Nobuo Otani

Fatigue life prediction of composites under two-stage loading

In order to predict the fatigue life under two-stage loading by formulating a cumulative fatigue damage rule for composite materials, the fatigue process in SiC–Al and glass-fibre reinforced plastics were investigated. A microcrack occurred within the composites which resulted in cumulative fatigue damage that increased linearly with the number of cycles. The mechanical conditions of damage growth and failure were determined by characterizing the microdamage governing the fatigue. The ultimate failure is shown to occur when the product of the stress amplitude ratio and microdamage density is beyond a critical value and an expression for the remaining fatigue life is derived.

Approximate estimation of fatigue strength of polymer matrix composites by material properties

To estimate the fatigue strength (life) of polymer matrix composites (PMC) by material properties, the fatigue process of satin woven carbon fabric laminate composites with two different matrix resins (CF/epoxy, CF/PEEK) was investigated from a microscopic viewpoint and the failure mechanism was discussed in relation to the local stress states and fatigue strength. It was found that fatigue strength can be approximately estimated by the function of material properties, κ. Moreover, it was revealed that the main fatigue damage changes from transverse crack in transverse fiber bundle to fiber fracture in longitudinal fiber bundle in dependence on the value of κ.

Fatigue life prediction of cross-ply composite laminates

Abstract To predict the fatigue life of fiber reinforced composites, fatigue process of CFRP laminates of [0°/90°] s is investigated and the influence of damages occurring at fiber, matrix and fiber/matrix interface on the various critical strengths and the relationship between residual critical strength and failure are discussed. As a result, it was shown that fatigue strength (i.e. fatigue life) consisted of residual critical strength and stresses occurring at each layer (0° and 90° layers) and interlayer. Moreover, the fatigue failure occurred because the residual critical strength of each layer and interlayer decreased with dependence of their microdamage densities, so that the fatigue life can be predicted by evaluating microdamage behavior in fatigue process.

Experimental Characterization of Matrix Cracking Behavior in Thermally Cycled CFRP Laminates

The effect of thermal cycling on the mechanical properties of composite materials is an important issue in engineering, especially in their applications to the space environment. The present study concerned with the experimental study of both the thermal cycling induced matrix cracking and the effect of thermal cycling on the matrix cracking behavior under tensile loading in CFRP laminates. Two kinds of carbon/epoxy systems, T800H/3631 and T300/2500, are used for the laminate configurations of (0/90)s and (90/0)s. The specimens are thermally cycled between −196 and 100°C. Thermal cycling tests are performed up to 1000 cycles. The polished edge surfaces of specimens are examined by the replica technique, and then the matrix crack density is measured as a function of the number of thermal cycles. It is found that the first matrix cracking in (0/90)s and (90/0)s laminates occurs at almost the same numbers of thermal cycles. It is also found that the matrix crack density increases more rapidly in (0/90)s laminates than in (90/0)s laminates in both material systems. To investigate the effect of thermal cycling on matrix cracking behavior under tensile loading, a series of tensile tests on thermally cycled specimens are performed. The effect of thermal cycling on matrix cracking under tension is evaluated in terms of the change in the critical energy release rate and the critical stress for matrix cracking.

Fatigue Damage Evaluation of PMC by Time Series Model

To detect quantitatively transverse cracks, the new technique was developed from the viewpoint of the time series model. The transfer function of PMC (polymer matrix composites) was determined using the mechanical model and the time series model. The response of the system depending on the internal damage was examined from the change of the time series signal passing through PMC. Moreover, the relationships among ARMA (auto regressive moving average) parameter, transverse crack density and mechanical constants were discussed. As a result, it was found that the increase of transverse cracks can be evaluated by using AR (auto regressive) parameter. It is also suggested that this detecting technique is applicable to the health-monitoring.

Study of electrical properties of meridian on human body surface

This paper presents the study of the subcutaneous electrical impedance on the human body surface. Measurements of the electrical impedance on five adult male subjects were carried out and analyzed for the possible detection of the acupuncture meridian lines of ancient Chinese medicine on the human body. The distribution of electrical impedance measured at 40 points over the volar side of the right upper limb of the subjects. The results show that electrical impedance varies at different locations of the human body surface, and the locations with lower electrical impedance coincide with the locations where the meridian is believed to exist.