<p>Bending fracture is a critical failure mechanism in many engineering structures such as rock beams, slabs and bridge components. This is particularly the case when prefabricated cracks are present. In order to accurately simulate such failure processes, a method is required that can capture both continuous deformation and the transition to discontinuous fracturing. The hybrid finite-discrete element method (HFDEM) is particularly well-suited for this purpose, as it integrates the advantages of continuum-based finite element methods with those of discontinuum-based discrete element methods. However, conventional HFDEM approaches are computationally intensive and often impractical for large-scale problems. This paper introduces a general-purpose graphic processing unit-parallelized HFDEM that is general-purpose and achieves execution speeds exceeding 128 times those of sequential code. The model is employed to simulate the bending fracture processes of sandstone specimens with prefabricated cracks under varying test configurations, revealing the mechanism by which loading rate influences fracture mechanics behavior. The results demonstrate good agreement with existing research findings, thereby validating the model. The core contribution, however, lies in revealing the intrinsic mechanisms behind the strongly rate-dependent fracture behavior. The simulations elucidate how higher loading rates weaken the guiding effect of prefabricated cracks and promote a transition to complex mixed-mode fracture, thereby establishing that the apparent toughness increase is primarily driven by inertial effects and exhibits fundamental divergence between different fracture modes.</p>