Effects of Temperature on the Adhesive and the CFRP–Steel Interface in Strengthened Structures

The bonding interface serves as a vulnerable point in steel structures strengthened with carbon fiber–reinforced polymer (CFRP), significantly influenced by the mechanical properties of adhesives and temperature. To understand the mechanisms through which temperature changes affect the properties of adhesive materials and the CFRP–steel interface, 70 adhesive tensile specimens and 28 CFRP–steel double-lap joint specimens were prepared based on the self-developed high-performance adhesive G3 and the typical commercial adhesive Sika30. Tests were conducted at seven different temperatures (−20°C, −5°C, 10°C, 25°C, 40°C, 55°C, and 70°C). The results indicate that compared with 25°C, an increase in temperature leads to a decrease in adhesive strength, whereas the ultimate bearing capacity of CFRP–steel double-lap joints increases when the temperature is below a certain value but still lower than the glass transition temperature (Tg) of the adhesive. However, a rapid decline in both adhesive and CFRP–steel joint performance occurs when the temperature approaches or exceeds the Tg of the adhesive. Decreasing the temperature results in a reduction in the ultimate bearing capacity of CFRP–steel double-lap joints. Therefore, when reinforcing with adhesive-bonded CFRP, it is essential to consider the adverse effects not only of adhesive softening at high temperatures but also of embrittlement in low-temperature strengthening systems.