Machine Design_slide 2

Machine - Week1,Lecture2 Material Strength and Stiffness P = A0 l l0 = l0 Hookes Law strain = E Material Strength and Stiffness Stress-strain

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ENGR 3220 – Machine Design ENGR 3220 – Machine Design Week 1, Lecture 2 Week 1, Lecture 2

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Material Strength and Stiffness ε σ E l l l A P = - = = 0 0 0 strain Hooke’s Law
Material Strength and Stiffness Stress-strain diagram obtained from the standard tensile test (a) Ductile material; (b) brittle material. pl marks the proportional limit; el, the elastic limit; y, the offset- yield strength as defined by offset strain a; u, the maximum or ultimate strength; and f, the fracture strength.

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Material Strength and Stiffness Tension specimen after necking The diagrams shown in slide 2 are called engineering stress-strain diagrams because these strain and stresses are not true values. These calculations are based on the original area before the load is applied. In reality, as the load is applied and the area is reduced, the actual stress is larger than the engineering values.
Material Strength and Stiffness True stress-strain diagram plotted in Cartesian coordinates 0 ln 0 l l l dl l l = = ε

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Shear and Torsion Strength J Tr l Gr = = θ τ 0 max Where θ is in radians, r is the radius of the specimen G is called shear modulus, l 0 the gauge length, T is applied torque, J is the polar second moment of area of the cross section
Strength and Cold Work Cold working is the process of plastic straining below the re-crystallization temperature in the plastic region of the stress-strain diagram.

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Hardness kpsi H S or MPa H S B u B u 5 . 12 23 . 0 86 58 . 1 - = - = Rockwell Hardness (H R ) Brinell Hardness (H B ) The resistance of a material to penetration by a pointed tool is called hardness. For cast iron = Mpa H kpsi H S B B u 41 . 3 495 . 0 For steels
Impact Properties The strain rate increases, as it is under impact conditions, the strength increases. Actually when strain rates are very high, the yield strength seems to approach the ultimate strength. But since the elongation has little change, that means the ductility remains the same.

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This note was uploaded on 04/16/2011 for the course ENGR 3220 taught by Professor Turro during the Spring '11 term at UOIT.

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Machine - Week1,Lecture2 Material Strength and Stiffness P = A0 l l0 = l0 Hookes Law strain = E Material Strength and Stiffness Stress-strain

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