Flexural failure mechanism and damage early warning of cracked timber beams: integrating acoustic emission time-varying multifractal and DIC full-field analysis

Cracks parallel to the grain in timber components significantly reduce structural mechanical properties and increase fracture vulnerability, yet the multi-scale fracture mechanisms and quantitative early warning strategies for cracked timber beams remain not fully elucidated.This study conducted four-point bending tests on six groups of Mongolian pine beams with varying crack geometries. By integrating mechanical testing, acoustic emission (AE), digital image correlation (DIC), RA-AF classification and time-varying multifractal theory, the multi-scale fracture evolution was quantitatively characterized.Results show that relative crack depth (h/H) dominates mechanical properties and failure modes: increasing crack size reduces ultimate bearing capacity and initial flexural stiffness by up to 30% and 50%, respectively; when h/H ≥ 0.25, failure shifts from tension-dominated to grain-parallel shear-dominated fracture. RA-AF and DIC mutually verified fracture characteristics across scales. A three-level Δα-based early warning system was established: 0.40–0.60 indicates linear-elastic stability; a 116.2% increase signals early damage propagation; a 308% increase corresponds to critical instability. Time-varying multifractal parameters enable full-process damage quantification. Shear-dominated specimens showed 21.8% higher mean Δα and 18.5% higher Δf(α) than tension-dominated ones, revealing greater shear damage heterogeneity.The proposed multi-scale framework provides a reliable quantitative basis for health monitoring and safety assessment of defective timber components.