A DISCUSSION ON THE RELATIONSHIP BETWEEN THE IMPACT TOUGHNESS AND THE FRACTURE TOUGHNESS

The impact value (AK) measured with classic impact experiment of mechanical properties is a normal index from the five important basic mechanical properties for engineering, but it still has no definite physical meaning. By use of the advanced impact experimental machine, the load-deflection signals under impact loading have been measured. According to the load-deflection curve of 30CrMnSiA steel obtained by this method, the AK value could be divided into the elastic deformation work, the plastic deformation work, the crack forming work, the work of crack propagation and the work of unstable crack propagation. Therefore, we can obtain the expression for the continuous changing of stress and strain under impact load. In this experiment, specimens with three different breaches, U type, V type and linear grooving, are used and the dynamic fracture toughness value (KDIC) of 30CrMnSiA steel is treated and calculated which follows the measuring method of material static fracture toughness, as ASTM E399. The experimental results verify that these impact specimens with three different breaches are satisfactory for plane strain relation under impact load. For high strength 30CrMnSiA steel, the maximum load of load-deflection curve under impact load can represent the unstable propagation load of crack. The precision of the calculated fracture toughness (KDIC) meets the engineering requirement of materials.

Impact of the fracture on low-order guided waves in long bone

Using ultrasonic guided waves to assess long bone fractures and fracture healing has become a promising diagnostic issue. But the multimode overlap of the guided waves challenges the quantitative evaluation and clinical application. In the preformed study, in order to simplify the signal interpretation, the low-frequency sinusoidial signals were used to only excite SO and A0 modes in fractured long bones. The amplitudes of SO and A0 modes were numerically analyzed with variation in crack width and fracture angle. Numerical simulation, based on the two-dimension finite-difference time-domain （2D-FDTD） reveals that both SO and A0 amplitudes decrease with the fracture widening. However, the increase in fracture angle gradually enhances the A0 amplitude, while with respect to the SO mode, its amplitude shows a non-monotonic trend to the variation in fracture angle with a turning point around 45°. The amplitude ratio between S0 and A0 can reflect the variations in crack width and fracture angles. The simulation illustrates that ultrasonic guided SO and A0 modes are sensitive to the degree of both vertical and oblique fractures in the long cortical bone. These findings may be helpful for fractures diagnosis and healing evaluation of the long bone.