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This work presents an experimental method to measure the compressive crack resistance curve of fiber-reinforced polymer composites when subjected to dynamic loading. The data reduction couples the concepts of energy release rate, size effect law and R-curve. Double-edge notched specimens of four different sizes are used. Both split-Hopkinson pressure bar and quasi-static reference tests are performed. The full crack resistance curves at both investigated strain rate regimes are obtained on the basis of quasi-static fracture analysis theory. The results show that the steady state fracture toughness of the fiber compressive failure mode of the unidirectional carbon-epoxy composite material IM7-8552 is 165.6 kJ/m 2 and 101.6 kJ/m 2 under dynamic and quasi-static loading, respectively. Therefore the intralaminar fracture toughness in compression is found to increase with increasing strain rate. [ABSTRACT FROM AUTHOR]

The effect of through-thickness compressive stress on mode II interlaminar fracture toughness is investigated experimentally and replicated numerically. The modified Transverse Crack Tensile specimen recently proposed by the authors is used, together with an experimental device designed to apply a constant transverse compressive stress on the surface of the specimen. Experiments are conducted using IM7/8552 specimens for different compressive stresses, ranging from 0 to 100 MPa, covering all the practical applications commonly encountered in the aeronautical industry (e.g., tightened filled holes or bolted joints). It is shown that mode II interlaminar fracture toughness increases with the applied compressive through-thickness stress. Finally, experiments are replicated using appropriate numerical models based on cohesive elements that take into account frictional effects. A good agreement between numerical predictions and experiments is found. [ABSTRACT FROM AUTHOR]

This paper presents a consistent anisotropic damage model for laminated fiber-reinforced composites relying on the 3D-version of the Puck failure criterion. The current model is based on ply failure mechanisms (fiber and inter-fiber failures) incorporating energetic considerations into the progressive damage evolution. The proposed formulation is implemented into the implicit FE commercial package ABAQUS using the user-defined capability UMAT . Additionally, the current damage model is combined with a locking-free solid shell formulation via the user-defined subroutine UEL in order to account for geometrical nonlinear effects in thin-walled applications. The reliability of the current formulation is first examined by means of several benchmark applications, and subsequently, the obtained numerical predictions are compared with experimental data corresponding to an open hole tension test. These applications show the practicability and accuracy of the proposed methodology for triggering damage in composite laminates, providing a robust modeling framework suitable for general specimens and loading conditions. [ABSTRACT FROM AUTHOR]

The effect of ply thickness on the onset of intralaminar and interlaminar damage is extremely important for the structural response of laminated composite structures. This subject has gained particular interest in recent years due to the introduction in the market of spread-tow, ultra-thin carbon-fibre reinforcements with different configurations. In the present paper, an experimental test campaign was carried out to study the structural response of aerospace-grade plain weave spread-tow fabrics (STFs) of different areal weights. The results showed that, in spite of an apparent superior longitudinal tensile strength of the thick STF, the multidirectional thin-STF laminate exhibited an improved tensile unnotched strength over the thick-STF laminate, attributed to its damage suppression capability. However, damage suppression was also responsible for similar tensile notched strengths. In compression, the thin-STF laminate performed substantially better than the thick-STF laminate in both unnotched and notched configurations. Finally, a similar bearing response was obtained in both STF laminates, in spite of a slightly higher resistance of the thin-STF laminate to the propagation of subcritical damage mechanisms. [ABSTRACT FROM AUTHOR]

Thin ply laminates are a new class of composite materials with great potential for application in the design of thinner and highly optimized components, resulting in potential weight savings and improved mechanical performance. These new composites can stir the development of lighter structures, overcoming current design limitations as well as notably reducing the onset and development of matrix cracking and delamination events. This paper presents the application of two recent modeling methods for the failure analysis and strength prediction of open-hole thin ply laminates under tensile loading, which exhibit a brittle response upon failure: (i) the analytical coupled energy-stress Finite Fracture Mechanics (FFMs) technique, and (ii) the FE-based Phase Field (PF) approach for fracture that is incorporated into an enhanced assumed solid shell element. The predictions obtained using both strategies are compared with experimental data. These correlations exhibit a very satisfactory level of agreement, proving the robustness and reliability of both methods under consideration. [ABSTRACT FROM AUTHOR]

• Assessment of CLC fixture constraint on notched composite specimens. • Experimental characterisation using UD thermoplastic and 2D woven thermoset composites. • Original constraint assumption resulted in significant underestimation of compressive fracture toughness. • DENC specimen testing also shown to provide an underestimation of this value. • New interpretation of CLC constraint effects results in higher fracture toughness estimates. This study reassesses the constraint effect provided by a Combined Loading Compression (CLC) fixture on notched specimens used to measure the compressive intralaminar fracture toughness of composite materials. An experimental campaign is carried out to determine whether a variation in the specimen gauge section length would result in a significant change in the nominal strength of Double Edge Notch CLC specimens. Following the testing of six sets of different specimen geometries for both a uni-directional thermoplastic and a bi-directional woven thermoset composite, the effect of the gauge length is analysed using the size-effect method. Results show that nominal notched strength variation is minimal, suggesting that the CLC clamp does not effectively constrain the gripped sections against in-plane deformations. This assessment proves that the correction factor used in the determination of the R -curve needs to be computed considering the entire specimen geometry, including the gripped regions. Furthermore, the results reveal that previous characterisation campaigns conducted using end-loaded Double Edge Notch Compression specimens could have led to a significant underestimation of the steady-state fracture toughness. [ABSTRACT FROM AUTHOR]

Longitudinal tensile failure of unidirectional fibre-reinforced composites remains difficult to predict accurately. The key underlying mechanism is the tensile failure of individual fibres. This paper objectively measured the relevant input data and performed a detailed experimental validation of blind predictions of six state-of-the-art models using high-resolution in-situ synchrotron radiation computed tomography (SRCT) measurements on two carbon fibre/epoxy composites. Models without major conservative assumptions regarding stress redistributions around fibre breaks significantly overpredicted failure strains and strengths, but predictions of models with at least one such assumption were in better agreement for those properties. Moreover, all models failed to predict fibre break (and cluster) development accurately, suggesting that it is vital to improve experimental methods to characterise accurately the in-situ strength distribution of fibres within the composites. As a result of detailed measurements of all required input parameters and advanced SRCT experiments, this paper establishes a benchmark for future research on longitudinal tensile failure. [ABSTRACT FROM AUTHOR]

This work represents the first step towards the application of machine learning techniques in the prediction of statistical design allowables of composite laminates. Building on data generated analytically, four machine algorithms (XGBoost, Random Forests, Gaussian Processes and Artificial Neural Networks) are used to predict the notched strength of composite laminates and their statistical distribution, associated to the uncertainty related to the material properties and geometrical features. This work focuses not only on the so-called Legacy Quad Laminates (0°/90°/ ± 45°), typically used in the design of composite aerostructures, but also on the newer concept of double-double (or double-angle ply) laminates. Very good representations of the design space, translating in low generalization relative errors of around ± 10%, and very accurate representations of the distributions of notched strengths around single design points and corresponding B-basis allowables are obtained. All machine learning algorithms, with the exception of the Random Forests, show very good performances, with Gaussian Processes outperforming the others for very small number of data points while Artificial Neural Networks have better performance for larger training sets. This work serves as basis for the prediction of first-ply failure, ultimate strength and failure mode of composite specimens based on non-linear finite element simulations, providing further reduction of the computational time required to virtually obtain the design allowables for composite laminates. [ABSTRACT FROM AUTHOR]

Many models for prediction of longitudinal tensile failure of unidirectional (UD) composites have been developed in the last decades. These models require significant assumptions and simplifications, but their consequences for the predictions are often not clearly understood. This paper therefore presents a blind benchmark of seven different models applied to two virtual materials. Reliably capturing the localisation of stress concentrations was vital in predicting the effect of matrix stiffness and strength on composite failure strain and strength as well as fibre break and cluster development. Although the models have different assumptions regarding stress redistributions around fibre breaks, the 2-plet (clusters of two fibre breaks) development was similar. Distance-based criteria were shown to be inadequate for monitoring cluster development. The discussions provide detailed insight into how the model assumptions are linked to the differences in the predictions. Image 1 [ABSTRACT FROM AUTHOR]

• The apparent intralaminar fracture toughness scales linearly with the ply thickness. • This increase is related to the appearance of split cracks near the notches. • The intralaminar fracture toughness can only be accurately measured using thin plies. • The intralaminar fracture toughness used in mesomodels should not be scaled with the ply thickness. The apparent crack resistance curves associated with longitudinal tensile failure of carbon fibre reinforced epoxy composite systems with ply thicknesses of 0.075 mm, 0.134 mm and 0.268 mm are determined experimentally from the size effect law of geometrically similar double edge notch tension cross-ply specimens. An increase in notched strength as a function of the ply thickness and a corresponding increase of the measured intralaminar fracture toughness is observed. This increase is shown to be a consequence of the appearance of split cracks in the thicker 0° plies in the vicinity of the notches. The numerical models developed demonstrate that if the notch blunting mechanisms are properly represented, the laminate strength is well predicted for a constant value of the ply intralaminar fracture toughness. This supports the hypothesis that these mechanisms are responsible for the higher strength of the thicker plies, and that the intralaminar fracture toughness of the 0° plies should not be scaled with the ply thickness. [ABSTRACT FROM AUTHOR]

Typical composite structures are characterized by having brittle failure and thus higher safety factors. Alternatively, hybrid composites with pseudo-ductile behaviour can contribute to safer and more efficient designs. The objective of this work is to achieve a "pseudo-ductile" behaviour in the response of unidirectional hybrid composites when subjected to uniaxial traction. To understand under what circumstances such behaviour is obtained, optimization problems are formulated and solved here. A Spring Element Model is used to predict the hybrid composite failure with relatively low computational cost. This cost is an important issue since optimization requires several analyses of the composite response. Two different types of optimization problems are considered. Firstly, one finds out the optimal properties of fibres to hybridize and get the pseudo-ductile behaviour. Once an optimal hybridization is found, another optimization problem is solved in order to understand the influence of the fibre dispersion on the composite response. Both optimization problems are solved using a Genetic Algorithm. The optimal results obtained show hybrid composites having a considerable pseudo-ductile behaviour. It is also shown that the degree of spatial fibre dispersion at the composite microstructure level greatly affects the composite response. High fibre dispersion favours the pseudo-ductile behaviour. [ABSTRACT FROM AUTHOR]

Fibres Reinforced Polymers (FRPs) are found in several applications in aeronautics, space and in the automotive industry. These applications are exposed to loading conditions, including impact, which results in a complex mechanical response that is vital to accurately predict. This is particularly important for a new generation of thermoplastic-based composites. The model proposed in this work is an invariant-based approach to represent viscous effects in polymer composites. The model developed only requires two viscous parameters to calibrate the viscoelastic behaviour. A good correlation between the simulations and experimental data obtained in off-axis tests in tension and compression is obtained. [ABSTRACT FROM AUTHOR]

The development of numerical tools to complement the experimental determination of structural design parameters is of key importance to hasten the certification process of new materials and structures. In this work, a methodology to simulate elastic and inelastic deformation of composite laminates at the subcomponent level based on finite element analysis is proposed. A modified version of a continuum damage model proposed in the literature combined with a frictional cohesive zone model is used to capture the intralaminar and interlaminar damage and failure of composite laminates in general loading conditions. The methodology is validated for three aerospace-grade carbon fibre reinforced (epoxy) polymer composite material systems and coupon configurations with increasing level of complexity, including unnotched tension/compression, open-hole tension/compression and filled hole compression. The predictions obtained are in good agreement with the experimental results for all the test cases, oftentimes within the standard error of the tests, with a maximum relative error of 13%. [ABSTRACT FROM AUTHOR]