Characterizing Mode II Delamination Cracks in Stitched Composites

This paper deals with characterizing the bridging mechanisms developed across delamination cracks by through-thickness reinforcement, using stitched carbon/epoxy laminates under mode II loading as a prime example. End Notched Flexure (ENF) tests are performed which show that stitching can provide stable crack growth. The bridging law, which characterizes the bridging action of the stitches, is deduced from both crack profile measurements and load vs. deflection curves. Consistent results are obtained from the two methods. The inferred laws imply that delamination cracks will commonly grow in conditions that are neither accurately nor properly described by linear elastic fracture mechanics. Large scale bridging calculations are required, in which the essential material property is the bridging traction law. The level of detail in which the law must be determined can be inferred from the sensitivity of predicted crack growth to variations in the law. It is recommended that the required parametric traction law be deduced in engineering practice from load vs. deflection data from the standard ENF (or similar) test, with due regard to selecting the notch size and other specimen dimensions to ensure that crack growth is stable in the test. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

A new specimen geometry to determine the through-thickness tensile strength of composite laminates

The tensile strength in thickness direction is one of the dimensioning parameters for composite load introductions, which are exposed to complex three-dimensional stress states, like e.g. composite lugs. In the present paper a simple test setup which introduces the load into the specimen by a form fit was chosen to determine the through-thickness tensile strength of quasi-isotropic carbon/epoxy laminates. By means of detailed finite element analyses a new quadrilaterally waisted specimen geometry was developed and validated by mechanical testing. The influence of the manufacturing process on the location of failure was investigated and recommendations for future tests are made. Compared to alternative state of the art methods the proposed test method leads to higher accuracy and reproducibility of the determined through-thickness tensile strength.     

Experimental characterization of fracture of glass fiber reinforced composites laminates subjected to freeze‐thaw cycles

The simultaneous effect of moisture and freeze‐thaw cycle on the mechanical behavior of glass/epoxy composites laminates is experimentally investigated. The study is planned in order to simulate the detrimental presence of humidity due to rainfall in surface damage of composite structures operating in cold weather. Different mechanisms governing the monotonic response of specimens subjected to freeze‐thaw cycles are pointed out by taking advantages of SEM images. Comparing SEM images taken from dry and wet specimens shows that the failure mechanisms such as matrix cracking and delamination are vastly activated around the notched region when the material is exposed to humidity and freeze‐thaw cycle. The load‐displacement response of examined specimens, namely the linear response, is remarkably altered under these conditions. A reduction of approximately 40% in ultimate load and 30% in slope of tangent line of load‐displacement curves is identified after 100 cycles of freeze‐thaw as well as more than 20% decrease of strain energy release rate.     

Delamination tolerance studies in laminated composite panels

Determination of levels of tolerance in delaminated composite panels is an important issue in composite structures technology. The primary intention is to analyse delaminated composite panels and estimate Strain Energy Release Rate (SERR) parameters at the delamination front to feed into acceptability criteria. Large deformation analysis is necessary to cater for excessive rotational deformations in the delaminated sublaminate. Modified Virtual Crack Closure Integral (MVCCI) is used to estimate all the three SERR components at the delamination front from the finite element output containing displacements, strains and stresses. The applied loading conditions are particularly critical and compressive loading on the panel could lead to buckling of the delaminated sublaminate and consequent growth of delamination. Numerical results are presented for circular delamination of various sizes and delamination at various interfaces (varying depth-wise location) between the base- and the sub-laminates. Numerical data are also presented on the effect of bi-axial loading and in particular on compressive loading in both directions. The results can be used to estimate delamination tolerance at various depths (or at various interfaces) in the laminate.     

Fracture test for double cantilever beam of honeycomb sandwich panels

A modified double cantilever beam (DCB) test geometry is designed to investigate the fracture behavior of honeycomb sandwich panels containing embedded artificial pre-crack, and measure the strain energy release rate of the laminate facesheet/honeycomb cores interface. However, in terms of our DCB fracture test, owing to that the pre-crack does not propagate expectedly along the interface of facesheet/honeycomb core, a new fracture mode, namely IKP (initiation of interlaminar delamination, kinking into facesheet and propagation of interlaminar delamination), has been found.     

On the Method of Determination of Strain Energy Release Rate During Fatigue Delamination in Composite Materials

In this paper, the problem of calculation of the energy release rate for a fatigue test on composite material has been investigated. The application of the Linear Elastic Failure Mechanics (LEFM) leads to the use of varation of the energy release rate ( G). As the energy release rate is a function of the load squared, the variation of G becomes either a function of variation of the load squared ( G = f((P²))) or a function of the square of the load variation ( G = f(( P)²)).In this paper, we determine, by different fatigue tests, which of the two theoretical results is the best to describe the experiments. These fatigue tests have been made on DCB test-specimen in mode I with different R ratios (R = P_max / P_min) and different maximum loads. The material was a unidirectionnal glass-epoxy.The results show that considering G as a function of ( P)² seems more appropriated to describe a cracking test in fatigue.     

Interaction between transverse cracks and delamination during damage progress in CFRP cross-ply laminates

In previous papers the microscopic failure process of (0/90_n/0) (n = 4,8,12) cross-ply laminates was investigated. Progressive damage parameters, such as the transverse crack density and the delamination ratio, were measured. A simple modified shear-lag analysis including the thermal residual strains was conducted to predict the transverse crack density and the delamination length. The analysis did not consider the interaction between the transverse cracks and the delamination. In the present paper, a prediction is presented for the transverse crack density including the effect of delamination growth. The prediction shows better agreement with the experimental results, especially for laminates with thicker 90 ° plies in which extensive delamination occurs.

Loading/unloading tests have also been performed to obtain the Young's modulus reduction and the permanent strain as functions of the damage state. The shear-lag predictions of the Young's modulus reduction and the permanent strain are compared with the experimental data. Better agreement is obtained when the interaction between transverse cracks and delamination is considered.     

Tension fatigue analysis and life prediction for composite laminates

A tension fatigue life prediction methodology for composite laminates is presented. Tension fatigue tests were conducted on quasi-isotropic and orthotropic glass epoxy, graphite epoxy, and glass/graphite epoxy hybrid laminates. Edge delamination onset data were used to generate plots of strain energy release rate as a function of cycles to delamination onset. These plots were then used along with strain energy release rate analyses of delaminations initiating at matrix cracks to predict local delamination onset. Stiffness loss was measured experimentally to account for the accumulation of matrix cracks and for delimination growth. Fatigue failure was predicted by comparing the increase in global strain resulting from stiffness loss to the decrease in laminate failure strain resulting from delaminations forming at matrix cracks through the laminate thickness. Good agreement between measured and predicted lives indicated that the through-thickness damage accumulation model can accurately describe fatigue failure for laminates where the delamination onset behaviour in fatigue is well characterized, and stiffness loss can be monitored in real time to account for damage growth.     

Performance of Two Distinct Cohesive Layer Models for Tracking Composite Delamination

Two distinct cohesive layer models are developed for numerical simulation of delamination growth in composite layered specimens tested under static loading. One of these designated as the UMAT (user supplied material) model has a small, but finite thickness and the other designated as the UEL (user element) model has zero initial thickness. Crack growth in double cantilever beam specimens as well as in two test configurations of a composite plate carrying some discontinuities subjected to lateral load are studied using the models. It turns out that UEL model, though slightly more involved, is able to predict both crack initiation and large crack growth with sufficient accuracy. The UMAT model too consistently predicts crack initiation, but is unable to predict the crack growth accurately. It gives consistently higher loads for given crack extensions and predicts that the crack growth shuts off prematurely. Careful examination of the stresses in the cohesive layer of the UMAT model, in the upstream of the crack tip indicates that a ‘neck’ develops due to compressive stresses at some distance from the crack tip. Apparently it is the formation of this neck that ‘locks’ the crack from growing and is the cause of the inaccurate results given by the model.     

Fatigue crack growth in double cantilever beam specimen with an adhesive layer

A test method based on fracture mechanics concepts is applied to measure fatigue crack growth rates for an adhesive material in a bondline double cantilever beam specimen containing a cohesive crack. Tension–tension tests are conducted with a stress ratio of 0.5 and at 5 Hz. Debond growth rates are measured using a compliance method. Corresponding changes in J-integral are computed based on the beam on elastic–plastic foundation analysis of the specimen. There are three bondline thicknesses that are evaluated. When computed, ΔJ are plotted against the measured debond growth rates, the results showing a power law relationship which characterizes the debond behavior for a given bondline thickness. The increase of bond line thickness has a significant effect on fatigue crack growth. The larger the bond line thickness, the larger is the fatigue crack growth resistance.     

Z-pin增强复合材料层合板断裂韧性试验研究

针对Z-pin增强复合材料层合板, 开展了断裂韧性的试验研究。研究选取了3种Z-pin直径(0.28、 0.52、 0.80mm)且每种直径下分别以3种分布形式(5×5、 8×8、 10×10)排布Z---pin的增强方式, 为了确定比较基准, 试验中同时测试了不含Z-pin的复合材料层合板试样。通过Z-pin拔出试验测试了3种直径Z-pin从基体拔出过程中的载荷位移关系。利用双悬臂梁试验和端部开口弯曲试验分别测试了不含Z-pin和含Z-pin试样的Ⅰ型断裂应变能释放率G_ⅠC、 Ⅱ型断裂应变能释放率G_ⅡC。试验结果表明:? 与不含Z-pin的结构相比, Z-pin增强试样的Ⅰ型断裂应变能释放率G_ⅠC增大了83%~1110%, Ⅱ型断裂应变能释放率G_ⅡC增大了23%~438%; 在相同Z-pin体积含量下, 与增大Z-pin直径相比, 增大Z-pin分布密度能更有效地提高复合材料层合板的断裂韧性。      

Assessment of the delamination hazard of the core face sheet bond in structural sandwich panels

Delamination of the adhesive bond between face sheets and cellular core of structural sandwich panels is a major problem in sandwich construction. Due to incompatibilities in the modes of deformation associated with the face sheets and the cellular core, stress concentrations and singularities can occur even in absence of cracks. These stress concentrations are assumed to govern the onset of delamination. In the present study, a mesoscale concept for a first-order assessment of the delamination hazard induced by the incompatibility in the modes of deformation at the interface between core and face sheets is presented. The approach is based on a fourth order tensor which can easily be derived from the effective elasticity tensor for the cellular core. Due to the general formulation, the concept is applicable to all types of two dimensional cellular sandwich cores irrespectively of cell geometry and loading conditions. The approach is illustrated by an analysis of three examples concerning commercial sandwich core geometries as well as a more general non-orthotropic cellular structure.     

Off-axis fatigue crack growth and the associated energy release rate in composite laminates

Stable matrix crack growth behaviour under mechanical fatigue loading has been studied in a quasi-isotropic (0/90/-45/+45)_s GFRP laminate. Detailed experimental observations were made on the accumulation of cracks and on the growth of individual cracks in +45° as well as 90° plies. A generalised plain strain finite element model of the damaged laminate has been constructed. This model has been used to relate the energy release rate of growing cracks to the crack growth rate via a Paris relation.     

Crack growth induced delamination on steel members reinforced by prestressed composite patch

ABSTRACT Prestressed composite patch bonded on cracked steel section is a promising technique to reinforce cracked details or to prevent fatigue cracking on steel structural elements. It introduces compressive stresses that produce a crack closure effect. Moreover, it modifies the crack geometry by bridging the crack faces and so reduces the stress intensity range at the crack tip. Fatigue tests were performed on notched steel plate reinforced by CFRP strips as a step toward the validation of crack patching for fatigue life extension of riveted steel bridges. A crack growth induced debonded region in the adhesive‐plate interface was observed using an optical technique. Moreover, the size of the debonded region significantly influences the efficiency of the crack repair. Debond crack total strain energy release rate is computed by the modified virtual crack closure technique (MVCCT). A parametric analysis is performed to investigate the influence of some design parameters such as the composite patch Young's modulus, the adhesive thickness and the pretension level on the adhesive‐plate interface debond.     

A generalized micromechanical approach for the analysis of transverse crack and induced delamination in composite laminates

The previously developed micromechanical approaches for the analysis of transverse cracking and induced delamination are limited for laminates with specific lay-ups such as cross-ply and specific loading conditions. In this paper a new micromechanical approach is developed to overcome such shortcomings. For this purpose, a unit cell in the ply level of composite laminate including transverse cracking and delamination is considered. Then, the governing equations for the stress and displacement fields of the unit cell are derived. The obtained approximate stress field is used to calculate the energy release rate for the propagation of transverse cracking and induced delamination. To show the capability of the new method, it is employed for the analyses of general laminates with [0/90]_s, [45/−45]_s, [30/−30]_s and [90/45/0/−45]_s lay-ups under combined loadings to calculate the energy release rate due to the transverse cracking and induced delamination. It is shown that the obtained energy release rates for transverse cracking and delamination initiation are in good agreement with the available results in the literature and finite element method. Furthermore, the occurrence priority of further transverse cracks and/or delamination at each damage state of the laminates will be discussed.     

Prediction for progression of transverse cracking in CFRP cross-ply laminates using Monte Carlo method

This study predicted transverse cracking progression in laminates including 90° plies. The refined stress field (RSF) model, which takes into account thermal residual strain for plies including transverse cracks is formulated, and the energy release rate associated with transverse cracking is calculated using this model. For comparison, the energy release rate based on the continuum damage mechanics (CDM) model is formulated. Next, transverse cracking progression in CFRP cross-ply laminates including 90° plies is predicted based on both stress and energy criteria using the Monte Carlo method. The results indicated that the RSF model and the CDM model proposed in this study can predict the experiment results for the relationship between transverse crack density and ply strain in 90° ply. The models presented in this paper can be applied to an arbitrary laminate including 90° plies.     

Cohesive layer models for predicting delamination growth and crack kinking in sandwich structures

There are potentially two types of fracture that sandwich structures with strong and stiff facing sheets and lightweight cores are liable to suffer. These are the delamination growth at the face-sheet core interface and crack kinking into the sandwich core, respectively. The paper proposes computational models to simulate these failure mechanisms. The models employ the cohesive layer concept and are so constructed as to ensure that the crack advance is controlled by the critical value of strain energy release rate in mode I fracture. Of these, the first model can treat only delamination along a predetermined plane and is designated as CLD (cohesive layer delamination model). The performance of this model is thoroughly investigated in the light of experimental results. The influence of the key parameters of the model, viz. the thickness of the cohesive layer and the strength and stiffness of the cohesive layer material, have been studied. It is found that the model, as developed in this study, is fairly robust and is not sensitive to changes in parameters other than the critical strain energy release rate. The second model can track crack growth which is not predetermined in its direction. This it does by identifying the element in which the maximum principal tensile stress exceeds a critical value; once a crack is nucleated, the stress across the crack is relieved so that the right amount of energy is released when the crack is fully developed - much in the same manner as in a cohesive layer model. This model is designated as CLDK (Cohesive Layer Delamination and Kinking) model as it deals with interfacial delamination and crack kinking- whichever is the preferred mode of fracture. Experimental results of three sandwich specimens, viz. bottom restrained beams with 0° and –10° tilt angle, respectively, and a compressed beam, were used for comparison. The results indicate that the both the models are able to capture the initiation and track the growth of the interfacial delamination. The CLDK model is capable in addition to track the crack kinking into the core, and its subsequent return to the face sheet-core interface.     

Microscopic damage behavior in carbon fiber reinforced plastic laminates for a high accuracy antenna in a satellite under cyclic thermal loading

For a high accuracy antenna in next radio astronomy satellite, a candidate material is carbon fiber reinforced plastics (CFRP), because negative longitudinal coefficient of thermal expansion (CTE) for unidirectional CFRP enables a laminate with 0 CTE through appropriate laminate design. This enables high structural accuracy under large temperature fluctuation like space. On the other hand, when the laminate is subjected to thermal cycles, cyclic thermal stress occurs and causes microscopic damages. In this study, we characterized damage progress in CFRP laminates and resultant variation in mechanical properties under cyclic thermal loading. Three types of matrices, such as polycyanate ester, polyimide and epoxy resin were used to prepare CFRP laminates. Specimens were subjected to thermal cycles from ?197°C to 120°C. The test was periodically stopped for surface observation and flexural loading. Transverse cracks in 90° plies accumulated with thermal cycles, whereas flexural modulus remained constant. We also numerically evaluated temperature gradient and resultant thermal stress distribution during cooling by finite element analysis. The result indicates higher transverse stress appeared in the surface of the specimen and saturated to constant value which corresponded with the value calculated based on classical lamination theory.     

复合材料裂纹敏感性的评定──（Ⅱ）HTA／6376和T300／M10的Ⅱ型分层扩展行为

应用ＥＮＦ试验研究了ＨＴＡ／６３７６和Ｔ３００／Ｍ１０两种碳／环氧复合材料的静态与疲劳层间断裂行为。在静态载荷下，两种材料均呈现脆性不稳定和稳定的裂纹扩展特性。在Ｒ＝０．１且△Ｇ＿Ｉ大幅度变化的疲劳加载过程中，两种材料呈现稳定的裂纹扩展。采用位移控制方法，确定了裂纹扩展速率与循环应变能释放率的关系和应变能释放率门槛值。与Ｔ３００／９１４Ｃ相比，ＨＴＡ／６３７６和Ｔ３００／Ｍ１０具有较高的抗裂纹扩展能力。