Lecture 6 - 8th Ed. Ch. 6

# Lecture 6 8th Ed. - Chapter 6 Mechanical Properties 1 Mechanical Properties ISSUES TO ADDRESS Stress and strain What are they and why are they used

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1 Chapter 6 Mechanical Properties

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ISSUES TO ADDRESS. .. Stress and strain : What are they and why are they used instead of load and deformation? Elastic behavior: When loads are small, how much deformation occurs? What materials deform least? Plastic behavior: At what point does permanent deformation occur? What materials are most resistant to permanent deformation? Toughness and ductility : What are they and how do we measure them? Mechanical Properties
Elastic means reversible ! Elastic Deformation 1. Initial 2. Small load 3. Unload F d bonds stretch return to initial F d Linear- elastic Non-Linear- elastic

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Plastic means permanent ! Plastic Deformation (Metals) F d linear elastic linear elastic d plastic 1. Initial 2. Small load 3. Unload p lanes still sheared F d elastic + plastic bonds stretch & planes shear d plastic
Stress has units: N/m 2 or lb f /in 2 Engineering Stress Shear stress, t : Area, A F t F t F s F F F s t = F s A o Tensile stress, s : original area before loading Area, A F t F t s = F t A o 2 f 2 m N or in lb =

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Simple tension: cable Note: t = M / A c R here. Common States of Stress A o = cross sectional area (when unloaded) F F o s = F A o t = F s A s s M M A o 2 R F s A c Torsion (a form of shear): drive shaft Ski lift (photo courtesy P.M. Anderson)
(photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o s = F A Simple compression: Note: compressive structure member ( s < 0 here). (photo courtesy P.M. Anderson) OTHER COMMON STRESS STATES (1) A o Balanced Rock, Arches National Park

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Bi-axial tension: Hydrostatic compression: Pressurized tank s < 0 h (photo courtesy P.M. Anderson) (photo courtesy P.M. Anderson) OTHER COMMON STRESS STATES (2) Fish under water s z > 0 s q > 0
Tensile strain: Lateral strain: Shear strain: Strain is always dimensionless. Engineering Strain q 90º 90º - q y x q g = x / y = tan e = d L o - d e L = L w o d /2 d L /2 L o w o

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Stress-Strain Testing • Typical tensile test machine specimen extensometer • Typical tensile specimen Adapted from Fig. 6.2, Callister 7e. gauge length
Linear Elastic Properties Modulus of Elasticity, E : (also known as Young's modulus) Hooke's Law : s = E e s Linear- elastic E e F F simple tension test

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Poisson's ratio, n Poisson's ratio, n : Units: E : [GPa] or [psi] n : dimensionless n > 0.50 density increases n < 0.50 density decreases (voids form) e L e - n e n = - L e metals : n ~ 0.33 ceramics : n ~ 0.25 polymers : n ~ 0.40
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## This note was uploaded on 02/20/2012 for the course MSE 14:635:407 taught by Professor Dr.e.korayakdoğan during the Spring '12 term at Rutgers.

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Lecture 6 8th Ed. - Chapter 6 Mechanical Properties 1 Mechanical Properties ISSUES TO ADDRESS Stress and strain What are they and why are they used

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