The behavior of cancellous bone from quasi-static to dynamic strain rates with emphasis on the intermediate regime.

Previous studies, conducted using quasi-static and dynamic compression tests, have shown that the mechanical strength of cancellous bone is strain rate dependent. However, these studies have not included the intermediate strain rate (ISR) regime (1/s to 100/s), which is important since it is representative of the loading rates at which non-fatal injuries typically occur. In this study, 127 bovine bone specimens were compressed in 3 regimes spanning 8 distinct strain rates, from 0.001/s to 600/s, using three different devices: a conventional quasi-static testing machine, a wedge-bar (WB) apparatus and a conventional split Hopkinson pressure bar (SHPB) implemented with a cone-in-tube (CiT) striker and a tandem momentum trap. Due to the large sample size, a new robust automated algorithm was developed with which the material properties, such as the apparent Young׳s modulus and the yield and ultimate values of stress and strain, were identified for each individual specimen. A statistical summary of the data is presented. Finally, this study demonstrates that results obtained at intermediate strain rates are essential for a fuller understanding of cancellous bone behavior by providing new data describing the transition between the quasi-static and dynamic regimes.

Constant strain rate compression of bovine cortical bone on the Split-Hopkinson Pressure Bar.

Cortical bone is a visco-elastic material which implies that strain rate will affect its response. Although the Split-Hopkinson Pressure Bar is an accepted technique for determining the dynamic compressive properties of cortical bone it has been shown that the strain rate of compression does not remain constant throughout the duration of a classical experiment with a uniform striker. This raises concerns as to the measurement of smeared responses. This paper presents a shaped striker technique whereby the incident pulse can be shaped to attain a constant strain rate experiment for bovine bone. Shaped strikers offer benefits such as re-usability and increased test repeatability. A comparison of the stress-strain-strain rate responses attained through classical and constant strain rate experiments shows that the shape of the stress-strain curves from conventional experiments is adversely affected in the portion where the strain rate varies. The dynamic response corridors for the two tests are similar, however the ultimate properties are affected. It is concluded that the strain rate history should be presented with dynamic stress-strain responses since the instantaneous strain rate is a likely contributor to potential constitutive models.

Hopkinson bar techniques for the intermediate strain rate testing of bovine cortical bone.

Detailed knowledge of the dynamic viscoelastic properties of bone is required to understand the mechanisms of macroscopic bone fracture in humans, and other terrestrial mammals, during impact loading events (e.g. falls, vehicle accidents, etc.). While the dynamic response of bone has been studied for several decades, high-quality data remain limited, and it is only within the last decade that techniques for conducting dynamic compression tests on bone at near-constant strain rates have been developed. Furthermore, there appears to be a lack of published bone data in the intermediate strain rate (ISR) range (i.e. 1-100 s(-1)), which represents a regime in which many dynamic bone fractures occur. In this paper, preliminary results for the dynamic compression of bovine cortical bone in the ISR regime are presented. The results are obtained using two Hopkinson-bar-related techniques, namely the conventional split Hopkinson bar arrangement incorporating a novel cone-in-tube striker design, and the recently developed wedge bar apparatus. The experimental results show a rapid transition in the strain rate sensitive behaviour of bovine cortical bone in the ISR range. Finally, a new viscoelastic model is proposed that captures the observed transition behaviour.