Generally, the viscosity of thermoplastic resin is considerably higher than that of thermosetting resin, even if it melts at a high temperature. Furthermore, because the thermoplastic resin is difficult to dissolve in a solvent, it is difficult to prepare a prepreg by the solvent methods used for preparing a thermosetting resin. Therefore, when molding fiber-reinforced thermoplastic (FRTP), impregnation of the thermoplastic resin into the reinforcing fibers has become a serious problem. In this study, we propose a novel method for manufacturing the FRTP prepreg, in that the thermoplastic resin is dissolved with an ionic liquid and impregnated with a reinforcing fiber. Unlike organic solvents, ionic liquids have a low environmental burden and can sufficiently dissolve thermoplastic resins; thus, ionic liquids are suitable for manufacturing FRTP prepregs. In this study, a prepreg was prepared by dissolving polyamide and polyethersulfone resins in an ionic liquid and impregnating it with a glass fiber, and the FRTP was molded using this prepreg. The optimum conditions for polymer dissolution in ionic liquids were found by prototyping and evaluating the prepreg under various polymer-dissolution conditions. It was confirmed that the FRTP using the prepreg showed a good resin impregnation state and mechanical properties superior to those of FRTP produced by the conventional film-stacking method.
Robotic Automated Fiber Placement (AFP) and Automatic Tape Lay-up (ATL) technique are well-developed and applied to practical producing process for enhancing productivity and reducing costs. The robotic process enables steering fiber or prepreg tape, which makes it possible to produce structures without fiber ends at the edge of open holes or cut-outs. It can reduce the stress concentration and prevent failure from the fiber ends located at the hole surfaces. The strength of the structure can significantly improve with the presence of cut-outs and open holes. In this study, a simplified model of a laminate comprising steered layers with a circular hole is proposed that can be produced by employing the steering technique. The thickness distribution and the elastic properties were theoretically derived on the basis of a geometrical consideration and a classical lamination theory. The stress and strain distributions of axisymmetric laminated plates were semi-analytically derived for an axisymmetric load and a uniaxial uniform load to understand the mechanism and to evaluate the degree of reduction in stress concentration.
In this study, impact and CAI (compressive after impact) tests, with varying impact energies, were conducted using quasi-isotropic laminates with different ply thicknesses (0.02, 0.12, 0.24mm). It consisted of a 0.02-mm-thick thin-ply prepreg sheet. The effects of ply thickness and impact energy on damage characteristics and CAI strength during impact were evaluated. Furthermore, specimens were meticulously inspected after impact using X-ray CT to investigate the effect of ply thickness on damage mode, especially the locations of matrix cracks and delamination in thickness direction. Results confirmed that fiber breakages became a predominant damage mode upon decreasing the ply thickness due to the constraining effect of neighboring layers, while most of the failures in thick-ply laminates, such as matrix cracks and delamination, were resin failure. Moreover, when the ply thickness was decreased, the CAI strength was lower than that in thick-ply laminates due to the propagation of fiber breakage in the thickness direction at a high impact energy. The CAI strength improved when the impact energy was below 5 J/mm.