Carbon fiber reinforced plastics (CFRP) has been used in aerospace and expensive automobile applications, because of its light weight, high strength and rigidity, design flexibility, long material life, etc. However, CFRP is not used for a general-purpose material yet. CFRP has the issues of capital importance to cost reduction and recycling. Thus, we have examined to develop a low cost CFRP with easy recycling. In this paper, we propose a resolution to improve the flexural and impact properties of CFRPP by the use of a carbon fiber-polypropylene (i-PP) matrix system. As a result, we found that the CFRPP in our study is about 750 MPa of the flexural strength, about 89 GPa of the flexural modulus and about 115 kJ/m² of the izod impact strength, in particular UD8 improved 1.5, 1.1 and 1.1 times in the strength, modulus and impact strength, repectively compared with UD4. CFRPP is one of promising material to make the structure light.

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Effects of water absorption and thermal environment on compression after impact (CAJ) charac-teristics of CFRP laminates are studied. Specimens were immersed in 71°C water for 10, 000 hours (410 days) to quantify the ratio of water absorbed in the CFRP laminates. Water content of the specimen was 1.4 wt% after the water absorption test. Numerical simulation using FEM code was conducted to predict moisture diffusion based on the measured orthotropic diffusion constants. Then impact tests by the drop-weight were carried out at room temperature at normalized energy level of 3.3 J/mm. Compression tests after impact tests were carried out at various temperatures, i.e., RT, 32°C, 177°C, for water absorbed specimens, and -54°C, RT, 82°C, 121°C, 149°C, 177°C for dry specimens. Delamination area and the number of transverse cracks of the water absorbed CFRP were smaller than those of dry specimens. Such smaller delamination area of wet specimens provides higher CAI strength than dry specimens. CAI strength of the water absorbed CFRP decreased significantly at high temperature because the glass-transition temperature deteriorates seriously due to the water absorption.

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The primary objective of this study is to address a key research challenge for improving tradi-tional joint design, which develop functions and structures that can monitor their own structural integrity. A self-sensing technique using the bolt itself to monitor the bearing damage in bolted composite or metallic joints is proposed. The basic concept is based on making Bolt Gauge (BG) measurements. That is, have only to bond a strain gauge onto the surface of bolt head to reflect changes in the bolt strain, which would identify the state of bearing damage. Novel system can maintain not only the peculiar structures and functions of the mechanical fastener, but more importantly, also can provide the self-diagnostic function for component damage. And, it can be contributed directly to technology development for structural health monitoring (SHM) to address these issues, and to meet demanding goals in increasing aerospace vehicle safety and reliability while reducing operating costs. Extensive tests using double lap bolted joint specimen are performed to verify this technique in the various joint cases. The investigations include the effects of the bolt clamp-up, material response, joint geometry and loading history on the detection sensitivities for the bearing damage. The onset and progress of the bearing damage monitored by BG measurements are also compared with acoustic emission (AE) measurements that used in traditional NDE method. A further consideration of the detection mechanisms by BG including an axial loading test and a three-dimensional finite element analysis is performed. The behavior of bolt deformation and stiffness degradation due to the bearing damage initiation and progression are estimated using damage mechanics approach.

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