ch7.2-S09 - Chapter 7: Mechanical Properties structure...

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MY2100 – Ch 7, Part 2, Slide 1 Chapter 7: Mechanical Properties structure processing properties Performance Mechanical behavior of metals - plastic deformation
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MY2100 – Ch 7, Part 2, Slide 2 ISSUES TO ADDRESS… Plastic behavior: At what point do dislocations cause permanent deformation? What materials are most resistant to permanent deformation? Yield strength and ductility : What are they and how do we measure them? • What is a hardness test and why is it used so often in industry?
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MY2100 – Ch 7, Part 2, Slide 3 Mechanical behavior of metals - elastic deformation As we have been discussing, metals exhibit elastic (recoverable) deformation at low stresses. Stress Strain σ = E ε Load Unload σ σ ε ε
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MY2100 – Ch 7, Part 2, Slide 4 Metals - plastic behavior Stress Strain Load Unload σ σ ε ε ε p l At sufficiently high stresses, metals undergo plastic (permanent) deformation in addition to elastic deformation. If this is a part, it has just lost its shape.
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MY2100 – Ch 7, Part 2, Slide 5 What causes plastic (permanent) deformation? planes still sheared F δ ελαστιχ+πλαστιχ βονδσ στρετχη σηεαρ πλαστιχ Plane shearing is caused by bonds breaking and reforming - this is easier (as we discussed in Ch. 5) using dislocations.
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MY2100 – Ch 7, Part 2, Slide 6 What is a yield strength? Stress Strain The stress level at which a material starts to deform permanently is called the yield strength , abbreviated σ y . For engineering design, this is the most important strength parameter. If the stress is kept below σ y , the behavior is elastic. (Sometimes it is called the “proportional limit” - since below σ y , σ is proportional to ε .) σ y
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MY2100 – Ch 7, Part 2, Slide 7 Metals - Yield behavior Many metals begin to yield gradually, and it is not possible to objectively define a “yield strength” or “proportional limit.” Because of this, and the desire to have reproducible test procedures that do not depend on subjective operator input from a curve, the yield strength is almost always defined as a “yield strength at 0.2% offset”. Stress Strain Strain
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MY2100 – Ch 7, Part 2, Slide 8 Offset yield strength A line is drawn parallel to the elastic portion of the σ-ε plot, but “offset” from the origin by a strain of 0.2% ( ε = 0.002). The intersection of that line with the σ-ε curve is defined as the yield strength ( σ y ) of the material.
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MY2100 – Ch 7, Part 2, Slide 9 What happens after yielding occurs? Metals
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This note was uploaded on 03/16/2011 for the course MY 2100 taught by Professor Staff during the Spring '08 term at Michigan Technological University.

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ch7.2-S09 - Chapter 7: Mechanical Properties structure...

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