Comparative study on modes I and II fracture characteristics of bituminous coal using asymmetric semi-circular bend specimen

•The differences in the modes I and II fracture characteristics of coal was studied by using asymmetric semi-circular bend specimen.•The notch length has a significant impact on peak load and fracture toughness, but it has a slight influence on crack propagation path.•A method based on acoustic emission parameters is proposed to determine the real-time microscopic failure mechanism of specimens.•Coal specimens experience tensile fracture (mode I) initiation under both mode I and mode II loading.