On FPZ for mixed mode I-II fracture in concrete after sustained loading using DIC technique

This paper investigates the fracture process zone (FPZ) of mixed mode I-II fracture in concrete after sustained loading, employing the digital image correlation (DIC) technique.Concrete specimens were subjected to sustained three-point bending (TPB) and four-point shearing (FPS) loading for 90 days, with the sustained load level at 80% of the initial cracking load.Afterward, the specimens were unloaded and tested under static loading until failure.The DIC technique was employed to monitor the fracture process of concrete beams after sustained loading.The crack mouth opening displacements obtained from the DIC technique were consistent with those measured using clip gauges, validating the accuracy of the DIC technique.The crack opening displacement field derived from DIC technique was used to determine the size and shape of the FPZ.The lengths of FPZ (lFPZ) for all specimens were measured at peak load.The results indicated that as the ratio of mode II to mode I stress intensity factor (KII/KI) increased, the lFPZ increased.The lFPZ increase in became more pronounced with higher KII/KI ratios at peak load.The lFPZ of creep specimens were shorter than those of aging specimens.The increase in lFPZ for creep specimens was less than that for aging specimens.The findings indicated that that the crack propagation resistance improved with a higher mode II component in mixed mode I-II fracture.Additionally, the brittleness of the concrete increased after sustained loading. INTRODUCTIONConcrete gravity dams play a crucial role in flood control, environmental protection, and power generation.However, factors such as construction methods, maintenance conditions, and hydrostatic pressure can result in cracks over time.Due to the random orientation of inherent flaws and discontinuities in concrete, combined with the complexity of loading