Chapter 6 Slides

# Chapter 6 Slides - Chapter 6 Lectures begin Feb 26 08 1...

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1 Chapter 6 Chapter 6 Lectures begin Feb. 26, 08

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2 ISSUES TO ADDRESS. .. Stress and strain : What are they and why are they used instead of load and deformation? Elastic behavior : Occurs for small loads. 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: Chapter 6: Mechanical Properties Mechanical Properties
3 Elastic means Elastic means reversible reversible ! Tensile Elastic Deformation Tensile Elastic Deformation 2. Small load F δ bonds stretch 1. Initial 3. Unload return to initial F δ Linear- elastic Non-Linear- elastic

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4 Plastic means Plastic means permanent permanent ! Tensile Plastic Deformation (Metals) Tensile Plastic Deformation (Metals) linear elastic δ plastic F δ 1. Initial 3. Unload planes still sheared δ plastic linear elastic & planes shear F δ elastic p l a s t i c δ plastic F 2. Small load bonds stretch Note: The sample is still deformed elastically when it is being deformed plastically. The elastic deformation is not converted to plastic deformation.
5 Force Force Displacement Diagram Displacement Diagram Stress Stress Strain Diagram Strain Diagram ε = Strain (unitless) = normalized displacement ε ε σ = Stress (Pa) = force per unit area σ Force Force Displacement Diagram Displacement Diagram

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6 Elastic region in Elastic region in σ σ - - ε ε diagram diagram Slope = Modulus of elasticity This is also called Young’s modulus = E Units are force per unit area (N/m 2 = Pa or pounds per square inch (psi)) Hooke’s law σ = E · ε
7 What happens to the material when we pull on it? What happens to the material when we pull on it? Material lengthens in direction in which it is pulled Material shrinks in the perpendicular direction(s)

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8 Pull a little harder Pull a little harder get a little plastic deformation get a little plastic deformation Yield point - σ Y Easy to define – point where plastic deformation begins (i.e., where curvature begins) Difficult to determine exact point where curvature (plastic deformation) begins Practical way to determine yield point - use 0.002 strain offset (0.2%) to determine yield point - σ Y0.002 = σ Y - we will use σ Y in class Construct line parallel to lower portion of σ - ε curve Begin line at ε = 0.002, σ = 0 MPa Draw line parallel to elastic region σ Y is where line hits σ - ε curve Real yield point – called the proportional limit This point is very difficult to determine 0.002 offset yield point = σ Y
9 Pull harder until the material breaks Pull harder until the material breaks Metals : occurs when noticeable necking starts.

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Chapter 6 Slides - Chapter 6 Lectures begin Feb 26 08 1...

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