Year: 1993
Type: dissertation
Abstract: Many aspects of bone's tensile loading response are well documented.However, the processes that cause this response are poorly understood.The nomenclature commonly used to describe this response includes the terms elastic, plastic, yield and Young's modulus.This nomenclature implies that bone is considered to be an elastic.plastic material, and that the knee in the loading curve is due to yielding.In many published studies of its time-dependent mechanical behaviour bone is viewed as a viscoelastic material.However, a few workers have attributed the creep and failure of bone to another process: damage accumulation.This thesis extends this latter approach.To gain a better understanding of bone's mechanical behaviour and the associated processes, a comparative study has been conducted.Two bones with different mechanical responses were used: bovine bone (mostly femoral) and antler (from red deer and reindeer).Only tentative suggestions of the cause of this difference are found in the literature.The results of impact, tension, creep and notch sensitivity tests on these materials are described.In some cases the rate-dependence of these results is examined.During many of these tests optical changes that are indicative of damage were observed and recorded.It is concluded that damage accumulation is the main controlling process of the mechanical behaviour for both materials.Anelasticity and fracture also contribute to the mechanical response.The relative importance of these three processes depends on a number of factors, those considered are: the material in question, the aspect of the response examined, and the loading rate.The difference between the mechanical response of bovine bone and that of antler is attributed to the different rates at which these materials accumulate damage.It is concluded that bone fails by a damage related fracture -process, and antler by a damage coalescence process.2 That the Theory of Physic has been much improved, and many truths discovered by the industry of the Moderns, of which the Ancients were intirely ignorant, is a position nobody, I believe, will dispute.But that a great many more relating to every branch of this useful Art lie still buried in obscurity, is equally certain.And what is yet more to be lamented than real ignorance itself, is, false maxims and positions are many times introduced as facts, and assumed as Data, whereon to build future inquiries.Anatomy itself, tho' it has been more cultivated, and received greater improvements, than any other branch, as lying most obvious to our senses, is not free from this error, no not even the largest vessels; what then shall we think of such as are more remote?The fatal spring from whence most of the errors in the Theoretical part of physic have flowed, is, the deducing general conclusions from a few particular Experiments, and on this foundation erecting some pompous plausible Hypothesis, which like Descartes's Vortices may indeed shew how the one would have framed the Universe, and the other the Animal body; but give us very little light into the real structure of either.This can only be obtained by a laborious search, and the strictest examination imaginable, and that too by a vast variety of Experiments; as any one may be convinced by the few methods of trial here made use of, wherein only the different Densities, Strengths and Thicknesses of some few of the Solids are considered; but much more so, if he will give himself the trouble of such a variety of Experiments, as are requisite fully to illustrate any branch of this complicated Machine, or thoroughly explain any one particular Secretion; wherein it is impossible to advance one step part 4, summary 4.3.7.21.279 Closing remarks on the general correlation approaches suggested by Conway (1967) as applied to data for antler and bovine bone S 4.3.8.281 Creep-rupture results: The damage approach 4.3.8.1.281 Kachanov's 'rupture by an idealised ductile process' 4.3.8.2.283 Kachanov's 'rupture by an idealised brittle process' 4.3.8.3.285 Kachanov's 'rupture by an combined idealised brittle and ductile process' 4.3.8.4.285 Odqvist's correction to Kachanov's approach 4.3.8.5.286 The NTDF model approach: part 1 normalised by bending stiffness 4.3.8.6.288 The NTDF model approach: part 2 normalised by tensile stiffness 4.3.8.7.290 The NTDF model approach: part 3 instantaneous strain 4.3.8.8.291 The NTDF model approach: part 4 steady state instantaneous strain 4.3.8.9.293 The NTDF model approach, part 5: Summary 4.3.8.10.293 Summary of the damage approach 4.3.9.294 Creep-rupture results: modelling the creep curve using constitutive damage equations 4.3.9.1.308 Closings remarks on the modelling the creep curve using constitutive damage equations 4.3.10.309 Creep-rupture results: closing remarks and conclusions 4.4.311 Closing remarks on the time-dependent properties of bone and antler.5.The next plateau is due to the work of Burstein et al. (1972).Until the publication of their paper the mechanical response of bone was viewed as being that of a brittle material.The tensile load-deformation plots obtained were essentially a straight line up to the point of failure.(Although this is generally true there are exceptions, such as the work of Sedlin (1965), reproduced here as figure 3.008.) In their 1972 paper entitled The Ultimate Properties of Bone Tissue: The effects of Yielding, Burstein, Currey, Frankel and Reilly report a different shaped loading curve.After the initial straight region the curve bends over, a greater amount of elongation is then obtained per increment in load.The authors report, that 'there was more than twice as much deformation produced after the initial yielding as there was in the elastic zone'.They also say 'our findings showing the large amount of plastic strain in bone tissue are probably the result of keeping the bone surface wet during testing'.The nomenclature these authors used; elastic, yield and plastic have remained within the literature.Burstein et al. (1973) present a figure showing loading-unloading cycles of a specimen of bone.They use this figure as evidence that the 'behaviour is truly plastic'.However, they also note a reduction of the specimen's stiffness between successive loading cycles.It will be shown later that this observation contradicts their description of the process as plastic deformation.This observation is related to what I consider to be the next plateau in the description of the mechanical properties of bone.It is my opinion that the next, and most recent, important advance in the research of the mechanical properties of bone is the application of the theory of damage accumulation to bone.This idea was first applied to the mechanical testing of bone by Carter and Caler (1983).This theory is reviewed in chapters 2 and 3 and features prominently in this thesis.The concept of damage accumulation is based on the idea that the material fails as a result of the continual degradation of its mechanical properties, caused by an increasing density of voids or cracks, rather than due to the flow of material as in plasticity.
Author: Andrew J. Sedman
Language: English
Subfield: Mechanics of Materials
Field: Engineering
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2565471910