Mode‐II Contribution Under Nominal Mode‐I Loading in Cementitious Material–Rock Interface TPB Tests

Material mismatch and loading‐point deviations can induce Mode‐II contributions in nominal Mode‐I three‐point bending (TPB) tests of cementitious–rock interfaces. Finite element modeling using the interaction–integral method quantified the Mode‐II‐to‐Mode‐I stress intensity factor (SIF) ratio. The ratio was governed primarily by the elastic‐modulus ratio and modified by Poisson's ratio pairing, reaching 15.0% under centered loading at an elastic‐modulus ratio of 3.0. Load shifts toward the more compliant material increased the ratio, whereas shifts toward the stiffer material reduced it. The ratio varied nonmonotonically with initial crack‐height, peaking at an initial crack‐height ratio of approximately 0.5. Contour maps integrating material mismatch, loading‐point deviation, and initial crack‐height ratio identified initial crack‐height ratios of 0.2–0.3 and slight load shifts toward the stiffer material as preferable for reducing Mode‐II contributions. These findings support assessing crack‐tip mode mixity in nominal Mode‐I TPB tests of concrete–granite, concrete–marble, and sandstone–concrete interfaces.