Abstract

Wrinkles (out-of-plane fiber waviness) are, unfortunately, a common phenomenon during the manufacture of composite materials. Because these internal defects affect the mechanical properties, all composite structures are inspected in order to ensure they meet the high quality standards that are demanded by the industry. During the desing process, parts are generaly oversized to mitigate the risk of undetected wrinkles and defects. Delaminations are widely characterized using traditional ultrasound techniques whereas  wrinkles have traditionally been more difficult to detect using such techniques. This is due to the fact that wrinkles don’t represent a change in material density or a strong boundary for the elastic wave. The aim of this work is to detect and characterize wrinkles in thick laminates of composite material using ultrasonic non-destructive tests. Different parts with induced wrinkles have been inspected with ultrasonic techniques. The ideal frequency to perform the inspections was determined using an analytical model in MATLAB. The amplitude of the wrinkles was measured visually on the samples and compared with compared with the ultrasonic measurements to determine the accuracy of the proposed method.

Abstract

This work focuses on analyzing the uniaxial response of carbon fiber reinforced polymer matrix laminates with the configurations [±45]_2S under tension and [±45]_4S under compression. These present non-linear stress-strain evolutions that allow them to withstand large deformations before losing their load-carrying capacity. Both responses are characterized by a first linear stage, followed by a plateau in which the strain grows without increasing the stress level, and finally by a re-stiffening phase. But there are quantitative differences that lead to different failure patterns, which are explained with the help of the state of stress in the plane of maximum shear stress. For a better understanding of the process, tensile load-unload-reload tests are performed to verify if the energy recovered in each cycle could be related to the loss of the apparent stiffness. During the first two stages of the mechanical response, the laminate suffers a progressive damage with a reduction in stiffness related to the dissipated energy, but this pattern is not repeated in the last stage of strain hardening. Based on the experience of other authors, it is assumed that the re-stiffening stage follows a different pattern due to possible microstructural changes in the matrix during the plateau. These are promoted by a narrowing process under tensile loads and a local widening of the test zone under compression. The dimensional changes perpendicular to the load direction are observed thanks to the strain fields obtained by Digital Image Correlation.

Abstract

The transformation processes used for thermoplastic composites, have been increasing their complexity, while incorporating new high demanding materials. From the aeronautic and medical sector, with their restrictions and specific demanding needs. The PAEK materials have been promoted and deeply applied due to their high temperature stability and their chemical resistance. In this research, the work has focussed on the use of the reinforced 40%CF PEEK process through injection moulding. In these over injection moulding processes the joint between 40%CF PEEK and other materials such as Aluminium, or reinforced PA and PEEK has been studied. As well as the adhesion between them and the variation generated on the mechanical properties. It has been observed that this joint is far better between thermoplastic materials than between metals and thermoplastic, this is due to the fact that even tough the PEEK has high processing temperatures, these are not relevant chemically speaking for metals such as Aluminium. Limiting the forces of the interphase to the physical and mechanical interaction between both materials. Nevertheless, in the case of the thermoplastic over injected parts several parameters have been observed to be affecting the final properties of the joint. Firstly, the over injected angle, obtaining higher mechanical properties as the angle increases as well as the contact surface. And the effect of the roughness of the overinjected part, having these a direct impact on the final mechanical properties.