This work presents a novel numerical methodology to model the degradation and failure of composite materials like GFRP submitted to monotonic and high cycle fatigue loads. This is done by using the Serial–Parallel Rule of Mixtures homogenisation technique together with a proper mechanical characterisation of the constituent materials of the composite. This paper also proposes an efficient way of estimating the fatigue properties of each of the material constituents (fibre or matrix) to comply with the experimental results obtained at composite level; this enables to estimate the fatigue strength of any stacking/orientation of fibres with only one mechanical characterisation of the material properties. A comparison of the results obtained analytically and experimentally for GFRP is presented. The results show the applicability and accuracy of the proposed methodology in this field.

The experimental evaluation of the shear response of fibre-reinforced plies is a requirement for accurate material models predicting progressive damage. In the first part of the paper, the quality of the Iosipescu shear test is investigated with full-field strain measurements and finite element analyses. In the second part, the in-plane and through-thickness shear response of an orthotropic carbon/epoxy uni-weave non-crimp fabric composite are compared, and the stress–strain curves used as input for two continuum damage mechanics models. Both models were able to predict accurately the nonlinear shear behaviour of the material. The model parameters and the damage evolution laws could easily be extracted from cyclic Iosipescu tests.

The effect of using thin plies to increase the bearing strength of composite laminates has been investigated. A series of 5 laminates of theoretically identical stiffness with varying proportions of thin plies were manufactured using a single material system. Four specimens from each plate were tested for bearing strength and damage was subsequently characterized using an optical microscope. The results show that performance in terms of bearing stiffness, strength at onset of damage, and ultimate bearing stress increase proportionally with the increasing amount of thin plies within the stack. Shifting from a 100% conventional ply laminate to a 100% thin-ply laminate gave an increase of 47% in the strength at onset of damage. Placement of the thin plies within the stack was also shown to be important for strength at initial onset of damage. Microscopic examination of the failure modes for all samples showed fiber kinking, localized to the center of the hole, to be the dominant failure mode regardless of the stacking sequence. © 2020 The Authors

A newly developed physically based model for the longitudinal response of laminated fibre-reinforced composites during compressive damage growth is implemented in a Finite Element (FE) software. It is a mesoscale model able to capture the physics of kink-band formation by shear instability, the influence of the matrix in supporting the fibres and the rotation of the fibres during compression, resulting in more abrupt failure for smaller misalignments. The fibre kinking response is obtained by solving simultaneously for stress equilibrium and strain compatibility in an FE framework. Strain softening creates pathological sensitivity when the mesh is refined. To make the model mesh objective, a methodology based on scaling the strain with the kink-band width is developed. The FE implementation of the current model is detailed with focus on mesh objectivity, and generalized to irregular meshes. The results show that the current model can be used to predict the whole kinking response in a 3D framework and thus account for the correct energy dissipation.

This paper presents the development of a homogenised non-linear soft inclusion which captures the geometric and material non-linearity of impact damage zone loaded in tension and compression. The homogenised non-linear soft inclusion can present a conservative worst case damage zone or use experimental data to mimic the behaviour of a particular damage zone in a simple and computationally efficient way that can be used as a structural design tool for composite structures subjected to impact. The development of the non-linear soft inclusion, implemented in an ABAQUS/Explicit VUMAT, is presented at element and coupon level. The non-linear soft inclusion is validated against experimental coupon data and produces a conservative worst case estimate in all cases investigated. © 2010 Elsevier Ltd.

To develop reliable and physically based models for the crash behaviour of composite laminates, a thorough understanding of the failure mechanisms is crucial. Compression tests of corrugated Non-Crimp Fabric (NCF) laminates, made of carbon fibre unidirectional (UD) fabric with a [0/90]3S stacking sequence and epoxy, have been performed to study the energy absorbing damage mechanisms. Samples from the specimens have been studied with optical microscopy and Scanning Electron Microscopy (SEM) to identify the mechanisms involved in the crushing process. The specimens tested fail partly in bending and partly in pure compression with a mode I delamination separating these two regions. In the region failing in pure compression, the main damage mechanisms are kink band formation and matrix cracking of transverse bundles, whereas in the part failing in bending mixed mode delaminations, intralaminar shear fracture of axial bundles and kink band formation through parts of bundles are identified.

This paper presents a cost-efficiency study of part integration with respect to reduced assembly effort within aeronautical composite structures. The study is performed through the use, and continuous improvement upon, a previously developed cost model. Focus are on the assembly and basic inspection a wing box, part of a section of a full wing, where involved parts are all considered to be manufactured from carbon fibre reinforced plastic (CFRP). Treated cases range from traditional, mechanical joining, to high integration either through co-curing or co-bonding of composite structures. The outcome of presented cost study shows that increased integration decreases the overall production cost of said considered wing box. In general it is shown that co-curing or co-bonding reduces a number of cost-expensive assembly steps in comparison to mechanical joining

This paper presents a new discrete parametrization method for simultaneous topology and material optimization of composite laminate structures, referred to as Hyperbolic Function Parametrization (HFP). The novelty of HFP is the way the candidate materials are parametrized in the optimization problem. In HFP, a filtering technique based on hyperbolic functions is used, such that only one design variable is used for any given number of material candidates. Compared to state-of-the-art methods such Discrete Material and Topology Optimization (DMTO) and Shape Function with Penalization (SFP), HFP has much fewer optimization variables and constraints but introduces additional non-linearity in the optimization problems. A comparative analysis of HFP, DMTO and SFP are performed based on the problem of maximizing the stiffness of composite plates under a total volume constraint and multiple manufacturing constraints using various loads, boundary conditions and input parameters. The comparison shows that all three methods are highly sensitive to the choice of input parameters for the optimization problem, although the performance of HFP is overall more consistent. HFP method performs similarly to DMTO and SFP in terms of the designs obtained and computational cost. However, HFP obtains similar or better objective function values compared to the DMTO and SFP methods. © 2021 The Author(s)

We propose a new deterministic robust design optimization method for composite laminate structures under worst-case material uncertainty. The method is based on a simultaneous parametrization of topology and material and combines a design problem and a material uncertainty problem into a single min–max optimization problem which provides an efficient approach to handle variation of material properties in stiffness driven design optimization problems. An analysis is performed using a design problem based on a failure criterion formulation to evaluate the ability of the proposed method to generate robust composite designs. The design problem is solved using various loads, boundary conditions and manufacturing constraints. The designs generated with the proposed method have improved objective responses compared to the worst-case response of designs generated with nominal material properties and are less sensitive to the variation of material properties. The analysis indicates that the proposed method can be efficiently applied in a robust structural optimization framework. © 2023 The Author(s)

We propose a method to analyze effects of material uncertainty in composite laminate structures optimized using a simultaneous topology and material optimization approach. The method is based on computing worst-case values for the material properties and provides an efficient way of handling variation in material properties of composites for stiffness driven optimization problems. An analysis is performed to evaluate the impact of material uncertainty on designs from two design problems: Maximization of stiffness and minimization of a failure criteria index, respectively. The design problems are solved using different loads, boundary conditions and manufacturing constraints. The analysis indicates that the influence of material uncertainty is dependent on the type of optimization problem. For compliance problems the impact on the objective value is proportional to the changes of the constitutive properties and the effect of material uncertainty is consistent and predictable for the generated designs. The strength-based problem shows that material uncertainty has a significant impact on the response, and the effects of material uncertainty is not consistent and changes for different design requirements. In addition, the results show an increase of up to 25% of the maximum failure index when considering the worst-case deviation of the constitutive properties from their nominal values. © 2022 The Author(s)

Simulation of damage in composite laminates using currently available three-dimensional finite element tools is computationally demanding often to the point that analysis is not practical. This paper presents an enriched shell element that can provide a computationally efficient means to simulate low-velocity impact damage in a composite. The enriched element uses the Floating Node Method and a damage algorithm based on the Virtual Crack Closure Technique that is capable of simulating progressive damage growth consisting of delamination and delamination-migrations from ply to ply during a dynamic impact load. This paper presents results from the shell model in a test-analysis correlation for impact testing of 7-ply and 56-ply laminates. Analysis results from a separate high-fidelity three-dimensional finite element analysis are included also for comparison in the case of the 7-ply laminate, but not in the case the 56-ply laminate due to excessive computational demand. This paper serves as the first application of both models in low-velocity impact simulation. The shell model is considerably more computationally efficient than the high-fidelity model by at least an order of magnitude and is shown to produce results, while not as accurate as the high-fidelity model, potentially sufficiently accurate for a wide range of engineering applications including structural design and rapid prototype assessments.

Three dimensional (3D) fibre-reinforced composites have shown weight efficient strength and stiffness characteristics as well as promising energy absorption capabilities. In the considered class of 3D-reinforcement, vertical and horizontal weft yarns interlace warp yarns. The through-thickness reinforcements suppress delamination and allow for stable and progressive damage growth in a quasi-ductile manner. With the ultimate goal of developing a homogenised computational model to predict how the material will deform and eventually fail under loading, this work proposes candidates for failure initiation criteria. It is shown that the extension of the LaRC05 stress-based failure criteria for unidirectional laminated composites, to this class of 3D-reinforced composite presents a number of challenges and leads to erroneous predictions. Analysing a mesoscale representative volume element does however indicate, that loading the 3D fibre-reinforced composite produces relatively uniform strain fields. The average strain fields of each material constituent are well predicted by an equivalent homogeneous material response. Strain based criteria inspired by LaRC05 are therefore proposed. The criteria are evaluated numerically for tensile, compressive and shear tests. Results show that their predictions for the simulated load cases are qualitatively more reasonable. 

Core shear cracking induced by impact on sandwich panels is a critical failure mode causing severe loss of structural performance. This paper reviews previous experimental and theoretical work in the area and derives improved closed form expressions for initiation of skin rupture and core shear cracking during impact on sandwich panels with foam cores. The criterion for skin rupture is also applicable to laminates without a core. It is shown that the skin rupture load limits the achievable core shear load, and that core shear cracking can be prevented by selecting a core thickness above a certain threshold value. The criteria are successfully validated by comparison with experimental results for a range of thicknesses of skins and cores in panels with carbon/epoxy skins and a Rohacell foam core. The criterion for skin rupture is also validated for plain laminates.

Hybrid bolted joints between composite materials and metals are an example of composites’ sensitivity to notches. The current research paper presents an experimental test setup on single-lap shear bolted joints of a unidirectional (UD) composite laminate with steel plate exposed to a tensile load. A 3D explicit finite element (FE) simulation is developed using ABAQUS/Explicit and a 3D physically-based progressive damage model in the VUMAT subroutine. The model can identify the locations of delamination represented by the fully damaged plies among other degrading properties. Then “damage and cohesive” contact surfaces are defined in the respected damaged plies, for the material interface, to simulate interlaminar damage and delamination. The predicted failure load was significantly influenced by the combination of a 3D physically-based progressive damage model and damage and cohesive surfaces. The combined model can reproduce with accuracy the experimental load–displacement test curves up to the point where bearing damage occurs. Experimental results using DIC measurement are presented to demonstrate the feasibility of the methodology. The study shows that numerical models can be used to help in the stiffness and strength design of the bolted joints of composites and metals, but further improvements are necessary to predict the energy absorption. 

Experimental and numerical studies were carried out to characterize hybrid thin- and thick-ply composite laminates and assess modelling capabilities. Five different composite laminates were manufactured using a single material system with varying proportions of thin plies (0%, 50%, and 100% thin-ply). Bearing tests were performed and the results from the tests were investigated. The results showed that performance, in terms of bearing strength at onset of damage and ultimate bearing stress, increased proportionally with the increasing amount of thin plies within the laminate. Microscopic examination of the failure modes for all laminates was performed at the center of the hole to determine the dominant failure mode. The numerical investigation uses a highly detailed mesoscale model previously validated for crash simulations but never used successfully to bearing damage areas. The results showed a good correlation regarding both the load response and the morphology of damage. 

Three dimensional (3D) textiles are finding their way into fibre reinforced composite applications, and for good reasons; they can eliminate the hazard of delamination and enable complex reinforcement shapes. There is therefore a need for engineering methods to simulate these advanced textile structures during the product development phase. This is many times challenging since the textile architecture is truly 3D and not built by layers as in conventional laminated composites. The overall approach is similar as in a method previously presented by the authors, but some steps are changed that enable modelling of textiles containing strongly curved yarns, yet with very good geometric representation. That is essential for reliable simulations of all parts of the 3D reinforced composite materials, which could then be performed at close to authentic meso level resolution. The resulting textile geometries are very similar to the real materials they represent, both in terms of variation of yarn cross section area and shape along the length of the yarns. This is demonstrated by comparison of details between the real materials and the numerical implementations of their geometry.

In this paper we apply a recently developed methodology to assess the compressive strength of an aero-engine component based on measured fibre misalignment angles. The component is a fan outlet guide vane made from a carbon fibre reinforced polymer, which was manufactured and tested by GKN Aerospace in Sweden. The main novelty with this work is that kink-band formation is predicted from measurements of fibre misalignment angles with high spatial resolution in a real component. In addition, we validate the recently published “defect severity model” on unidirectional specimens of the same material system as the aerospace component. We confirm high accuracy of the model for prediction of compressive strength on unidirectional composite laminates. Further work is however needed to extend the methodology for cases where progressive damage leads to final failure. 

A novel sandwich element design consisting of two facings made of carbon reinforced Textile Reinforced Concrete (TRC), a low density foamed concrete (FC) core and glass fibre reinforced polymer (GFRP) connecting devices was experimentally investigated according to quasi-static and cyclic quasi-static fourpoint bending. Optical measurements based on Digital Image Correlation (DIC) were taken during testing to enable a detailed analysis of the bending behaviour and level of composite action. A model, verified by the experiments, was developed based on non-linear finite element analysis (NLFEA) to gain further insight on the failure mechanisms. Under both loading conditions, the bending behaviour of the TRCFC composite elements was characterized by favourable load bearing capacity, partial composite action, superior ductility and multiple fine cracking. The connecting devices were found to be the critical elements causing the initial failure mechanism in the form of localized pull-out within an element.

Sandwich panels with unidirectional core stiffeners are known for their relatively high bending stiffness at low weight, stability under compressive and shear loads and energy absorption capability. In this study, 3D printing was used to screen biocomposite sandwich panels easily and preliminarily with different unidirectional core stiffener designs. Thermomechanical pulp (TMP) fibre-reinforced poly(lactic acid) (PLA) was used in this study. A corrugated, trapezoid and arched cell structure were tested experimentally and numerically using a bimodular material model, accounting for different behaviour in tension and compression. The trapezoid structure showed the best flexural properties of the three 3D-printed sandwich beams. It was chosen to be explored further, manufacturing it by extrusion. Extrusion is a production process likely to be used in industry on a larger scale. Basic material properties of the biocomposites were obtained from injection moulded dogbone specimens. The flexural properties of the extruded panels were measured experimentally and simulated using finite element analysis. Simulations were done with a hyperelastic material model. Predictions and experiments were in adequate agreement, allowing such kind of simulation to be used for extruded biocomposite sandwich panels. © 2023 The Author(s)