Quasi-isotropic Composite Laminates Research Articles

Detection of edge delamination in composite laminates using edge waves

Detecting near-edge damage in composite structural elements using guided wave-based techniques can be challenging, primarily due to the complexity of wave analysis arising from material anisotropy. Furthermore, scattered waves containing damage information can be contaminated by waves reflected from the edges, which makes detecting near-edge damage difficult to implement. In the literature, studies showed that in elastic materials, the edge of a structure can serve as a waveguide, enabling the existence of typical edge modes with concentrated energy at the edge. However, studies regarding edge waves in composite structures have received limited attention. This paper aims to explore the potential of detecting edge delamination damage in composite laminates using edge waves. The modal properties of edge waves in [(0/90)2]s composite laminates are investigated using the Semi-Analytical Finite Element (SAFE) method. Additionally, dispersion curves for quasi-isotropic composite laminates are calculated. Following this, numerical and experimental studies were conducted to investigate the sensitivity of edge waves in detecting edge delamination in the [(0/90)2]s composite laminates. The outcomes of this study offer physical insights into the modal properties of edge waves and confirm their effectiveness in detecting damage near the edges.

Characterization of multidirectional carbon-nanotube-yarn/bismaleimide laminates under tensile loading

Unidirectional, cross-ply, and quasi-isotropic composite laminates are made from continuous carbon nanotube (CNT) yarns with bismaleimide (BMI) resin. The laminates are highly graphitic and have low resin content. Elastic modulus and strength of CNT/BMI laminates and IM7/8552 carbon-epoxy laminates are measured using a scaled-down tensile test method. For CNT/BMI laminates, the variation in the measured tensile modulus is high and the laminates fail in a more gradual manner than IM7/8552 laminates. Microscopy of the failed specimens indicates that intra-yarn splitting is a common feature in all CNT/BMI laminates tested. The results of this investigation will inform the development of CNT yarn reinforced composites for structural applications.

Analysis of Dielectric Properties of Carbon-Epoxy Composite Laminates

This study highlights the investigation of different dielectric properties of non-interleaved carbon-epoxy and interleaved carbon-epoxy-nylon 66 nanofiber composite laminates. Carbon-epoxy quasi-isotropic composite laminates were developed in an autoclave using A 24-ply AS4/3501-6. Nylon-66 solution of 12 wt.% was prepared in dissolving crystals of Nylon-66 in the 90% formic acid and chloroform mixture with the weight ratio of 75/25, respectively. The nano fabric was developed by electro-spinning technique using prepared Nylon-66 solution. The fabric of 0.7g/m2 average aerial density and composite ply (AS4/3501-6) of 260 g/m2 average aerial density were observed. The interleaved laminates were developed using a nylon-66 nanofiber layer between the adjacent laminate flies. Measurement of some of the dielectric properties like dielectric constant, tan δ, and ac conductivity of the interleaved composite laminates was done at the frequency of range from 20 Hz to 10 MHz. Improvement in the different dielectric properties of the interleaved composite laminates was observed with the addition of a small wt.% of Nylon-66 nano fabric between the alternate layers of carbon-epoxy composite. A possible mechanism involved in the improvement of dielectric properties in the nanocomposites was reported in this paper.

Postbuckling Strengths of Composite Laminates with a Central Circular/Elliptical Cutout Under In-Plane Combined Loads

This paper deals with the effect of cutouts (i.e., circular and elliptical) aspect ratio on the buckling and postbuckling strengths, and failure characteristics of a simply-supported quasi-isotropic composite laminate subjected to in-plane shear and uni-axial compression combined with in-plane shear loads. The present study is carried out using self developed finite element program. The finite element formulation is based on the first order shear deformation theory and von Karmans assumptions to incorporate geometric nonlinearity. The resulting nonlinear algebraic equations are solved by Newton-Raphson method. The postbuckling strength of the laminate is defined in terms of the first-ply failure load of the lamina predicted by tensor polynomial form of the 3-D Tsai-Hill criterion. It is observed that for maximum buckling and first-ply failure strengths of the laminate with a cutout, it is preferred to have elliptical shape of the cutout as compared to the circular shape of the same area. It is also found that irrespective of load conditions and cutout shapes and sizes, shear load direction has significant effects on buckling and postbuckling stiffness of the laminate; although buckling stiffness is greater under negative shear, but the laminate shows stiffer postbuckling response under positive shear load.

Experimental and numerical research on opening size effect of novel short fiber reinforced composite laminates

This study investigates the opening size effect of a novel short fiber reinforced quasi-isotropic composite laminate with unidirectionally arrayed chopped strands (UACS) under tensile load by approaches of simulations and experiments. The layup of [45/0/-45/90]s was used to fabricate novel carbon fiber laminates with various central hole diameter from 4 mm to 10 mm. Low notch sensitivity and strength reduction rate were observed in UACS open-hole laminates. It was found that when the width diameter ratio (d/w) of UACS laminate with open-hole is less than 20%, 80% of the original strength can be maintained, which can greatly reduce the design safety factor. Compared with continuous fiber laminates, high increase rate of strength reduction rate was induced by bi-angled slits with the increasing of open hole diameter. The strain distribution around the hole was affected significantly by bi-angled slits, which also inhibited the initiation and propagation of delamination effectively. The strain concentration factor of UACS laminates increases with the increasing open hole diameter. The damage progression of notched and unnotched UACS laminates were analyzed by simulation method, which are consistent well with experimental results.

EXPERIMENTAL CHARACTERIZATION OF THE STRENGTH OF HAND-LAMINATED AND VACUUM BAGGED QUASI-ISOTROPIC GLASS-EPOXY COMPOSITES AGAISNT THE ISO 12215-5

Glass fibre reinforced polymer composites are the most common materials employed for the manufacturing of small crafts. Their hull construction and scantlings are governed by the ISO 12215-5. The latest version of the standard introduced a new methodology to assess the ultimate strength of composites laminates. The regression equations based on the fibre weight fraction of the former version have been replaced by a more detailed ply-by-ply analysis. However, no validation data is available to ascertain the relevance of the default ultimate strengths provided by the ISO 12215-5:2019. This paper employs destructive testing to experimentally characterize the ultimate flexural, tensile and compressive strength of a quasi-isotropic glass-epoxy laminate. Experiments were undertaken in accordance with the ISO 178 for flexural properties, the ISO 527-4 for tensile properties, and the ISO 14126 for compressive properties. Two manufacturing techniques are investigated, namely hand lamination and vacuum bagging. The former represents a cheaper and therefore more common manufacturing option, while the latter is representative of an advanced manufacturing process. The key results show that the ISO 12215-5:2019 default ultimate strengths for the quasi-isotropic composite laminate tested are (i) conservative for the ultimate flexural strength, (ii) appropriate for the ultimate tensile strength, and (iii) optimistic for the ultimate compressive strength, especially for vacuum bagged samples, with the main cause identified as the value of the ultimate compressive breaking strain for chopped strand mat. These findings provide validation data for the ISO12215-5:2019. They may inform designers, compliance assessors and policy makers in the selection of relevant factors of safety for composite structures, and it is anticipated the results may contribute to future improvements in small craft regulations.

Efficient finite element simulation of compression after impact behaviour in quasi-isotropic composite laminates

A simple and efficient finite element modelling approach in LS-DYNA is presented for the simulation of residual strength in compression after impact tests of quasi-isotropic IM7/8552 carbon fibre reinforced composite laminates. The modelling strategy has been previously validated for transverse impact test simulations with impact energies ranging from 6 J up to 20 J. These results are used as input to analyse residual strength, quantitative and qualitative damage patterns as well as effects of prediction errors caused by the transverse impact simulations. It is found that residual strength in [45/0/90/−45]4s sub-laminate scaled laminates can be predicted reasonably well with a maximum error of 10%, whereas simulations of [452/02/902/−452]2s ply scaled laminates yield errors of up to 30%.

Influence of kerosene flame on fire-behaviour and mechanical properties of hybrid Carbon Glass fibers reinforced PEEK composite laminates

This work examines the influence of kerosene flame exposure on the residual mechanical behavior (tension and compression) of hybrid quasi-isotropic composite laminates consisting of carbon/glass fibers and a PEEK thermoplastic matrix. The influence of a kerosene flame exposure (116 kW/m2 and 1100 °C), on the composites structural integrity was examined as a function of exposure time (5–10-15 min). The changes in the tensile and compressive properties (axial stiffness and strength) were compared with respect to the virgin materials (experiencing no prior flame exposure). The discussions on fire- and mechanically-induced damage mechanisms are supported by fractographic analysis of specimens. It is therefore possible to better understand how the fire-induced damages within the laminates micro- and meso-structures modify the mechanical behavior of flame-exposed laminates.Regardless the exposure time, the kerosene flame exposure involves in-plane and through-thickness temperature gradients, which ultimately create two well defined areas within the laminates: an extensively delaminated one (the one close to the exposed surface) and a relatively well preserved one (close to the back surface). From the present work, it is possible to conclude that the mechanical properties in tension are severely affected (-50% in stiffness and −70% in strength) by prolonged exposures to kerosene flame (15 min) with respect to as-received specimens. When compared to the 5 min case, flame exposure time (ranging from 5 to 15 min) seems to have little very influence on tensile properties and the effect is moderate on compressive properties. The barrier formed by an extensive thermally-induced delamination contributes to relatively preserve the structural integrity of the plies close to the back surface. The mechanical loading is taken up by the 0° fibers in non-delaminated areas of the specimens, resulting in preserving the residual mechanical in tension and in compression.

Minimum variance Lamb wave imaging based on weighted sparse decomposition coefficients in quasi-isotropic composite laminates

Lamb wave is a promising means for active structural health monitoring and nondestructive evaluation of laminated composite plates. The inevitable reflections from structural geometric boundaries and other interferences limits the imaging performance of many existing imaging methods. The image generated through the well-known delay-and-sum method holds a wide mainlobe width and high level of sidelobes. Although the sparse reconstruction method is free from those deficiencies, it is sensitive to the regularization parameter. To overcome those limitations, a Lamb wave imaging method to locate anomalies or damage in laminated composite plates is proposed. The scattering signals are sparsely decomposed one by one with a prior weights penalized on the undetermined sparse coefficients, and the minimum variance distortionless response algorithm is adopted to process those sparse coefficients so as to generate the image. Experimental results on a quasi-isotropic laminated composite plate demonstrate the effectiveness of the proposed method.

Guided wave scattering at a delamination in a quasi-isotropic composite laminate: Experiment and simulation

Carbon fibre composite laminates are increasingly being used for aerospace structures due to their low weight and improved mechanical performance. Impact damage can cause delaminations below the visible surface of the structure due to limited interlaminar strength. Guided ultrasonic waves can detect and characterize delaminations in composite laminates. The scattering of the A0 Lamb wave mode at an artificial delamination, located at an asymmetric depth in a quasi-isotropic laminate, was investigated. Full field non-contact laser measurements were used to visualise wave trapping and scattered waves. A three-dimensional finite element model was developed and validated against the experiments. The influence of delamination shape and depth on guided wave scattering were studied. Small variations in delamination shape significantly affected the interference pattern on top of the delamination, but had limited effect on the scattered wave outside the delamination. Delamination depth was found to strongly influence the angular direction and amplitude of scattered waves. Implications for structural health monitoring were discussed.

Experimental Study on Compression Failure of Composite Laminates with Prefabricated Surface Cracks.

A new compression test fixture was designed in the present work to study the damage tolerance of composite laminates with surface cracks or notches. The compression failure behaviors of CCF300/5228A quasi-isotropic composite laminates with prefabricated surface cracks were studied experimentally. Through the size design of the test fixture and specimens and an application of a simple test method, the complex crack growth process was captured. The experimental results showed that the compression failure modes were mainly affected by crack angles and depths, and there were two typical failure modes, which were local intra- and inter-laminar damage propagating from the crack tips and delamination growth induced from the crack leading edge. This study verified the validity of the test fixture and test method, and revealed the compression failure mechanisms of composite laminates with surface cracks.

An experimental investigation on low-velocity impact response and compression after impact of a stochastic, discontinuous prepreg tape composite

The present paper investigated the impact and compression after impact behavior of thin carbon fiber epoxy plates manufactured from prepreg platelet molding compound (PPMC) with two different discontinuous fiber length, 25.5 mm and 12.7 mm, and compared it with the behavior of continuous fiber quasi-isotropic composite laminate with a similar plate thickness. The results of dynamic testing showed that PPMC/12.7 mm exhibited larger amount of absorbed energy, which correlated with larger damage area measured by ultrasonic inspection. Micro-CT analysis showed that a discontinuous fiber composite developed significant amount of through-thickness damage. C-scan measured damage area correlated well with the compression after impact testing results, which indicated that longer fibers provide for better damage tolerance.

Bearing strength performance of mixed thin/thick-ply, quasi-isotropic composite laminates

The effect of using thin plies to increase the bearing strength of composite laminates has been investigated. A series of 5 laminates of theoretically identical stiffness with varying proportions of thin plies were manufactured using a single material system. Four specimens from each plate were tested for bearing strength and damage was subsequently characterized using an optical microscope. The results show that performance in terms of bearing stiffness, strength at onset of damage, and ultimate bearing stress increase proportionally with the increasing amount of thin plies within the stack. Shifting from a 100% conventional ply laminate to a 100% thin-ply laminate gave an increase of 47% in the strength at onset of damage. Placement of the thin plies within the stack was also shown to be important for strength at initial onset of damage. Microscopic examination of the failure modes for all samples showed fiber kinking, localized to the center of the hole, to be the dominant failure mode regardless of the stacking sequence.

Simulation of Quasi-Isotropic E-Glass Composite Laminate at Low Velocity Impact with Cohesive Interface Elements

This paper examines the expediency of interface elements in modeling of impact damage analysis for Eglass composite laminate under low velocity impact test. Numerical modelsare built adopting cohesive interface behavior to authenticate the cross-ply damage response; and successively used the strategy to model the impact response of quasi-isotropic composite laminate. Impact test are performed to characterize the induced-damage behavior in quasi-isotropic composite laminate at different impact energy test in terms of impact force, displacement and damage size as well as the stress failure trajectory. Numerical result shows reliability of the model for structural impact analysisin damage initiation and progression in laminated composite plates. The simulation result though reveals large deformation, yet, did not yield in total fracture. This development shows the importance of adopting interface elements in structural impact damage criterion to trigger constraints effect on initiation phase.The study also reveals that the bottom most surface suffers huge deformation compare to the impact surface. It divulges that the extent of damage area in each ply of the composite laminate orients in the fiber direction in ‘star-shaped contour. The main novelty is the capability of using this model for structural impact analysis on both cross-ply and quasi-isotropic composite laminate.

Progressive failure analysis of CFRP composite laminates under uniaxial tension using a discrete element method

This study presents a 3D Discrete Element Method (DEM) model for the progressive failure analysis of Carbon Fiber Reinforced Polymer (CFRP) composite materials subjected to uniaxial tensile loading. Particles in the model are packed and bonded in regular patterns (hexagonal or square). The relationship between the bond stiffness and material properties is established based on the average strain energy method. The random distribution of bond strengths calibrated from experiments with a variation of 30% and 10% following a normal distribution law is assigned to the bonds in 0∘ and 90∘ plies to capture random cracks, respectively. Tsai-Hill failure criterion is utilized for the calibration of bond strength of [Formula: see text] plies to predict their failures in composite laminates. Quantitative and qualitative analyses were conducted for predicting the damage initiation and propagation of the cross-ply and Quasi-Isotropic (QI) composite laminates under tensile loading, respectively. Two interface stiffnesses were utilized in the failure prediction of cross-ply composite laminates, and it was found that the numerical results with the interface stiffness calculated from fracture energy are in good, quantitative agreements with the experiments. All the four stages of the failure process of QI composite laminates are well captured by the 3D DEM model, including isolated cracks, inner delamination cracks, outer delamination cracks and final failure.

On the instability of multiple annular delaminations of axisymmetric laminates with arbitrary boundary conditions subjected to transverse load

Herein, the damage problem of quasi-isotropic composite laminates subjected to a quasi-static transverse load has been studied analytically. The damage was modeled as multiple equal-sized annular delaminations of equal-interval in the thickness direction without any opening during transverse loading. This simplified damage shape was adopted according to the typical damage morphology of impact-tested laminates. Closed-form expressions of the responses, such as a deflection and an energy release rate, were developed in terms of the applied force, dimensions of the plates, and damage under arbitrary boundary conditions based on the Mindlin plate theory. Though their applicability is limited owing to an assumption of linearity, the closed-form expressions provide fundamental information, particularly at the initiation and early growth of the damage, on the effects of the boundary conditions, dimensions, fundamental material properties, and ply thickness of the composite laminates.

Theory-guided machine learning for damage characterization of composites

A novel approach for damage characterization through machine learning is presented where theoretical knowledge of failure and strain-softening is linked to the macroscopic response of quasi-isotropic composite laminates in over-height compact tension tests. A highly efficient continuum damage finite element model enables the training of a system of interconnected Neural Networks (NNs) in series solely based on the macroscopic load-displacement data. Using experimental results, the trained NNs predict suitable damage parameters for progressive damage modeling of IM7/8552 composite laminates. The predicted damage properties are validated successfully using experimental measurements obtained through cumbersome non-destructive data analysis. The proposed strategy demonstrates the effectiveness of machine learning to reduce experimental efforts for damage characterization in composites.

A comparative numerical study of standard 3D composite plate using virtual crack closure technique and cohesive zone modelling method

A standard square plate specimen made up of Carbon Fibre Reinforced Polymer (CFRP) and Glass Fibre Reinforced Polymer (GFRP) laminates with an artificially-introduced delamination was considered for evaluation of fracture parameters using Abaqus-Standard software. Though there are several methods to evaluate the initiation of delamination using software, Virtual Crack Closure Technique (VCCT) and Cohesive Zone Modelling (CZM) method are commonly used. By using the above-mentioned methods, the Quasi-Isotropic (QI) composite laminates were analysed for varying compressive loads. The Load-Displacement responses were reported and seen to be in overall good statement with each other, while the modelling effort and computational time required were quite higher in CZM method compared to VCCT. Onset of delamination growth for CFRP and GFRP laminates was studied using VCCT by evaluating Strain Energy Release Rates (SERR). The results for both the specimens were compared and reported.

Mode Conversion and Scattering of Lamb Waves at Delaminations in Composite Laminates

AbstractThis paper presents a study of Lamb wave scattering and mode conversion at a delamination in quasi-isotropic (QI) composite laminates. The study employs the fundamental antisymmetric mode (...

A micromechanics-based damage model for compressive behavior analysis of impacted composite laminates

The compressive strength of composite laminates decreases seriously after being subjected to impact loading, which is an important item to be considered in the usage of composite material. In this paper, a micromechanics-based damage model is proposed to study the compressive behavior of impacted composite laminates. The micro stresses of fiber and matrix are calculated by stress amplification factors and then used to judge the failure mechanisms according to corresponding physical failure criteria. A progressive damage model based on different failure statuses of constituents is established to study the degradation of material properties. The bi-linear cohesive model is used in the research of delamination onset and propagation. The compressive behaviors of quasi-isotropic composite laminates subjected to different impact energies are investigated by this proposed method. Good agreements in terms of structure responses, failure mechanisms, and residual compressive strengths are obtained between numerical results and experimental data. The matrix cracking and delamination caused by impact loading are responsible for the initiation and propagation of buckling, which leads to the final collapse of entire laminates. Based on the numerical investigations of material parameters, the increment of mode II interlaminar fracture toughness is capable of improving the residual compressive strength significantly.