UD Composites Research Articles

Evaluation of second-order correlations adjusted with simulated annealing on physical properties of unidirectional nonoverlapping fiber-reinforced materials (UD Composites)

The focus of this paper is on aligned fiber-reinforced composites, where fiber centers were randomly distributed in their cross-sections. The volume fractions of fibers were [Formula: see text]% and [Formula: see text]%. Samples were built with the help of the simulated annealing technique according to the chosen Radial Distribution Functions (RDFs). For each sample, the fields of local stresses and heat fluxes were simulated by finite element method. Then, homogenization by volume averaging was performed in order to investigate both the effective mechanical and thermal properties. The effect of RDF shape on elastic and thermal properties was quantified along with the influence of the probability of near neighbors of fibers on the physical properties. The more the fiber distributions deviate from Poisson’s Law, the higher the results compared to the lower bound of Hashin–Shtrikman.

Machining induced damage in orthogonal cutting of UD composites: FEA based assessment of Hashin and Puck criteria

FE models offer a promising virtual alternative to study machining responses of composites, thereby allowing an informed selection of favorable cutting parameters. Appropriate mathematical schemes are needed to predict damage initiation in fibrous composites; Hashin and Puck failure criteria are the most commonly used for this purpose. This work focusses on the assessment of these criteria to predict ply-level damage in orthogonal cutting of unidirectional composites. A novel algorithm accounting for strain-softening after damage initiation is also proposed. Efficacy of the developed FE model is shown by simulating effects of the cutter tool on the damage of underlying workpiece.

Predicting damage accumulation and fatigue life of UD composites under longitudinal tension

Unidirectional composites under cyclic longitudinal tension develop damage through the accumulation and clustering of fibre–breaks, and through fibre–matrix interface debonding growth; these processes lead to a reduction of the material’s load–carrying ability with increasing loading cycles, which raises a challenge to predict the fatigue response of composite structures. This paper proposes the first model in the literature to predict the kinetics of fibre–breakage and their effect on the macroscopic response of unidirectional composites under cyclic longitudinal tension. The model couples (i) a statistical hierarchical scaling law to predict fibre failure with (ii) a Paris law to predict interfacial fatigue damage propagating from broken fibres; due to its analytical formulation, the model predicts the response of composite bundles up to virtually any size and for their entire fatigue life in less than one minute. Model predictions for the accumulation and clustering of fibre–breaks show a good correlation with experiments from the literature; the model also predicts that, although the critical cluster size does not vary significantly between static, low–cycle/high–stress fatigue, and high–cycle/low–stress fatigue, the material can withstand the highest amount of softening under high–cycle/low–stress fatigue.

Extension of Puck's inter fibre fracture (IFF) criteria for UD composites

Puck's action plane failure criteria have already proven their capability in the first and second world-wide failure exercises (WWFE-I and WWFE-II). However, Puck and his co-workers have only recommended inclination parameters for GFRP/Epoxy and CFRP/Epoxy. These UD composites generally have high transverse compressive-to-tensile strength ratios YC/YT, and are regarded by them as intrinsically brittle materials. Therefore, Puck's inter fibre fracture (IFF) criteria might not be directly applied to other types of UD composites with low YC/YT ratios. In particular, Puck's original IFF criteria will result in unrealistic predictions if YC/YT is extremely low. In the present study, Puck's original IFF criteria are extended to all types of UD composite materials to solve the problem. Composites are divided into three categories, namely semi-brittle materials, brittle materials, and intrinsically brittle materials. Depending on the material category, three different algorithms are proposed for determination of the two parameters in Puck's IFF criteria, namely the resistance of the action plane against transverse tensile stressing, R⊥At, as well as the inclination parameter of contour lines of the fracture body, p⊥⊥. The present criteria are theoretically evaluated for composites with low YC/YT ratios, while the predictions of the present criteria are compared with the test results of composites with high YC/YT ratios such as thermoset GFRP and CFRP. Theoretical and experimental assessment demonstrates the reasonableness of the extension of Puck's IFF criteria.

Numerical study of the effects of irregular pores on transverse mechanical properties of unidirectional composites

For investigating the effect of the irregular pores on transverse elastic properties of the unidirectional Carbon/Carbon (C/C) composites, a new strategy is proposed to generate the representative volume element (RVE) on the basis of scanning electron microscope (SEM) images, in which these pores are approximated as the polygons and the identical fibers are generated by using the RSA algorithm. A good agreement is achieved by comparing the effective elastic properties obtained from the FEM-homogenization techniques to those predicted by experimental tests and two-step Mori-Tanaka method. FEM results indicate that the convergence of the results is seen when the number of vertices of the polygon reaches 40 and the RVE edge length is eight times of the maximum pores size (L/lmax = 8). Meanwhile, the average departure from isotropy is below 5% and the UD composites can be considered as transverse isotropic. The effective transverse elastic properties would decrease with increase of the porosity and with increase of the pores clustering ηp.

The Tsai-Wu failure criterion rationalised in the context of UD composites

This paper is to rationalise the empirical aspect of the Tsai-Wu failure criterion in the context of UD composites associated with the determination of the interactive strength property F12 based on the analytic geometry. It reveals that the condition of closed failure envelope cannot be satisfied by all UD composites and hence the restriction should be abandoned. Depending on the way the failure envelope opens, UD composites can be classified into two categories. (a) F12 can be determined uniquely using the conventional strength properties with an additional assumption that the material exhibits very high or infinite strength under triaxial compression at a specific stress ratio; or (b) The Tsai-Wu criterion leads to one of the two scenarios: either allowing infinite strength for an in-plane stress state or allowing infinite strength under triaxial stresses involving tension along fibres.

Effect of span length on micromechanics of fracture under flexure load in CFRP composites

ABSTRACTUnidirectional (UD) and bidirectional (BD) woven carbon fiber reinforced plastic (CFRP) composites were tested under three-point flexure at different span lengths ranging from 20 to 60 mm. Expectedly, flexure load decreases with increasing span length. A sharp change in flexure load is observed at a span length of 30 mm with a transition in failure mechanism. Analysis suggests that crack initiation occurs at compression side and tension side, respectively, in UD and BD woven composites. In case of UD composites, at a span length of 20 mm, the failure was governed by kink band initiation (two fractures per fiber) followed by tension side delaminations, and at other span lengths, microbuckling (multiple fiber fractures) together with compression side delaminations led to failure. On the other hand, the failure in BD composites at a span length of 20 mm is controlled predominantly by thickness crack propagation with tensile fiber pullouts, whereas for other span cases the failure occurs due to the development of delaminations at interlace regions. The stress analysis by finite element modeling (FEM) also supports the experimental observations in an attempt to correlate the failure mechanisms under flexure loads in CFRP composites.

Mechanical performance of hybrid thermoset composites: Effects of matrix and reinforcement hybridization

Hybrid epoxy/vinyl ester (EP/VE) and epoxy/unsaturated polyester (EP/UP) resins were used as matrices to prepare unidirectional carbon fibre (CF) and carbon/glass fibre (CF/GF) reinforced composites targeting toughness of improvement. Hybrid resins were produced simultaneously (one-pot) and sequentially. (Thermo)mechanical properties of hybrid resins were determined in surface hardness, three-point bending, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC) tests. Hybrid matrix composites with CF and CF/GF hybrid reinforcements were characterized with quasi-static mechanical (three-point bending) tests performed in 0° (longitudinal) and 90° (transverse) directions. In addition, flexural fatigue tests were run on UD composites. Interlaminar properties were deduced from in-plane shear strength (IPSS) test and fractographic inspection in a scanning electron microscope. The EP/VE hybrid resin exhibited improved energy absorption compared to neat constituent resins in contrast to EP/UP. Using hybrid resins as matrix highly improved the toughness and durability of the corresponding composites. Improved energy absorption was attributed to the phase structure of the hybrid resins, which also positively affected the IPSS.

Influence of PA11 and PP thermoplastic polymers on recycling stability of unidirectional flax fibre reinforced biocomposites

In this study, we explore the interest of recycling PA11 and PP unidirectional flax fibre composites. After describing the mechanical performance of the various components of the composite, its tensile properties are investigated before and after recycling. The results indicate a good stability of the modulus and strength of the composites during the multiple recycling steps, which also applies to the fibre properties, as highlighted by nanoindentation tests throughout the recycling process. However, interestingly, the elongation at break of the PA11 composites shows a significant increase with the number of injection cycles, ranging from 5.2 to 14.7%. Further analyses show that the PA11 matrix has highly stable thermal, rheological and mechanical properties, so the matrix cannot be involved in this increased elongation at break. Flax fibers display a steady decrease in length with recycling, especially in the case of PA11 matrix compared to PP matrix. This drastic decline is explained by the high viscosity of the matrix, but above all by the presence of defects (kink bands) in the flax fibers, spaced at intervals of 60 μm, which is consistent with the residual lengths measured at the end of recycling. PA11 and PP flax UD composites prove to be very efficient after recycling, while PA11 biocomposites exhibit an increase of elongation after recycling that makes it a material of first choice in the biocomposites industry.

Effect of fibre shape and interphase on the anisotropic viscoelastic response of fibre composites

We perform Monte Carlo analyses of the anisotropic viscoelastic response of random RVEs representing the microstructure of UD fibre composites. Both fibres and matrix are taken as isotropic viscoelastic solids; the fibres have different shapes including circular, elliptical, Reuleaux and star-shaped; they are separated from the matrix by interphase regions of different mechanical properties and thicknesses. The analyses allow determining the sensitivity of the transversely isotropic, viscoelastic response of UD composites to fibre volume fraction, fibre shape, interphase volume fraction and interphase properties.

Investigating the in-plane mechanical behavior of single-ply quasi-unidirectional glass fiber/polypropylene composites

Abstract The objective of this study was to determine the engineering constants and off-axis tensile stress-strain relation of single-ply quasi-unidirectional (UD) glass fiber (GF)/polypropylene (PP) composites using the new approach. A series of off-axis tensile tests of quasi-UD composites were carried out. In this study, Puck’s interfiber fracture criterion was expanded for the first time to estimate the off-axis tensile stresses of UD composites. With the experimental values, the shear properties were obtained through the curve-fitting methods. Damage mechanisms were demonstrated to evolve with the loading angle. By comparison to experimental data, the Hahn-Tsai equation, together with the transformation equation, was found to be adequate to describe the off-axis tensile stress-strain relation of single-ply quasi-UD GF/PP composites.

Effect of fiber clustering on debond growth energy release rate in UD composites with hexagonal packing

Steady-state energy release rate (ERR) for fiber/matrix interface debond growth originated from fiber break in unidirectional composite is calculated using 3-D FEM models with hexagonal fiber arrangement. In the model the debonded fiber is central in the hexagonal unit which is surrounded by effective composite. The effect of neighboring fibers focusing on local fiber clustering on the ERR is analyzed by varying the distance between fibers in the unit. The steady-state ERR is calculated from potential energy difference between a unit in the bonded region far ahead of the debond front and a unit in the debonded region far behind the debond front. The ERR for different modes of crack propagation is obtained from a FEM model containing a long debond by analyzing the stress at the debond front.Results show that in mechanical axial tensile loading fracture Mode II is dominating, it has strong angular dependence (effect of closest fibers) but the average ERR is not sensitive to the local fiber clustering. In thermal loading the Mode III is dominating and the average ERR is highly dependent on the distance to neighboring fibers. However, for realistic loads the thermal ERR is much smaller than the mechanical.

Effect of fibre orientations on the mechanical properties of kenaf–aramid hybrid composites for spall-liner application

This paper presents the effect of kenaf fibre orientation on the mechanical properties of kenaf–aramid hybrid composites for military vehicle's spall liner application. It was observed that the tensile strength of woven kenaf hybrid composite is almost 20.78% and 43.55% higher than that of UD and mat samples respectively. Charpy impact strength of woven kenaf composites is 19.78% and 52.07% higher than that of UD and mat kenaf hybrid composites respectively. Morphological examinations were carried out using scanning electron microscopy. The results of this study indicate that using kenaf in the form of woven structure could produce a hybrid composite material with high tensile strength and impact resistance properties.

A Material Model for FE-Simulation of UD Composites

Composite materials are being increasingly used for industrial applications. CFRP is particularly suitable for lightweight construction due to its high specific stiffness and strength properties. Simulation methods are needed during the development process in order to reduce the effort for prototypes and testing. This is particularly important for CFRP, as the material is costly. For accurate simulations, a realistic material model is needed. In this paper, a material model for the simulation of UD-composites including non-linear material behaviour and damage is developed and implemented in Abaqus. The material model is validated by comparison with test results on a range of test specimens.

Synchrotron radiation computed tomography for experimental validation of a tensile strength model for unidirectional fibre-reinforced composites

Synchrotron radiation computed tomography has been used to analyse fibre break accumulation in unidirectional composites loaded in tension. The data are compared to model predictions. The model only slightly overestimated the composite failure strain, but predictions of fibre break density were too high, which can be mainly attributed to errors in the Weibull distribution. Both the number and percentage of interacting fibre break clusters were under-predicted by the model. This was attributed to an underestimation of stress concentrations in the model. While the experimental observations revealed mainly co-planar clusters, the model predicted mainly diffuse clusters. The experiments showed that the clusters did grow any further after their formation, while the model predicted a gradual development. Both local and dynamic stress concentrations were hypothesised to be key features for further exploration. The discrepancies identified, inform suggestions for directions advancing the state-of-the-art strength models of UD composites.

Tensile behaviour of stainless steel fibre/epoxy composites with modified adhesion

In this study we investigate the tensile behaviour of unidirectional and cross-ply composites reinforced with ductile stainless steel fibres and modified adhesion to the epoxy matrix. Results show that annealed stainless steel fibres have a potential in designing tough polymer composites for structural applications. The stiffness of the UD composites made from these fibres is 77GPa combined with the strain-to-failure between 15% and 18% depending on the level of adhesion. Silane treatments were used to modify the adhesion. By treating the stainless steel fibres with different silane coupling agents, an increase of 50% in the transverse 3-point-bending strength was realised. Increasing the adhesion by 50% leads to a higher tensile strength and strain-to-failure in both UD and cross-ply laminates and a higher in-situ strength of the 90° plies. It also delays formation of matrix cracks and hinders growth of debonding.

Exploring the potential of interleaving to delay catastrophic failure in unidirectional composites under tensile loading

Abstract This work investigates the potential of interleaving to delay catastrophic translaminar failure in unidirectional Carbon Fibre Reinforced Polymers (CFRPs) under tensile load. A Finite Element Model of a damaged Polymer Interleaved Composite (PIC) specimen was built by considering an initial translaminar crack in a CFRP ply, and cohesive zones across the neighbouring interleaves and interleaf/CFRP interfaces. Under tensile load, two competing damage processes (delamination and interleaf yielding/cracking) are predicted, leading to different stress concentrations in the neighbouring CFRP plies for different interleaf geometries and material properties. The probability of unstable through-the-thickness crack propagation is then calculated, considering the strength variability of CFRP plies. Results showed that: (i) a high strength variability in CFRP plies leads to a more stable crack propagation in PICs, which can be achieved by using a low number of fibres in each CFRP ply, and (ii) the geometry and mechanical properties of the PIC can be designed to promote controlled delamination, thereby decreasing the stress concentration at the neighbouring plies. The potential of interleaving to delay catastrophic translaminar failure lies in this last aspect, which permits a larger critical cluster of broken fibres than that of classical UD composites.

Stab resistance of UHMWPE fiber composites impregnated with thermoplastics

Compared with aramid fabrics, ultrahigh molecular weight polyethylene fiber unidirectional (UHMWPE UD) composites have higher ballistic limited velocity at the same areal density, making UHMWPE UD composites better candidate materials for the design of body armor against ballistic and stab impact. In this study, UHMWPE UD composites impregnated with thermoplastic films were prepared, and a novel methodology was proposed to evaluate the stab‐resistant performance of these composites using the concept of stab impact. Stab‐resistant performance is strongly affected by stab impact parameters, such as stab velocity or impact energy, layers of UD, and stab angle. Results show that the stab‐resistant performance of composites can be significantly improved by thermoplastic films, particularly polyethylene terephthalate and polypropylene films. The structure of UD composites functions importantly in enhancing stab resistance. Copyright © 2014 John Wiley & Sons, Ltd.

A new efficient and reliable algorithm to determine the fracture angle for Puck’s 3D matrix failure criterion for UD composites

This work presents an enhanced method to accelerate the detection of inter-fibre fracture (IFF) in unidirectional fibre reinforced polymer composites. Subject of enhancement is Puck’s IFF failure criterion for 3D stress states. To compute the IFF stress exposure, the orientation of the IFF fracture plane needs to be determined by a time consuming fracture angle search algorithm. In this work, the characteristics of occurring stress exposure over fracture angle curves are determined to find a reliable basis for accelerating the procedure of fracture angle search. Based on the obtained knowledge, a new, much more efficient search algorithm has been developed. Its performance and its significance are demonstrated by a finite element analysis of a cap-profile under 3-point bending. A previously existing enhanced fracture angle search algorithm is also evaluated, but found to be less reliable in predicting the correct fracture angle and corresponding stress exposure.

A Note on mori-tanaka's method

Explicit expressions of Mori-Tanaka's tensor for a transversely isotropic fiber reinforced UD composite are presented. Closed-form formulae for the effective elastic properties of the composite are obtained. In a 3D sense, the resulting compliance tensor of the composite is symmetric. Nevertheless, the 2D compliance tensor based on a deteriorated Mori-Tanaka's tensor is not symmetric. Nor is the compliance tensor defined upon a deteriorated 2D Eshelby's tensor. The in-plane effective elastic properties given by those three approaches are different. A detailed comparison between the predicted results obtained from those approaches with experimental data available for a number of UD composites is made.