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A large quantity of engineering plastics are used in electric industrial products. It is necessary to estimate the life of the product made of engineering plastics because of its shorter life and lower strength than some of ceramics or metals. Polybutylene terephthalate is one of the hydrolyzed engineering plastics. Their test pieces were submitted to accelerated stress test of elevated temperature and humidity. Their strength was evaluated by three-point bending test. It was found that the strength degradation was related to water vapor pressure, temperature and retention time. It was possible to estimate the time required for strength degradation in service environment.

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