V. Mollón

A new method for testing composite materials under mode III fracture

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.

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.

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.