Off-axis Layers Research Articles

The Influence of Different Lay-Up Parameters on the Fatigue Response of Carbon/Epoxy Laminates under Internal Multiaxial Stress States.

The inherent anisotropy of composites complicates their damage response. The influence of multiaxiality, particularly in carbon-based composites, is not thoroughly understood due to obstacles related to damage monitoring during loading. In this study, the response of different carbon/epoxy laminates under fatigue is examined through dedicated in situ microscopic observations. By varying the orientation of off-axis layers, the impact of multiaxiality on the mechanical and damage response is evaluated. Furthermore, balanced and unbalanced laminates are compared, considering the limited information for the latter. The influence of the number of off-axis layers is finally assessed leading to important conclusions about optimal fatigue response. The fatigue response is evaluated in all cases considering both the mechanical properties and the damage characteristics. Significant conclusions are drawn, especially for the benefits of unbalanced laminates and the impact of shear stresses, allowing for the utilization of the obtained data as important input for the establishment of reliable fatigue damage models.

Effect of Off-Axis Ply on Tensile Properties of [0/θ]ns Thin Ply Laminates by Experiments and Numerical Method.

The effect of off-axis ply on the tensile properties of unbalanced symmetric [0/θ]ns laminates was explored through experimental and numerical analysis. Six CFRP [0/θ]2s plies with different off-axis angles θ were fabricated for tensile tests. In situ observations of the damage to the laminates were conducted to investigate the initiation and progressive growth of the laminates during the tension tests. The fiber fractures, crack initiation, and progressive propagation were analyzed by observing the free edge of the laminates, and the difference in damage behavior caused by different off-axis angles was investigated. All the six [0/θ]2s plies with off-axis angles θ ranging from 15° to 90° showed approximate linear stress–strain responses in the tensile tests. Matrix cracks were not observed prior to the final catastrophic failure in the off-axis layers of the [0/θ]2s laminates with a θ in the range of 15–60°. Finite element analysis (FEA) of the [0/θ]s plies was conducted using a 3D micromechanical model, in which matrix cracking and fiber-matrix debonding in the off-axis layer were simulated using a cohesive interface element. Three micromechanical crack-free, cohesive interface, and initial crack models were analyzed to predict the influence of the matrix cracks inside the off-axis layer on the damage behavior of the [0/θ]s laminates. The numerical results from the initial crack micromechanical model show a lower bound of the tensile strength of the [0/θ]s plies. A high stress concentration is observed adjacent to the cracked off-axis layer, inducing a tensile strength loss of about 20%.

Effect of variable amplitude block loading on intralaminar crack initiation and propagation in multidirectional GFRP laminate

Studies have shown that the fatigue life of multidirectional laminated fibre composites is reduced for variable amplitude block loading and random load spectra compared to constant amplitude loading. One of the hypotheses in the literature for this is a cycle mix effect that causes extra damage each time the load increases i.e. load amplitude or mean load. However, no explanation for this effect has been provided in the literature. This work aims at identifying the reasons for the reduced fatigue life of variable amplitude block loading tests compared to constant amplitude loading tests. This is done through white light imaging which is used to detect when intralaminar cracks occur in the off-axis layers and from this to determine crack density evolution, crack density rates, as well as initiation and propagation of intralaminar cracks contributing to the total crack density. Combined, these parameters show that there is a slow transition from a high crack density rate in the high load blocks to a lower crack density rate in the low load blocks. The crack density rate in the low load blocks in a variable amplitude loading test is higher than in a constant amplitude loading test at similar damage states. Finally, the high load blocks initiate new cracks in each block but they also cause all existing cracks to continue their propagation in subsequent low load blocks at increased rates. In conclusion, there is evidence that the block changes from high to low loads cause increased crack propagation in subsequent low load cycles, which is an important contribution to explain the damaging effect of cycle mixing.

Numerical analysis of stresses in double-lap adhesive joint under thermo-mechanical load

A numerical study for the double-lap adhesive joint made of similar adherends subjected to tensile and thermal loads is presented. A novel displacement coupling conditions which are able to correctly represent monoclinic materials (off-axis layers of composite laminates) are used to build a comprehensive numerical model. Two types of double-lap joints are considered in this study: metal–metal and composite-composite. In case of composite laminates, four lay-ups are evaluated: unidirectional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/−45]S and [90/45/0/−45]S). The effect of different parameters (adherend stiffness, ply stacking sequence, adherend thickness, one-step or two-step manufacturing of the joint) on peel and shear stress distribution in the middle of the adhesive is studied. The comparison of the behaviour of single-lap and double-lap joint in relation to these parameters is made. The maximum peel and shear stress at the ends of the overlap with respect to the axial modulus of the adherends are presented in a form of the master curves. The analyses of results show that: the maximum peel and shear stress concentration at the overlap ends is reduced with the increase of the axial modulus of the adherend; the stress distribution in the adhesive layer can be improved (lower stress concentrations and level-out the curve) by changing the fibre orientation (which affect the stiffness) in plies connected to the adhesive layer.

Local Delaminations Induced by Interaction Between Intralaminar Cracking and Specimen Edge in Quasi-Isotropic CF/EP NCF Composites in Fatigue Loadings

Experimental results are presented on the onset and propagation of local delaminations caused by the interaction between specimen edges and intralaminar cracks in fiber bundles of 90° layers in quasi-isotropic [–45/90/45/0]s CF/EP noncrimp fabric (NCF) laminates subjected to tension-tension fatigue loadings. It is confirmed that the first damage mode is intralaminar cracking in 90° layers, which consists of intrabundle cracks and cracks in the matrix between bundles (often beginning from stitches). This damage mode triggers cracking in off-axis layers and local delaminations in positions where the 90° layer crack meets an adjacent layer. The process of local delamination is significantly enhanced at specimen edges, where the out-of-plane edge stresses contribute to the local delamination. During cyclic loadings, delaminations grow and coalesce along the edge and propagate towards the specimen center. These processes are quantified experimentally at different levels of cyclic load. In a low-stress fatigue, a very high number of cycles is required to detect small edge delaminations, and they stay at the edge. In high-stress cyclic tests, delaminations grow faster inside the composite: about 20% of the interface in the central zone can be delaminated. It is found that the reduction in the axial modulus is proportional to the relative delaminated area, proving that delamination is the major stiffness reduction factor in these laminates.

Effect of multiaxiality, stacking sequence and number of off-axis layers on the mechanical response and damage sequence of carbon/epoxy composite laminates under static loading

The mechanical response and damage sequence of composite materials are nowadays still a topic of ongoing research. However, many parameters influencing their overall behavior are still not thoroughly taken into consideration. The effect of multiaxial stresses, the distinction between balanced and unbalanced configurations and the influence of the number of off-axis layers are just a few to mention. Experimental data regarding the effect of all these parameters on the damage progression in composites is of great importance, since it is proven that commonly used failure criteria, neglecting the occurring damage mechanisms, cannot always predict the material response. In this work, a study of the influence of all these parameters is attempted, by testing carbon/epoxy laminates with different off-axis angles to account for different multiaxiality. Both balanced and unbalanced laminates are taken into account, considering the lack of experimental evidence in literature regarding the latter case, and significant differences between the two lay-ups are reported for the first time. Finally, the influence of the number of the off-axis layers on the mechanical response in conjunction with the previous parameters is also studied through elaborate damage observations.

Tensile behavior of on- and off-axis carbon fiber reinforced polymer composites incorporating steel wire mesh

This paper experimentally investigates the tensile behavior of hybrid composites made with on- and off-axis carbon fiber reinforced polymer that incorporates stainless steel wire mesh. Various layers of off-axis carbon fiber, layers of steel wire mesh, and levels of curing temperature were used as test parameters. The test results indicated that the test specimens using an intermediate amount of carbon fiber layers (three and five off-axis layers) showed an inclined fracture of carbon fiber layers at failure, whereas the specimens using a large amount of carbon fiber layers (ten off-axis layers) exhibited a significant lateral contraction at failure. In particular, the use of a large amount of off-axis carbon fiber layers could significantly increase the initial stiffness, peak load, and residual load of the hybrid composites, in particular, when using steel wire mesh layers. In addition, the tensile behavior of the hybrid composites was governed by the combined effect of the steel wire mesh layers and the number of off-axis carbon fiber fabric layers. A model was proposed to predict the mechanical properties of the hybrid composite specimens, and the prediction results showed good agreement with the test results.

Effects of the off-axis layer on the tensile failure of carbon fiber reinforced polymer [0/θ]ns laminates

Off-axis plies have a great influence on the tensile failure stress of carbon fiber reinforced polymer (CFRP) multidirectional laminate. Matrix cracks in the off-axis plies will cause negative effect on the layers, leading to fatal failure of the laminate. A microscale cohesive zone model is proposed to explore the influence of matrix cracks in the off-axis plies on the tensile failure of [0/θ]ns laminates. Simulations of tensile failure of [0/θ]s laminate were performed by varying the off-axis ply angle θ ranging from 15° to 90° and the cohesive properties to investigate the effects of off-axis plies to the failure of the laminates. Tensile tests of [0/θ]2s laminates with various off-axis ply angles were conducted to validate the simulations of microscale model. The simulation results with matrix cracking are in high agreement with the experimental results, illustrating the influence of off-axis plies on the tensile failure of [0/θ]ns laminates.

Numerical stress analysis in adhesive joints under thermo-mechanical load using model with special boundary conditions

A numerical study of the adhesivejoint made of similar and dissimilar adherends subjected to thermo-mechanical loading is presented. A comprehensive numerical model was used for this purpose with the novel displacement coupling conditions which are able to correctly represent monoclinic materials (off-axis layers of composite laminates). The geometrical nonlinearity as well as nonlinear material model are also taken into account. Three different types of single-lap and double-lap adhesive joints are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). This paper focuses on the parameters which have the majoreffect on the peel and shear stress distribution within adhesive layer at the overlap ends. The comparison of behaviour of single-and double-lap joints in relation to these parameters is made. The master curves for maximum stress (peel and shear) at the ends of the overlap with respect to the bending stiffness and axial modulus of the adherends are constructed by analysing stress distributions in the middle of the adhesive.The main conclusions of this paper are: the maximum peel stress value for SLJ is reduced with increase of the adherend bending stiffness and for DLJ,similar behaviour was observed at the end next to the inner plate corner, while, at the end next to the outer plate corner peel stress is reduced with increase of adherend axial modulus.

Influence of Reversed Fatigue Loading on DamageEvolution of Cross-Ply Carbon Fibre Composites.

Microcrack formation and delamination growth are the main damage mechanisms in the fatigue of composites. They lead to significant stiffness loss, introduce stress concentrations and can be the origin of subsequent damage events like buckling or fibre breakage, especially in case of shear and compression stresses during load reversal. Fatigue experiments of carbon fibre reinforced laminates were conducted at several stress ratios and analysed in terms of crack and delamination growth. These investigations were accompanied by microscopic imaging, digital image correlation and finite element modelling to take into account the effects of residual stresses and crack closure. It was found that residual stresses significantly change the local stress ratio in off-axis layers and lead to residual crack opening of inter fibre cracks. These cracks remain open and close under high compression loadings only. Furthermore, crack formation under pulsating compression loading turned out to be driven by residual stresses leading to perpendicular cracks as observed under pure tension loading. The experimental findings further confirm the severe detrimental effect of tension-compression loading on crack formation and delamination growth compared to pulsating tension-tension or compression-compression loads.

Investigation of end and edge effects on results of numerical simulation of single lap adhesive joint with non-linear materials

This paper presents systematic numerical study of stresses in the adhesive of a single-lap joint with the objective to improve understanding of the main material and geometrical parameters determining performance of adhesive joints. For this purpose a 3D model as well as 2D model, optimized with respect to the computational efficiency by use of novel displacement coupling conditions able to correctly represent monoclinic materials (off-axis layers of composite laminates), are employed. The model accounts for non-linearity of materials (adherend and adhesive) as well as geometrical non-linearity. The parameters of geometry of the joint are normalized with respect to the dimensions of adhesive (e.g. thickness) thus making analysis of results more general and applicable to wide range of different joints. Optimal geometry of the single-lap joint allowing to separate edge effect from end effects is selected based on results of the parametric analysis by using peel and shear stress distributions in the adhesive layer as a criterion. Three different types of single lap joint with similar and dissimilar (hybrid) materials are considered in this study: a) metal-metal; b) composite-composite; c) composite-metal. In case of composite laminates, four lay-ups are evaluated: uni-directional ([08]T and [908]T) and quasi-isotropic laminates ([0/45/90/-45]S and [90/45/0/-45]S). The influence of the abovementioned parameters on peel and shear stress distributions in the adhesive layer is examined carefully and mechanical parameters governing the stress concentrations in the joint have been identified, this dependence can be described by simple but accurate fitting function. The effect of the used material model (linear vs non-linear) on results is also demonstrated.

Effective permeability averaging scheme to address in-plane anisotropy effects in multi-layered preforms

In Liquid Composite Molding (LCM) processes, fabric layers are stacked in a mold which may be a few meters long and wide to build up a thickness of not more than a few millimeters. Resin is introduced to fill all the empty spaces between the fibers. As the in plane dimensions are a few orders of magnitude larger than through the thickness, flow of resin through the preform can be modeled using the two-dimensional Darcy’s law, neglecting the through-thickness velocity and assigning the preform an arithmetic averaged permeability from the layers. However, there are situations in which the through-thickness flow is significant where this assumption is no longer valid or justified. To address such cases, a modified averaging scheme was proposed by Calado and Advani (1996) to account for the transverse flow between adjacent layers of a preform and consequently derive an homogenized one-dimensional value of effective permeability. In the current work, such a model is extended to account for the effect of anisotropic off-axis layers in the stack. The result is a generalized scheme for effective permeability averaging layers of heterogeneous preforms, capturing both through-thickness and in-plane effects into a one-dimensional permeability value. This methodology was validated and a parametric study was conducted with different combinations of in-plane and through-thickness permeability values to identify the influence of preform in-plane dimensions and thickness and to define a criteria that relates the material and geometric parameters to the transverse flow.

Fluid diffusion in cracked composite laminates – Analytical, numerical and experimental study

Composite structural components are often subjected to service loads that cause cracks in off-axis layers, without compromising the load bearing capability of the structure. In marine or humid environments fluid ingress in the material can be accelerated by the presence of such cracks, leading to further strength degradation.Several studies have been dealing with the effect of cracks on fluid diffusion in composites, sometimes with contradictory results. In this work the difference between cracks on the external and internal plies of the laminate has been addressed both experimentally and analytically, showing that the first ones have a strong influence on diffusivity, while the second have a negligible effect.

Analysis of Microdamage in Thermally Aged CF/Polyimide Laminates

Microdamage in the layers of CF Thornel® T650 8-harness satin-weave composites with a thermosetting NEXIMID® MHT-R polyimide resin, designed for high service temperatures, is analyzed. After cooling down to room temperature (RT), a multiple intrabundle cracking due to tensile transverse thermal stresses was observed in the [(+45/–45)/(90/0)]2s laminates studied. Then, the composite was subjected to two ramps of thermal cycling quantifying the increase in crack density in its layers. A comparison of two ramps with the same lowest temperature showed that the highest temperature in the cycle where thermal stresses were low had a significant detrimental effect on the thermal fatigue resistance of the composite. The effect of holding it at 288°C for 40 days was also studied: many new cracks formed in it after cooling down to RT. During the time at the high temperature, the mechanical properties degraded with time, and the crack density versus aging time was measured at RT. Then, both aged and nonaged specimens were tested in uniaxial quasi-static tension quantifying the damage development in layers of different orientation. Cracking in the layers was analyzed using fracture mechanics arguments and probabilistic approaches: a) a simple one, not considering crack interaction; b) Monte Carlo simulations. It is shown that cracking in the off-axis layers which are not in contact with the damaged 90°-layer can be predicted based on the Weibull analysis of the 90°-layer, whereas in the off-axis layer contacting the 90°-layer, the crack density is much higher due to the local stress concentrations caused by cracks in the 90°-layer. The thermal treatment degraded the cracking resistance in the surface and adjacent layer, whereas the composite close to the midplane was not changed.

Microdamage analysis in thermally aged CF/polyimide laminates

Microdamage in layers of CF Thornel® T650 8-harness satin woven composite with thermosetting polyimide NEXIMID® MHT-R resin was analysed. After cooling to room temperature multiple intra-bundle cracking due to tensile transverse thermal stresses was observed in the studied [(+45/-45)/(90/0)]2s composite. The composite was subjected to thermal cycling quantifying the increase of crack density in layers. Comparison of two ramps with the same lowest temperature shows that the highest temperature in the cycle has a significant detrimental effect. Exposure for 40 days to 288°C caused many new cracks after cooling down to room temperature. Both aged and not aged specimens were tested in uniaxial quasi-static tension. Cracking was analysed using fracture mechanics and probabilistic approaches. Cracking in off-axis layers was predicted based on Weibull analysis of the 90- layer. The thermal treatment degraded the cracking resistance of the surface layer and of the next layer.

High-resolution computed tomography in resin infused woven carbon fibre composites with voids

Tomographic imaging using both microfocus radiation and synchrotron radiation was performed to assess the void defects in resin transfer moulded woven carbon fibre composites. The focus of this study is on characterising the void homology (e.g. local void size and spatial distribution) in relation to weave orientation, infusion direction and potential effects on damage formation in tensile loading. As the orientation angle between the fibre direction of unidirectional layer in the laminate and the direction of the global resin flow increases, from parallel to perpendicular, larger voids and a greater volume fraction of voids were observed, which led to increased damage formation upon loading. Significant accumulation of voids around both the layer interfaces and yarn fibres were also observed. With regard to yarn design, it is recommended to balance the benefits (e.g. fabric handling, structural integrity of preform) and drawbacks (e.g. lower fibre content, more voids) of the supporting yarn. Also, sensible placement of resin inlets and outlets could reduce the amount of deleterious voids, i.e. by promoting resin flow along the fibre direction in the most defect-sensitive off-axis layers.

Off-axis deposition of Al layer for low resistance tunnel barrier

In order to improve the magnetic and electrical properties of low resistance alumina based magnetic tunnel junctions, an off-axis method of sputtering has been investigated. It is shown that the tunnel magnetoresistance ratio can be greatly increased when there is an offset between the target and the wafer axes during the deposition of the ultrathin aluminum layer (off-axis sputtering) prior to its natural oxidation. The ferromagnetic coupling between the pinned and the free layer through the alumina barrier is also reduced compared to a classical on-axis deposition. This observation is interpreted as an improvement of the barrier quality, reducing both the roughness and the pinholes density. We assume that when the Al layer is sputtered off-axis, the magnetic and aluminum layers are protected from energetic neutralized Ar atoms bombardment.

Growth and Properties of SiC On-Axis Homoepitaxial Layers

Homoepitaxial growth has been performed on 3” Si-face on-axis 4H–SiC substrates using standard gas system in a horizontal Hot-wall chemical vapor deposition system. Substrate surface damages are found to act as preferential nucleation sites for 3C inclusions also, the surface morphology after in-situ etching is found to largely influence the polytype stability in the epilayer. Different in-situ etching conditions were studied where Si-rich conditions are found to be better. Growth parameters and starting growth conditions are refined to obtain stable polytype in the epilayer. High quality homoepitaxial layers with 100% 4H–SiC are obtained on 3” substrates. Different optical and structural techniques are used to characterize the layers and to understand the growth mechanisms. The layers are found to be of high quality and no epitaxial defects typically found on off-axis epitaxial layers are observed. A high surface roughness is observed in these layers, however higher growth rate significantly lowers the surface roughness without affecting the polytype stability in the epilayer.

Analysis of matrix cracking in GFRP laminates using Raman spectroscopy

A Raman spectroscopic technique was used to investigate the elastic modulus degradation and the residual strain development in cross-ply and [0/45 2/0] T glass fibre/epoxy laminates. Glass has a poor Raman signal and, therefore, embedded aramid fibres were used as strain sensors for mapping of the internal strain development within the 0° ply due to matrix cracking in the off-axis ply. The elastic modulus and the residual strain were derived from the changes of the distance between well identified strain “peaks” on the aramid fibres. The elastic modulus reduction and the residual strain development are described by a closed form model. The modulus reduction predictions are accurate, whereas the experimental residual strain is higher than that predicted. Since both phenomena are governed by the same elastic parameters, the discrepancy cannot be explained by purely elastic analysis. Development of inelastic strains in the off-axis layer is suggested as a possible cause.

Raman spectroscopy investigation of stiffness change and residual strains due to matrix cracking

The matrix cracking models developed for cross-ply composite laminates have been poorly extended in the past to more complex geometries used in practice, and they are still under development. In this paper, a new detailed analysis of the effect of matrix cracking on the behaviour of cross-ply and [0/45]s laminates under uniaxial tension is attempted. The model used in this work is applicable both to cross-ply laminates and unbalanced systems. It gives exact closed-form expressions for all thermomechanical properties of a general symmetric laminate with cracks in arbitrary layers. The theoretical approach is backed by experimental data obtained by microscopic strain-state variation measurements within a specimen, with using the technique of laser Raman spectroscopy. Glass-fibre-reinforced epoxy systems were investigated. Embedded aramid fibres-sensors within the 0° ply and near the 0°/θ ° interface were necessary due to the poor Raman signal of glass. Using experimental Raman data, the residual strain and the stiffness reduction are determined as functions of increase in crack density. The stiffness reduction is predicted with a high accuracy, whereas the measured residual strains are larger than predicted. The good results for the reduction in the elastic modulus show that the basic assumption of the model is accurate. The difference is explained by the viscoelastic-viscoplastic behaviour of the off-axis layer in shear, which in creases the “apparent” residual strain.