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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.

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...

This paper describes a method for characterizing composite materials subjected to mode III delamination fracture using a custom-designed testing device and test equipment which allows loads or displacements to be applied to the test specimen in two directions, one axial and the other torsional. To verify the method’s functionality experimentally, a composite material made up of an epoxy matrix and unidirectional carbon-fiber reinforcement was used in conjunction with an image analysis device for the purpose of determining the displacement field in the crack front of a double cantilever beam test specimen. According to the results, this test method permits almost pure mode III fracture tests to be carried out, as the mode II component is practically negligible. Another feature of the method is the improvement in the quality and ease of inserting the specimen in the device, thus permitting more repetitive results to be obtained with less dispersion.

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 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 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.

In this work, the Longitudinal Half Fixed Beam test (LHFB) for mode III characterization is analysed by means of Finite Element Analysis (FEA) and optical microscopy. The obtained results were compared with experimental data and analytical formulations obtained in previous works. The objective of this study is to determine the energy distribution across the crack front and to understand the micromechanics that give rise to the delamination failure in unidirectional carbon/epoxy composites. It was found that for samples with long initial crack lengths (i.e. a0 = 30 mm), pure mode III takes place in the central part of the delamination front. Nevertheless, these samples present a significant contribution of mode II at the edges of the specimen. As a0 decreases, pure mode III increases in extension across the delamination front and mode II decreases at the sample edges. When the initial crack length (a0) is quite small, the sample presents pure mode III practically on the entire length of the crack front. Nevertheless, when the initial crack is very small, the applied force exercises a local influence on the tip of the crack at the edge of the sample and mode III distribution loses its uniformity across the crack front. The concordance between the analytical results and the numerical results obtained by the Finite Element Method (FEM) was variable depending on the length of the crack and if the comparison was made with GIII or GT as reference. Intralaminar cracks at approximately 45° with respect to the midplane were observed in planes perpendicular to the direction of delamination propagation (planes perpendicular to the fibre direction). Other authors also found this type of intralaminar cracks in other mode III test configurations.

This article analyzes the influence of temperature on the delamination phenomenon in two composites with the same carbon-fiber reinforcement, but different epoxy matrices. Interlaminar crack initiation and propagation under Mode I with static and fatigue loading are experimentally assessed for different test temperatures: 20, 50, and 90°C. The materials under study are made of AS4 unidirectional carbon fibers and different matrices: one is made of a 3501-6 epoxy matrix (AS4/3501-6), whereas the other is made of an 8552 epoxy matrix modified to increase its toughness (AS4/8552). Both composites have a symmetric laminate configuration [0°]16s. In the experimental program, double cantilever beam specimens were tested under static and fatigue loading. A fractographic study was also performed using a scanning electron microscope on samples obtained from specimens previously tested under static and fatigue loading. A comparison was made of the fracture surfaces at the different test temperatures and the experimental tests' results obtained. POLYM. COMPOS., 36:747–755, 2015. © 2014 Society of Plastics Engineers

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.

The simulation of delamination using the Finite Element Method (FEM) is a useful tool to analyse fracture mechanics. In this paper, simulations are performed by means of two different fracture mechanics models: Two Step Extension (TSEM) and Cohesive Zone (CZM) methods, using implicit and explicit solvers, respectively. TSEM is an efficient method to determine the energy release rate components GIc, GIIc and GIIIc using the experimental critical load (Pc) as input, while CZM is the most widely used method to predict crack propagation (Pc) using the critical energy release rate as input. The two methods were compared in terms of convergence performance and accuracy to represent the material behaviour and in order to investigate their validity to predict mode I interlaminar fracture failure in unidirectional AS4/8552 carbon fibre composite laminates. The influence of increasing the loading speed and using mass scaling was studied in order to decrease computing time in CZ models. Finally, numerical simulations were compared with experimental results performed by means of Double Cantilever Beam specimens (DCB). Results showed a good agreement between both FEM models and experimental results.

In this study, the delamination toughness under pure modes I and II and a 2/1 mode mixity ratio and the associated fractures morphology of AS4/8552 and AS4/3506-1 unidirectional carbon fibre/epoxy composites have been related to the principal tensile stresses at the crack front. Pure modes I and II and a GI/GII = 2/1 mixed-mode ratio tests were respectively carried out by means of the following methods: DCB (Double Cantilever Beam), ENF (End Notch Flexure) and ADCB (Asymmetric Double Cantilever Beam). ADCB specimens, in which the crack plane is outside the laminate midplane, are simple as well as useful test configurations to produce a mixed-mode load state at the crack tip of samples. As the ADCB test is not still covered by international standards, the calculations were performed by means of FE models and analytical formulations developed in previous studies. The Finite Element Modelling (FEM) method was used to analyze the stress state ahead of the crack to better explain the fracture micromechanisms acting under different loading modes and their influence on delamination fracture toughness. It has been demonstrated that the direction of the principal tensile stresses at the crack front has a major influence on the orientation of the so-formed microcracks, on microcrack coalescence and, hence, on the failure mode description reflected in the fracture morphology.

In this paper unidirectional carbon epoxy End Notch Flexure samples (ENF) with different crack plane positions have been analyzed in order to study the influence of the degree of asymmetry over the load state at the crack tip. This study has been performed from different points of view: experimental, numerical approaches (Finite Element Models) and fractographic observations. From these studies it has been concluded that the deviation of the crack plane from the specimen midplane has no influence or little influence over the corresponding critical value of the energy release rate in mode II (GII). In other words, the asymmetry of the crack plane does not induce mode I load state at the crack tip while the energy release rate related to the mode II load remains almost constant as the crack plane moves from the midplane. Fractographic observations have shown “hackle markings” structures typical from mode II fracture. Finally, a good agreement was found between experimental and numerical procedures as errors were always lower than 7%.

In this paper, a modified mechanical grip fitting is used to perform double cantilever beam (DCB) tests. The advantage of using mechanical grips instead of piano hinges or end blocks lies in the fact that the use of adhesive bonding is not required to fix the sample. Adhesive bonding can be an important source of uncertainty and unexpected debondings in fatigue tests. Mechanical fittings are also well suited for high temperature applications where adhesive bonds usually undergo premature failure. An experimental programme has been developed in order to compare the performance of the modified mechanical hinges with classical hinges and end blocks. The highest GIc values and scattering were found for the end block system, while hinges and mechanical hinges furnished similar results. The material used to perform the experimental study was a Hexcel AS4/3501-6 unidirectional laminate.

This paper deals with the numerical determination of mode II energy release rate in AS4/8552 unidirectional carbon fibre/epoxy laminates. The Two-Step Extension Method is reviewed as an alternative procedure to perform Finite Element Analysis (FEA) on mode II delamination tests. The influence of some variables such as element length, type of element and 2D versus 3D models on the results was studied. Results were compared with empirical data obtained from End Notched Flexure (ENF) tests carried out on unidirectional carbon/epoxy laminates. In order to detect outliers, two statistical methods were used to validate the experimental results. Finally, the influence of the friction forces on the numerical results has been addressed.

Asymmetric DCB specimens (ADCB), 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. This test configuration is as simple as pure mode I tests. However, the analytical procedures developed in the ASTM D5528-01 standard to calculate G I are no longer valid to analyze the ADCB specimen. In this test configuration, the position of the crack plane controls the mode I and mode II load levels at the crack tip. In this work an empirical formulation, based on Finite Element Modelling (FEM), has been developed in order to calculate G I / G and G II / G (where G = G I + G II ) in ADCB specimens. On the other hand, the Modified Beam Theory (MBT) has been adapted by means of experimental calibration to allow the calculation of G for this test configuration. These formulations have been validated by means of FE modelling for different crack plane positions. Finally, the formulations developed in this paper were used to calculate G I and G II in experimental results on glass fibre epoxy laminates and the obtained results were compared with formulations developed by other authors. A good agreement was found among the different analytical, empirical, and numerical methods studied in this paper.

The Asymmetric DCB test (ADCB) is an alternative configuration to the MMB test (Mixed Mode Bending) to produce a mixed mode load state at the crack tip. In this test, the crack plane is out of the laminate midplane. This test configuration is as simple as pure mode I tests. Nevertheless, the mixed mode load state at the crack tip cannot be controlled by means of the test fixtures and so G I and G II are not easily calculated. In this test configuration, the position of the crack plane controls the percentage of mode I and mode II load at the crack tip. In this paper, unidirectional carbon epoxy ADCB samples have been used to study the mixed mode load state at the crack tip from different points of view: experimental, numerical and analytical procedures. The MBT method has been adapted to allow the calculation of G in ADCB samples. Also, an empirical procedure has been proposed in order to calculate G I and G II .

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%.

It is well-known from the scientific literature that the asymmetric double cantilever beam (ADCB) specimen is subjected to mixed-mode I/II load at the crack tip. In these samples, the crack plane lies outside the laminate midplane. In this work, the energy release rate in modes I and II (GI and GII) are obtained by different approaches. The analytical determination of GI and GII in ADCB samples is not simple or straightforward and is usually based on partition methods. Numerical results obtained from finite element analysis (FE) are compared with the analytical local partition method (LP) for a carbon fibre epoxy AS4/3501-6 laminate. Both results are also compared with an empirical formulation obtained in previous works. Results obtained from all three methods are in good agreement.