With several attractive properties, nitrile butadiene rubber O-rings have been widely used as compression seal components in various critical engineering systems. Therefore, it is important to obtain an accurate lifetime estimation of the nitrile butadiene rubber O-ring. In this study, aiming to estimate the lifetime of a nitrile butadiene rubber O-ring under storage conditions, a reliability model is developed by considering two performance characters, namely, compression set and compressive stress relaxation. With a long lifetime in storage, a constant-stress accelerated degradation testing is conducted to obtain the degradation model for each performance character. In addition, the statistical dependency between the two performance characters is considered, and a bivariate time-varying copula model is used to capture this relationship. An efficient two-stage approach is proposed to construct a suitable time-varying copula function. By comparing with previously reported time-varying copulas, it can be proved that the time-varying copula constructed using the proposed approach achieves a better goodness of fit.
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