Time-dependent behaviour of steel/CFRP double strap joints subjected to combined thermal and mechanical loading

Degradation of structural adhesives at elevated temperatures makes the time-dependent behaviour of adhesively-bonded steel/CFRP joints a critical issue for safety considerations of CFRP strengthened steel structures. This paper reports the examination of specimens at different load levels (i.e. 80%, 50%, and 20% of their ultimate load measured at room temperature) and constant temperatures from 35 °C to 50 °C (i.e. temperatures below and above the glass transition temperature Tg, 42 °C of the adhesive). Furthermore, a scenario of cyclic thermal loading between 20 °C and 50 °C was included to represent more realistic exposure. Joint time-dependent behaviour was demonstrated by the stiffness and strength degradation as a function of not only temperature but also time. At the same temperature level close to or above Tg, a higher load level corresponded to a shorter time-to-failure. In addition, up to 47% of strength recovery was found for the specimens subjected to cyclic temperatures compared with those under constant 50 °C which failed at the same load level. Based on the proposed temperature and time-dependent material property models, the time-dependent failure time of steel/CFRP double strap joints was well described and validated by the experimental results.