Characterization of ultrasonically assisted orthogonal cutting of bone using digital image correlation analysis

Bone cutting with high performance material removal is critical for enhancing orthopedic surgery. Ultrasonically assisted cutting (UAC) is an advanced process with the potential to improve the material removal. However, strain and other intermediate variables in bone cutting are difficult to obtain because of the lack of suitable measurement methods, especially for high-frequency vibration-assisted cutting. In this study, digital image correlation (DIC) analysis was applied for the first time to investigate the mechanism of crack development during conventional cutting (CC) and ultrasonically assisted cutting of cortical bone. A novel method for calculating cutting and thrust forces under the mixed fracture mode of bone was also proposed. Extensive experimental results showed that the average strain and strain rate of cortical bone decreased after the application of UAC, but the maximum transient strain rate in UAC was greater than that in CC, and the crack-affected area and shear band width in UAC were smaller than those in CC. In addition, the strain parameters obtained by the DIC analysis were used to calculate the cutting and thrust forces in the hybrid fracture mode. The calculated values of forces matched well with the measured results, indicating the strong feasibility of DIC applications in orthogonal bone cutting research. This study has significant theoretical and practical value since it reveals the fracture mechanism of cortical bone in UAC, demonstrates a non-contact full-field measurement method for tissue strain calculation, and provides inspiration for optimizing the design of innovative orthopedic instruments.