Chapter 6- Mechanical Properties

# Chapter 6- Mechanical Properties - Chapter 6: Mechanical...

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Chapter 6: Mechanical Properties 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? Chapter 6 - 1

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Common States of Stress Simple tension: cable A o = cross sectional area (when unloaded) F Chapter 6 - 2 F M M A o 2 R F s A c Torsion (a form of shear): drive shaft Ski lift (photo courtesy P.M. Anderson)
Other Common Stress States (1) (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o σ = F A Simple compression: Note: compressive structure member ( σ < 0 here). (photo courtesy P.M. Anderson) A o Balanced Rock, Arches National Park Chapter 6 - 3

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OTHER COMMON STRESS STATES (2) Bi-axial tension: Hydrostatic compression: Pressurized tank σ < 0 h (photo courtesy P.M. Anderson) Fish under water (photo courtesy P.M. Anderson) σ z > 0 σ θ > 0 Chapter 6 - 4
Stress-Strain Testing Chapter 6 - 5 • Typical tensile test machine Adapted from Fig. 6.3, Callister 7e. specimen extensometer • Typical tensile specimen Adapted from Fig. 6.2, Callister 7e. gauge length • Other types of tests: - compression: brittle materials (e.g., concrete) - torsion: cylindrical tubes, shafts

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Engineering Stress Tensile stress, σ : original area before loading Area, A F t F t σ = F t A o 2 f 2 m N or in lb = Shear stress, τ : Area, A F t F t F s F F F s τ = F s A o Chapter 6 - 6 Stress has units: N/m 2 or lb f /in 2
Engineering Strain Tensile strain: Lateral strain: Shear strain: Strain is always dimensionless. Chapter 6 - 7 θ 90º 90º - θ y x θ γ = x / y = tan ε= δ L o δ ε L = L w o Adapted from Fig. 6.1 (a) and (c), Callister 7e. δ /2 δ L /2 L o w o Compressive strain is always negative. Note : torsion is a variation of shear

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Modulus of Elasticity, E : (also known as Young's modulus, or stiffness) F F simple tension test Hooke's Law : σ = E ε σ Linear- elastic E ε For elastic deformation only! For typical metals E is of the order of 10-100 GPa
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## This note was uploaded on 04/02/2008 for the course MSE 200 taught by Professor Wholedepartment during the Spring '08 term at N.C. State.

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Chapter 6- Mechanical Properties - Chapter 6: Mechanical...

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