No abstract

The approach for joining thermosetting matrix composites (TSCs) proposed in this study is based on the use of a low melting co-cured thermoplastic film, added as a last ply in the stacking sequence of the composite laminate. During curing, the thermoplastic film partially penetrates in the first layer of the thermosetting composite, leading to macro-mechanical interlocking as the main connection mechanism between the thermoplastic film and the underlying composite. After curing, the thermosetting composite joints with the thermoplastic modified surface can be assembled by welding. Welding of the TSC-TSC joints is performed by ultrasonic and induction welding. The weld strength is investigated by morphological characterization of cross sections and failure surfaces and by mechanical testing. The effect of the thermoplastic film thickness on the welding process and on its outcome is also analyzed. Both induction and ultrasonic welding mostly result in good-quality welded joints. The welding process used as well as the initial thickness of the 2 thermoplastic film are found to have a significant effect on the final thickness of the weld line and on the location of failure. Thicker thermoplastic films are found to ease the welding processes.

The aim of this study is the characterization of recycled carbon fibres, in view of their potential application in long-fibre reinforced thermoplastic composite. The fibres were obtained from epoxy matrix composite panels, applying a patented process that includes the pyrolisis of the matrix followed by an upgrading of the fibres. Then, recycled fibres were further subjected to thermal and acid treatments in order to modify their surface morphology and chemistry. Scanning electron microscopy and energy dispersive spectrometry were used to characterize the morphological and compositional changes of the fibre surface. The fibres were characterized in terms of mechanical properties and adhesion to an epoxy matrix. The fibres treated by thermal processes at high temperatures (600°C) were shown to be too severely damaged, making them unsuitable for the production of fibre-reinforced composites. A thermal treatment at lower temperatures (450°C) involved a very limited damaging without any evident chemical modification of the fibre surface, which in turn involved a limited increase of the adhesion properties to an epoxy matrix. Chemical treatment by nitric acid caused a very limited damage of fibres, coupled with a significant modification of surface chemistry, which in turn involved a further increase of the fibre/matrix adhesion properties.

During autoclave processing of composites for high‐performance applications, it is mandatory to limit the porosities, which mainly depend on the hydrostatic pressure in the resin. This pressure, which is not constant during heating being affected either by resin flow either by elastic stress in the fiber stack, can be significantly different from the autoclave pressure. Modeling of resin flow and stress in the fiber stack is a key issue for prediction of the resin hydrostatic pressure, which can be related to void development. Also, the viscosity of the thermosetting matrix is a relevant parameter since it is not constant but evolves during curing going through a minimum and then increasing to an infinite value at gel point. In this work, a viscoelastic model is adopted to calculate the evolution of resin pressure during an autoclave cycle up to gelation, accounting for viscosity and degree of reaction changes. Therefore, the model includes a kinetic and rheological model whose input parameters have been experimentally determined by Differential Scanning Calorimetry and rheological analysis. The predicted resin pressure for three case studies associated to different composite and bleeder thicknesses and reinforcement materials have been discussed. POLYM. ENG. SCI., 57:631–637, 2017. © 2017 Society of Plastics Engineers

A thermoplastic matrix composite flat panel stiffened by induction welded stringers, according to a typical aerospace configuration was tested. A Poly-Ether-Ether-Ketone (PEEK) matrix reinforced by carbon fibers was adopted.The aim of this study was to investigate the shear buckling, a load condition that is very common for the aerospace structures, of the panel obtained using a non-conventional approach for its fabrication, that is, induction welding. A finite element (FE) model capable to describe the buckling and the postbuckling behavior of the stiffened panel was also developed and compared with experimental results. Testing was performed using fixtures suitably designed to transmit a pure shear to the panel, and strain gauges were mounted on the external surfaces to measure the deformations.