Unidirectional Plies Research Articles

Mode I and mode II fracture behavior in nano‐engineered long fiber reinforced composites

AbstractDelamination still remains one of the common concerns in continuous fibers reinforced polymer composites. In the literature, the incorporation of nanoconstituents into constitutive composites plies has been suggested with the aim of improving interface toughness. In this study, vertically aligned carbon nanotubes forests (VACNTs) are transferred at composites interfaces, and mode I (DCB tests) and mode II (ENF tests) fracture tests carried out on composite samples. Microscopic analysis of reference and nano‐engineered composites help to understand fracture behavior and toughness. While the crack path in ENF samples is observed at interlaminar VACNTs‐resin interfaces, the crack path in DCB samples is intralaminar, located within the unidirectional carbon fiber plies. These microscopic observations give explanation for unstable crack propagation in nano‐engineered ENF samples and the unchanged toughness from the nano‐engineered to the reference zone in DCB samples.

Experimental characterization and phenomenological modeling of nonlinear viscoelasticity, plasticity and damage of continuous carbon fiber-reinforced thermoplastics

The presented work addresses the experimental characterization and phenomenological modeling of the complex rate dependent mechanical response of continuous fiber-reinforced thermoplastics (cFRTP). The theoretical basis is given by the concept of an equilibrium curve of the material which provides a comprehensive framework for separating the mechanical response into a rate dependent overstress and a rate independent equilibrium stress. In long-term creep and relaxation tests, it is shown that relaxation tests provide several advantages over creep tests when determining the equilibrium response. A stepwise loading–unloading relaxation (SR) test is proposed which makes it possible to extract nonlinear viscoelasticity, damage and plasticity present in the material from a single test. The application to cFRTP is exemplified by using the proposed method for the experimental characterization of a continuous carbon fiber-reinforced polycarbonate (CF-PC). Based on the obtained experimental data, a phenomenological material model is developed which describes the mechanical response of a unidirectional ply. The model consists of an overstress branch addressing the nonlinear viscoelastic material response and an equilibrium branch that utilizes a coupled damage-plasticity approach. After calibrating the model using the experimental data, it is shown that the proposed model is able to accurately reproduce the mechanical response of cFRTP.

An experimental evaluation of cryogenic machining in the drilling of carbon fiber reinforced plastics for aerospace

The present research aims at studying both dry machining and alternative solutions to wet machining in the drilling of carbon fiber-reinforced polymers (CFRPs). CFRPs are key materials in aerospace and their drilling is critical and challenging. Specifically, cryogenic fluids such as CO2 and N2 were investigated. Drilling tests were performed using three different CFRP laminates. These CFRP laminates are composed of various unidirectional plies of different orientations. Thus, the configurations to analyze were: 0/90/45, 30/60 and 0/90. As output variables, surface roughness, diameter deviation, delamination and thrust forces were analyzed. In general, the internal application of CO2 and dry machining were the best- and worst-performing techniques for surface roughness, respectively. Regarding the dimensional quality, both CO2 and dry machining offered the best results. In the case of LN2 or LN2 + CO2, the results showed that the CFRP configuration is critical for the obtained results. However, it was identified that cryogenics can improve the results of the dry machining process under certain conditions in terms of delamination. Finally, in general, CO2 provided the worst results when attending to the thrust forces.

Evaluation of the effect of fiber bridging on mode I quasi‐static testing

AbstractThe purpose of this investigation is to evaluate the contribution of fiber bridging to the energy release rate of a fracture resistance curve. Fiber bridging occurs when testing beam‐type specimens consisting of unidirectional plies in a laminate. Unidirectional double cantilever beam specimens composed of the carbon fiber reinforced polymer prepreg AS4/8552 were tested using standard methods. In addition, a cohesive zone model was developed and used to carry out finite element analyses on the tested specimens. It was employed to calculate the contribution of fiber bridging to the ‐curve which was determined from the tests. Fiber bridging in beam specimens increases the apparent fracture toughness of a composite laminate. A method has been proposed for evaluating its contribution to the energy release rate. In that way, an ‐curve may be determined for which the effect of fiber bridging is eliminated.

Influence of the lay-up and curing steps in the manufacturing process of thick laminate composites on their compressive strength

The influence of the lay-up and curing steps in the manufacturing process of CFRP is investigated in terms of compressive strength. Automated Fibre Placement (AFP) and manual lay-up (HLU), from the same pre-preg material , are compared as well as the use of a porous or non-porous consumable during curing. It is shown that the latter factor may result in heterogeneous mesostructures with gradients in fibre volume content from the top of the laminate to its center. This made it necessary to propose a specific data analysis of the four-point bending test, used to evaluate the compressive strength of the most loaded unidirectional ply. Experimental results suggest a better performance of AFP vis-à-vis HLU plates, especially in terms of reproducibility. The increase in performance for plates using non porous consumables is discussed with respect to the relevance of using such a strategy on thin laminates for designing thick parts.

Crashworthiness and Failure Analyses of FRP Composite Tubes under Low Velocity Transverse Impact

Currently, FRP composite tubes are drawing increasing attention in many industrial applications, due to their excellent mechanical and lightweight properties, with reduced energy consumption and enhanced sustainability. This study investigates the failure mechanisms and crashworthiness performance of glass and carbon fibre reinforced polymer (GFRP and CFRP) composite tubes under low velocity transverse impact. Finite element methods were developed to establish numerical models to predict the failure responses of FRP composite tubes with a complex ply sequence of both woven and unidirectional layers. In the modelling, continuum damage mechanics and cohesive zone method were used to calculate the intralaminar and interlaminar failure behaviours, respectively, in FRP composite tubes. The numerical models were validated by corresponding experiments, and the effects of the impact energy and material type were investigated. The experimental results show that the initial impact energy does not significantly affect the specific energy absorption (SEA) and peak force (PF) of GFRP composite tubes, and the SEA and PF are generally around 0.5 kJ/kg and 600 N, respectively, when the impact energy varies from 10 J to 50 J. Failure mechanism analyses show that GFRP tubes and CFRP tubes with totally unidirectional plies present global bending deformation with significant matrix damage, and CFRP tubes with “hybrid layer type” exhibit local penetration with severe fibre and matrix damage. The crashworthiness analyses indicate that CFRP tubes perform better in SEA while GFRP tubes possess smaller PF when subjected to low velocity transverse impact.

Effect of matrix and fibre cracks on static and fatigue strength of composite laminates

The effect of matrix and fibre cracks on the static strength and stiffness of uni-directional plies and cross-ply laminates under tensile loads was examined with the goal of understanding the main damage mechanisms. The size of matrix cracks emanating from cracked fibres was determined and found to be in excellent agreement with test results. This was used to predict static tensile failure of uni-directional laminates and was shown to be in good to excellent agreement with test results for six different materials. The approach was extended to fatigue loading by determining the number of cycles for the plastic zone ahead of a matrix crack tip to fail. It was combined with an approach to quantify the effect of matrix cracks under any in-plane loading previously developed by the authors to predict changes in laminate stiffness as a function of cycles. Comparisons with test results showed good to excellent agreement between predicted and measured laminate stiffness as a function of fatigue cycles.

Finite element based interfacial failure analysis of PVC and laminated FRP composite pipe socket joints subjected to internal pressure

Present study reports a comparison of stress, failure and failure growth behaviour of polyvinyl chloride (PVC) and fiber reinforced plastic (FRP) made pipe socket joints. Finite element based Ansys codes have been developed and validated with experiment and literature based analytical results. Various stress based failure criteria (parabolic yield criterion, and Tsai-Wu failure criteria), and Strain Energy Release Rates (SERRs) as the fracture parameters determined using Virtual Crack Closure Technique (VCCT) have been used to simulate the onset of failure and its growth, respectively. Stress and failure analysis suggested, the pipe-adhesive interface at the junction of the joints to be the most vulnerable location for failure under internal pressure. Pre‐embedded adhesion failure growth along the axial length and circumferential width has been simulated. Adhesion failure growth in axial direction was found more susceptible to mode I failure whereas mode III was dominating in circumferential direction. It was also evident from the studies that axial direction is the dominating direction for adhesion failure growth. Unidirectional ply [90]4 was observed to be the optimum stacking sequence for adhesion failure growth resistance.

Analysis of laminated composites composed of coupled layers by polar method

Composite laminated plates are made of two or more unidirectional plies (or layers) stacked together at various orientations, the layers are composed of two phases: the fibers and the matrices. The material property and orientation of the fibers inside of the matrix determines the elastic characteristics of the layers. If the conventional layers used to fabricate laminate, there is no elastic coupling between bending and extension, other types of layers (the satins for example) have coupled behaviors. The classical lamination theory (CLT) formulates the elastic behavior of a laminate composed of classical layers, but it should be modified to be able to include these coupled layers that is the aim of the first part of this article. The second aspect of coupled layers must be considered: when they turned over, some of their elastic characteristics change, this phenomenon is studied in a second part and a method is proposed, including the reversal into the CLT equations, give more useful equations describing the laminates made of coupled layers. Finally, some cases of uncoupled and quasi-homogeneous stacking sequences are studied.

The importance of accounting for large deformation in continuum damage models in predicting matrix failure of composites

The work presented in this paper investigates the ability of continuum damage models to accurately predict matrix failure and ply splitting. Two continuum damage model approaches are implemented that use different stress–strain measures. The first approach is based on small-strain increments and the Cauchy stress, while the second approach account for large deformation kinematics through the use of the Green–Lagrange strain and the 2nd Piola–Kirchhoff stress. The investigation consists of numerical benchmarks at three different levels: (1) single element; (2) unidirectional single ply open-hole specimen and (3) open-hole composite laminate coupon. Finally, the numerically predicted failure modes are compared to experimental failure modes at the coupon level. It is shown that it is important to account for large deformation kinematics in the constitutive model, especially when predicting matrix splitting failure modes. It is also shown that continuum damage models that do not account for large deformation kinematics can easily be adapted to ensure that the damage modes and failure strength are predicted accurately.

Modelling and evaluation of carbon fibre composite structures using high-frequency eddy current imaging

The mechanical performance of carbon fibre reinforced polymer (CFRP) composites is determined by the stacking sequence of specifically orientated ply lamina layers. In these multi-layer structures, global ply misalignment and local fibre waviness can occur during the manufacturing process, leading to reduced performance and structural failure during operation. High-frequency eddy-current testing (ECT) has previously demonstrated its capability for detecting orientation-related features, including fibre orientation and waviness. However, accurate sensor optimisation and inversion of orientation & material structure cannot be achieved without proper, validated modelling techniques to simulate CFRP features. The lack of suitable models is in part due to CFRP exhibiting highly-complex electromagnetic interactions between fibres, lamina and the ECT sensors, which are challenging to integrate. This work proposes a novel finite element modelling approach to simulate the ECT response to planar multi-layered CFRP components. The fibre tow structure of each unidirectional ply is modelled using orientation dependant 2D conductivity tensor waveforms, and virtual 2D ECT scans are simulated by shifting the waveforms within the model mesh. The results demonstrate that idealised electromagnetic characteristics of the CFRP structure can be successfully modelled compared with experimental data and that 2D ECT data of complex CFRP layers structures can be simulated with improved computational speeds, up to 5x faster compared to standard approaches. Automated data-analysis tools, including Radon transform (RT) and 2D FFT, are employed to validate the simulated 2D scan data through the characterisation of fibre orientations and simulated 2D scans used to evaluate the orientation inversion techniques. The results demonstrate that RT analysis detects fibre orientations with better accuracy, precision and consistency than equivalent 2D FFT analysis techniques. The simulation also demonstrates the reduced resistivity losses compared with isotropic materials caused by the different heterogeneous and multi-layer structures. It predicts high current densities at interfaces of plies with orthogonal orientations, resulting in an effective interface skin-depth.

A unified physically-based finite deformation model for damage growth in composites

Two 3D homogenized models for damage growth in a unidirectional (UD) composite ply are simplified and merged into a unified model. The fibre kinking behaviour is based on fibre kinking theory handled in a finite deformation framework. The nonlinear shear behaviour is pressure dependent and is modelled by combining damage and friction on the fracture plane. Fibre kinking growth and transverse behaviour are modelled with a single damage variable. This allows both modes to occur simultaneously and mutually influence each other in an efficient and physically-based way.For validation the model is tested against micro-mechanical Finite Element (FE) simulations under pure longitudinal compression and influenced by shear. The results show nearly perfect agreement for stiffness, strength and crushing stress. The model validation is performed against two different components under three-point bending and a quasi-static crash scenario. Both simulation show good correlation with experiments, validating thus the present unified model.

Experimental Analysis of Residual Stresses in CFRPs through Hole-Drilling Method: The Role of Stacking Sequence, Thickness, and Defects

Carbon fiber reinforced plastics (CFRPs) gained high interest in industrial applications because of their excellent strength and low specific weight. The stacking sequence of the unidirectional plies forming a CFRP laminate, and their thicknesses, primarily determine the mechanical performance. However, during manufacturing, defects, e.g., pores and residual stresses, are induced, both affecting the mechanical properties. The objective of the present work is to accurately measure residual stresses in CFRPs as well as to investigate the effects of stacking sequence, overall laminate thickness, and the presence of pores on the residual stress state. Residual stresses were measured through the incremental hole-drilling method (HDM). Adequate procedures have been applied to evaluate the residual stresses for orthotropic materials, including calculating the calibration coefficients through finite element analysis (FEA) based on stacking sequence, laminate thickness and mechanical properties. Using optical microscopy (OM) and computed tomography (CT), profound insights into the cross-sectional and three-dimensional microstructure, e.g., location and shape of process-induced pores, were obtained. This microstructural information allowed for a comprehensive understanding of the experimentally determined strain and stress results, particularly at the transition zone between the individual plies. The effect of pores on residual stresses was investigated by considering pores to calculate the calibration coefficients at a depth of 0.06 mm to 0.12 mm in the model and utilizing these results for residual stress evaluation. A maximum difference of 46% in stress between defect-free and porous material sample conditions was observed at a hole depth of 0.65 mm. The significance of employing correctly calculated coefficients for the residual stress evaluation is highlighted by mechanical validation tests.

Characterization and modeling of continuous carbon fiber-reinforced polycarbonate under multiaxial loads

The presented work considers the characterization of a continuous fiber-reinforced thermoplastic (FRTP) with unidirectional plies under quasi-static, multiaxial loading conditions and the modeling of the non-linear mechanical behavior. For the experimental investigation, hoop wound tubular specimens fabricated from carbon fiber-reinforced polycarbonate (CF-PC) tape were tested on an electromechanical static testing machine with an axial and torsional drive. The multiaxial material response was determined for an extensive set of tests performed at several load ratios of in-plane shear to transverse stress. For modeling the non-linear mechanical response, an elasto-plastic framework is used which is kept preferably simple in order to facilitate the application in numerical analysis. Based on the generated experimental data, suitable criteria for fracture and yielding are proposed. The defined yield criterion respects the special characteristics of a FRTP such as the different mechanical behavior under transverse tensile and compressive loading as well as a linear material response in the direction of the fiber reinforcement . Furthermore, the implementation of the proposed model as a user-defined material (UMAT) routine for implicit finite element analysis (FEA) using Abaqus /Standard is described and the calibration of the plasticity approach using only the data of the in-plane shear and transverse compression tests is discussed. The model predictions for multiaxial loads are validated using the determined biaxial material data. • Multiaxial experimental data for a continuous fiber-reinforced thermoplastic • Full dataset of the arithmetic mean stress–strain curves is provided • Selection of a suitable fracture criterion and definition of a yield criterion • Development and implementation of a non-linear elasto-plastic material model • Calibration and validation of the material model using the determined multiaxial data

Fracture toughness resistance curves for a delamination in CFRP MD laminate composites, Part I: Nearly mode II deformation

In this investigation nearly mode II initiation and resistance energy release rate values, required for delamination propagation, were determined based on quasi-static calibrated end loaded split (C-ELS) fracture tests. Two multi-directional (MD) carbon fiber reinforced polymer (CFRP) material systems were examined. The first was manufactured as a wet-layup, with an initial delamination between a unidirectional ply and a plain woven ply. The second was manufactured from a prepreg, with an initial delamination along an interface between two plain woven plies oriented differently. Two-dimensional finite element analyses (FEAs) of the tested specimens were performed. Based upon the FEA results, with use of the displacement extrapolation (DE) method, as well as the virtual crack closure technique (VCCT), stress intensity factors were calculated. The obtained values were used to determine the in-plane mixed-mode phase angle for each test, which indicated nearly mode II deformation. Fracture toughness resistance curves or R-curves were generated as a function of the delamination extension. The critical initiation and resistance energy release rate values were obtained from the stress intensity factors, as well as with the J-integral, which are local methods. In addition, the global experimental compliance method (ECM) was used. Small differences were observed between the results obtained by means of the two methods. From a comparison between the R-curves of the two material systems, it was seen that the initiation energy release rate values were higher for the prepreg by 25.5%. A greater difference was found in the increasing portion of the R-curves, as well as the steady state energy release rate values.

Design guidelines for hybrid continuous/discontinuous carbon fibre laminates

Combining discontinuous carbon fibre moulding compounds with unidirectional (UD) non-crimp fabric has been studied as a potential solution for producing cost effective composite structures, with short cycle times via compression moulding. The tensile stiffness and strength of a composite with a hybrid fibre architecture were found to be up to 60% and 110% higher, respectively, than the baseline discontinuous fibre moulding compound, when 20% (by vol.) of UD fibres were aligned in the primary loading direction. However, stress concentrations arising from the dissimilar material properties can result in the strength of the hybrid architecture being reduced to below that of the baseline composite if the UD fibres are not incorporated effectively. This paper focusses on optimising the stress transfer within the transition zone (inter-material interface) between the UD material and the discontinuous fibre moulding compound, when local isolated patches of continuous fibre are used rather than full coverage plies. This is key if hybrid architectures are to be adopted for structural applications, minimising the stress concentration effects from the UD ply ends, whilst maintaining the low cycle times and costs associated with compression moulding. Following aerospace laminate design guidelines, a range of step sizes and joint configurations have been considered for different ply drop strategies, minimising the magnitude of the stress concentration at the UD ply ends. Results indicate that the step length must be at least 20 times the thickness of the UD ply to ensure the bending strength at the joint is the same as the baseline discontinuous fibre material. An ‘alternating joint' yields 18% higher bending strengths than a conventional ‘stepped joint’ design, as the stress concentrations at the dropped ply ends are reduced in comparison to the inherent stress concentrations within the discontinuous fibre material.

Investigation on graphene addition on the quasi-static and dynamic responses of carbon fibre-reinforced metal laminates

In this paper, graphene nanoplatelets (GnPs) were used with carbon fibre/epoxy composite to enhance the mechanical performance of fibre metal laminates (FMLs). The designed FMLs are composed of thin aluminium alloy layers alternating with carbon-fibre epoxy/GnP composite plies with a chemical treatment performed on the aluminium laminate to promote adhesion strength with the epoxy composite. Two different configurations of FMLs were investigated (i) composite plies with unidirectional carbon fibres (U-FMLs) and (ii) composite plies with plain carbon woven (W-FMLs). Samples were tested under dynamic loading (Charpy impact) and quasi-static loading (three-point flexure). The experimental results showed that FMLs with 0.3 wt% GnPs logged the best impact performance; the impact strength of W-FMLs and U-FMLs are respectively 18.2% and 25.2% higher than FMLs without GnPs. FMLs with 0.5 wt% GnPs recorded the highest enhancement in flexural strength, fracture strain and flexural modulus recording increments 23.1%, 19.3% and 48% for the W-FMLs, and 60.3%, 34.5% and 61.4% for the U-FMLs, respectively. Also, an in-depth microscopic analysis was conducted to understand the reinforcing mechanism of GnPs into FMLs. Moreover, a numerical model of a three-point flexural test was developed to show the ability of numerical tools to predict material behaviour at optimized costs. Johnson–Cook and Hashin damage models were used respectively for aluminium alloy and carbon fibre epoxy/GnP composite to accurately predict their deformation and damage modes.

Testing and numerical investigations of hybrid composite for flight control surfaces in manufacturing variability contest

The design and certification of composite structures is based on the building block approach (BBA), following a bottom-up procedure. Starting from the bottom level of Rouchon’s pyramid of test, thousands of coupons need to be produced to determine design allowables and capture manufacturing, geometrical and microstructure variability values. The numerical approach provides desirable data to be confirmed by a reduced testing campaign, especially for the preliminary design process stages and material selection.In this paper a carbon fibers reinforced composite with epoxide resin used for aerospace application has been analyzed, in form of both fabric and unidirectional tape to validate the numerical model considering Salver (of MA Group company) manufacturing process variability data. Hybrid laminates composed of unidirectional plies and woven fabric layers have been tested to pursue a tailored approach based on design guidelines and stress requirements for aircraft flight control surfaces. Benchmarking the experimental-numerical test results allows to assess the statistical reliability of the proposed method.

Comparative Analysis of the Effect of Fibre Architecture on the Tensile Properties of Sisal Fibre Reinforced Polyethylene Polymer Composite

This work investigates the effect of fibre architecture on the mechanical properties of composite laminates fabricated from natural sisal fibres reinforced with polyethylene polymer matrix. The main objective for this is to determine which architectural pattern produces the best material in terms of mechanical properties that can be employed for use in the fabrication of engineering components to replace the more expensive components produced from the highly expensive synthetic fibre materials. Four fibre architectural patterns were chosen and tailor-made for the study namely;±45° angled ply, 0°/90° cross ply, 90° and 0°unidirectional plies. A five layered composite laminates were manufactured from the fabrics and subjected to mechanical tensile tests using Instron universal testing machine (model 4467) having 30kN load cell attached to it and an extensometer gauge length 75mm at a crosshead speed of 200mm/min. The result of the mechanical tests revealed that the 90° laminate gives the best mechanical properties such as; elastic modulus, yield strength and tensile strength amongst the fabricated composite laminates. It was followed by the ±45° angle plied laminate, the 90° unidirectional fibre laminate and lastly the 0°unidirectional fibre laminate has the lowest mechanical properties amongst all the laminates tested. SEM micrographs of the fractured surfaces reveal that the failure modes of the laminates are characterized mostly by fibre-matrix debonding, fibre delamination, fibre splitting, fibre cracking and so on.

Effect of fiber orientation on mechanical properties of jute/carbon/glass hybrid composite

The natural fibers are continuously gaining interest amongst the researchers due to comparative mechanical properties along with biodegradability. Hybrid composite with Carbon, Glass and natural Jute fiber is prepared with different fiber orientation to see the effect of fiber orientation on various mechanical properties. The effect of stacking sequence of the three fibers has been studied by various researchers but the effect of fiber orientation is not addressed so far. The effect of fiber orientation on Tensile, Flexural, shear strength and hardness were tested. It is observed that there is significant effect of fiber orientation on mechanical properties. Six natural fiber hybrid composite samples (unidirectional &angle ply) were prepared and tested. The tensile strength and flexural strength were following the similar trends. It was observed that G6 has maximum tensile & flexural strength 427.700 MPa & 549.50 MPa respectively and J6 has minimum tensile & flexural strength 31.300 MPa & 56.30 MPa respectively, whereas shear strength of C6 has the maximum value 135.40 MPa. There is a variation in hardness & strength of the different composite samples. Replacement of natural fiber with synthetic fiber and fiber orientation will affect the tensile, flexural strength shear strength and hardness of the hybrid composite.