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Various prediction methods have been proposed for evaluating the fatigue life of welded joints by combining finite element analysis (FEA) with an experimental database. However, to obtain more universal and accurate fatigue life predictions, it is necessary to have criteria for making integrated evaluations of the fatigue strength of welded joints. This paper presents a study that focuses on the local cyclic plastic zone size (ω*) as the criterion of fatigue strength and investigates its validity. The definition of ω* was given by the relationship between the stress state at the notch tip and the elastic strain which was defined along the strain-life fatigue curve (ε - N diagram) of a base metal. As a result of using ω*, it was found that an integrated fatigue life prediction was possible to a certain extent for notch and arc-welded joint specimens.

To improve performance of motorcycle 4 cycle 250 cc engine, we examined the following. (1) We improved crank balance so that friction loss of crank shaft was reduced. (2) We reduced air intake resistance and improved exhaust system so that volumetric efficiency was increased. (3) We reduced weight of valve system parts so that maximum RPM was increased. As these improvement, we have achieved 33.1 kw/15000 RPM and torque increase in low and middle RPM range for motorcycle 250 cc engine on 1989 year model.