Multidirectional Laminates Research Articles

Fluid-structure interaction modeling of the abrasive waterjet drilling of carbon fiber reinforced polymers

Abstract The major challenges facing engineers while fabricating high precision components from carbon fiber reinforced polymers (CFRP) especially when using an abrasive waterjet (AWJ) include the predominant interlaminar delamination and tapered kerfs. Therefore, in the present work, a comprehensive 3-D transient fluid-structure interaction (FSI) model is developed and numerically simulated to investigate mechanisms leading to delamination and hole-geometric defects while drilling autoclave cured aerospace grade multidirectional CFRP laminates. The FSI model comprised a two-way interaction between the computational fluid dynamics (CFD) flow model and structural finite element (FE) model. The CFD model explored the dynamic jet characteristics while impinging the anisotropic CFRP surface. The resulting forces from the CFD domain acted as loads in the transient FE model to predict the initiation of cracks and delamination. Subsequently, extensive experiments were conducted to validate the proposed numerical model. The results illustrated that during abrasive waterjet drilling of CFRP, delamination was initiated by the hydraulic impact as the abrasive waterjet penetrated the workpiece. Additionally, the study revealed that an increase in abrasive waterjet pressure and abrasive particle size resulted in a higher inter-ply debonding and crack propagation. Correspondingly, an increase in the standoff distance (SoD) resulted in reduced crack propagation and delamination. Further, the liquid-solid interface exhibited asymmetric cracks. The present findings not only provide requisite guidelines to achieving high-precision drilling of multidirectional CFRP but also provide technical guidelines to improving the performance characteristics of the abrasive waterjet machining (AWJM) process.

Finite element analysis of tensile notched strength of composite laminates

Stress concentration due to holes (notches) on FRP laminates, a widely used structural material in aerospace applications, often substantially reduces their strength and durability. Some analytical and experimental studies in literature evaluated strength of FRP laminates in the presence of a notch. This work considers stress concentrations factor (SCF), damage progression and tensile notched strength for both GFRP lamina and laminates using finite element (FE) models based on progressive damage analysis. In this work, D/W ratio for holes on 0° GFRP lamina varied from 0.1–0.5, by changing hole size from 2.5–12.5 mm. Further, GFRP lamina with four different fibre orientations, and GFRP laminates with three different layups were considered. FE results show that the stress concentration factor (SCF) increases from 3.91 to 5.32 and the tensile notched strength reduces from 435 MPa to 192 MPa, as D/W ratio varies from 0.1–0.5 for 0° GFRP lamina. It is found that laminates can be designed with SCF (<3) that is lower than even an infinite isotropic plate. Further, the axial stiffness of multi-directional laminates degrades during loading due to early damage initiation observed in off-axis plies. Consequently, they show lower axial stiffness and tensile notched strength as compared to unidirectional 0° lamina.

Probabilistic bending behaviour of a symmetric multi-directional composite laminate subjected to moisture induced material property asymmetry

In practical applications, moisture diffusion in a composite laminate occurs one dimensional, which results in moisture content asymmetry through-the-thickness of the laminate. The effect of such moisture content asymmetry on the bending behaviour of the laminate is investigated in this study. The bending response of the laminate is numerically simulated by following the first-order shear deformation theory and is experimentally validated. The probabilistic simulation was conducted by developing a surrogate model for the laminate bending response using the Polynomial Chaos Expansion method, and concurrently applying the Monte Carlo simulation. The necessity for a probabilistic simulation is also discussed in detail. The moisture diffusion through the thickness of the laminate is predicted using the Fick diffusion model, and the extent of moisture asymmetry is established for different ageing periods. The moisture asymmetry in the laminate resulted in the unwanted bending-extension coupling, which diminished as the laminate approached moisture equilibrium. Further, the moisture asymmetry introduced a material property asymmetry in the laminate, which affected the ply stresses. The study concludes that the ply stresses are dependent on the moisture content, position of the ply in the laminate, and the moisture induced stress redistribution.

An experimental analysis of the end-notched flexure composite laminates beams with elastic couplings

This paper presents an experimental study of the composite beams with elastic couplings subjected to the end-notched flexure (ENF) test. The object of research were carbon/epoxy composite laminates with specific stacking sequences exhibiting the bending–extension and the bending–twisting couplings. In addition, multidirectional laminates with different delamination interfaces were tested. Influence of elastic coupling phenomena on behavior of laminates subjected to the bending moment as well as on fracture toughness and delamination surfaces after the ENF tests was discussed. All experiments were performed according to the ASTM D7905 Standard. Determination of crack initiation was supported by the acoustic emission technique. Data analysis of the ENF test was performed by using both the direct beam theory and the corrected beam theory. Results obtained by using the classical data reduction schemes were compared with the compliance-based beam method (CBBM). Greater values of the fracture toughness obtained by using the non-standardized methods can be explained by the effect of the fracture process zone which was taken into account in the CBBM calculation scheme. Delamination surfaces of coupled laminates after the ENF tests were investigated by using the scanning electron microscopy. Results revealed that the effect of elastic couplings might influenced on intense bridging phenomena, more extensive fracture process zone and contribution of the mode III in total delamination resistance. Nevertheless, application of the standardized ENF procedure to determination of the fracture toughness of laminates exhibiting elastic couplings is still possible, provided assistance of additional data reduction schemes and techniques or numerical analysis.

Experimental and numerical investigation of interface damage in composite L-angle sections under four-point bending

Curved laminates in aero-structures, such as the L-angle sections where webs and flanges meet, are prone to delamination due to high interlaminar stresses in these regions. Some efforts to investigate delamination in these structures can be found in the literature but commonly structures are limited to unidirectional layups or modelling approaches are constrained to the cohesive element based methods. In this work, multi-directional L-angle laminates were manufactured using unidirectional prepregs and tested under four-point bending load conditions to examine the interface damage. Acoustic emission technique was used to assist the capture of damage initiation and propagation. Three interface modelling strategies for predicting delamination, namely cohesive element, cohesive surface and perfectly bonded interface were used in the numerical study. The interface damage behaviour was successfully predicted by the simulation methods and differences among the strategies were compared.

An Experimental and Numerical Investigation to Characterize an Aerospace Composite Material with Open-Hole Using Non-Destructive Techniques.

In this study, the open-hole quasi-static tensile and fatigue loading behavior of a multidirectional CFRP thick laminate, representative of laminates used in the aerospace industry, is studied. Non-destructive techniques such as infrared thermographic (IRT) and digital image correlation (DIC) are used to analyze the behavior of this material. We aim at characterizing the influence of the manufacturing defects and the stress concentrator through the temperature variation and strain distribution during fatigue and quasi-static tests. On the one hand, the fatigue specimens were tested in two main perpendicular directions of the laminate. The results revealed that manufacturing defects such as fiber waviness can have a major impact than open-hole stress concentrator on raising the material temperature and causing fracture. In addition, the number of plies with fibers oriented in the load direction can drastically reduce the temperature increment in the laminate. On the other hand, the quasi-static tensile tests showed that the strain distribution around the hole is able to predict the crack initiation and progression in the external plies. Finally, the experimental quasi-static tests were numerically simulated using the finite element method showing good agreement between the numerical and experimental results.

Determination of stresses, strains and failure types in multidirectional laminates under pure bending

Curved structures made of fibre-reinforced plastics tend to show an out-of-plane failure type, when loaded under bending. One method to determine the related tensile strength in thickness direction, is the standardised unfolding test related to ASTM D6415 with L-profile specimens. However, the standard shall be used for unidirectional materials only, which is not the case for multidirectional laminates. Therefore, this publication presents methods to determine the layer-wise stress and strain state as well as occurring failure types of multidirectional laminates. An extension of the Lekhnitski formulae is presented and its results are validated by comparison with high resolution strain measurements by Digital Image Correlation. Based on the analytical solution, a failure analysis using Cuntze’s Failure Mode Concept is conducted. It was revealed, that the failure load and position can be predicted accurately, if a failure type related strength –in-plane or out-of-plane failure– is used. In the presence of [Formula: see text]-layers higher deviations occur between test and prediction and therefore limits the validity of the presented analytical failure analysis.

Investigation on interlaminar delamination tendency of multidirectional carbon fiber composites

In this investigation carbon fiber reinforced laminates with different orientation layups are prepared and studied under tensile loading condition. Multiple strain measurement techniques, namely, resistive strain gauges, embedded optical sensors and digital image correlation are used to analyze stress-strain behavior simultaneously through the thickness of composite materials, and to determine the sequence of failure in different plies. Inconsistencies of strains measured through different methods is correlated with the tendency for interlaminar delamination, therefore demonstrating the ability of multi-instrument approach to describe damage progress through the thickness of multidirectional laminates. Complementary analysis through acoustic emission methods reveals that the angle of off-axis surface plies can influence the sequence of failure under tensile loading condition, and damage monitoring capabilities of acoustic emission system is directly affected by delamination tendency of surface plies. Remarkably, the delayed failure of off-axis plies is shown to be related to reorientation of these layer towards loading direction using infrared thermography method.

Effect of out-of-plane wrinkles in curved multi-directional carbon/epoxy laminates

Defects such as out-of-plane wrinkles are known to strongly affect in-plane strength but there has been very little research on their effect on out-of-plane properties. Experimental and numerical studies of multi-directional curved-beam laminates were thus carried out to understand the effects of out-of-plane wrinkles on through-thickness tensile strength. The initially selected layup saw free-edge delamination interacting with transverse cracking, which is undesirable. After suppressing the free-edge delamination by dispersing the plies near the specimen surfaces, through-thickness tensile failure was observed near the mid-plane. The effects of out-of-plane wrinkles could be studied with this appropriate layup, showing a 16% reduction in strength. A High-fidelity Finite Element Method (Hi-FEM) has been used to distinguish between the different failure modes and to understand the effects of wrinkles. Good agreement was achieved between the numerical and experimental results in terms of through-thickness tensile strengths and delamination locations.

Multiscale image-based modelling of damage and fracture in carbon fibre reinforced polymer composites

This paper is the first to predict and then validate the overall stress-strain curve and the damage sequence comprising matrix cracking, interface debonding and fibre fracture against X-ray Computed Tomography (CT) observations for a multidirectional laminate. Until recently, numerical modelling of multi-directional multi-ply composites required idealised continuum mechanics models or idealised unit cell approaches (or homogenisation method) that cannot reliably capture property variations and the complex sequence of damage events that occur upon tensile loading. Here a multiscale 3D image-based model is used to simulate stochastic crack growth in a double-notch (-45°/90°/+45°/0°/-45°/90°/+45°/0°)s carbon fibre reinforced polymer (CFRP) composite specimen subjected to tensile loading monitored by time-lapse X-ray CT. The data integration approach involves: (1) parallel simulations of meso-scale elements (MeEs) for each ply for which the orientation of the individual fibres has been extracted from an X-ray CT image, (2) local hierarchical coupling of the MeEs into a macro-scale mechanical model of the test piece, and (3) the use of a random variation in material properties where microstructural details are not revealed by the X-ray CT characterisation method.Cohesive interface elements (CIEs) are used at both scales to predict the accumulation of interface damage and crack growth. The fibre-level modelling captures the detailed damage sequence and crack morphology including fibre/matrix debonding, sliding, matrix cracking and fibre fracture events. The multiscale model is validated by comparison with the measured tensile loading curve and the damage evolution recorded by the X-ray CT. This approach can reduce the reliance of certification on extensive heirarchical structural testing schemes from test-piece to full-scale component.

Mixed-mode fracture response of anti-symmetric laminates: Experiments and modelling

Multidirectional laminates are very often used in advanced structures. However, the existing data in the literature regarding their fracture response is not conclusive. In this work, the fracture response of a +45∘//−45∘ interface is investigated under remote mixed mode loading and compared to a 0∘//0∘ layout. Several experiments were conducted under three mode mixities φ=0.15, 0.35 and 0.65 as well as pure modes I (φ=0) and II (φ=1.0) following the established standard procedures. Energy release rates were calculated using an appropriate reduced form of the J integral. Fracture toughness was independent of the interface at initiation but increased with mode mixity. Subsequent fracture resistance was negligible for the 0∘//0∘ layout, but increased significantly for the +45∘//−45∘ interface. Mechanistic investigations were carried out using X-ray computer tomography at 5 different load levels as well as transverse cross sections. The results showed lack of bridging in the case of 0∘//0∘ layout for all values of φ. For the +45∘//−45∘ interface, fracture was dominated by delamination, ply splitting and crack migration with their extent dependent upon φ. Numerical analyses based on the virtual crack closure technique demonstrated that the antisymmetric local fracture modes, interlaminar longitudinal shear vs. transverse shear, are responsible for delamination and crack migration, respectively. Based on the experimental observations, a mesoscale FE model was able to reproduce not only the failure mechanisms, i.e. delamination, transverse cracking and crack migration resulting in the zig-zag fracture pattern, but also give a reasonable approximation of the energy release rate obtained for each of the different mode mixities considered in this study.

Influence of stress ratio and stress concentration on the fatigue behaviour of hygrothermal aged multidirectional CFRP composite laminate

The fatigue behaviour of carbon fibre reinforced polymer (CFRP) under the dry and hygrothermal aged conditions was investigated at two stress ratios, 0.1 and 0.5. Two types of layup, named as the unconstrained [06], and constrained [±45/03]S laminates were investigated to understand the ‘constraining effect’ on the zero-degree plies, imposed by the ±45 plies of the constrained laminate. The experiments revealed that the constraining effect significantly deteriorated the fatigue behaviour of the constrained laminate at low stress ratio due to stress concentration. In contrast, the laminate had better fatigue behaviour at the high stress ratio, which deteriorated after hygrothermal ageing.

The influence of material shear nonlinearity on predictive failure envelopes of multidirectional laminates

In present study, laminates with the type of [±θ°]S and [0°2/±θ°]S are investigated in order to clearly show the effect of material shear nonlinearity, and then theoretical predictions are compared with experimental data. In the whole process of generating failure envelopes, secant shear modulus instead of initial shear modulus is used for accurate shear stress-strain relation. The modulus values are recalculated every time when the combining load is changed. Final failure envelopes are then compared between numerical models. It is found that the variation in the size of safe regions from nonlinear analysis is closely related to the way of combing load and fiber directions. So in the process of improving failure theories or in the applications of failure theories, the effect of material nonlinearity can be considered appropriately.

Experimental and numerical multiscale approach to thermally cycled FRP

Due to the different thermal expansion of the constituent materials, cyclic thermal loading of FRP induces alternating stresses in the material at two scales: at the micro scale (level of fibre–matrix-interaction) and at the macro scale (level of the multidirectional laminate). Especially the micro scale effect is not comprehensively investigated yet. Additionally, computational investigations mostly neglect this effect due to the homogenous modelling of the composite material. As this effect is assumed to significantly contribute to the fatigue of FRP at thermal loads, the present paper suggests an experimental and numerical multiscale approach including experiments at the different involved material scales to separately observe the effects acting at these scales. The approach also includes numerical modelling for each scale to complement the knowledge gained from the experiments and to create a basis for the consideration of the micro effect even in macroscopic fatigue models treating homogeneous modelled composites. The main focus of the contribution is to bring the overall approach up for discussion, rather than to present the multiscale modelling details.

Crack closure in the fatigue delamination of composite multidirectional DCB laminates with large-scale fiber bridging

The stress ratio is commonly known to have an effect on the fatigue delamination growth (FDG) behavior of composite laminates. The crack closure phenomenon during delamination growth may change the value of stress ratio. Whether the stress ratio effect on the FDG behavior is resulted by the crack closure or not is investigated in this study and experimental evidences are presented in order to obtain a reliable conclusion. Using the quasi-static unloading approach, the crack closure loads during fatigue tests are firstly determined. The effect of crack closure on the mode I FDG in multidirectional composite laminates, which are widely used in engineering field, was experimentally studied. Test results show that the crack closure has an obvious effect on the minimum fatigue load and a linear relationship exists between the crack closure load and the delamination length. However, the influence of crack closure on the FDG rate is negligible. Further discussions illustrate that the crack closure does not fully explain the stress ratio effect in composite multidirectional laminates with large-scale bridging fibers.

Mixed mode testing of a multi-directional woven laminate

Mixed-mode fracture toughness tests were carried out on a multi-directional laminate making use of a Brazilian disk specimen. The composite laminate consisted of plain, balanced woven plies with tows in the $$0^{\circ }/90^{\circ }$$ -directions and the $$+45^{\circ }/-45^{\circ }$$ -directions. The delamination was placed between two of these plies by means of a polytetrafluoroethylene (PTFE) film. Tests at various loading angles were performed in order to obtain a wide range of mode mixities. Each ply was homogenized by means of a micro-mechanical model to obtain its effective properties. The specimens were analyzed by means of the finite element method. An interaction energy or M-integral was extended for this interface to obtain the stress intensity factors. The latter were used to determine the critical interface energy release rate and two phase angles. Employing this information, a three-dimensional failure criterion proposed elsewhere was presented. The criterion may be used to predict failure of a composite structure containing a delamination along the interface and material studied here.

Effects of hygrothermal aging and fiber orientations on constant amplitude fatigue properties of CFRP multidirectional composite laminates

Carbon fiber reinforced polymer (CFRP) multidirectional composite laminates are vital for moisture-prone structures where directional fatigue properties can be optimized. In this work, the tension-tension(T-T) and tension-compression(T-C) fatigue properties of hygrothermal aged [±454]S and [+45,−45, 0, 90]2S laminates were compared with virgin properties. The effect of fiber orientation and hygrothermal aging on moisture diffusivity, FTIR spectra, glass transition temperature, and static strength was also investigated. The fiber orientation had a minor impact on moisture diffusivity, static strength, and fatigue strength. Aging caused degradation in T-C fatigue life was higher as compared to T-T loading due to fiber-matrix debonding and matrix properties deterioration.

Ply thickness and fibre orientation effects in multidirectional composite laminates subjected to combined tension/compression and shear

The effects of laminate lay-up and multiaxial loading on the failure of multidirectional laminates can be significant. Thus, ply thickness and fibre orientation effects in quasi-isotropic carbon/epoxy laminates subjected to combined tension-shear and compression-shear loading are investigated. Three laminate lay-ups with equivalent thickness and homogenised elastic properties, but with different ply thicknesses and ply orientation angles are studied using open-hole specimens. Combined tension/compression-shear loading is applied using a new Modified Arcan Fixture (MAF). A methodology for identifying the failure behaviour based on stereo Digital Image Correlation (DIC) is devised. The results show that ply thickness has a strong effect on the failure behaviour in combined tension-shear, whereas the effect is small in compression-shear loading. No significant effect of the relative fibre orientation angles is observed under either loading regime. The experimental approach provides a new tool to investigate composite laminates under the full tension/compression-shear loading regime.

A physically-based failure analysis framework for fiber-reinforced composite laminates under multiaxial loading

Fiber reinforced composites are widely accepted as efficient alternatives for designing light-weight and high-performance structures, yet theoretical prediction of failure for such composites is still a challenging task with uncertainties and controversies. In this work, a new physically-based failure analysis framework is proposed to predict both intralaminar failure onset and strengths for composite laminates under general stress states, with interactive and coupling effects of stresses fully considered. The in situ strengths are introduced using the simplified fracture mechanics-based approximation formula where the constraining effects of both the adjacent plies and embedded laminar thickness are considered. The proposed framework is validated by comparing predictions with existing experimental data. Both initial and final failure envelopes are well predicted for unidirectional and multi-directional laminates under multiaxial loads. Stress-strain responses are also well captured, further illustrating the influence of in situ strengths on failure initiation.

A modified mode I cohesive zone model for the delamination growth in DCB laminates with the effect of fiber bridging

Fiber bridging has a significant influence on the delamination propagation behavior in multidirectional composite laminates. Traditional pure mode I bilinear cohesive zone models (CZM) do not consider the effect of fiber bridging and result in an inaccurate simulation on the delamination behavior. This study proposed a physical-based three-linear CZM superposed by two bilinear CZMs, which represent two different phenomena including the quasi-brittle matrix fracture characterized by a higher peak stress and a shorter critical opening displacement, and the fiber bridging characterized by a lower peak stress and a longer critical opening displacement, respectively. The three-linear CZM was implemented in the commercial FE software using a user-subroutine UMAT. Double Cantilever Beam (DCB) tests on the multidirectional composite laminates with 0°/5° and 45°/−45° interfaces conducted in our previous studies are shown to have large-scale fiber bridging in mode I delamination and are used to provide experimental data for calibrating the new CZM. Good agreements between the predicted and tested results can be achieved by adopting this new CZM, demonstrating its applicability on predicting the mode I delamination behavior in composite laminates with the effect of fiber bridging.