MSE230 Tensile Properties Aisling Coughlan 1 STRESS STRAIN Tensile stress s s Shear stress t t 2 Stress s for tension and compression Strain e for tension and compression Shear stress 3 COMMON STATES OF STRESS Simple tension cable Ski lift Simple shear drive shaft 4 OTHER COMMON STRESS STATES Simple compression Ao Canyon Bridge Los Alamos NM photo courtesy P M Anderson Balanced Rock Arches National Park photo courtesy P M Anderson Note compressive structure member s 0 here 5 OTHER COMMON STRESS STATES Bi axial tension Pressurized tank Hydrostatic compression Fish under water s 0 sz 0 s h 0 6 ENGINEERING STRAIN Tensile strain Lateral strain 2 wo L 2 Lo 2 L 2 7 Stress Strain Testing Typical tensile test machine extensometer Typical tensile specimen specimen gauge length 8 9 Linear Elastic Properties Modulus of Elasticity E F also known as Young s modulus Hooke s Law s Ee s E e Linearelastic F simple tension test 10 Poisson s ratio n Poisson s ratio n eL eL n e metals n 0 33 ceramics n 0 25 polymers n 0 40 Units E GPa or psi n dimensionless e n n 0 50 density increases n 0 50 density decreases voids form 11 12 Mechanical Properties 13 14 c07f08 MECHANICAL BEHAVIOR METALS 15 ELASTIC DEFORMATION 1 Initial 2 Small load 3 Unload bonds stretch return to initial F 16 II PLASTIC PERMANENT DEFORMATION Simple tension test 17 PLASTIC DEFORMATION METALS 1 Initial 2 Small load 3 Unload 18 Tensile properties YIELD STRENGTH sy Stress at which noticeable plastic deformation has occurred tensile stress s sy engineering strain e ep 0 002 19 TENSILE STRENGTH TS Maximum possible engineering stress in tension Adapted from Fig 6 11 Callister 6e Metals Ceramics Polymers 20 Ductility Plastic tensile strain at failure smaller EL Engineering tensile stress s larger EL Lo Ao Af Lf Engineering tensile strain e 21 Stress strain of iron at several temperatures c07f14 22 RESILIENCE Capacity to absorb energy when deformed elastically and then upon unloading to have this energy recovered Modulus of Resilience ey U r sde 0 For a linear elastic region 1 U r s ye y 2 23 TOUGHNESS Ability to absorb energy up to fracture Engineering tensile stress s smaller toughness ceramics larger toughness metals PMCs smaller toughnessunreinforced polymers Engineering tensile strain e 24 True stress strain HARDENING An increase in sy due to plastic deformation Curve fit to the stress strain response s T Ke n T 25 Elastic Recovery After Plastic Deformation 26 Hardness Measure of material resistance to localized plastic deformation Early tests Mohs scale 1 for talc and 10 for diamond Depth or size of an indentation Tests Mohs Hardness Rockwell Hardness Brinell Hardness Knoop Vickers Microindentation Hardness 27 Hardness Resistance to permanently indenting the surface Large hardness means resistance to plastic deformation or cracking in compression better wear properties apply known force measure size of indent after removing load e g 10 mm sphere D most plastics brasses Al alloys Smaller indents mean larger hardness d easy to machine steels file hard cutting tools nitrided steels diamond increasing hardness 28 Hardness Measurement Rockwell No major sample damage Each scale runs to 130 but only useful in range 20 100 Minor load 10 kg Major load 60 A 100 B 150 C kg A diamond B 1 16 in ball C diamond HB Brinell Hardness TS psia 500 x HB TS MPa 3 45 x HB 29 c07tf05 30 Correlation between Hardness and Tensile Strength Tensile strength and Hardness measure metal resistance to plastic deformation For example TS Mpa 3 45 HB or TS psi 500 HB 31 Property Variability and Design Safety Factors Variability of Material Properties Average and standard deviation 32
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