Hiroshi Aoyama

Mode I and II delamination fatigue crack growth behavior of alumina fiber/epoxy laminates in liquid nitrogen

Mode I and II interlaminar fracture toughness and delamination fatigue crack growth behavior were investigated with unidirectional alumina fiber (ALF)/epoxy laminates at 77 K in liquid nitrogen. The mode I fracture toughness values at 77 K were higher than those at room temperature in laboratory air (RT). Although initial values of the fracture toughness under mode II loading was higher at 77 K than those at RT, the propagation values of the fracture toughness was insensitive to the test temperature. The fatigue crack growth threshold at 77 K under mode I loading was higher than that at RT. The stress ratio dependency under mode II loading at 77 K was completely different from that at RT. Then, the increase of the fatigue crack growth resistance at 77 K from that at RT was observed only under stress ratio, R=0.1. The difference of the fracture mechanism due to the test temperature and the loading mode was discussed on the bases of fracture mechanics and microscopic fracture mechanism consideration.

Health-monitoring technologies for alumina-fiber-reinforced plastics

Abstract Transmitted light could detect matrix cracks that occurred in the early stages of fracture of alumina-fiber-reinforced plastics (alumina-FRPs); these cracks could not be detected by acoustic emission (AE). An optical-fiber Bragg grating (FBG) sensor could detect the matrix cracks but it must be embedded in the FRPs. To determine which wavelength is sensitive to the defects in the FRP, the spectra of the transmitted and reflected light were measured. The intensity of the transmitted light, especially in the visible region (wavelength: 400–800 nm), decreased as the bending stiffness of the test piece decreased. It is thus concluded that to monitor the decrease in bending stiffness of FRPs, a simple sensor using a visible ray is good enough. And this transmitted-light NDE technique can work in the strong electromagentic field associated with a superconductor. It will therefore be useful for detecting defects in the FRP of the load-support system in service.

Effect of molding processes on multiaxial fatigue strength in short fibre reinforced polymer

This study concerns the multiaxial static and fatigue strength properties. Short-glass-fibre-reinforced phenolic-resin composites (SGP) molded by injection and compression processes were subjected to tensiontorsion combined static and fatigue tests at room temperature under various test conditions. Tension – torsion combined static strength well agreed with Tsai-Hill failure criteria without depending on processes. Relationships between the maximum principal stress, ?p1, max, and the number of fracture cycles, Nf, were approximately linear in the whole range of up to 106 cycles. For a unified evaluation of multiaxial fatigue life for SGP, non-dimensional effective stress, ?*, defined by modifying Tsai-Hill failure criteria was applied. The slopes of ?*- Nf curves according to Baskin’s law were almost identical to the injection (n = 26.3) and compression (n = 26.2). We finally confirmed that the multiaxial fatigue life of SFRP could be predicted by using ?* with a unique Wohler curve without relying on molding processes. KEYWORDS. Short-fibre reinforced plastics; Multiaxial; Fatigue; Tsai-Hill.

Nondestructive Evaluation of Alumina-Fiber-Reinforced Epoxy

Alumina-fiber-reinforced epoxy (alumina/epoxy) is often used in making the thermal insulating structure of superconducting magnet because it is stiff and strong and has a low thermal conductivity. The stiffness of the alumina/epoxy structure is one of the factors determining the soundness of a superconducting magnet, so it is important that the relation between the degradation in stiffness and the growth in the size of defects in alumina/epoxy is determined. A new defect-monitoring method using light transmitted through alumina/epoxy has therefore been developed. Defects inside a semitransparent alumina/epoxy laminate will scatter the light and decrease its intensity. The spectra of the transmitted and reflected light were measured using a spectrometer in order to identify the wavelengths most sensitive to defects in alumina/epoxy. With decreasing stiffness, the intensity of transmitted light decreased and that of the reflected light increased. These changes were most obvious at wavelengths from 400 to 800 nm, so it seems that measuring the intensity of transmitted visible light is a promising method for assessing the structural integrity of alumina-reinforced epoxy.

Nondestructive Evaluation Technology Using Transmitted Light through FRP.

Alimina-fiber-reinforced epoxy(alumina/epoxy)is eften used to fabricate to the thermal insulating structire of a superconducting magnet because it has low thermal conductivity and high bending stiffness and strength. The stiffsess of the alumina/epoxy structure is one of the governing factors when evaluating the soundness of a supercenducting magnet. So, to evaluate the soundness of this system, the relation between the degradation in stiffness and the growth in the size of defects in alumina/epoxy must be determined. For this purpose, a new defect-monitoring method using transmitted light through alumina/epoxy was developed. Any defect inside a semi-transparent alumina/epoxy laminate will scatter the light and decrease its intensity. The spectra of the transmitted light were measured using a spectrometer and a halogen lamp. The amplitude of the transmitted light decreased as the stiffness decreased, and the amplitude decreasing ratio was approximately constant from 400nm to 800 nm.