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4.2.3. Time-Dependent Properties (1) Creep plastic deformation under constant load over time at specified temp. strain vs. time curve a) primary creep: repositioning of aspects of the material with loading b) secondary creep: equilibrium & minimum creep rate c) tertiary creep: rapid elongation to material failure parameters for creep: (steady state creep rate), r (time to rupture) stress and temp : , r Slide 2 Creep curve for polymers negative strain w/ compressive loading even at low temp & stress (2) Molecular causes of creep Metals sliding of grain boundaries or vacancy migration a) stress-induced vacancy diffusion grain elongation along the line of applied stress (Nabarro-Herring creep) cf) Coble creep b) dislocation climb diffusion of an entire row of vacancies Slide 3 (3) Molecular causes of creep Ceramics more resistant to creep deformation to maintain electroneutrality, difference in diffusivities, fewer point defects grain boundary sliding (4) Molecular causes of creep Polymers movement of chains in the amorphous regions via viscous flow % crystallinity and Tg crystallinity --- creep TTg: time-dependent deformation (polymer chain movement) viscous flow (creep and stress relaxation) (5) Stress relaxation and its causes creep: stress --- strain stress relaxation: decrease in stress over time under constant strain strain --- stress chain movement in the amorphous regions % crystallinity and T>Tg (viscous flow) Slide 4 (6) Mathematical models of viscoelastic behavior T>Tm: viscous liquid T tension lower localized stress amplification Slide 10 4.4 Fatigue & Fatigue Testing 4.4.1. Fatigue repeated loading --- fracture at stresses less than the tensile or yield strength fatigue fracture: brittle with no plastic deformation repeated stress --- # of dislocations --- imperfections in crystals --- flaws ---- cracks cf) strain hardening a) crack initiation b) crack propagation c) final failure N f = N i + N p 4.4.2. Fatigue testing rotating-bending apparatus & uniaxial tension-compression machine stress (S) & # of cycles (N) to failure S = ( max min )/2 Slide 11 Fatigue limit or endurance limit no fatigue failure below a certain level of stress ex) titanium Fatigue strength: stress level causing the failure Fatigue life: # of cycles for the fracture kinetics of crack propagation 4.4.3. Factors that affect fatigue life a) regional stress concentrator b) amplitude of the applied stress c) impurity in the surface region d) other stress raisers e) biodegradable materials f) environment of the implant corrosive fatigue Slide 12 4.5 Methods to Improve Mechanical Properties dislocation glide or slip --- plastic deformation slip reduction 1) inclusions of additives a) metal alloys (impurity cancel the lattice strain) b) polymers (fillers entanglement and X-linking) 2) processing a) polycrystalline materials grain boundaries ---- dislocation movement smaller grain more grain boundaries stronger b) cooling rate rapid cooling --- low % crystallinity --- low overall strength thermal history of materials Slide 13 4.6 Techniques: Introduction to Mechanical Analysis dynamic mechanical analysis (DMA) mechanical properties during oscillatory loading 4.6.1 Mechanical testing (1)Basic principles uniaxial loading at a controlled amplitude and rate sample shape tensile testing smaller X-section reproducible region of breakage (2) Instruments stress and strain curve a) grip/actuator; b) load cell; c) extensometer; d) processor (3) Information provided stress vs strain stress/strain vs time modulus, yield & tensile strength, fatigue life, etc.