J. Bonhomme

Influence of Resin Type on the Delamination Behavior of Carbon Fiber Reinforced Composites Under Mode-II Loading

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.

Influence of the loading system on mode I delamination results in Carbon-Epoxy composites

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.

Compliance Correction for Numerical and Experimental Determination of Mode I and Mode II Composite Fracture Failure

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 than 10% in both modes I and II.

A study of the effects of the matrix epoxy resin and graphene oxide (GO) manufacturing process on the tensile behaviour of GO-epoxy nanocomposites

ABSTRACT 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, resulting from the use of two epoxy matrices (a mono-component system and a bi-component system) and different types of GOs, at different weight percentages were studied and tensile tests were performed on specimens of these materials in order to quantify the variations in their elastic constants and tensile strength. The GO reinforcements used were obtained by means of the modified Hummers method followed by thermal reduction at different temperatures. The aim was to quantify the effect of carbon/oxygen ratio on the reinforcement capacity of GO in order to optimise the manufacturing process. The stiffness of the nanocomposites improved with the addition of TRGO for both matrices, but the tensile strength depended on the matrix.

Fracture behavior under mixed mode I/II static and dynamic loading of ADCB specimens

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.

A Method to Determine the Rolling Resistance Coefficient by Means of Uniaxial Testing Machines

The characterization of the rolling resistance is an important issue when designing mechanical systems in order to predict the force needed to run a given mechanism. Frictional forces in bearings and rolling resistance in rollers and wheels are the most important factors contributing to the overall resistance to the movement. As the rolling coefficient varies as a function of the material nature, normal load, and rolling velocity, it is desirable to test the designed system under real conditions. Mechanical testing machines, present in most mechanical laboratory tests, usually work in uniaxial mode. In this work a simple testing device to be used with uniaxial mechanical testing machines is presented.

Influence of the Matrix Toughness in Carbon-Epoxy Composites Subjected to Delamination under Modes I, II, and Mixed I/II

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. Finally, experimental and numerical results showed a good agreement as the differences obtained from both procedures were generally lower than 10%.

Proposals to improve the durability assessment after ageing tests for wood-based sandwich panels

This work deals with the durability assessment, after ageing, of wood-based sandwich panels, carried out according to the European guideline ETAG 016, Annex C7 Climatic testing cycles. ETAG 016-2 is a guideline to assess the wood-based sandwich panels for roofs. It has tests and criteria for evaluating the results of these tests. As regards durability and performance under the weather, it proposes to choose one of three different climatic cycles. The choice depends on the material of the core. Each climate cycle has assessment criteria to measure whether the cycle has been successfully overcome or not. This assessment is related to the standard EN 14509, paragraph 5.2.3.1 that contains the same climatic test cycles. In this work we subjected several types of non-metallic sandwich panels to this test and criteria. Then, a critical review of these acceptance criteria was done. As a result, some recommendations are proposed regarding the acceptance criteria and testing procedure.