Experimental research on bond behavior of fiber-matrix subjected to monotonic and cyclic loading

The fatigue pullout behavior of steel fibers from plain ultra-high-performance concrete matrix was investigated by experiments for the first time hitherto. Specifically, the influence of two critical factors stress level S (defined as the ratio of the maximum fatigue load to the quasi-static strength L_max/P_max, low 0.7, medium 0.8 and high 0.9) and stress characteristic factor R (defined as the ratio of the minimum fatigue load to the maximum fatigue load, L_min/L_max, 0.1, 0.2 and 0.3) on fatigue pull-out properties was studied by double-sided pullout tests. The results indicated that the rate of interfacial crack growth was affected by S, while the fatigue pullout damage was decided by both S and R. Meanwhile, based on the law of stiffness coefficient and energy dissipation with fatigue load cycles increasing, the evolution law of interface under cyclic loading was studied. To enhance the interfacial fatigue resistance, a sol-gel modification method for steel fibers was innovatively proposed, which significantly increased the residual average bond strength and pullout energy by 103.36 % and 61.06 %. Furthermore, the effectiveness of modification method was verified by SEM, EDS and AFM tests. The morphology and surface roughness were modified and the product composition was verified as nano-SiO₂. The residual average bond strength and pullout energy were both significantly improved by this sol-gel steel fiber modification method. The post-cracking behavior of fiber-reinforced composites is controlled by the properties of interface bonding. To investigate how fiber-matrix interface evolves under fatigue loads is helpful to understand the failure mechanism of steel fiber reinforced composites subject to cyclic loads.