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Biomechanics glossary 1

 
Topic: Def: Notes:
0.1% Proof Stress the stress which results in a 0.1% plastic strain. For materials where the yield stress is not easily identified (aluminium). (proof stress not usually quoted for polymers) Line drawn on force-elongation/ stress-strain graph parallel to the linear part of graph & passing through the 0.1% strain value(=0.1% gauge length).
Alloy A substance containing two or more metals mixed in the liquid phase.
Annealing Process involving heating to & holding at a temp. high enough for recrystallization to occur and then cooling slowly. Results in a softened state (more ductile), to facilitate cold-working, improved machine-ability and mechanical properties.; eg. Orthop. wires (stainless steel 316L, annealed)
Barba's Law Takes into consideration the effect of csa on % elongation in Tensile testing. (For test-pieces of the same csa, % elong. of a shorter gauge length will be greater than a longer gauge length). % Elong.= {[a x sq.rt.(csa)/gauge length] + b} x100
Bone Fracture
*Bone fails in TENSION. Shear failure is a tension failure, but crack propagates in spiral because of the ANISOTROPY of bone.; *Haversion canals help to prevent crack propagation.
Boyle's Law Pressure= Force/Area
Brittle Fracture Break a material, & the broken ends fit together perfectly (i.e. no reduction in csa).
Ceramics A substance chemically comprised of metallic and non-metal elements/molecules (eg. ZnO, SiO, TiO2)) Properties determined by ionic bonds, stronger than covalent bonds of polymers & metallic bonds of metal.; Very High Melting Point. High chemical resistance. High Elastic Modulus. Highly Crystalline -> Brittle. Hard -> High wear resistance. Inert/ Biocompatible. Bioactive (eg. calcium hydroxyapatite). Can withstand high stresses, but cannot produce high strains. ; NB- because of high melting point large ceramics are prepared by compressing together small powder particles(sintered ceramics). This always has small defects -> stress risers -> Brittle + very WEAK.
Coefficient of Friction the resistance encountered in moving one object over another. Normal joints= 0.008-0.02; metal-on-metal= 0.8; metal-UHMWPE= 0.02; metal-bone= 0.1-0.2; ceramic-ceramic= v. low; ceramic-UHMWPE= v. low; metal-ceramic= v. high
Composite Bone Mechanical Properties
UTS(MPa) Etens.(GPa) UCS(MPa) Ecomp.(GPa); Canc. 3-20 0.2-5 1.5-50 0.1-3; Cort. 107-146 11.4-19.1 156-212 15.1-19.7; Sawbone 172 18.6; Tufnol 120 8.0; (6F/45); Last-A-Foam 10.2 0.354 14.7 0.36
Composites = a multiphase material. The constituents must be chemically dissimilar & seperated by a distinct interface. (matrix & dispersed phases). It should provide distinctive properties that cannot be obtained by the individual components alone. High strength to weight ratio. Types:; 1) Particle Reinforced: a) Large particle (concrete), b) dispersion strenthened (atomic); 2)Fiber Reinforced: whisker, fiber, wire; continuous, discontinuous; aligned(anisotropic),random(isotropic); 3) Structural: a) Laminar(wood), b) Sandwich panels.; For fiber-reinforced: longit: Ec=EfVf+EmVm,
Corrosion Destruction of metal by electrochemical action Types:; 1) Pitting; 2) Crevice; 3) Galvanic; 4) Intergranular; 5) Stress (Corrosion Fatigue); 6) Fretting.;  mechanical or chemical (chloride) breakdown; Corrosion rate can be reduced by passivation.; Surface stresses/ fatigue can be reduced by peening, polishing & heat treatment.
Corrosion Mechanism
*Metals with different chemical reactivities or the same metal in different environment -> electrical currents by destruction of the more reactive metal.; *Differences in oxygen concentration often leads to corrosion (low oxygen beneath screw head= anode).
Coulombs Law of Friction The shear stress is always parallel to the relative velocity & equal to the product of the contact pressure & the dynamic friction coefficient as determined from measurements on particular combinations of materials. [Shear Stress= Compressive stress × Coefficient of Friction]
Creep Continued straining of a material under constant stress. It is stress, time & temp. dependent (fatigue is stress & time dependent only).
Definitions
Kinematics= Analysis of motion w/out reference to forces.; Kinetics= Analysis of motion under the action of given forces or moments. (= static / dynamic); Statics= study of forces & moments acting on a body in equilibrium (at rest or constant speed).; Dynamics= study of forces & moments acting on a body in motion (accelerating/ decelerating)
Ductility/ Brittleness Materials which develop significant permanent deformation before they break are called 'Ductile'. Measures of Ductility:; 1. Percentage Elongation =[final-initial length]/[initial length] x 100.; Ductile material (mild steel)= >20%; Brittle material (cast iron)= <1%; 2. Percentage Reduction in cross-sectional area= [Ao-A]/Ao.; Percentage elong./red. csa for polymers is measured at the moment of fracture, for metals it is measured after elastic recovery (rupture) has occurred. (due to large elastic recovery in polymers). On force-elong. curve: elong. used for plastics is that value at rupture, & for metals it is the value at a line drawn from fracture point parallel to elastic range.; 3. Bend tests; 4. Cupping tests(Erichson); 5. Impact tests(Charpy) - ductile materials absorb appreciable energy (higher energy to # than brittle material).
Dynamics Linear Motion v=u+at; v²=u²+2as; s=ut+0.5at²
Elastic Limit The stress at which the material starts to behave in a non-elastic manner. Yield Stress = stress at the elastic limit (yield point)
Energy (Joules) The ability to do work. Potential E=mgh (energy a body possesses); Kinetic E=½mv² (energy a body has due to motion); KE=½I.w² (angular motion- I=mass moment of inertia, w=angular velocity); Energy cannot be created or destroyed.
Euler's Column Law Determines critical load for scoliosis. Pcrit = C.(E.I/L²); where Pcrit is the critical load, C is the end conditions, E is the modulus of elasticity, I is the cross sectional moment of inertia, L is the column length.
Euler/ Cardan angles Describe the pathway thro which a joint segment moves from one position to another in terms of three independent rotations. These angles describe the attitude/ orientation of the second position w/ respect to the first. Euler angles= the situation where the first & last axes about which a rotation take place are the same.; Cardan angles= when all three axes are different. ; Some literature sources use the term Euler angles to describe both.
Failure When a material loses its ability to satisfy the original design function. Types(6): fatigue, corrosion, fracture, creep, plastic buckling, alteration of properties & characteristics.
Failure
* Ductile metals may fail in a brittle manner at low temps, in thick sections, at high strain rates or where thre are flaws.
Fatigue The reduction of strength by the application of cyclic loads below the tensile strength of the material. Low cycle fatigue = max. stress in a cycle > yield stress. High cycle fatigue = max. stress in a cycle < yield stress.; Fatigue limit for polished steels = half the tensile strength. This is reduced by surface scratches.; Peening= light hammering of the surface with a round-nosed hammer -> increased Fatigue Life by inducing residual compressive stresses in material.; 
Fatigue Fracture
Occurs in 3 steps:; 1) Nucleation of a crack- occurs at locations of highest stress & lowest local strength. These are usually at or near the surface & include surface defects, such as scratches or pits, sharp corners, inclusions, grain boundaries or dislocation concentrations.; 2) Propagation of a crack- towards lower stres regions. The crack propagates a little bit further each cycle, until the load-carrying capacity of the metal is approached.; 3) Catastrophic failure- in a brittle manner. ; Beach marks are formed when the load is changed during service. Striations, which are on a much finer scale, may show the position of the crack tip after each cycle.


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Biomechanics glossary 1
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