Lecture11 - Plastic (Permanent) Deformation (at lower...

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Chapter 6 - 1 (at lower temperatures, i.e. T < T melt /3) Plastic (Permanent) Deformation • Simple tension test: engineering stress, engineering strain, Elastic+Plastic at larger stress permanent (plastic) after load is removed p plastic strain Elastic initially Adapted from Fig. 6.10 (a), Callister 7e.
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Chapter 6 - 2 Typical Stress-Strain Behavior
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Chapter 6 - 3 Elastic Strain Recovery Adapted from Fig. 6.17, Callister 7e.
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Chapter 6 - 4 How does plastic deformation proceed at the atomic level? entire planes of atoms move past each other Slip systems: (slip) plane and (slip) direction along which slip occurs most easily. Active slip system is the one for which shear stress is greatest. Zn single crystal
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Chapter 6 - 5 Role of dislocations in slip • Slip occurs through the motion of dislocations along the slip plane. • The motion of planes of atoms relative to each other is broken down into discrete steps ->lower stress required.
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Chapter 6 - 6 Room T values Based on data in Table B4, Callister 7e . a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt Yield Strength : Comparison Graphite/ Ceramics/ Semicond Metals/ Alloys Composites/ fibers Polymers Yield strength, y (MPa) PVC Hard to measure , since in tension, fracture usually occurs before yield. Nylon 6,6 LDPE 70 20 40 60 50 100 10 30 200 300 400 500 600 700 1000 2000 Tin (pure) Al (6061) a Al (6061) ag Cu (71500) hr Ta (pure) Ti (pure) a Steel (1020) hr Steel (1020) cd Steel (4140) a Steel (4140) qt Ti (5Al-2.5Sn) a W (pure) Mo (pure) Cu (71500) cw Hard to measure, in ceramic matrix and epoxy matrix composites, since in tension, fracture usually occurs before yield. HDPE PP humid dry PC PET ¨
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Chapter 6 - 7 Yield in different material classes: Ceramics Dislocation motion extremely difficult: Ionic crystals - strong repulsive forces between similar ions Covalently-bonded ceramics: strong, highly directional bonds -> very difficult to move atoms from lattice positions Ceramics generally fracture before yield stress can be reached.
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Chapter 6 - 8 Yield in different material classes: Polymers Process complicated by the presence of chain molecules. Yield occurs by drawing out the chain molecules, rather than dislocation motion
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Chapter 6 - 9 Yield in different material classes: Metals: yield occurs through dislocation motion. Yield stress can be increased by creating obstacles to dislocation motion. Ceramics: dislocation motion very difficult. Can be produced under carefully- controlled circumstances, but in practice, ceramics fracture before plastic deformation can occur. Polymers: yield occurs, but not through dislocation motion processes
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Chapter 6 - 10 Tensile Strength, TS Metals : occurs when noticeable necking starts.
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This note was uploaded on 11/25/2008 for the course MATSCI 104 taught by Professor Lan during the Spring '08 term at UCLA.

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Lecture11 - Plastic (Permanent) Deformation (at lower...

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