# ch2 - CH 2: Materials Choosing the appropriate material is...

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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 2 Page 1 of 6 CH 2: Materials Choosing the appropriate material is an important step in mechanical design. Material Strength and Stiffness Standard tensile test is used to determine many of the mechanical properties of the materials. - Load, ܲ , and deflection, ߜ ൌ ሺ݈ െ ݈ , are recorded and from that the engineering stress vs. strain curve is determined. Engineering stress: ߪ ൌ Strain: ߳ ൌ ௟ି௟ Elastic region : no permanent deformation takes place (if the load is removed, the specimen goes back to its original length). - Stress & strain are related linearly by Hook’s law: ߪ ൌ ܧ߳ ܧ : Young’s modulus or modulus of elasticity (slope of the linear part) Proportional limit : the curve starts to deviate from straight line. Yield point (yield strength, ܵ ) : the end of elastic deformation, plastic (permanent) deformation starts. - Usually determined using the 0.002 yield criterion. Ultimate strength , ܵ : is the maximum stress reached on the (engineering) stress strain diagram. Necking starts after ܵ (for ductile materials) until fracture at point “ ݂ ”. The elongation at fracture is referred to as ( %EL ). ݀ : original diameter ݈ : gauge length (original length)

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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 2 Page 2 of 6 h Strength of a material usually refers to the yield strength. h Stiffness refers to the amount of deformation the material shows under applied load. The lesser the deformation, the stiffer the material. - In the elastic region, “stiffness” refers to Young’s modulus. The higher the ܧ , the higher the stiffness. Ductile and Brittle Materials Ductility measures the degree of plastic deformation sustained at fracture. A Ductile material is one that exhibits a large amount of plastic deformation before failure. - It shows necking before fracture. Example : Steels %EL > 20% A Brittle material exhibits little or no yielding before fracture. -
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## This note was uploaded on 10/25/2010 for the course MECHINCAL 2010 taught by Professor علاءحجازي during the Spring '10 term at Hashemite University.

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ch2 - CH 2: Materials Choosing the appropriate material is...

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