L04_Stress and Strain-fall 10

L04_Stress and Strain-fall 10 - EAS 209-Fall 2010...

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EAS 209-Fall 2010 Instructors: Christine Human 8/24/2010 1 04-Mechanical Properties Lecture 4: Mechanical Properties of Materials: Stress and Strain under Axial Loading ( Chapter 2) Design of machines and structures that are safe and function properly requires an understanding of the mechanical behavior of materials. Today’s Objectives: Stress was defined in chapter 1. After defining strain today’s objectives are to understand: Stress-strain curves Elastic versus plastic deformation Brittle versus ductile behavior Engineering stress-strain versus true stress- strain Tension versus compression Fatigue failure Today’s Homework: EAS 209-Fall 2010 Instructors: Christine Human 8/24/2010 2 04-Mechanical Properties Stress –Strain Under Axial Loading Laboratory tests are conducted in accordance with standard protocols. In the U.S. one of the major standards organizations is the American Society of Testing and Materials Uniaxial Tension Test : ASTM E8-04: Standard Test Methods for Tension Testing of Metallic Materials. Standard coupon D=0.505 in L (gage length) = 2.0 in Apply P, measure elongation δ The elongation is measured over the gage length, L using an extensometer
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EAS 209-Fall 2010 Instructors: Christine Human 8/24/2010 3 04-Mechanical Properties Recall, we can normalize the load P by dividing by the initial area of the specimen, this is the stress, σ A P = σ We can also normalize the elongation by dividing by the gage length, this is the strain, ε L δ ε = Each material has a unique stress-strain curve Typical Stress-Strain for Low Carbon Steel NOT TO SCALE Ductile Behavior EAS 209-Fall 2010 Instructors: Christine Human 8/24/2010 4 04-Mechanical Properties Elastic Region (OB) Stress is proportional to strain up to the proportional limit, σ PL . Hooke’s Law = E For steel, E=29x10 6 psi (200 GPa) A PL L A P E = = = / / Rearranging AE PL = In the elastic region strains are recoverable if the load is removed – no residual deformation. Between
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L04_Stress and Strain-fall 10 - EAS 209-Fall 2010...

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