The poly(butylene succinate) (PBS) biodegradable composites reinforced with short coir fibres were fabricated by compression molding method. Effect of different fibre contents varying from 10 to 50 wt% on the mechanical and thermal properties of coir/PBS composites have been studied in terms of tensile and flexural properties, thermal stability, thermal expansion, dynamic mechanical properties, and microscopic observations. Tensile and flexural moduli of the composites were improved with the increase of fibre content up to 50 wt%. The results of thermogravimetric analysis showed that thermal stability of coir/PBS biodegradable composites is remarkably to be intermediate between the PBS resin and coir fibre depending on the fibre content. The thermomechanical stability of PBS resin is significantly improved by the addition of reinforcing coir fibres into the composite matrix. Dynamic mechanical analysis results also confirmed that the storage modulus of the composites was increased with increase in the loading of coir fibre, showing the stress transfers from the matrix to the fibre. The greatest value of storage modulus and lowest coefficient of thermal expansion obtained at 50 wt% fibre content in the present study. The fracture surface morphology of the composite specimens observed by scanning electron microscope (SEM) showed the weak bonding between untreated coir fibre and PBS matrix.

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In order to investigate the mechanism for the intergranular fatigue crack propagation of a low carbon steel in a low pressure hydrogen gas environment, two kinds of examinations were carried out. One examined the effect of the cyclic pre-strain on the crack growth behavior. The other was the in-situ observation of intergranular fatigue crack propagation in a hydrogen gas environment. The main results are as follows. (1) SEM observation of the specimen surface morphology of a plain specimen fatigued in hydrogen gas showed that a gap at the grain boundary was induced by slip behavior, but not in nitrogen gas. (2) A specimen cyclic-prestrained in hydrogen gas showed slight influences on the increase in the crack propagation rate. (3) Mating intergranular facets on the fatigue fracture surfaces showed the matching of a striation-like pattern in the manner anticipated from the damage mechanism of (1). (4) The environment-change test from hydrogen to nitrogen during the fatigue test showed that intergranular facets appeared even in nitrogen. Results (3) and (4) suggests that the damaging process of (1) is valid in an actual fatigue crack. (5) In-situ observation of the crack propagation behavior in hydrogen gas showed that a crack propagated faster along the grain boundary. However, no visible discontinuous crack advances appeared as a large number of load repetitions was required. Phenomena considered to be a damage process (1) appeared ahead of a crack tip. Based on these results, a convincing mechanism for the intergranular fatigue crack propagation process is as follows. Grain boundaries just ahead of a crack tip are damaged due to the large number of hydrogen-enhanced slip repetitions, therefore, the fatigue crack becomes easier to propagate along the grain boundary in hydrogen.

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In order to investigate the hydrogen gas effect on the fatigue limits of austenitic stainless steels, SUS304 and SUS316L, with small twin blind holes, metal fatigue tests were carried out in low pressure hydrogen gas, in air and in inert gases; i.e., argon or nitrogen. The fatigue limit of SUS304 is defined as the crack propagation limit not only in air, but also in hydrogen and nitrogen. The fatigue limit of SUS304 in hydrogen gas is slightly higher than that in air. The extremely low Fatigue Crack Growth Rates (FCGR) of not only SUS304, but also SUS316L in hydrogen gas is lower than that in argon gas in the FCGR region of less than 10^-9m/cycle. This seems to be due to the plasticity-induced crack closure, and supports the higher fatigue limit in hydrogen.

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In this study, in order to calculate the dangerous volume under various stress distributions, the dangerous volume quantification method is proposed. Also in order to predict the stress distribution of the spray coating, an easy calculation method of the residual stress distribution based on the first report was proposed. The calculated dangerous volume in the alumina spray coating on titanium layer is different from an alumina spray coating single layer. The difference is based on the stress distribution in the alumina spray coating. The fracture strengths predicted by the proposed method and the third report are comparable to the experimental results obtained by 4-point bending tests. Therefore, the dangerous volume calculation for the thermal stress distribution that is different from the mechanical stress distribution also becomes possible.

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