Vinylidene fluoride [VDF] oligomer is a novel substance with a smaller number of VDF units and lower molecular weight than poly(vinylidene fluoride) [PVDF]. We previously reported that the piezoelectric coefficient of VDF oligomer was greater than that of vinylidene fluoride/trifluoroethylene copolymer [P(VDF/TrFE)], and that a film composed of VDF oligomer was thin and uniform. However, the piezoelectric coefficients measured with our experimental setup using pneumatic pressure were very large. In the present study, we investigated the effects of the pyroelectricity and the substrate constraints on the piezoelectric effects of VDF oligomer by means of both experimental analyses and mathematical simulations. First, we showed that the output charge of the pyroelectric effect is large, according to our measurements. Second, we showed using the finite element method that the piezoelectric effects in the thickness direction of the film are weakened when the film is constrained by the substrate. We then estimated the piezoelectric coefficient of VDF oligomer and P(VDF/TrFE). The estimated value for P(VDF/TrFE) is similar to that reported in other previous studies.

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At a tee junction point in a piping system, hot and cold water are mixed with each other in a whirl. The vibrating mixing boundary between the hot and cold water causes a temperature fluctuation on an inside surface of the pipe just after the connection point. The temperature fluctuation yields a cyclic thermal stress near the pipe surface and results in the crack initiation. In this study the thermal fatigue tests using disk-like specimens in simulated BWR environment were performed and the thermal fatigue crack initiation strength was compared with the mechanical fatigue strength. Furthermore, the thermal fatigue crack arrest depth was analyzed using linear elastic fracture mechanics. The arrest depth was found to be in proportion to the reciprocal root of the frequency of temperature fluctuation, and the experimental results agreed well with the analysis

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