Effect of loading rate on mode I/mode II/mixed mode I-II fracture performance of engineered cementitious composites

In this paper, the influence of loading rate on crack initiation and propagation of engineered cementitious composites (ECC) is studied, which is conducive to evaluate the applicability of ECC when the structure is subjected to seismic load for improving the structure safety. However, most of the researches focus on the mode I fracture under static load and fatigue load till now, and there are few researches on the influence of loading rate on the mode I and mixed mode I-II fracture properties. Therefore this paper uses three-point bending beams and four-point shear beams to study the effects of five loading rates (0.0004 mm/s, 0.001 mm/s, 0.01 mm/s, 0.1 mm/ s, 1 mm/s) on mode I, mode II and mixed mode I-II fracture of ECC respectively. The loading rate from quasi -static to seismic level is adopted, which provides a theory basis for the practical application of ECC in earthquake engineering. Four-point shear beams have four kinds of offset crack positions (0 mm, 25 mm, 40 mm, and 60 mm) under each loading rate. The results show that multiple cracking occurs in three-point bending and four -point shear beams under different loading rates, and there is an "inclined fracture process zone" in the cracks in the four-point shear beams. With the increase of loading rate, the peak load, initial cracking load and Mode I initial fracture toughness KiniIC all increase, while the dissipated energy UI and fracture energy GI of mode I decrease, which indicates that the toughness of ECC decreases as the loading rate increases. In addition, Mode II initial fracture toughness KiniII increases, while the dissipated energy UII and UCOD of mode II and mixed mode I-II decreases as the loading rate increases. The sensitivity of loading rate of mode II and mixed mode I-II dissipated energy is greater than that of mode I, while there are no specific law of mode II fracture energy GII and mixed mode I-II fracture energy GI-II with loading rate, thus, dissipated energy is more suitable for evaluating the toughness of ECC.