ABSTRACT The fracture behavior of concrete was studied by using four‐point bending (4PB) tests. Analytical expressions for the crack extension resistance ( K R ) of 4PB beam were formulated. Experiments were conducted on 4PB specimens with initial crack–depth ratios of 0.2, 0.3, 0.4, 0.5, and 0.6. The crack growth during loading was monitored using the digital image correlation (DIC) technique. Strain gauges were employed to capture the initial cracking load. The evaluations of fracture process zone (FPZ), K R ‐curves, and crack tip opening displacement ( CTOD ) during the whole fracture process were analyzed. The results indicated that with the crack–depth ratio increasing from 0.2 to 0.6, the initial load and maximum load decreased by 57.2% and 63.9%, respectively, while the maximum FPZ length decreased by 44.6%. The K R ‐curves started from the initial crack toughness, which showed a slight fluctuation for each group, and increased with the effective crack extension. Coupling with the DIC results, the horizontal opening displacement decreased as the section moved upward, and increasing the span–depth ratio resulted in a decrease of the extended crack length and CTOD . The measured CTOD values and crack extension length showed good consistency with the calculated ones.
Yin et al. (2026) studied this question.