MECH 341 Notes for Final, Spring 2016

Mech 341 Final Notes Lab 1/2: Tensile Testing of Metals  Scope : (1) Yield point stress σy (2) Ultimate Tensile Strength σult (3) Failure Point  Terms : - Discontinuous Yielding: a fluctuation of force at the onset of plastic deformation due to localized yielding. - Lower yield strength- the minimum stress recorded during discontinuous yielding. - Upper yield strength- the first stress maximum associated with discontinuous yielding. - Uniform elongation, Elu - the elongation determined at the maximum force sustained by the test specimen prior to necking, or fracture, or both.  Procedure: Prepare to dimensions ~ grip on UTM w/ rate 0.05 in/min till fracture  Affecting Variables: (1) test method (2) fabrication (3) bias (4) calibration (5) structure inhomogeneity  Graphs: a- Annealed mild steel b- Aluminium alloy 5454 c- PVC d- Silica glass e- Fiberglass (30%) Lab 3: Brinell Hardness of Metallic Material  Scope : (1) Determination of HBW through ball contact indentation (tungsten carbide)  Terms : - HBW: quantitative test of force vs. area of indentation -Hardness: resistance to indentation  Material: Cast Iron ~ Low carbon ~ High carbon ~ Heat treated ~ Brass  Hardening procedure for the High-Carbon heat treated: 1. Heat up specimen (in oven) to a temperature of 600o C (Time = 1hr) 2. Raise temperature to 800o C in 10 minutes 3. Keep specimen at 800o C for 25 minutes 4. Quench specimen in oil at a temperature of 30o C  Advantage of HBW: ~simple & inexpensive ~ nondestructive ~ derives mech properties (tensile stress)  Procedure: ~fixed load for 30s ~ measure indentation via average ~ calculate via HBW formula  Graphs: Lab 4: Charpy (Notched) Bar Impact Testing For Metals  Scope : (1) Measure absorbed energy before fracture CV (2) measure toughness under dynamic loading  Material: ~ Low carbon ~ High carbon ~ Heat treated ~ Brass  Dimensions: 55 x 10 x 10 mm - 45 V notch + 2 mm deep  Terms: - Shear Lip % : ratio of the fractured specimen area subject to brittle fracture over the whole fracture area. -Transition Temp: temperature below which the impact fracture energy of the specimen decreases drastically. -Toughness: of a metal is its ability to absorb energy in the plastic range especially when subjected to dynamic loading (area under TS-strain curve)  Effect of temperature on the toughness: direct relation  Effect of carbon% on toughness: direct relation  Procedure: ~ post impact, measure initial - final energy (=absorbed) ~ measure shear lip of fracture  Fracture VS Ductility: ~ Fibrous surface = shear fracture (ductile) ~ Granular surface = cleavage fracture (brittle)  Graphs: Lab 5: Fatigue Testing of a (Notched) Aluminum Rod  Scope : (1) Measure fatigue life (nb. of cycles) (2) effect of notching on fatigue life  Material: ~ Aluminum  Dimensions: diameter 9.5mm ~ notch diameter 8.5mm ~  Terms: - Completely reversed stress cycle: max stress = min stress on curve -Repeated stress cycle: max≠min stress, both in tension -Fatigue failure: Failures occurring under conditions of dynamic loading after a period of service.  Factors influence fatigue life: Loading ~ Geometry ~ Material ~ Manufacturing  Effect of notch: reduces fatigue life ~ makes stress concentration factor Kf  Procedure: -rotate motor 3460 rpm ~ 30 kg load in bending mode till failure  Graphs: Lab 6: Preparation of Metallographic Specimens  Scope : (1) Observe microstructure (2) Calculate grain size  Material: ~ Low carbon ~ High carbon ~ Heat treated ~ Cast iron  Terms: - Sectioning: cutting of the metallographic samples to the area of interest and for easier handling. -Planar Grinding: planarizing of the specimen surface to reduce the damage created by sectioning. -Rough Polishing: first smoothing operation of the grinded surface to remove the damage produced during cutting and planar grinding. - Final Polishing: final smoothing operation of the grinded surface to remove only surface damage. It should not be used to remove any damage remaining from cutting and planar grinding. -Etching: immersion in a chemical material that selectively reacts with the specimen surface optically enhance microstructural features. Removes intergranular material.  2% Nital Etch: composed of 98 ml ethyl alcohol and 2 ml - Nitric acid for a time of 30 to 45 seconds.  Procedure: 1. Cut: sectioning & grinding 2. Rough Polishing: 3 abrasive papers  320 grit  400 grit  600 grit 3. Final Polishing: abrasive rotating disk 4. Etching: 2% natal etch for 30-45s 5. Microscope on 100x for ASTM plus 10x  1000x mag.  ASTM# N=2n-1 Or Hyen’s intercept method: nb. of grains intersected per unit line x magnification NO=0.735(NL)2  Graphs: Lab 7: Tensile Testing Polymers  Scope: (1) Yield point (2) Ultimate tensile strength (3) Failure point  Dimensions: g70 x 10 x 2 mm  Material: Polymeric material “polyamide”  Terms: -Strain rate: variation of applied strain as a function of time - Energy at Yield : area under plastic portion of deformation curve -Energy at Break: area under the entire load-deformation curve -Proportional Limit Stress: stress value at which the stress-strain curve goes nonlinear -Modulus of Resilience: area under the elastic portion of the stress-strain curve  An increase in temperature initiates: ~ Decrease in the elastic modulus ~ Reduction in tensile strength ~ Enhancement of ductility  An increase in operation strain rate initiates: ~ Increase in the elastic modulus ~ Increase in tensile strength ~ Enhancement of brittleness  Procedure: ~use pneumatic grips on UTM ~ rate of 100mm/min  Graphs:  Lab 8: Viscoelasticity Creep & Stress Relaxation of Polymers  Scope: viscoelastic properties (e.g. modulus) of plastics under creep and relaxation conditions  Dimensions: -Relaxation: g25 x 10 x 2 -Creep: cross section 18 x 3 mm  Terms: -Creep: constant stress applied while measuring elongation as a function of time (ε(t)  Ec ) -Relaxation: constant elongation (strain) applied while measuring the forces w.r.t. time. (F  σ  Ec ) -Viscoelastic materials: their stress-strain relationship depends on time ~ stiffness depends on rate of load application -Relaxation modulus: time dependent elastic modulus for viscoelastic materials -Creep modulus: a time dependent elastic modulus for viscoelastic materials -Kelvin-Voigt creep model: yields that if the material is subjected to a constant stress, change in strain is found by -Maxwell stress relaxation model: if the material is subjected to a constant strain, the change in stress is found by  Effect of temperature on the mechanical properties: - as temp properties  elastic - as temp properties  viscous  Stages in creep strain curve: a-Primary creep (curve is concave down), is related to the transfer from the elastic to inelastic deformation. b-Secondary creep (strain is proportional to time) – true measure of time-dependent properties c-Tertiary creep (concave up and where deformation is accelerated until rupture occurs). Marks the acceleration to failure.  Procedure: -Creep: ~ constant 5kg load ~ measure strain in intervals -Relaxation: ~ initial L= 4mm ~ constant displacement 10mm ~ measure force in intervals  Graphs: Viscous s Viscoelastic Elastic Lab 9: Izode (Un-notched & Notched) Bar Impact Testing of Plastic Material  Scope: (1) Absorbed energy (2) Effect of notching  Dimensions: 55 x 13 x 13 mm  Material: Plexiglass (V-notch 45°, U-notch, Unnotched)  Observations: ~ Unnotched  straight clean crack ~U-notch  crack diagonal (greater incline than V-notch) ~V-notch  crack diagonal  Breaking energy: V < U< No notch