Lecture10

Lecture10 - Chapter 6 Mechanical Properties ISSUES TO...

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Chapter 6 - 1 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: Mechanical Properties

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Chapter 6 - 2 Why do we care about mechanical properties of materials? An oil barge fractured in a brittle manner by crack propagation around its girth
Chapter 6 - 3 Why care? Structural failure of a ladder Failure can result from poor design – choice of material versus load Structural failure - is driven by repeat loading, which is a necessary requirement of a ladder, a drive shaft on an automobile, and the wing on an airplane. Heverill Fire Department aerial ladder failure.

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Chapter 6 - 4 Elastic means reversible ! Elastic Deformation 1. Initial 2. Small load 3. Unload F δ bonds stretch return to initial F δ Linear- elastic Non-Linear- elastic
Chapter 6 - 5 Plastic means permanent ! Plastic Deformation (Metals) F δ linear elastic linear elastic δ plastic 1. Initial 2. Small load 3. Unload planes still sheared F δ elastic + plastic bonds stretch & planes shear δ plastic

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Chapter 6 - 6 Simple tension: cable τ Common States of Stress A o = cross sectional area (when unloaded) F F o σ = F A o τ = F s A σ σ M M A o 2 R F s A c Torsion (a form of shear): drive shaft Ski lift (photo courtesy P.M. Anderson)
Chapter 6 - 7 (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM o σ = F A Simple compression: Note: compressive structure member ( σ < 0 here).

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