A preliminary investigation of the dynamic viscoelastic relaxation of bovine cortical bone

T.J.C Loete,G Paul,E.B Ismail,E Cadoni

doi:10.1051/epjconf/20159403004

Abstract

A new experimental approach is proposed to characterize the dynamic viscoelastic relaxation behaviour of cortical bone. Theoretical models are presented to show that a linear viscoelastic material, when allowed to relax between two long elastic bars, will produce stress, strain and strain rate histories that contain characteristic features. Furthermore, typical experimental results are presented to show that these characteristic features are observed during split Hopkinson bar tests on bovine cortical bone using a Cone-in-Tube striker. The interpretation of this behaviour in the context of a standard linear viscoelastic model is discussed.

Highlights

The design of protective structures to prevent injury during impact loading events, such as sporting accidents and vehicle collisions, requires a detailed understanding of the mechanisms of dynamic bone deformation and fracture
Note that after the passage of the reflected wave the stress signals in both Hopkinson bars show the same decay behaviour. This is due to the specimen exerting equal but opposite forces on the bar faces during the unloading phase of the test and shows that the specimen is in quasi-equilibrium
There was less variation in the main portion of the loading curves where all five experiments resulted in gradients of 15.2 ± 0.7 GPa. This implies that the latter part of the loading curve represents the actual mechanical properties of the bone specimen, while the initial gradient represent transient “settling” behaviour as the specimen experiences slight adjustments until it is in full contact with the Hopkinson bar faces

Summary

Introduction

The design of protective structures to prevent injury during impact loading events, such as sporting accidents and vehicle collisions, requires a detailed understanding of the mechanisms of dynamic bone deformation and fracture. Traditional approaches to quantify responses of the human body to impacts have typically included full scale experimental testing involving the use of a post mortem human subject (PMHS) [14] Such tests are expensive and time consuming which has lead to the increasing use of numerical modelling [6], for which detailed material models of bone are required. Bone is a complex composite material composed of mineral crystals (hydroxyapatite) embedded in an organic matrix (primarily collagen fibres). This combination of organic and mineral components gives rise to the viscoelastic quasi-brittle behaviour of bone, i.e. bone displays highly rate sensitive viscoelastic behaviour but fails via progressive brittle fracture. Additional viscous contributions may arise during dynamic loading events due to the fluid component contained within the bone cavities

Results

Discussion

Conclusion