Influence of seismic strain stress on evolution law of microcracks in concrete TPB tests using AE technology

Since the fracture properties and failure modes of concrete are deeply rate-dependent, changes in crack resistance and failure mechanism of concrete, under seismic strain rates, can provide critical insights for structural safety withstanding seismic load. In this study, the dynamic fracture characteristics and micro-mechanisms of concrete under a wide of strain rates range from the static to the seismic (10^- 6s^- 1~ 10^-2s^- 1) were explored using three-point bending (TPB) tests combined with Acoustic Emission (AE) monitoring. The results reveal a significant positive correlation between strain rate and mechanical performance. As the strain rate increases, the peak load increases by up to 37.1%, and the fracture energy rises by up to 36.7%, demonstrating a distinct pseudo-strengthening effect. Microscopically, the failure mechanism transitions from ductile interfacial cracking, where cracks deflect along the interface transition zone (ITZ), to brittle transgranular cracking, where aggregates are fractured directly. AE analysis further indicates a shift in the dominant fracture mode from Mode I (tensile) to Mode II (shear), with the proportion of shear cracks increasing from 16.2% to 53.8%. In addition, the spatial distribution of AE events becomes highly concentrated near the pre-crack tip, signifying a transition to brittle failure. These findings provide critical insights into the dynamic fracture mechanisms of quasi-brittle materials, highlighting the inherent trade-off between fracture strength enhancement and ductility reduction under rapid loading conditions, which is essential for seismic engineering and structural safety assessments.