2010-02-08 Chapter 06 Mechanical Properties of Metals

# 2010-02-08 Chapter 06 Mechanical Properties of Metals -...

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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 do dislocations cause permanent deformation? What materials are most resistant to permanent deformation? 1 Toughness and ductility : What are they and how do we measure them? CHAPTER 6: MECHANICAL PROPERTIES

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Tensile (Pulling) Compression (Pushing) Elongation or, extension Compression or, contraction TYPES OF FORCES/LOADS (I) Tensile and compressive forces are normal (perpendicular) to the cross sectional area. Change in dimension is referred as deformation. Tension – elongation Compression – contraction
Shear Angular deformation Shear forces are parallel to the cross sectional area. TYPES OF FORCES/LOADS (II)

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F δ bonds stretch return to initial 2 1. Initial 2. Small load 3. Unload Elastic means reversible ! F δ Linear- elastic Non-Linear- elastic ELASTIC DEFORMATION
1. Initial 2. Small load 3. Unload Plastic means permanent ! planes still sheared F δ elastic + plastic bonds stretch & planes shear δ plastic PLASTIC DEFORMATION (METALS) F δ linear elastic linear elastic δ plastic Elastic strain recovery

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HOW DO WE MEASURE MAX. CAPACITY OF THE MATERIAL BEFORE IT FRACTURES (BREAKS)? Can we measure that capacity in terms of force/load? NO. F1 F2 For same material, Rod with larger diameter will take more load before breaking. F2 >F1. It doesn’t mean larger diameter rod is stronger! Let F1 & F2 be loads required to break the rod made of same material. 1 2
CONCEPT OF STRESS If we compute quantities and in earlier example, then these quantities are equal. It is called “Ultimate Strength of the Material” i.e. max. capacity of the material before it breaks. Therefore, it is convenient to define the term called “Stress”. Stress is ‘force’ divided by ‘area on which force is acting’. When the stress reaches the maximum value, the material breaks (fractures). The corresponding stress is called “Ultimate Strength of the Material”. 1 A 1 F 2 A 2 F A F = σ

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ULTIMATE STRENGTHS Tensile load – ‘Ultimate tensile strength’ (UTS) or, ‘Tensile strength (TS)’, σ ut Compressive load – ‘Ultimate compressive strength (UCS) or, “Compressive strength’, σ uc Shear load – ‘Ultimate shear strength’ (USS) or, ‘Shear strength’, τ u In this course we ONLY deal with tensile loads so we will refer TS simply as σ u instead of σ ut
Simple tension: cable o σ = F A A o = cross sectional Area (when unloaded) F F σ σ Ski lift (photo courtesy P.M. Anderson) Canyon Bridge, Los Alamos, NM Simple compression: o σ = F A (photo courtesy P.M. Anderson) ENGINEERING STRESS When original area is considered for calculation, stress is called ‘engineering stress’.

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Bi-axial tension: Hydrostatic compression: Fish under water
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2010-02-08 Chapter 06 Mechanical Properties of Metals -...

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