Delamination growth mechanisms in woven glass fiber reinforced polymer composites under Mode II fatigue loading at cryogenic temperatures

The purpose of this research is to characterize the cryogenic delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates subjected to Mode II fatigue loading. Mode II fatigue delamination tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) using the four-point bend end-notched flexure (4ENF) test method, and the delamination growth rate data for the woven GFRP laminates were obtained. The energy release rate range was determined by the finite element method. Microscopic examinations of the specimen sections and fracture surfaces were also carried out. The present results are discussed to obtain an understanding of the fatigue delamination growth mechanisms in the woven GFRP laminates under Mode II loading at cryogenic temperatures.     

Mode III fatigue delamination growth of glass fiber reinforced polymer woven laminates at cryogenic temperatures

This paper investigates the cryogenic fatigue delamination behavior of glass fiber reinforced polymer woven laminates under Mode III loading. Fatigue delamination tests were conducted using split cantilever beam specimens at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A finite element analysis was also employed to calculate the energy release rate. The temperature dependence of the fatigue delamination growth rate vs. energy release rate range is discussed. Fracture surfaces were examined by scanning electron microscopy to identify the delamination mechanisms under fatigue loading. The important conclusion we reach is that the Mode III fatigue delamination growth rates of woven laminates at cryogenic temperatures are lower than that at room temperature.     

Influence of the matrix constituent on mode I and mode II delamination toughness in fiber-reinforced polymer composites under cyclic fatigue

Mode I and mode II fracture behaviour under static and dynamic loading was analyzed in two composites made up of the same reinforcement though embedded in two different matrices. Specifically, the delamination energy under static and dynamic loading was obtained for both materials and both fracture modes, i.e. the number of cycles necessary for the onset of fatigue delamination. Subsequently, the crack growth rate (delamination rate) was obtained for different percentages of the critical energy rate. The main goal of the study was to ascertain the influence of the matrix on the behaviour of the laminate under fatigue loading.From the experimental results for the onset of delamination, similar fatigue behaviour was observed at a low number of cycles for both matrices and both fracture modes, while in fatigue at a high number of cycles, a higher fatigue limit was obtained in the composite with the modified resin (higher toughness) for both fracture modes. From the point of view of crack growth rate, both materials behaved similarly for different levels of stress under fatigue and the two fracture modes for small crack lengths (initial growth zone < 5 mm), although the growth rate increased for large crack lengths. This behaviour was the same in both loading modes.     

Fatigue delamination growth in woven glass/epoxy composite laminates under mixed-mode II/III loading conditions at cryogenic temperatures

We investigate the cryogenic delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode II/III fatigue loading. Fatigue delamination tests were conducted with six-point bending plate (6PBP) specimens at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), and the delamination growth rate data for various mixed-mode ratios of Modes II and III were obtained. The energy release rate was evaluated using the three-dimensional finite element method. In addition, the fatigue delamination growth mechanisms were characterized by scanning electron microscopic observations of the specimen fracture surfaces.     

Acoustic Emission-Based Methodology to Evaluate Delamination Crack Growth Under Quasi-static and Fatigue Loading Conditions

The aim of this study was to investigate the applicability of acoustic emission (AE) technique to evaluate delamination crack in glass/epoxy composite laminates under quasi-static and fatigue loading. To this aim, double cantilever beam specimens were subjected to mode I quasi-static and fatigue loading conditions and the generated AE signals were recorded during the tests. By analyzing the mechanical and AE results, an analytical correlation between the AE energy with the released strain energy and the crack growth was established. It was found that there is a 3rd degree polynomial correlation between the crack growth and the cumulative AE energy. Using this correlation the delamination crack growth was predicted under both the static and fatigue loading conditions. The predicted crack growth values was were in a good agreement with the visually recorded data during the tests. The results indicated that the proposed AE-based method has good applicability to evaluate the delamination crack growth under quasi-static and fatigue loading conditions, especially when the crack is embedded within the structure and could not be seen visually.     

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。     

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。     

Mode I fatigue delamination growth in GFRP woven laminates at low temperatures

The cryogenic fatigue delamination behavior of glass fiber reinforced polymer woven laminates under Mode I loading has been investigated experimentally and numerically. Fatigue delamination tests were conducted using double cantilever beam specimens at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). Fracture surface examination using scanning electron microscopy revealed delamination mechanisms under fatigue loading. A finite element analysis was also employed to calculate the J-integral range and damage distributions. The effects of temperature and loading condition on the fatigue delamination growth rates were discussed.     

Effect of stress ratio on near-threshold propagation of delimination fatigue cracks in unidirectional CFRP

The effect of the stress ratio on near-threshold growth of delamination fatigue cracks was investigated with unidirectional laminates made from Ciba Geigy 914C prepegs (T300/914) and from Toray P305 prepegs (T300/#2500). Tests of delamination fatigue crack propagation were carried out under mode I opening loading by using double cantilever beam specimens. The normalized gradient of energy release range was controlled in load-shedding tests. In the region of crack growth rates above about 5 × 10⁻¹⁰ m/cycle, the growth rate was expressed as a power function of fracture mechanics parameters. Below this region, there existed a growth threshold. The influence of the stress ratio became smaller when the rate was correlated to the energy release rate range than when the rate was correlated to the stress intensity range or the maximum energy release rate. A controlling fracture mechanics parameter is discussed on the basis of fractographic observation and mechanism consideration. A new phenomenological law of fatigue crack propagation is derived.     

Cryogenic Interlaminar Fracture Properties of Woven Glass/Epoxy Composite Laminates Under Mixed-Mode I/III Loading Conditions

We characterize the combined Mode I and Mode III delamination fracture behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. The eight-point bending plate (8PBP) tests were conducted at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) using a new test fixture. A three-dimensional finite element analysis was also performed to calculate the energy release rate distribution along the delamination front, and the delamination fracture toughnesses were evaluated for various mixed-mode I/III ratios. Furthermore, the microscopic examinations of the fracture surfaces were carried out with scanning electron microscopy (SEM), and the mixed-mode I/III delamination fracture mechanisms in the woven GFRP laminates at cryogenic temperatures were assessed. The fracture properties were then correlated with the observed characteristics.     

Hygrothermal influence on delamination behavior of graphite/epoxy laminates

The hygrothermal effect on the fracture behavior of graphite/epoxy laminates has been investigated as part of an overall effort to develop a methodology for damage-tolerance predictions in advanced composite materials. Several T300/934 laminates were tested using a number of specimen configurations (double cantilever, compact tension, and cracked lap shear) in order to evaluate the effects of temperature and humidity on delamination fracture toughness under Mode I and Mode II loading. The specimens were exposed to different humidity levels and temperatures for varying periods of time prior to testing. The preexposed specimens were tested under room conditions, and fracture energies during initiation and propagation were estimated. Acoustic emission was used to detect crack initiation. It was indicated that moisture has a slightly beneficial influence on fracture toughness or critical strain energy release rate during Mode I delamination but a slightly deleterious effect on Mode II delamination and Mode I transverse cracking. The failed specimens were examined by scanning electron miscroscope (SEM) and topographical differences due to fracture modes were identified. The effect of moisture on fracture topography could not be distinguished.     

Computation analysis of interlaminar fracture of laminated composites

The interlaminar fracture behavior of laminated composites has been investigated. Contact and friction along the crack surfaces is taken into account in the finite element modeling of the delamination crack growth. Mode I, mode II and mixed mode loading conditions at the crack tip have been analyzed. For the cracks with contact and friction along the crack surfaces the virtual crack closure integral method is used in order to calculate separated energy release rates. Computational modeling and analysis of cross-ply laminates in three-point bending has been performed. Contact elements were used in order to prevent the material interpenetration along the crack surfaces. Comparison of the results obtained with and without using contact elements has been carried out and significant differences between the correlated values of the energy release rates have been found. The influence of the coefficient of friction on the energy release rates was found to be significant for short delamination crack lengths but insignificant for long cracks. Numerical analyses of experimental data obtained for unidirectionally reinforced glass fiber composites by double cantilever beam tests and by notched flexure tests have been carried out. For the double cantilever beam test geometric linear and nonlinear finite element analyses have been performed and critical energy release rates were calculated. For the end notched flexure test the contact problem has been solved taking into account that adjacent to the support contact and friction will occur. For the double cantilever beam test the critical energy release rates obtained by linear and nonlinear finite element solution has been compared with those from four different analytical data reduction methods (the area method, the Berry method, the modified beam analysis, the compliance method). For the end notched flexure test the critical energy release rates, calculated by the finite element analysis and taking into account contact and friction along the crack surfaces, have been compared with those obtained by conventional beam analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

Self-healing of delamination fatigue cracks in carbon fibre–epoxy laminate using mendable thermoplastic

This article examines the self-healing repair of delamination damage in mendable carbon fibre–epoxy laminates under static or fatigue interlaminar loading. The healing of delamination cracks in laminates containing particles or fibres of the mendable thermoplastic poly[ethylene-co-(methacrylic acid)] (EMAA) was investigated. The results showed that the formation of large-scale bridging zone of EMAA ligaments along the crack upon healing yielded a large increase (~300%) in the static mode I interlaminar fracture toughness, exceeding the requirement of full restoration. The mendable laminates retained high healing efficiency with multiple repair cycles because of the capability of EMAA to reform the bridging zone under static delamination crack growth conditions. Under fatigue loading, healing by the EMAA was found to restore the mode I fatigue crack growth resistance, with the rates of growth being slightly less than that pertinent to the unmodified laminate. The EMAA bridging zone, which generated high toughness under static loading conditions, does not develop under fatigue loading because of rapid fatigue failure of the crack bridging ligaments. Similar to the multiple healing capability of EMAA under static loading, multiple healing of delamination fatigue cracks is confirmed, with the fatigue crack growth rates remaining approximately unchanged. This study shows that EMAA was capable of full recovery of fatigue crack growth resistance and superior healing efficiency for static loading.     

Fatigue fracture of AMg6N alloy under loading at sound frequencies

Cyclic crack growth resistance tests of AMg6N alloy under loading at frequencies of 20 to 10 kHz have demonstrated that the rate of fatigue crack propagation decreases with increasing frequency and the threshold stress intensity factors increase exponentially with the frequency of strain cycling. Fractographic observations of fracture surfaces of the specimens have revealed that an increase in the loading frequency is accompanied by a decrease in the fatigue striation spacing and in the size of the striation microzones by intnsifying the processes of secondary cracking and the formation of fretting products. This leads to a decrease in the rate of fatigue crack propagation and an increase in the threshold values of the stress intensity factors. An increase in the asymmetry of the loading cycles reduces the contribution of delamination and the formation of fretting products to the process of fracture of the alloy and results in a smaller fraction of the striation relief and in an earlier occurrence of the elements of quasistatic fracture by dimples, which is the cause of the reduction in characteristics of the cyclic crack growth resistance under asymmetrical loading. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 94–105, March–April, 1999.     

Experimental Investigation of the Interface Behavior of Balanced and Unbalanced E-Glass/Polyester Woven Fabric Composite Laminates

The aim of this work is to study the influence of weave structure on the crack growth behavior of thick E-glass/polyester woven fabric composites laminates. Two different types of laminates were fabricated: (i) balanced: plain weave (taffetas T)/chopped strand mat weave (M) [T/M]₆ and (ii) unbalanced: 4-hardness satin weave (S)/chopped strand mat weave [S/M]₇. In order to accurately predict damage criticality in such structures, mixed mode fracture toughness data is required. So, the experiments were conducted using standards delamination tests under mixed mode loading and pure mode loading. These tests were carried out in mode II using End Load Split (ELS) tests and in mixed-mode I+II by Mixed Mode Flexure (MMF) tests under static conditions. The test methodology used for the experiments will be presented. The experimental results have been expressed in terms of total strain energy release rate and R-curves. The fracture toughness results show that the T/M interface is more resistant to delamination than the S/M interface.     

Influence of temperature on the delamination process under mode I fracture and dynamic loading of two carbon–epoxy composites

This paper experimentally analyzes the influence of temperature and type of matrix on the delamination process of two composites subjected to fatigue loading through the study of their fracture under mode I behavior. The materials were manufactured with the same AS4 unidirectional carbon reinforcement and two epoxy matrices with different fracture behavior. The chosen temperatures for the experiments were 20 (room temperature), 50 and 90 °C.The experimental study carried out under dynamic loading enabled the authors to determine the influence that temperature has on the onset of delamination for the entire range of fatigue life of the material, from the low number of cycles zone to the high number of cycles zone. That is, it enabled the plotting of fatigue curves, represented as G_Imax–N (number of cycles required for the onset of delamination given a certain energy release rate) for an asymmetry coefficient of 0.2 (the ratio between the maximum and minimum fracture energies applied during the dynamic tests).The experimental data obtained were treated with a probabilistic model based on a Weibull distribution which allowed the identification of relevant aspects of the fatigue behavior of the materials such as the estimation of fatigue strength for periods greater than the tested values and the analysis of the reliability of the results.     

Analysis of the fracture behaviour of a stitched single-lap joint

The effect of stitching on the fracture response of single-lap composite joints was studied by a combined experimental and numerical analysis. Unstitched and Kevlar stitched joints were tested under static and fatigue loading to characterize damage progression and failure modes; a three-dimensional finite element analysis was carried out to evaluate the influence of stitches on strain energy release rates as a function of damage and to identify the role of various stitching parameters on the fracture behaviour of joints.It was observed that the failure of the joints occurs as a consequence of the propagation of delamination at the interface between the adherends; the propagation is stable under fatigue loads and unstable under static loads. Stitching does not improve the static strength of joints but significantly prolongs the duration of the crack propagation phase under fatigue loading.The results of finite element modelling indicate that the incorporation of stitches reduce G_I to zero after the delamination front passes the stitch line, but it is not effective in reducing mode II energy release rate. They also show that strain energy release rates are not greatly affected by the length of stitch-laminate debonding, which, conversely, does influence stitch tensioning. Moreover, 3D analysis reveals that stitches become less efficient in reducing the crack driving force with increasing stitching steps.     

The effect of mode ratio and bond interface on the fatigue behavior of a highly-toughened epoxy

The fatigue threshold and the cyclic crack growth of a highly-toughened epoxy adhesive were studied under mode I and several mixed-mode loading cases and compared with the quasi-static critical fracture energies. Four different adhesive systems were examined using steel and aluminum substrates having different surface roughness, and surface treatment. The effect of increasing the amount of mode II (increasing the phase angle) on the fatigue threshold strain energy release rate and the cyclic crack growth rate was found to be insignificant at low phase angles. However, a significant increase in the fatigue threshold and decrease in the cyclic crack growth rate was observed at higher phase angles. These trends were similar to that seen in adhesive joint fracture. Adherend surface roughness and surface preparation affected the fatigue behavior significantly, particularly at low crack speeds and high phase angles. The fatigue properties were essentially the same for both steel and aluminum adherends provided that the crack paths were cohesive. A general observation was that the fatigue crack path moved progressively closer to the more highly strained adherend under mixed-mode loading as the applied strain energy release rate and hence the crack speed, decreased. This caused mixed-mode cracks to be nearly interfacial in the threshold region.     

Interlaminar fatigue crack growth of cross-ply composites under thermal cycles

The fatigue growth of a fiber reinforced composite laminate was characterized under thermal cycling using a combined experimental and computational investigation. Twenty-four ply composite laminates ([0°₁₂/90°₁₂]) are fabricated with a pre-existing delamination, and subjected to thermal cycling in an environmental chamber. The large mismatch in the coefficients of thermal expansion is used to grow an interlaminar crack at the interface of the 0° and 90° laminae. This thermal fatigue crack growth behavior is investigated for different amplitudes of temperature change (ΔT = 30–140 °C). The inspection of fracture surfaces, after completion of the fatigue tests, reveals an angled or kinked crack front growth with greater propagation distances near the free-surfaces/edges. Due to the non-uniform crack growth across the specimen thickness, three-dimensional finite element analyses are performed to investigate the fatigue growth mechanisms under thermal load. From the analysis, the energy release rate as well as the mixed-mode stress intensity factors is calculated and the variations of these fracture parameters are found to be consistent with the observed crack front configuration. Using the computed results, the experimentally measured crack growth rates are also correlated with the amplitude of energy release rate, and a power law form of the fatigue law is established. The relevant coefficients as well as the threshold energy release rate are also determined. The present analysis is useful for not only understanding the fatigue delamination mechanisms under thermal cycling but also for estimating the threshold temperature variation that is needed to drive crack growth.     

缝合复合材料II型层间断裂特性研究

分别采用测量ENF试样加载点位移与测量其端部剪切位移CSD(Crack Shear Displacement)的试验方法,研究了缝合复合材料层合板的II型层间断裂韧性以及缝合密度,缝合线的直径等缝合参数对于缝合复合材料层合板II型层间断裂韧性和分层模式的影响。结果表明,缝合降低了层合板初始分层韧性G_IIi,但对于分层的扩展有良好的抑制作用。缝合参数对此有较大影响。