Thermal evaluation of the mean fatigue limit of a complex structure

摘要

The study deals with the long-term reliability of a high precision pressure sensor using bellows mainly made of electroplated Ni. Bellows are expected to stay in service for many decades. Their high cycle fatigue behavior has to be known to assess the probability of airtightness loss. A specific high cycle fatigue setup, put in a resonant machine that is displacement-controlled, has been designed. An infrared thermographic technique is used to determine the mean fatigue limit of bellows. Increases in the mean temperature of the bellows with the displacement range are monitored. Several authors empirically relate the mean fatigue limit of a flat specimen to a rapid temperature change. A similar analysis is performed in the present case by using the bellows mean temperature. Finite element computations allow us to estimate a mean fatigue stress threshold for electroplated Ni. This result is compared with those obtained mechanically in a Woehler diagram.