lecture_8_-_material_properties_-_ch_3

lecture_8_-_material_properties_-_ch_3 - ME 350 – Lecture...

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Unformatted text preview: ME 350 – Lecture 8 – Chapter 2, 3 Nature of material and material properties Crystal structure Defects Stress & Strain Crystalline vs noncrystalline Stress‑Strain Relationships Hardness Effect of Temperature on Properties Viscoelastic Behavior of Polymers 1 Crystal Structures in Metals 1. Body-centered cubic (BCC) e.g. Chromium, Iron, Molybdenum, Tungsten 1. Face centered cubic (FCC) e.g. Aluminum, Copper, Gold, Lead, Silver, Nickel 1. Hexagonal close-packed (HCP) e.g. Magnesium, Titanium, Zinc How many atoms in each unit cell? BCC: 2 FCC: 4 HCP: 6 2 Imperfections (Defects) in Crystals • Point defects: • Line defects: • Surface defects: – Grain boundaries or the surface of a crystal 3 Elastic & Plastic Strain When a crystal experiences a gradually increasing stress, it first deforms elastically , then atoms change lattice positions, and the deformation is plastic , or a permanent change. 4 Effect of Dislocations on Strain • As compared to a perfect lattice, the stress required for plastic deformation of a material with dislocation is: lower Figure 2.12 Effect of dislocations in the lattice structure under stress 5 Slip on a Macroscopic Scale • Slip occurs many times over throughout the metal when subjected to a deforming load, thus causing it to exhibit its macroscopic behavior in the stress-strain relationship • Dislocations are a good‑news‑bad‑news situation – Good news in manufacturing – the metal is easier to form – Bad news in design – the metal is not as strong as the designer would like • HCP has the fewest slip directions, then FCC , and BCC has the most. 6 Ch3: Tensile Test Figure 3.2 Example tensile test: (1) no load; (2) uniform elongation and reduction of cross‑sectional area; (3) continued elongation, maximum load reached; (4) necking begins, load begins to decrease; and (5) fracture. If pieces are put back together as in (6), final length can be measured. 7 Stress & Strain • Stress is defined as force divided by area: A F = σ Where σ = stress, F = applied force, and A = instantaneous or initial cross-sectional area Engineering stress: A = original area (used in safety calculations) True stress: A = instantaneous area (used in deformation calcs) where e = engineering strain;...
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lecture_8_-_material_properties_-_ch_3 - ME 350 – Lecture...

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