More fundamental test methods are needed to assist the development of physically based and truly predictive simulation tools for composite materials under crash conditions. In this paper, a unidirectional flat specimen that can be used to validate the predicted behaviour from a simulation to the physical behaviour in the experiment is developed. A systematic experimental investigation is conducted to evaluate the influence of the trigger geometry on the crushing response by selecting two trigger types and different trigger angles. For longitudinal crushing, the traditional bevel trigger leads to out-of-plane failure by splaying with a limited amount of in-plane fracture, while the proposed trigger achieves a high amount of compressive fragmentation failure. For transverse crushing, the symmetry of the proposed new trigger improves the specimen stability during the crushing process. It is also observed that the weft threads of the unidirectional fabric reinforcement used for the tests have a strong influence on the longitudinal crushing response. The boundary conditions of the test and the information on the specimen failure gleaned from video recordings and microscopic inspections are discussed in order to facilitate a future correlation with modelling results. © 2017, © The Author(s) 2017.

In this paper a rapid method for residual cure stress analysis from composite manufacturing is presented. The method uses a high-fidelity path-dependent cure kinetics subroutine implemented in ABAQUS to calibrate a linear elastic model. The path-dependent model accounts for the tool-part interaction, forming pressure, and the changing composite modulus during the rubbery phase of matrix curing. Results are used to calculate equivalent lamina-wise coefficients of thermal expansion (CTE) in 3 directions for a linear temperature analysis. The goal is to accurately predict distortions for large complex geometries as rapidly as possible for use in an optimization framework. A carbon-epoxy system is studied. Simple coupons and complex parts are manufactured and measured with a 3 D scanner to compare the manufactured and simulated distortion. Results are presented and the accuracy and limitations of the rapid simulation method are discussed with particular focus on implementation in a numerical optimization framework. © The Author(s) 2021.

This paper details a complete crush model for composite materials with focus on shear dominated crushing under a three-dimensional stress state. The damage evolution laws and final failure strain conditions are based on data extracted from shear experiments. The main advantages of the current model include the following: no need to measure the fracture toughness in shear and transverse compression, mesh objectivity without the need for a regular mesh and finite element characteristic length, a pressure dependency of the nonlinear shear response, accounting for load reversal and some orthotropic effects (making the model suitable for noncrimp fabric composites). The model is validated against a range of relevant experiments, namely a through-the-thickness compression specimen and a flat crush coupon with the fibres oriented at 45° and 90° to the load. Damage growth mechanisms, orientation of the fracture plane, nonlinear evolution of Poisson's ratio and energy absorption are accurately predicted.

In the present study, non-crimp fabric (NCF) composite face sheet sandwich panels have been tested in compression after impact (CAI). Damage in the face sheets was characterised by fractography. Compression after impact loaded panels were found to fail by plastic fibre microbuckling (kinking) in the damaged face sheet. Studies of panels for which loading was interrupted prior to failure revealed extensive stable kink band formation at several positions and in numerous plies. Kink bands initiated and propagated within a wide region close to the point of impact. In addition, kink bands initiated in zones with high shear stresses, away from the impact centre line. Consequently, the fractographic results from this investigation do not support the assumption of modelling the impact damage as an equivalent hole. To achieve accurate predictions of kink band initiation, the stress field must be known. The results from this study imply that bending effects caused by remaining dent or material eccentricities in the damaged region must be considered.

In this study, an assessment of the composite processing-related properties of a newly developed 6-FDA-based phenylethynyl-terminated polyimide (available under the tradename NEXIMID®MHT-R) is presented. Processing schemes, used for preparing high quality carbon fibre-reinforced composites by the use of conventional resin transfer moulding are developed and presented. The influences of manufacturing parameters on glass transition temperature of the composites are presented. The results confirm that composites with exceptionally high Tg, in the range between 350 and 460℃ can be achieved. A manufacturing scheme that yields in composites with Tg of 370℃ is presented and proposed as a good candidate to serve as baseline for further studies.

Multifunctional composites which can fulfil more than one role within a system have attracted considerable interest. This work focusses on structural supercapacitors which simultaneously carry mechanical load whilst storing/delivering electrical energy. Critical mechanical properties (in-plane shear and in-plane compression performance) of two monofunctional and four multifunctional materials were characterised, which gave an insight into the relationships between these properties, the microstructures and fracture processes. The reinforcements included baseline T300 fabric, which was then either grafted or sized with carbon nanotubes, whilst the baseline matrix was MTM57, which was blended with ionic liquid and lithium salt (two concentrations) to imbue multifunctionality. The resulting composites exhibited a high degree of matrix heterogeneity, with the ionic liquid phase preferentially forming at the fibres, resulting in poor matrix-dominated properties. However, fibre-dominated properties were not depressed. Thus, it was demonstrated that these materials can now offer weight savings over conventional monofunctional systems when under modest loading.

A material model for unidirectional fibre-reinforced composites coupling damage to the friction acting on newly created microcracks is developed. While existing material models accounting for progressive damage assume that microcracks remain traction free under compressive load, the present model accounts for contact and friction at microcrack closure. The model is validated against experimental data and it is shown that friction can account for part of the non-linear response and the hysteresis loops typically observed in the shear response of composites. Further validation against simple crushing tests is performed and shows that the physics behind crushing is well captured.

Understanding the damage mechanisms of fiber-reinforced polymer matrix composite materials under high voltage conditions is of great significance for lightning strike protection and high voltage insulation applications of composite structures. In this paper, we investigated effects of the lightning impulse (LI) voltage and high voltage alternating current (HVAC) puncture on damage modes of the electrically nonconductive glass fiber-reinforced polymer (GFRP) matrix composite materials through experimental tests and numerical simulations. The LI and HVAC tests represent the lightning strike and high voltage insulation cable puncture conditions, respectively. Our experimental examinations showed that GFRP composite specimens subjected to the LI voltage test exhibited distinct damage modes compared with those in the HVAC puncture test. The GFRP composite material suffered more charring and fiber vaporization in the HVAC puncture test, whereas less matrix charring and fiber vaporization but severe fiber breakage and delamination in response to the LI voltage tests. The findings indicate that the thermal effect dominates the damage of GFRP composites inflicted by the HVAC puncture test, whereas the mechanical impact effect governs the GFRP composite damage in the LI voltage test. In addition, the electric arc plasma formation during the puncture of the GFRP composite material was modeled through solving Maxwell’s equations and the heat generation equations using finite element analysis. Simulation results provided insights on the effects of duration and intensity of the high voltage electric discharge on the composite damage. © The Author(s) 2020.

The concept of the “effective stiffness” for plies in laminates containing intralaminar cracks is revisited presenting rather accurate fitting expressions for the effective stiffness dependence on crack density in the ply. In this article, the effective stiffness at certain crack density is back-calculated from the stiffness difference between the undamaged and damaged laminate. Earlier finite element method analysis of laminates with cracked 90-plies showed that the effective longitudinal modulus and Poisson’s ratio of the ply do not change during cracking, whereas the transverse modulus reduction can be described by a simple crack density dependent function. In this article, focus is on the remaining effective constant: in-plane shear modulus. Finite element method parametric analysis shows that the dependence on crack density is exponential and the fitting function is almost independent of geometrical and elastic parameters of the surrounding plies. The above independence justifies using the effective ply stiffness in expressions of the classical laminate theory to predict the intralaminar cracking caused stiffness reduction in laminates with off-axis plies. Results are in a very good agreement with (a) finite element method calculations; (b) experimental data, and (c) with the GLOB-LOC model, which gives a very accurate solution in cases where the crack face opening and sliding displacements are accurately described.

An apparent permeability of an idealized unidirectional reinforcement is derived for power-law fluid flow perpendicular to the fibers. The reinforcement consists of regularly ordered, parallel fibers arranged in a quadratic or hexagonal pattern. The expression is obtained starting from first principles for low Reynolds number, incompressible and isothermal flow and is expressed in terms of the radius of the fibers, the fiber volume fraction, and the power-law index. The model is successfully verified with computational fluid dynamic simulations. © 2006 SAGE Publications.

The effect of wood fiber anisotropy and their geometrical features on wood fiber composite stiffness is analyzed. An analytical model for an N-phase composite with orthotropic properties of constituents is developed and used. This model is a straightforward generalization of Hashin's concentric cylinder assembly model and Christensen's generalized self-consistent approach. It was found that most macro-properties are governed by only one property of the cell wall which is very important in attempts to back-calculate the fiber properties. The role of lumen (whether it filled by resin or not) has a very large effect on the composite shear properties. It is shown that several of the unknown anisotropic constants characterizing wood fiber are not affecting the stiffness significantly and rough assumptions regarding their value would suffice. The errors introduced by application of the Hashin's model and neglecting the orthotropic nature of the material behavior in cylindrical axes are evaluated. The effect of geometrical deviations from circular cross-section, representing, for example, collapsed fibers, is analyzed using the finite element method (FEM) and the observed trends are discussed. © 2008 SAGE Publications.

In this paper, a set of failure criteria for transverse failure in non-crimp fabric-reinforced composites is presented. The proposed failure criteria are physically based and take into account the orthotropic character of non-crimp fabric composites addressing the observed lack of transverse isotropy. Experimental data for transverse loading out-of-plane in combination with in-plane loads are scarce. Therefore, to validate the developed criteria, experimental data are complemented with numerical data from a representative volume element model using a meso-micromechanical approach. The representative volume element model also provides a deeper understanding of how failure occurs in non-crimp fabric composites. Strength predictions from the developed set of failure criteria show good agreement with the experimental and numerical data. © The Author(s) 2015.

Intralaminar cracking in relatively thick 90-plies of [(Formula presented.)]s laminates is analyzed using experimental data for two Glass fiber/Epoxy (GF/EP) material systems. Weibull parameters for transverse failure stress of the 90-ply are obtained from experimental intralaminar crack density versus applied strain data, showing that a reliable analysis requires sufficient amount of data in so called noninteractive crack density region. Monte Carlo simulations of cracking were performed using stress distribution between two cracks calculated using two models: Hashin’s model and a novel model that ensures that the average stress is exactly the same as in FEM solution. Due to its features, the Hashin’s model predicts too low intralaminar crack density (it predicts too strong interaction between cracks). The results emphasize the importance of having a proper stress distribution model when performing Monte Carlo simulations. Simulations were used not only to simulate intralaminar cracking in high and very low crack density regions but also for improving the procedure of Weibull parameter determination.

Effect of degree of cure on irreversible (viscoplastic) shear strain development in layers of glass fibre/ epoxy resin (LY5052 epoxy resin) [+45 °/−45 °]s laminate is studied performing a sequence of constant stress creep and viscoelastic strain recovery tests. For fixed values of degree of cure in range from 79.7% to 100%, the viscoplastic strains were measured as dependent on time and stress and Zapa's integral representation was used to characterize the observed behaviour. It is shown that at all degrees of cure the viscoplastic behaviour can be described by Zapa's model with parameters dependent on degree of cure. It is shown that for degree of cure lower than 80% the viscoplastic strains grow much faster and are much more sensitive to the increase of the applied shear stress. These irreversible strains developing in the final phase of the curing can significantly alter the residual stress state in the composite structure.

Stress relaxation and strain recovery phenomena during curing and changed thermal conditions are analyzed using a viscoelastic model developed for thermorheologically complex materials (VisCoR). By making several simplifying assumptions regarding the material behavior, the incremental form of the VisCoR model is reformulated to a version describing thermorheologically simple material and presented in one-dimension for simplicity. The model (called VisCoR-simple) is used to analyze material behavior under various conditions, including stress relaxation behavior at varying temperatures and time scales; tensile loading and unloading tests at high temperatures; stress build up and “frozen-in” strains during curing and following cool-down and strain recovery during the next step of heating. Furthermore, the differences between the so-called “path-dependent” model, which is a linear elastic model with different elastic properties in glassy and rubbery regions, and the presented viscoelastic model are studied. The path-dependent model is an extreme case of the viscoelastic model presented. The importance of considering viscoelasticity when considering temperature and curing effects on polymers and the shortcomings of the path-dependent model are revealed and discussed. © The Author

A novel intralaminar model has, for the first time, been applied and validated for the rate-dependent failure of multidirectional carbon/epoxy laminates. Quasi-static compressive failure is evaluated by the growth of intralaminar rate-dependent damage combined with the interaction of cohesive zones for interlaminar delamination. A special feature of the intralaminar model is the homogenised ply response, allowing simultaneous damage-degradation of the polymer matrix combined with the fibres. To model the observed quasi-brittle failure response of the plies under finite deformation, we have used a viscoelastic-viscoplastic matrix combined with damage and isotropic hardening behaviour. Elastic transverse isotropy is used to model the fibre reinforcement of the plies. Standard cohesive surfaces are used to model the initiation and propagation of delamination. Numerical simulations using ABAQUS/Explicit are performed to predict the growth and delamination of intralaminar damage under compression in different laminates with 56 plies of IM7/8552 carbon/epoxy. Predictions of stress versus strain and damage growth are shown to agree well with experimental results for a range of strain rates and stacking sequences. 

The effect of maleic anhydride grafted polypropylene (MAPP) coupling agents on properties of a new composite made of recycled carbon fibers and recycled polypropylene (rCF/[rPP + MAPP]) was studied experimentally. This new material presented significantly improved properties, compared to the previous generation, without the addition of MAPP (Giannadakis K, Szpieg M and Varna J. Mechanical performance of recycled carbon fibre/PP. Exp Mech 2010; published online.). This was mostly attributed to improvement of the fiber/matrix interface. The inelastic and time-dependent behavior of the MAPP modified composite material in tension was analyzed. A series of quasi-static tensile and creep tests were performed to identify the material model, which accounts for: (a) damage-related stiffness reduction, (b)development of stress and time-dependent irreversible strains described as viscoplasticity, (c) nonlinear viscoelastic behavior. The damage-related stiffness reduction was found to be less than 10%. Although damage-dependent stiffness was not the main source of nonlinearity, it was included in the inelastic material model. In creep tests, it was found that the time and stress dependence of viscoplastic strains follows a power law, which makes the determination of the parameters in the viscoplasticity model relatively simple. The viscoelastic response of the composite was found to be linear in the investigated stress domain. The material model was validated in constant stress rate tensile tests. © The Author(s) 2011.

A fully recycled carbon fiber reinforced maleic anhydride grafted polypropylene (MAPP)-modified polypropylene (rCF/rPP) composite material has been developed and characterized. This new composite was manufactured employing papermaking principles, dispersing the recycled carbon fibers (rCF) in water, and forming them into mats. Two layers of the recycled polypropylene (rPP) films manufactured using press-forming were sandwiched between three rCF preform layers in a stack. The stack was heated and press-formed resulting in a composite plate with a nominal thickness of 1.20 mm and a fiber volume fraction of 40%. A series of tensile tests using rectangular specimens cut in four different directions (0°, 90°, ± 45°) in the composite plate were performed to confirm in-plane material isotropy. Models to predict stiffness and strength of the short fiber rCF/rPP composite were also employed and validated using experiments. The models were found to be in good agreement with experimental results. Fiber length distribution measurements were performed before (unprocessed) and after (processed) composite manufacturing to investigate the influence of processing on fiber degradation. The results revealed a significant reduction in fiber length by the press-forming operation. To model the viscoelastic and viscoplastic responses of the composite an inelastic material model was employed and characterized using a series of creep and recovery tests. From the creep tests, it was found that the time and stress dependence of viscoplastic strains follows a power law. The viscoelastic response of the composite was found to be linear in the investigated stress range. The material model was validated in constant stress rate tensile tests and the agreement was good, even close to the rupture stress. © The Author(s) 2011.

In this study, the mechanical performance assessment of a newly developed carbon fibre-reinforced polyimide composite system T650/NEXIMID (R) MHT-R is presented. This system was subjected to a series of mechanical tests at ambient and elevated temperature (320?) to determine basic material properties. Moreover, an additional test was conducted, using a T650/NEXIMID (R) MHT-R laminate in which the fibre sizing was thermally removed prior to laminate manufacturing, to investigate the effect of fibre treatment on mechanical performance. The experimental results indicated that the T650/NEXIMID (R) MHT-R composites along with exceptionally high T-g (360-420?) exhibited competitive mechanical properties to other commercially available polyimide and epoxy-based systems. At elevated temperature, the fibre-dominated properties were not affected whilst the properties defined by matrix and fibre/matrix interface were degraded by approximately 20-30%. Finally, the fibre sizing removal did not affect the tensile and compressive strength, however, the shear strength obtained from short-beam shear test was deteriorated by approximately 15%.

The old concept of the effective stiffness of a 90-layer with intralaminar cracks is revisited performing 3-D FEM parametric analysis of symmetric and balanced laminates. It is shown, focusing on the effective transverse modulus, that the expected dependence of this property on composite elastic properties and laminate lay-up is very weak and follows very simple rules. Calculations show that the effective longitudinal modulus and Poisson’s ratio of the layer are not affected at all by intralaminar cracking. Simple fitting curve for effective transverse modulus change with normalized crack density is obtained from analysis of GF/EP cross-ply laminate. It is shown, comparing with FEM results and experimental data, that this expression can be used as a ‘master curve’ in laminate theory to predict macroscopic elastic property change with crack density in laminates with very different lay-ups and made of different unidirectional composites.