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In this paper, the phenomenon of delamination under static and dynamic loading of a composite made of an epoxy matrix and carbon fiber reinforcement has been studied, analyzing its fracture behavior under mixed mode I/II loading employing an asymmetric double cantilever beam test. Under static loading, some of the most representative formulations for calculating the energy release rate were analyzed, finding a good agreement between the results obtained by means of the different formulations. Under dynamic loading, the number of cycles necessary for the crack onset was determined (determination of ΔG − N fatigue curves and number of cycles necessary for the delamination onset for a given energy release rate). As regards the experimental results, apparent fatigue limits of the order of 38% of the critical fracture energy were obtained for an asymmetry coefficient of 0.1. Subsequent statistical analysis of the results enabled the fatigue limit to be defined more accurately. This was found to be 15% for this material, indicating the need to use these tools for the actual determination of the infinite fatigue life of the material. Finally, an optical study of the fracture surfaces was carried out which confirmed the presence of mixed mode fracture typologies. [ABSTRACT FROM AUTHOR]

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 G Ic 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. [ABSTRACT FROM AUTHOR]

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 energyGcand the scatter increased as the mode ratioGII/Gcincreased. Finally, experimental and numerical results showed a good agreement as the differences obtained from both procedures were generally lower than 10%. [ABSTRACT FROM PUBLISHER]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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 (G II ). 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%. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

In this study, the delamination fatigue behavior of two different aeronautical quality composites under mode-II notched flexure loading has been experimentally investigated with the aim of analyzing the effect of the resin type used. Both composites contain the same unidirectional AS4 carbon reinforcement, though embedded in two different matrices: an 8552 epoxy with improved toughness and an unmodified brittle 3501-6 epoxy. The end notched flexure test was found suitable for promoting fatigue delamination under mode-II loading. First, the ΔG-N fatigue curves were determined as the number of cycles necessary for crack onset at a given energy release rate. Then the crack growth rate was obtained for different fractions of the critical energy release rates, Gcr. Calculations of the shear toughness were carried out in accordance with the methods proposed in the ESIS protocol. Critical experimental details are reviewed and the influence of the different procedures proposed for the data assessment of results is analyzed. The results confirm, on the one hand, the influence of the manufacturing process on fatigue crack initiation, and, on the other, the improved performance of the modified resin, both in terms of crack initiation and growth rate. [ABSTRACT FROM PUBLISHER]

Abstract: 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%. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

This work deals with the determination of the critical strain energy release rate ([image omitted] 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 than 10% in both modes I and II. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Highlights: [•] Experimental data were treated with a model based on a Weibull distribution. [•] Fatigue failure was considered to occur when interlaminar propagation of crack starts. [•] The statistical model was used to analyze the variability in experimental results. [•] This study suggests a variable behavior depending on the type of fracture loading. [ABSTRACT FROM AUTHOR]

Abstract: 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<5mm), although the growth rate increased for large crack lengths. This behaviour was the same in both loading modes. [Copyright &y& Elsevier]

Abstract: In this paper, interlaminar crack initiation and propagation under mode-I loading of a composite material made of an epoxy matrix reinforced with unidirectional carbon fibres are experimentally assessed for different asymmetry ratios. In the experimental program, DCB specimens were tested and the number of cycles to failure necessary for the onset of crack growth was registered for the asymmetry ratios R =0.2 and R =0.5. The G–N curves under fatigue loading were determined, N being the number of cycles at which delamination commences for a given energy release rate. The ΔG–da/dN curves associated with the previously generated crack growth were also determined for different G cr rates (90%, 80%, 70%, etc.) and two different asymmetry ratios. For the asymmetry ratios, the fatigue limits show values in the order of 50% and 70%, respectively, of the critical fracture energy resulting from the static tests employing the modified beam theory. The fatigue limit is discretionally taken as the fatigue strength corresponding to 2.5 million cycles, i.e., the limit number of cycles considered for testing. The micrographs show that the morphology of the fracture toughness under mode-I loading are quantitatively but not qualitatively influenced by the stress ratios. [Copyright &y& Elsevier]