents a comparison between different configurations and modeling approaches of these panels. Finite element analyses are carried out to simulate the first-ply failure of tow-steered panels under tensile load. The simulations show how matrix cracking is affected by discontinuities in the fiber angle between adjacent courses and fiber-free areas resultant from manufacturing effects; whereas fiber tensile failure is directly influenced by the orientation angles of the fiber, whose distribution depends on the chosen course cutting method: on one side, or on both sides.

This paper presents finite element simulations of two standardized and sequential tests performed in polymer–matrix composite laminates reinforced by unidirectional fibers: the drop-weight impact test and the compression after impact test. These tests are performed on laboratory coupons, which are monolithic, flat, rectangular composite plates with conventional stacking sequences. The impact and the compression after impact tests are simulated using constitutive material models formulated in the context of continuum damage mechanics. The material models account for both ply failure mechanisms and delamination. Comparisons with experimental data are performed in order to assess the accuracy of the predictions.

Matrix cracking and delamination are the main initial forms of damage in advanced laminated composites manufactured by stacking unidirectional plies of fiber reinforced polymers. In this paper, the onset of matrix cracking is determined for in-plane stress states; in addition, delamination promoted by matrix cracks is analyzed. Taking into account that under in-plane shear stresses composite laminates show a non-linear response prior to the formation of a macro-crack, a plastic-damage model is proposed and implemented. The models predictions correlate well with published experimental data.

This paper presents a model to predict the propagation of transverse cracks in polymer matrix composite laminates. Different possibilities for the crack pattern are analyzed and the different stress–strain response are compared. Taking into account that matrix cracking promotes delamination between the plies, the propagation of delamination is also simulated. The model predictions are compared with experimental data obtained in composite laminates that accumulate transverse cracks and delaminations before failing catastrophically. The possibility and limitations of a general constitutive law applied at ply level, as a mesomodel, is analyzed and the bounds of applicability of the model are explained.

The dimensional accuracy of formed components is an important issue for stamping applications. The flexibility of the air bending process involve that for different bent angles, different final bent radius are obtained. For air bending parts, springback analysis and final angle measurement have usually been considered whereas final bent radius or curvature determination is pushed into the background. Even though this parameter is a non constant geometrical feature when punch bending radius is very small and there are few mechanical methods for measuring these non constant curvatures. This paper describes the measurement procedure of final bent radius through analysis and manipulation of scanned images of bent parts. The method fits different polynomial functions on bent profiles. Once the best adjustment is chosen, a zone curvature in an air bent part is defined and the radius on punch-sheet contact is quantified. For air bending conditions of small punch radius, this technique has given good results in order to determine the main geometrical features of an air bent part: final bent angle, final bent radius and curvature zone.

The mechanical behaviour of glass–fibre composites under fatigue or under long term stress is strongly influenced by the stress corrosion cracking of the fibres. In this paper, a stress corrosion model for composite materials reinforced with environment sensitive fibres has been developed and incorporated into a progressive damage model. The model is based on the definition of an effective stress, σ *( t ), which depends on the stress story, that permits the initial failure probability, given by a Weibull distribution, to be used at any later time. This methodology has been incorporated to a thermodynamically consistent constitutive model valid for early fragmentation stages. The model may be used to account for the fibre’s behaviour in numerical models based on the rule of mixtures. The ability of the extended progressive damage model to reproduce the stiffness loss of the composite under static or cyclic fatigue loads is demonstrated.

Constitutive damage models for fibre-reinforced composite materials should take into account the occurrence of the different damage mechanisms, their interaction and their influence on the resulting mechanical properties. Fibre breakage has usually been considered in damage models by means of deterministic failure criteria which thus leads to non-progressive behaviour or to a complete material collapse which is not realistic. This work presents a progressive damage model for fibre-reinforced composites based on the fragmentation analysis of the fibres. The stiffness loss of a unidirectional composite comes from the parameters of the Weibull distribution of the fibre strength and the mechanical properties of the fibre, matrix and the interface. The model has been developed for the initial stages of damage. The model is formulated in the framework of the mechanics of the continuous media. The constitutive model can be employed to simulate the contribution of fibres in damage models based on the rule of mixtures.

The present work develops an optimization procedure for a geometric design of a composite material stiffened panel with conventional stacking sequence using static analysis and hygrothermal effects. The procedure is based on a global approach strategy, composed by two steps: first, the response of the panel is obtained by a neural network system using the results of finite element analyses and, in a second step, a multi-objective optimization problem is solved using a genetic algorithm. The neural network implemented in the first step uses a sub-problem approach which allows to consider different temperature ranges. The compression load and relative humidity of the air are assumed to be constants throughout the considered temperature range.The mass, the hygrothermal expansion and the stresses between the skin and the stiffeners are defined as the optimality criteria. The presented optimization procedure is shown to yield the optimal structure design without compromising the computational efficiency The authors acknowledge the financial support of the Spanish Government under the Project DPI2009-08048

This article proposes a fully three-dimensional continuum damage model, developed at the sub-ply level, to predict in an integrated way both the intralaminar and the interlaminar failure mechanisms that occur in laminated fiber-reinforced polymer composites. The constitutive model is based on the assumption that the composite material is transversely isotropic, and accounts for the effects of crack closure under load reversal cycles. The damage model is implemented in an implicit finite element code taking into account the requirement to ensure a mesh-independent computation of the dissipated energy. The comparison between the model predictions and published experimental data indicates that the model can accurately predict the effects of transverse matrix cracks on the residual stiffness of quasi-isotropic laminates, the interaction between transverse matrix cracks and delamination, and final failure of the laminate.

Automated Fiber Placement is being extensively used in the production of major composite components for the aircraft industry. This technology enables the production of tow-steered panels, which have been proven to greatly improve the structural efficiency of composites by means of in-plane stiffness variation and load redistribution. However, traditional straight-fiber architectures are still preferred. One of the reasons behind this is related to the uncertainties, as a result of process-induced defects, in the mechanical performance of the laminates. This experimental work investigates the effect of the fiber angle discontinuities between different tow courses in a ply on the un-notched and open-hole tensile strength of the laminate. The influence of several manufacturing parameters are studied in detail. The results reveal that ‘ply staggering’ and ‘0% gap coverage’ is an effective combination in reducing the influence of defects in these laminates.

Thin-ply based laminates are a promising development in composite materials and are expected in the near future to outperform conventional laminates in mechanical performance. A rational design with thin plies requires understanding the effect of ply thickness on each damage mechanism. This paper presents an experimental investigation into damage occurrence in a quasi-isotropic laminate made from thin-ply, bi-axial, Non-Crimp-Fabric (NCF), under different off-axis uniaxial loadings. The NCF layers are positioned through the laminate thickness creating two regions, namely THICK and THIN (with and without ply clustering). Then, the onset and progress of three damage mechanisms (transverse matrix cracking, matrix crack induced delamination and free-edge delamination) for both regions are analyzed by monitoring the specimen's free-edge. The results show that the critical region where damage occurs is that with ply clustering (THICK), whereas delamination originating from matrix cracks or free edge effects are delayed or even suppressed in the THIN region The authors would like to acknowledge Chomarat (Ardeche, France) and Prof. S.W. Tsai (Stanford University, USA) for providing the material, Aldila (Poway, California, USA) for the epoxy system pre-preg and VX Aerospace (Leesburg, Virginia, USA) for manufacturing the panel. The authors also acknowledge the support of the Spanish government through the Ministerio de Economia y Competitividad under the contracts DPI2012-34465 and MAT2012-37552-C03-03. The first author especially acknowledges the support of the Generalitat de Catalunya with the pre-doctoral Grant FI-DGR (2013FI-801062)