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The authors are indebted to the Spanish Ministerio de Ciencia, Innovacion y Universidades (Proyecto RTI2018‐095290‐B‐I00) for funding this study.

The fatigue delamination process of three different aeronautical quality composites subject to mode III fracture loading is studied experimentally in this paper with the aim of analysing the effect of the type of matrix used under this fatigue fracture mode. In all cases, the main difference between the composite materials considered in this study is the type of epoxy resin used as the matrix.The test method used to produce the delamination process under mode III fracture is known as the Longitudinal Half Fixed Beam (LHFB) method, used in previous studies by the authors, which was shown to be a valid method for generating the delamination process in composite sheet materials subject to mode III fatigue loading.The experimental data from the fatigue tests were treated with a probabilistic model based on the Weibull distribution. This model facilitates the identification of relevant aspects of the fatigue behaviour of these materials, such as the estimation of fatigue strength for periods of time greater than those tested here, as well as the reliability of the results.The results thus obtained reflect important differences in the fatigue behaviour of the materials under study, the type of matrix being the predominant parameter of the fracture process under mode III loading. Keywords: Delamination, Fracture, Fatigue, Mode III

This work comprises a study of the reinforcement capacity provided by the addition of different types of nano-reinforcements of graphene oxide (GO) to epoxy matrices. A range of nanocomposites, res...

Composite materials manufactured by overlapping plies with certain specific geometries are likely to lose part of their strength due to the presence of internally delaminated regions. The aim of this paper is to experimentally evaluate the generation and propagation of these interlaminar cracks in a carbon-epoxy composite material subjected to fatigue loading under mixed mode I/II fracture. Two different test methods were used for this purpose: The standardized mixed-mode bending (MMB) test and the asymmetric double cantilever beam (ADCB) test, with the goal of exploring the viability of the ADCB test as a simpler alternative to perform than the MMB test, especially in fatigue testing. With this aim in mind and after prior static characterization of the material in which the critical values of the energy release rate were determined under both test methods, the levels of the energy release rate to be applied in fatigue tests were defined for two mode mixity ratios, GII/Gc = 0.2 and 0.4 (0.34 ADCB), and a fatigue loading ratio, R = Gmin/Gmax = 0.1. The G-N fatigue onset curves were subsequently obtained from these experimental data. The most relevant result of the study is that the fatigue limits obtained using the MMB method are generally more conservative than those obtained via the ADCB method.

The present paper studies the fatigue delamination behaviour of an epoxy/carbon composite material under mode III loading using a longitudinal half fixed beam (LHFB) device initially designed for mode III static tests of composite materials formed by the stacking of plies. For this purpose, a series of tests was carried out at different levels of loading representative of the fatigue behaviour of the material, from the crack onset phase through the delamination phase to final fracture. The experimental results were treated statistically, obtaining the values of the fatigue limit for probabilities of fracture of 5% and 50%. Finally, a fractographic analysis of the fracture surfaces was performed which allowed us to identify the same characteristic patterns of static mode III fracture, namely broken fibres, cusps and saw-teeth, in addition to a new morphology consisting of the formation of agglomerations of resin produced by the friction between the lips of the specimen in the fracture zone that point to dynamic mode III fracture. These agglomerations eventually crack and become detached from the fibres, leaving these free of resin.

This paper reports the results obtained after carrying out tests on specimens of two types of composite materials. These materials have the same polyetherimide (PEI) thermoplastic matrix, but a different type of reinforcement. In both cases, the reinforcement is fabric, one being made of carbon fibre and the other, of fibreglass. The specimens were tested after various periods of exposure to an outdoor environmental aging up to a maximum period of 20 years. Following static characterization tests, a significant decrease in mechanical properties was observed during the first eight years of exposure. From that time onwards, once what could be called the aging limit has been reached, these properties were maintained. From the fractographic point of view, there is a substantial loss of surface resin. This loss of resin is much more pronounced in the fibreglass composite, even being accompanied by fibre breakage.

The delamination phenomenon is one of the major failure mechanisms in composite materials formed by the stacking of successive layers. The fracture mode that has been studied the least within this phenomenon is mode III, due to the difficulty its simulation involves. A novel test device is presented in this paper that allows laminated composites to be subjected to this mode of loading. Characterization tests were carried out on two materials with different matrices in order to analyse the influence of the type of matrix in the fracture behaviour under mode III-fracture. Furthermore, two analytical models adapted to this test methodology have been used for the determination of the value of the energy release rate for this mode of loading in both materials. The load was applied at different distances from the front of the notch, this being another parameter that was analyzed. From the results obtained it follows that the device used in this work allows a dominant mode III and that this result holds for different points of load application studied.

This paper, analyses the mode I fracture behaviour under fatigue loading of two aircraft quality composites at different test temperatures. The materials under study are made of the same unidirectional carbon fibre but employ a different epoxy resin matrix. Both composites had the same symmetric configuration [0°]16/s. In the analysis, double cantilever beam specimens were tested at different temperatures: 20 °C (room temperature), 0 °C, −30 °C and −60 °C (different estimated in-flight temperatures aircraft). A fractographic study was carried using a scanning electron microscope (SEM) on samples extracted from specimens previously tested at the different test temperatures in order to analyse their fracture surfaces.

In this work, the fracture behavior under modes I, II, and mixed mode I/II has been studied for two different AS4 carbon fiber epoxy laminates. One of the laminates was produced with a Hexcel 3501-6 epoxy resin while the other was laminated with a tougher modified Hexcel 8552 epoxy resin. Both laminates were experimentally tested in modes I, II, and mixed I/II with different mixity ratios by means of DCB (double cantilever beam), ENF (end notch flexure), and MMB (mixed mode bending) specimens, respectively. Finite element modeling (FEM) was used in order to analyze modes I, II, and mixed I/II and to compare experimental and numerical results. The modified 8552 resin matrix presented the best behavior in mode I and mixed mode I/II as the critical energy release rate was higher than that for the 3501-6 matrix composites. In mode II, the best performance was reached for the 3501-6 matrix laminates. It was also found that the critical energy Gc and the scatter increased as the mode ratio GII/Gc increased. Fin...

In this paper, interlaminar crack initiation and propagation under mode-I with static and fatigue loading of a composite material are experimentally assessed for different test temperatures. The material under study is made of a 3501-6 epoxy matrix reinforced with AS4 unidirectional carbon fibres, with a symmetric laminate configuration [0°] 16/S . In the experimental programme, DCB specimens were tested under static and fatigue loading. Based on the results obtained from static tests, fatigue tests were programmed to analyse the mode-I fatigue behaviour, so the necessary number of cycles was calculated for initiation and propagation of the crack at the different temperatures. G – N curves were determined under fatigue loading, N being the number of cycles at which delamination begins for a given energy release rate. G ICmax – a , a – N and da / dN – a curves were also determined for different G cr rates (90%, 85%, 75%, etc.) and different test temperatures: 90 °C, 50 °C, 20 °C, 0 °C, −30 °C and −60 °C.

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

This work deals with the determination of the critical strain energy release rate ( by means of experimental and numerical methods in unidirectional carbon epoxy composite laminates in modes I and II, and the influence of the test configuration compliance on the results. In previous works, it was found that the determination of GIc by means of experimental procedures and numerical determination using the Finite Element Method (FEM), presented differences in the order of 20–30%. In order to improve the convergence of both numerical and experimental models, research was carried out about the points that could have influenced the results, i.e., FEM element type and size, material behavior law and testing compliance. From this research it was demonstrated that the testing machine compliance had a great influence over the obtained results. The introduction of a correction for testing machine compliance in the calculation has lowered the difference between numerical and experimental results from 20–30% to less ...

The mode I and mode II interlaminar fracture surfaces of a unidirectional AS4/8552 carbon/epoxy laminate were studied by means of SEM microscopy and numerical analysis in order to correlate the surface appearance with the stress state. SEM micrographs of fractured surfaces showed that the most predominant fractographic features in modes I and II were ‘river markings’ and ‘hackle markings’ respectively. Finite Element Analysis (FEA) showed that the principal tensile stress direction near the crack tip was perpendicular to the ‘hackle’ or ‘stacked lamellae’ microstructure and at 45° to the fracture plane. As was observed by other authors, these results suggest that matrix microcracking perpendicular to the principal stress direction is the driving mechanism forming the ‘hackle marking’ microstructure.

This work compares the numerical determination of the strain energy release rate in mode I for the interlaminar fracture of composite laminates by means of two different models: the virtual crack closure technique (VCCT) and the two-step extension method. Results were compared with empirical data obtained from double cantilever beam (DCB) tests carried out on unidirectional AS4/8552 carbon/epoxy laminates. The study showed that, in a pure mode I state, results obtained via the two-step method were in agreement with a straightforward calculation of the elastic energy variation in the system. Results from VCCT calculations were slightly lower than those obtained through the two-step method using four node elements in plane strain. As expected, results from both models converge as element length decreases.

The wear behavior of a thermoplastic polymer, polyetherimide, and of a composite with this polymer as matrix and a reinforcement of glass fiber fabric has been analyzed. The test was carried out according to the Standard ASTM G99 using a device pin-on-disk. Obviously, the reinforced material has higher wear strength than the nonreinforced material. Also, the evolution of the wear with the temperature has been studied at ambient temperature and at 50, 100, 150, and 200°C. The effect of the temperature is very important because when it was increased the wear was increased, except to 200°C. At 200°C there was an important decrease. The test temperature was measured in the inner of the furnace and when it was 200°C in this zone, the temperature in the contact point was higher and it would be close to glass transition temperature of the polymer (~217°C), this is the reason for an important micro-structural variation in the material.

This article addresses the experimental characterization of the mechanical properties of three types of woven fabric composites taking into account the effects of hygrothermal and hygrothermomechanical aging. Characterization was carried out using the mode I, double cantilever beam (DCB) and the mode II, end notched flexural (ENF) interlaminar fracture tests in order to determine the loss in crack propagation resistance. The materials used were two types of woven (2/2 Twill, 8-Harness Satin) glass fiber, and 8-Harness Satin carbon fiber. The matrix was polyetherimide (PEI). The critical values of the energy release rate in mode I and mode II were calculated using the corrected beam theory. The material reinforced with 8-Harness Satin glass fiber presented the best behavior in mode I and mode II. The decrease in fracture strength is more important in the material reinforced with carbon fiber.

The static and fatigue behavior of two fiber-reinforced composites was characterized after samples were subjected to both natural and accelerated aging. The composites consisted of a thermoplastic matrix (PEI: polyetherimide) reinforced with glass or carbon fabric. Natural aging involved exposing samples to the elements over a period of two years, while accelerated aging was conducted in a saline solution during 200 days. Subsequently, static (tensile and interlaminar shear) and fatigue tests were carried out with the aim of determining the mechanical properties of the materials after exposure. A negligible decrease in the value of these properties was observed, while different behavior was detected depending on the type of aging of the material.

The mechanical behavior of a composite with thermoplastic matrix and glass fiber reinforcement has been studied. It has been tested to constant stresses of a value equivalent to tensile strength divided by three to 50, 100 and 150 °C, during periods of time between 250 and 3000 minutes. A decrement of the tensile strength and deformation can be observed in the three temperatures. Also a group of specimens have been exposed to accelerated aging in an environmental chamber to 70 °C and 95% RH, during 30 days; and after they have been tested. In that case a decrement of the strength and deformation after 250 minutes has been observed, but in larger time of exposure these properties suffered an increment.

The way in which the time of exposure to the atmosphere modifies both the elastic properties and the ultimate strengths of sheet moulding compounds has been evaluated. Various plates of this material were exposed to the elements over a total period of 2 years, periodic tests being carried out every 6 months. As well as the static mechanical properties, the dynamic flexure properties have also been obtained for different frequencies and over a range of temperatures from 20 to 250°C.

espanolSe estudia el comportamiento mecanico de un compuesto constituido por polieterimida (PEI), reforzada con tejido de fibra de vidrio. El material es sometido a un envejecimiento acelerado en una camara higrotermica (70oC y 95% de humedad relativa), durante 10, 30, 90 y 180 dias, y luego a tension isotermica. La tension fue la correspondiente a un tercio de la resistencia y a tres temperaturas inferiores a la de transicion vitrea, durante periodos entre 250 y 2000 minutos. Luego las muestras fueron ensayadas a traccion. Las probetas originales, sin envejecer, presentan valores de resistencia y de deformacion (en los ensayos realizados a mas de 750 min.) menores que las no envejecidas sometidas a tratamiento isotermico. EnglishThe mechanical behavior of a composite of polyetherimide (PEI) reinforced with glass fiber fabric (8H Satin) was analyzed. Initially, the material was subjected to accelerated ageing (70oC and 95%R.H.) in a hygrothermal chamber for 10, 30, 90 and 180 day periods, after which it was subjected to isothermal stress. The value of the stress was a third of the tensile strength and at three different temperatures below the vitreous transition, applied over periods between 250 and 2000 minutes. The samples were then tested by traction. The original non-aged test samples, showed values of strength and deformation (in tests for 750 min. or more) lower than values of the non-aged specimens which had been subjected to isothermal treatment.

In this work, the fracture behaviour under modes I, II and different mixed mode I/II ratios has been studied for a AS4/3501-6 carbon fibre epoxy resin laminate. Mixed Mode tests were carried out by means of two different procedures: MMB (Mixed Mode Bending) and ADCB (Asymmetric Double Cantilever Beam). ADCB specimens, in which the crack plane is out of the laminate midplane, are simple and useful test configurations to produce a mixed mode load state at the crack tip of the samples. The ADCB test is not still covered by international standards, so the calculations were performed by means of the Finite Element Modelling (FEM) and analytical formulation developed in previous works. FEM was used in order to analyze modes I, II and mixed I/II and to compare the experimental and numerical results. It was found a good agreement between ADCB and MMB tests. On the other hand, it was also observed that the critical energy Gc increased as the mode mixity ratio GII/Gc increased. Finally, experimental and numerical results showed a good agreement as the differences obtained from both procedures were generally lower than 10%.

The behaviour of specimens from a composite made of a thermoplastic matrix PEI (polyetherimide) reinforced with glass-fibre fabric was studied. The material was exposed to accelerated aging in a climatic chamber at 70°C and a relative humidity of 95%, during periods of time of 10, 30, 60, 90, 180 and 360 days. The specimens were subsequently subjected to isothermal tests at 50, 100 and 150°C during 250, 750 and 2000 minutes with a constant stress of 40% of the material's tensile strength (obtained to ambient temperature). These three temperature values were selected from the results obtained in tensile tests carried out on specimens subjected to different temperatures ranging between ambient temperature and that of glass transition. In addition, the behaviour of the aged specimens was compared with the original material, using the same parameters of temperature, time and constant stress. An increment in tensile strength with regard to the original material was observed.