Strength Of Composite Laminates Research Articles

Critical Void Content for Thermoset Composite Laminates

An experimental program to characterize the effect of voids on the strength of composite laminates is presented. The adequacy of a fracture criterion to represent the experimental data for the effect of voids on the flexure strength, tensile strength, and interlaminar shear strength of composite laminates is assessed. The experimental program investigates the effect of different pressures and dwell times on the critical void content. Laminates produced with carbon fiber/epoxy resin unidirectional prepreg have been produced with an intentionally high void content. Short beam shear, three-point flexure, and tensile testing are used for mechanical evaluation and the results correlate to void volume fraction and ultrasonic absorption coefficient. The ultrasonic absorption coefficient is measured for all the specimens and its variation is approximately linear with the void content, corroborating previous experimental results. The effects of these factors on the strength of the composite laminates are discussed in terms of the fracture parameters involved in the fracture criterion. The critical void content is estimated for each case both in terms of void content and ultrasonic attenuation.

Influence of Compression and Shear on the Strength of Composite Laminates with Z-Pinned Reinforcement

The influence of compression and shear loads on the strength of composite laminates with z-pins is evaluated parametrically using a 2D Finite Element Code (FLASH) based on Cosserat couple stress theory. Meshes were generated for three unique combinations of z-pin diameter and density. A laminated plate theory analysis was performed on several layups to determine the bi-axial stresses in the zero degree plies. These stresses, in turn, were used to determine the magnitude of the relative load steps prescribed in the FLASH analyses. Results indicated that increasing pin density was more detrimental to in-plane compression strength than increasing pin diameter. Compression strengths of lamina without z-pins agreed well with a closed form expression derived by Budiansky and Fleck. FLASH results for lamina with z-pins were consistent with the closed form results, and FLASH results without z-pins, if the initial fiber waviness due to z-pin insertion was added to the fiber waviness in the material to yield a total misalignment. Addition of 10% shear to the compression loading significantly reduced the lamina strength compared to pure compression loading. Addition of 50% shear to the compression indicated shear yielding rather than kink band formation as the likely failure mode. Two different stiffener reinforced skin configurations with z-pins, one quasi-isotropic and one orthotropic, were also analyzed. Six unique loading cases ranging from pure compression to compression plus 50% shear were analyzed assuming material fiber waviness misalignment angles of 0, 1, and 2 degrees. Compression strength decreased with increased shear loading for both configurations, with the quasi-isotropic configuration yielding lower strengths than the orthotropic configuration.

A multi-scale progressive damage model for laminates

Predicting the strength of composite laminates subjected to complex multi-axial loading based on the properties of the constituents, fibres and matrix, remains a major technical challenge. The purpose of this paper is to present a multi-scale progressive damage model for composite laminates. At the lamina scale, stress-based failure criteria are proposed for the fibres and the matrix, with the stresses required in the failure criteria being determined by a micro-mechanics analysis. Linking the lamina scale with the laminate scale is achieved through a finite element micromechanics model and a progressive damage model. Comparison has been carried out with the experimental data generated during the world-wide failure exercise, indicating a good agreement for several materials over a wide range of loading conditions.

A Study on Low Velocity Impact of Woven Glass/Phenolic Composite Laminates Considering Environmental Effects

The objectives of this study are to evaluate the internal damage and compressive residual strength of composite laminate by impact loading. To investigate the environmental effects, as-received and accelerated-aged glass/phenolic laminates are used. UT C-Scan is used to determine the impact damage characteristics and CAI tests are carried out to evaluate quantitatively the reduction of compressive strength by impact loading. The damage modes of the woven glass/phenolic laminates are evaluated. In the case of the accelerated-aged laminates, as aging time increases, initial failure energy and residual compressive strength decrease.

Critical Void Content for Polymer Composite Laminates

An experimental program to characterize the effect of voids on the strength of composite laminates is presented, and the adequacy of a fracture criterion to represent the experimental data for the effect of voids on the compressive and interlaminar shear strength of composite laminates is assessed. The experimental program investigates the effect of the material system (epoxy matrix vs bismaleimide matrix), type of reinforcement (unidirectional tape vs woven fabric), and the type of loading (compression vs interlaminar shear) on the critical void content. Laminates produced with carbon fabric/epoxy, carbon tape/epoxy, and carbon fabric/bismaleimide were produced with an intentionally high void content. The ultrasonic absorption coefficient was measured for all specimens and shown to vary approximately linearly with the void content, with the exception of the carbon fabric/epoxy laminates that presented a bilinear relationship with void content, corroborating previous experimental results. The effects of these factors on the strength of composite laminates are discussed in terms of the fracture parameters involved in the fracture criterion. The critical void content is estimated for each case both in terms of void content and ultrasonic attenuation.

Comparison of fracture models to assess the notched strength of composite/solid propellant tensile specimens

Modifications are made in the well-known inherent flaw model as well as in the stress fracture models for accurate prediction of notched tensile strength of composite laminates containing holes or slits. To examine the adequacy of these modifications, fracture data on different composite materials reported in the literature are considered. The notched strength estimates from the present fracture models are found to be close to the existing test results. An attempt is made to correlate the fracture data of center crack tension specimens made of nitramine and HTPB-based propellant materials. The present analysis results from the above simple models are found to be in good agreement with test results. This study confirms the applicability of the above three fracture models to solid propellant materials having relatively low stiffness and strength. Since the notched strength estimates of composite/solid propellant tensile specimens are close to the test results, any one of them can be utilized while evaluating the notched tensile strength of specimens.

Damage zone characterization of composite panels with a modified crack growth model

In the present study a modified crack growth model (MCGM) is introduced to analyse the crack propagation status in the damaged zone of fibre reinforced composites. The damaged zone is modelled on the basis of a virtual crack being subjected to a cohesive closure stress. The residual strengths of composite laminates are evaluated characterizing the matrix crack density at the damaged zone and defining a relationship between the plate stiffness before and after damage. Experimental tests were performed employing epoxy/glass fibre reinforced composite laminates to determine the residual strength of centre cracked specimens. The residual strength of composite panels using the new model correlates well with experimental results when compared with previous models.

A New Method for Compression After Impact Strength Prediction of Composite Laminates

A new method to determine the compression after impact (CAI) strength of composite laminates was presented. In this method, an impact damage zone was modeled as an equivalent hole. The most outstanding characteristic of the method was that the simplification of the impact damage was based on the compressive failure mechanisms of impacted laminates. Such a simplification has not been discussed in the literature before. A technique was established for determining the shape and size of the equivalent hole. To the authors’ knowledge, it is the first practical method for simplifying the impact damage as a hole, and also the present work was the first attempt to predict the CAI strength by modeling the impact damage as a hole. The stress distribution around damage was calculated using the complex potential method and the classical lamination theory. A lay-up independent failure criterion was used to predict CAI strength. The predictions of the present approach were compared to test results and found to be in very good agreement over a wide variety of materials. The study indicated that the width of damage zone was a key factor governing CAI strength. The influence of damage area, impact energy, impactor shape and dimension, etc. on CAI strength can be essentially characterized by their effects on the damage width. The study provided a very simple and effective approach for CAI strength prediction compared with previous methods.

Characterization of Matrix Crack-Induced Laminate Failure—Part II: Analysis and Verifications

A two-part investigation was performed to study the stiffness response and strength of composite laminates subjected to uniaxial tension. In Part I, experimental testing was performed to characterize the matrix crack-induced damage progression and failure modes in composite laminates under uniaxial tension loading conditions. In Part II, analytical models were developed to account for the damage modes observed in the experimental investigation. Models for matrix crack-induced delamination, matrix crack-induced microcracking, and matrix crack-induced edge delamination were developed. Model predictions were compared with experimental data. The proposed models were integrated and implemented in the computer code, PDCell. The code was then used to predict the stiffness response and failure of composite laminates subjected to tensile loading. Code predictions were used to evaluate the effects of ply orientation, ply group thickness, loading direction, and stacking sequence on laminate tensile strength. The predictions from the code were compared with available test data in the literature.

Fracture strength of composite laminates containing surface notches

An empirical relationship developed recently between the failure stress and stress intensity factor at failure is examined for the estimation of fracture strength of thick graphite/epoxy laminates containing surface notches. The results are found to be reasonably in good agreement with the existing test results.

Notched strength evaluation of fabric laminates having a circular hole

modifications are suggested in the Whitney-Nuismer (WN) two stress fracture criteria known as the point stress criterion and average stress criterion for evaluating the notched strength of composite laminates having a circular hole.The linear relationship between the asymptotic value of the center cracked tension plate stress intensity factor and the notched strength of the infinite plate is used for the evaluation of the characteristic lengths to evaluate the notched strength of laminates. The analytical results based on the modified WN fracture models are compared well with the existing test results of fabric laminates of E-glass/epoxy.

Estimation of the Tensile Strength Reduction of a Composite Laminate with a Hole

The strength problem for a body with an artificial defect in the form of a hole is considered. To solve this problem, an approach is suggested according to which the local strength of a material in the area of stress concentration is assumed to depend on the size of this area. The scale of the problem is introduced via the ratio between the characteristic sizes of the deformed area and the characteristic length of the material. This approach is used to estimate the strength of composite laminates weakened by holes and notches. Expressions for the failure stress are obtained, which can be applied to isotropic and orthotropic laminates both in quasi-brittle failure and in failure associated with significant inelastic deformations. A comparison between the calculated σ c values and the known experimental data for glass/epoxy and graphite/epoxy laminates is presented.

Residual strength of composite laminates with a centre crack under tension

An empirical relationship between the failure stress and stress intensity factor at failure is examined for the estimation of residual strength of various types of laminates with different layup sequence and having centre cracks. The estimated fracture strengths for these laminates under tension are compared with existing experimental results. They are found to be in reasonably good agreement with each other.

Ultimate strength of composite laminates with free-edge delamination

The present paper deals with the free edge effect of composite laminates by using a generalized quasi-three dimensional analysis and experimental verification of an analysis performed for laminates with Teflon inserted on interfaces to simulate initial free-edge delamination. We performed tensile tests for laminates [302/−302/90]s carbon-epoxy laminates with free-edge delamination under uniaxial tension. The experiment reveals that extensional stiffness of the laminate decreases by the initiation of the delamination, and that strength of the laminate without delamination is smaller than that of the laminates with delamination. Generalized quasi-three dimensional finite element technique, which employs energy release rate and maximum stress criteria, is developed to estimate behavior of the laminate after initial delamination. The numerical result by use of this technique predicts the ultimate strength of the laminates with sufficient accuracy according as the comparison with an experimental stress-strain curve. In the experiment conducted both for the laminate with initial delamination and the laminate without initial delamination, an unexpected results were obtained that is the ultimate load of the laminate without initial delamination was lower than that of the laminate with initial delamination. We presented clear explanation on the phenomenon occurred and developed the method to predict the nonlinear behavior of the laminate with or without initial delamination.

Notched strength of carbon fibre/epoxy composite laminates with a circular hole

This paper improves the two stress fracture criteria proposed by Whitney and Nuismer (known as the point stress criterion and the average stress criterion) to predict the strength of composite laminates with a circular hole. In the point stress criterion, it is assumed that the failure occurs when the stress over some distance (d 0) away from the notch is equal to or greater than the un-notched laminate strength. In the average stress criterion it is assumed that failure occurs when the average stress over some distance (a 0) ahead of the notch equals the unnotched laminate strength. Both stress fracture criteria are two parameter models based on the unnotched strength (σ 0) and a characteristic dimension (d 0 ora 0). A simple relation is used for the characteristic length to improve the accuracy while evaluating the notched strength of carbon/epoxy composite laminates. The analytical results are compared well with the existing test results of AS4-carbon/948 Al epoxy [0/90]4s and [0/ ± 45/90]2S composite laminates with various hole diameters and specimen widths.

Strength of Composite Laminates Containing Holes and Subjected to Complex Loading Conditions

Stress and failure analyses of composite laminates containing holes are presented. Both the finite element method (FEM) and an analytical method, based on complex stress functions, are used to determine the stress distributions around the holes. The Point Stress Criterion (PSC) and the Damage Zone Criterion (DZC) are used to predict the tensile strength of test specimens subjected to complex loading conditions. A comprehensive test program was carried out to establish criteria parameters and to generate data to validate the stress and failure analyses. For this purpose a special test fixture was designed. Good agreement between predicted and measured results was found.

Strength of composite laminates under biaxial loads

Five well known failure criteria and one simple progressive model have been used in conjunction with laminate theory, which allows for nonlinear lamina shear behaviour, to predict the initial and final failure strengths of filament wound composite tubes. The predictions have been compared with experimental leakage and fracture stresses for ±75°, ±55° and ±45° filament wound GRP tubes subjected to a wide range of biaxial stress systems including biaxial compression. In some cases the fracture strengths were a factor of 10 higher than the initial failure predictions. The simple progressive failure theory predictions gave the best agreement with the experimental results.

A comparison between two optimization methods on the stacking sequence of fiber-reinforced composite laminate

This article is dealing with the study of optimization for the stacking sequence of composite laminates by two optimization methods of simplex and interior penalty. In the simplex method, both the applicable domain of magnifying factors (MFs) and the differences of stacking sequence optimization between symmetrical and unsymmetrical laminate composites are studied. When the values of MFs are 50, 70, 85, the converging of optimization process is faster and its result is more accurate. The strength of the unsymmetrical laminate with fewer layers is larger than that of the symmetric one by 8% only. But when the layers are increased, the strength of symmetric laminate is higher. In the interior penalty method, the error value can be kept within 5–6%, while in the simplex method the more layers the larger the error value. To sum up, proved by progressive failure simulation of finite element analysis, the best strength of composite laminate with optimal stacking sequence can be obtained.

Analytical method for prediction of the strength of composite laminates

The problem of prediction of the strength of composite laminates based on the properties of the individual laminae and the structure of the composite has been solved for the general case, where parametric equations were obtained for the limiting surface for a composite of arbitrary structure. However, application of these equations for practical computations requires a computer. Here the authors propose a technique allowing them to solve the considered problem by a simpler method for a composite with a structure in which there are identical layers with orientation angles (+{phi}{sub j}) and ({minus}{phi}{sub j}) where j = {bar 1},n, n is the number of layers. There are not other constraints on the structure surface of a composite of structure ({+-}{phi}){sub c} using the hypothesis that the strain rate field is uniform over the thickness of the packet of layers. The theoretical results are compared with the corresponding experimental results of other authors. For prediction of the strength of a composite or arbitrary structure, the authors have used the Voigt model in the theory of mixtures. As an example, they solve the problem of the supporting capacity of the composite tube of the drive shaft of an automobile.

Effect of impact damages on the compressive strength of composite laminates

Abstract By simplifying the impact damages as a single delamination near the surface with an elliptical boundary, the approximate solution of total strain energy release rates can be derived as a function of delamination major axis, minor axis, external compressive strain, Possion's ratio of parent medium, extensional and bending stiffnesses of sublaminate. A linear relation of residual strength versus strain energy release rate can be constructed by correlating the approximate solution with test data of compressive residual strain (strength) after impact (CSAI), indicating that the dimension between the delamination major and minor axis should be dependent. In addition, the delamination aspect ratio is found to be not only a function of the specimen geometry and the extensional stiffness, but also a function of laminate thickness. The approximate solution provides a method for predicting the post impacted strength of composite laminate for only either thick laminate or thin laminate with low impact energy.