Strength of carbon/epoxy composite single-lap bonded joints in various environmental conditions

Composite single-lap bonded joints were tested to study the combined effect of the environmental condition and the manufacturing method as they pertain to joint strength and failure conditions. Three different environmental conditions and four different manufacturing methods were considered. In terms of the manufacturing method, cocured joints without an adhesive (CCN) showed the highest strength in all environments. Comparing the effect of the environmental conditions, the elevated temperature and wet condition (ETW) gave the best results for all joints except co-bonded joints. In co-bonded joints, the highest strength was found in the cold temperature condition (CTD).     

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.     

Reliability of damage mechanism localisation by acoustic emission on glass/epoxy composite material plate

This investigation focuses on the analysis of the Acoustic Emission (AE) wave propagation and velocity evolution in glass/epoxy composite materials according to the fibre orientation. In this work, several piezoelectric sensors were placed on a large composite laminate in the same direction in order to analyse by one measure, the velocity and wave attenuation over a large distance. Then, the measured velocity was compared with the theoretical values of velocity that have been identified by the theoretical model analysis. This last velocity was used during a three-point bending test to analyse the damage localisation on the plate according to the amplitude distribution model developed in the “Laboratory of Roberval at the University of Technology of Compiègne”. The results of the damage localisation on the specimens were compared to the visually observed damage (delaminating, cracks…) and were corrected on the basis of the amplitude correction. Finally, the error ratio of localisation without correction was quantified. The originality of this study was to use the AE results obtained on a large laboratory specimen in order to improve the AE testing on an industrial structure scale.     

Acceptance testing of graphite /epoxy composite parts using an acoustic emission monitoring technique

A graphite/epoxy composite dome was proof-tested by pressurization. Acoustic emission (AE) data was obtained during the proof test. A low-frequency bandpass was used to minimize AE signal propagation losses. The AE system and test part were calibrated in situ by an instrumented pencil-lead break. Large energy AE signals provide a clear indication of a flawed composite part.     

Effects of temperature on delamination growth in a carbon/epoxy composite under fatigue loading

This paper presents a study of delamination growth in HTA/6376C carbon fibre/epoxy laminates. Tests were conducted under Mode I, Mode II and mixed-mode static and fatigue loading at both ambient conditions and elevated temperature. The results show that the strain energy release rate threshold values for delamination growth under fatigue loading are significantly lower than the critical energy release rates in static tests. At elevated temperature, the threshold values in the fatigue loading were only about 10% of the critical values in the static tests. A fractographic analysis of the delamination growth revealed that the fracture surfaces generated at elevated temperature generally were similar to the fracture surfaces generated at room temperature. Nevertheless, some differences in morphology of the fracture surfaces were observed, and their effect on the static and fatigue delamination growth is discussed in detail.     

Cryogenic delamination growth in woven glass/epoxy composite laminates under mixed-mode I/II fatigue loading

This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures.     

Experimental and numerical failure analysis of carbon/epoxy laminated composite joints under different conditions

The aim of this study is to investigate the effects of preload moment, moisture and interference-fit on bearing strength and failure mode in pin-jointed and bolted carbon–epoxy plates which were subjected to a traction force. Two different geometrical parameters, end distance to pin diameter ratio (E/D) and width to pin diameter ratio (W/D), were considered experimentally. E/D and W/D ratios were selected ranging from 1 to 4 and from 2 to 4, respectively. The test results showed that the ultimate failure loads were directly affected by the geometrical parameters, preload moments and interference-fit. Furthermore, for the specimens in wet condition, failure loads increased with preload moments while failure loads stayed almost the same in non-preloaded specimens. Finite element models of the specimens were also developed by using ANSYS software. Contact and stress analyses of the models were carried out for both free and interference-fit. Tsai-Wu criterion was used to determine bearing strength corresponding to first failure load. It was concluded that the numerical results are in good agreement with experimental results.     

The micromechanics of fatigue-induced delamination of a graphite/epoxy composite

Fatigue crack growth in the resin layer between 0 and 90 plies of an AS/3501-5A graphite fibre/epoxy composite is discontinuous. Regularly spaced extensions of the crack front occur after periods of arrest. Crack compliance and tip strain fields have been measured to determine how the local minimum (K _min ^l) and maximum (K _max ^l) crack tip stress intensities affect growth. Contact of the fracture surfaces and swelling of the 90° ply modify these local stress intensities by an amount sensitive to load ratio (R), and the resulting propagation rate depends strongly onR. A model capable of describing thisR effect relates the distance of each individual crack advance to K_max ^l and the duration of each arrest toK _mnax ^l -K _min ^l, i.e., to K _eff. We discuss the genesis of this model, and its explanation of the large Paris law coefficient which results if growth rates are instead expressed against the applied cyclic stress intensity.     

A comparison of fatigue damage detection in carbon and glass fibre/epoxy composite materials by acoustic emission

Ring specimens of four fibre composite materials, two containing carbon fibres (crfp) and two containing glass fibres (gfrp) have been monitored for acoustic emission (AE) during tension/ tension fatigue cycling.In cfrp continuous monitoring failed to provide warning of impending fatigue failure. The results are not consistent with the view that cessation of activity after initial shakedown guarantees stabilization and safe operation.In gfrp, if the activity is averaged over adequate periods, an underlying trend is revealed, short term deviations from which are attributed to the occurrence of a different damage process which determines final failure. It is shown that in the tests reported, the progressively increasing component of the activity can be related to the fatigue life. The distribution of events over the load cycle shows that a dominant part of this progressively increasing activity is associated with some stress independent process. Use of the appropriate part of the load cycle also offers the possibility of focussing on the events associated with the damage that leads to final failure.     

A study on the failure mechanisms of carbon fiber/epoxy composite laminates using acoustic emission

The complex failure mechanisms that are commonly considered as the distinctive characteristic of composites are being amenable to nondestructive test advance. This research adopts the acoustic emission technique to study the failure mechanisms and damage evolution of carbon fiber/epoxy composite laminates. Effects of different lay-up patterns and hole sizes on the acoustic emission response are studied to set up the mapping between the failure properties and the acoustic signal features such as the energy, counting and amplitude. Moreover, the microscopic properties of different composite specimens after fracture are watched and analyzed by scanning electron microscope (SEM). Based on the mapping conception, the controlling microscopic failure mechanisms of composites including the splitting matrix cracking, fiber/matrix interface debonding, fiber pull-out and breakage as well as delamination are identified. It is expected the influence of complex lay-up patterns and sizes on the damage and failure properties of composites is represented by creating true mapping based on the acoustic emission technique.     

Thermal damage effects on delamination toughness of a graphite/epoxy composite

The effects of overheating AS/3501-6 graphite/epoxy composite material have been examined after exposures in air up to 350°C. Degradation was assessed from measurements of the interlaminar fracture toughness in both Mode I tension and Mode II shear as well as the interlaminar shear strength and hardness. Exposures up to 30 minutes at 225°C and up to 15 minutes at 300°C did not significantly reduce the interlaminar toughness. However, longer exposure at 300°C and short exposures at 350°C produced catastrophic decreases in toughness values, shear strength and hardness. The primary origin of embrittlement was attributed to deterioration of the epoxy as opposed to any fibre or fibre-matrix interfacial weakening.     

Durability of hygrothermally aged graphite/epoxy woven composite under combined hygrothermal conditions

The objective of this study was to examine the effects of hygrothermal aging on the durability of a graphite/epoxy woven composite material system. The study was part of a larger project in which the objective was to evaluate and model the effects of moisture, temperature, and combined hygrothermal conditions on the strength and life of a graphite/epoxy woven composite material system. The results presented here represent an extension of the work by Patel and Case (Int. J. Fatigue 22 (2000) 809).

The hygrothermal aging consisted of cyclical temperature and moisture variations which were meant to simulate mission conditions for an advanced subsonic aircraft. Durability studies were carried out on the aged material system in the form of fatigue and residual strength testing under humid and elevated temperature environments. Damage mechanisms and failure modes were determined through fatigue testing, residual strength testing, and nondestructive evaluation.

Changes in physical appearance, thermal analysis results, fracture surfaces, and moisture diffusion behavior all supported the idea that the material was affected by the aging process. However, experimental testing also showed that the initial and residual tensile properties of the aged material were virtually unaffected by the imposed environmental aging (as compared to unaged material testing results), except when tested at elevated temperature. At elevated temperature, both the dynamic stiffness and residual strength were noticeably reduced from that at room temperature.     

Electron emission and acoustic emission from the fracture of graphite/epoxy composites

In past studies it has been shown that the fracture of materials leads to the emission of a variety of species, including electrons, ions, neutral molecules, and photons, all encompassed by the term fracto-emission (FE). In this paper we examine electron emission (EE) from the fracture of single graphite fibres and neat epoxy resin. We also combine measurements of EE with the detection of acoustic emission (AE) during the testing of graphite-epoxy composite specimens with various fibre orientation. The characteristics of these signals are related to known failure mechanisms in fibre-reinforced plastics. This study suggests that by comparing data from AE and FE meausrements, one can detect and distinguish the onset of internal and external failure in composites. EE measurements are also shown to be sensitive to the locus of fracture in a composite material.     

Temperature effects on mixed mode I/II delamination under quasi-static and fatigue loading of a carbon/epoxy composite

This paper addresses the effect of temperature on the mixed-mode interlaminar fracture toughness and fatigue delamination growth rate of a carbon-fibre/epoxy material, namely IM7/8552. Quasi-static and fatigue characterisation tests were carried out at −50 °C, 20 °C, 50 °C and 80 °C, using asymmetric cut-ply coupons. The experimental results show that temperature may have an accelerating or delaying effect on delamination growth, depending on the loading regime, i.e. either quasi-static or fatigue. Fractographic examinations were also carried out in order to assist the interpretation of the experimental data. A semi-empirical equation is introduced to describe the experimentally observed fatigue delamination growth rates at elevated temperatures.     

Fatigue testing of materials under extremal conditions by acoustic method

Increasing fuel cycle time requires fatigue testing of the fuel clad materials for nuclear reactors. The standard high-temperature fatigue tests are complicated and tedious. Solving this task is facilitated by the proposed acoustic method, which ensures observation of the material damage dynamics, monitoring of the experimental parameters, and determination of the dynamic yield stress. Ring samples cut from zirconium cladding tubes were irradiated with fast neutrons to a total fluence of 2.2×10²⁶ m^?2 and fatigue tested at temperatures up to 360–400°C, including the tests in an iodine vapor—a uranium fission product capable of inducing corrosion cracking of the fuel elements.     

Characterization of damage initiation and propagation in graphite/epoxy laminates by acoustic emission

A method of characterizing two major failure processes in graphite/epoxy compositestransverse cracking and mode I delamination, has been studied. Representative laminates were tested in uniaxial tension and flexure. The failure processes were monitored and characterized by acoustic emission (AE). The effects of moisture on AE were also investigated. Each damage process was found to have a distinctive AE output and to follow a definite trend which was significantly affected by moisture conditions. It was concluded that AE can serve as a useful tool for detecting and identifying these failure modes in composite structures in laboratory and in service environments.     

全湿热场下碳纤维/环氧树脂复合材料弯曲性能及寿命预测

以碳纤维/环氧树脂（T700/TR1219B）复合材料为研究对象,采用湿度场和温度场单一及耦合的方式,研究了不同湿热环境下其弯曲性能的变化,通过断口形貌和表面粗糙度表征,分析其湿热损伤机制。结果表明:T700/TR1219B复合材料的弯曲性能受湿度场和温度场影响明显,当吸湿率达到2%时,弯曲强度从干态的1 440.60 MPa下降到1 081.07 MPa;随温度的升高弯曲性能呈下降趋势,且在玻璃化转变温度T_g所在温度区间发生陡降,当环境温度为180℃时,弯曲模量和弯曲强度分别下降了71.18%和93.32%;高温高湿环境下弯曲性能陡降的温度区间前移,且性能衰减并非单一湿度场和温度场下衰减量的简单叠加。通过微观形貌分析发现,湿度场主要导致树脂水解脱黏,温度场下树脂形态破坏严重,而湿热耦合场对纤维与树脂均产生较大程度的损伤。考虑湿度场和温度及湿热耦合相关项,建立并验证了全湿热场下剩余弯曲强度模型,结合湿热老化时间、环境当量等参数提出T700/TR1219B复合材料的寿命预测模型。      

Dynamic delamination fracture toughness of a graphite/epoxy laminate under impact

Dynamic delamination fracture toughness in a [90/0]_5s T300/934 graphite/epoxy laminate was investigated using impact loading. Delamination cracks of three different sizes were embedded at the mid-plane of the composite specimen. The threshold impact velocity that causes propagation of the delamination crack was used in the dynamic analysis with the finite element method. From the finite element solution, the time-history of the strain energy release rate was calculated. The critical strain energy release rate was taken to equal the maximum value of the response history.     

A generic approach to constitutive modelling of composite delamination under mixed-mode loading conditions

A generic approach to constitutive modelling of composite delamination under mixed mode loading conditions is developed. The proposed approach is thermodynamically consistent and takes into account two major dissipative mechanisms in composite delamination: debonding (creation of new surfaces) and plastic/frictional deformation (plastic deformation of resin and/or friction between crack surfaces). The coupling between these two mechanisms, experimentally observed at the macro scales through the stiffness reduction and permanent crack openings, is usually not considered in depth in many cohesive models in the literature. All model parameters are shown to be identifiable and measurable from experiments. The model prediction of mixed-mode delamination is in good agreement with benchmarked mixed-mode bending experiments. It is further shown that accounting for all major dissipative mechanisms in the modelling of delamination is the key to the accurate prediction of both resistance and damage of the interface.