Dynamic Mechanical Properties and Fracture Mechanism of Cortical Bone Under Impact Loading

Cortical bone, as the core load-bearing component of the skeletal systems of humans and many other vertebrates, is frequently exposed to fracture risks under impact loading during daily activities. Existing research has predominantly focused on fracture behavior under quasi-static loading conditions, yet the influence of cortical bone’s notable strain rate sensitivity on its fracture mechanisms remains inadequately explored. Using a universal testing machine and a split Hopkinson pressure bar(SHPB), uniaxial compression tests were conducted on chicken femoral cortical bone under quasi-static (0.001 s-1) and dynamic (350 s-1 and 700 s-1) conditions. Chicken femoral cortical bone specimens were obtained via cutting and grinding, with high-contrast speckle patterns prepared on them. Images of cortical bone during the experiment were recorded synchronously with two cameras, with the acquisition frequency set to 5 Hz for the quasi-static test and 150 kHz for the dynamic test. Full-field strain distributions were obtained using digital image correlation (DIC), and fracture micro-morphology was observed via scanning electron microscopy (SEM) to analyze the effect of strain rate on fracture behavior. The experimental results show that the failure stress of cortical bone exhibits a non-monotonic trend with increasing strain rate, with average values of 99.6 MPa, 195.5 MPa and 171.3 MPa at 0.001 s-1, 350 s-1 and 700 s-1, respectively, demonstrating an “increase-then-decrease” tendency. A similar trend was observed for fracture strain. Under quasi-static loading, the macroscopic crack propagation path aligns with the maximum shear strain region, and the fracture surface exhibits typical shear failure characteristics (layered delamination and abundant debris), indicating shear strain as the primary fracture driver. In contrast, under dynamic loading, the macroscopic crack correlates more closely with the maximum tensile strain region, and the fracture surface appears smooth, characteristic of tensile fracture morphology, confirming tensile strain as the dominant mechanism. Strain rate exerts a significant influence on the fracture mechanism by regulating the fracture-inducing factor, which may account for one of the key reasons for the variation in fracture stress of cortical bone under dynamic conditions. This study reveals, from a macro-micro correlated perspective, the shift in fracture mechanisms of cortical bone from shear dominance to tensile dominance with increasing strain rate, which provides an experimental evidence for understanding the fracture mechanism of bone under impact loading and reference for the design of bone biomimetic materials and the development of orthopedic protective equipment.