Study on stress relaxation behavior of steel strands and its induced strengthening effect on strength properties

The long-term service performance of steel strands in bridge structures is crucial to ensure the durability of the structure. Under long-term loads, steel strands are prone to stress relaxation, but the mechanical behavior and microstructural evolution laws are still unclear. This study systematically analyzes the changes in strength properties of steel strands after stress relaxation at different initial stress levels. First, stress relaxation experiments at four initial stress levels were conducted to quantify the stress attenuation characteristics of steel strands over time and predict the long-term relaxation rate. Subsequently, tensile tests were performed to evaluate the changes in strength properties of the steel strands after relaxation. Transmission electron microscopy (TEM) was used to observe the microstructural evolution of the steel strands after relaxation, and the contributions of dislocation density and interlamellar spacing changes to strength were analyzed in combination with Taylor hardening theory and Hall-Petch relationship. The results show that the stress relaxation rate of the steel strand increases nonlinearly with the increase of the initial stress level, and shows a certain strengthening effect after relaxation, with the yield strength and ultimate tensile strength increasing by 3.7 % and 2.3 %, respectively. During the relaxation process, the dislocation density increases significantly (from 10 ¹⁴ m⁻² to 10 ¹⁵ m⁻²), the cementite lamellae are significantly bent and broken, and the interlamellar spacing increases. At moderate stress levels, dislocation accumulation mainly drives the strength improvement, while at higher stress levels, the destruction of the lamella structure weakens the grain boundary strengthening effect, limiting further strength improvement. The results of this study can provide important basis for the practical design of steel strands and the evaluation of stress relaxation behavior under long-term service conditions. • The stress relaxation behavior of high-strength steel strands at four initial stress levels for 120 h was studied. • The relaxation rate increases nonlinearly with the initial stress, and recommended values for 50 years are provided. • Relaxation enhances yield and tensile strength via dislocation strengthening and cementite lamellae fragmentation.