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Materials Engineering – Day 2

Materials Engineering – Day 2. Deriving Information from Stress Strain curve of a ductile metal. We will do an exercise. Review ductility and brittleness. Contrast in terms of fracture appearance. Remove a common source of confusion. Stiffness is one thing and strength is another.

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Materials Engineering – Day 2

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  1. Materials Engineering – Day 2 • Deriving Information from Stress Strain curve of a ductile metal. We will do an exercise. • Review ductility and brittleness. Contrast in terms of fracture appearance. • Remove a common source of confusion. Stiffness is one thing and strength is another. • The Concept of Hardness. Discussion of Hardness testing. • A sad fact. Performance on the tension test is one thing. But sometimes surprising things happen in the service environment.

  2. Exercise • The exercise is a sample quiz question. Please plan to participate. • We will be comparing answers as we go.

  3. Ductile vs. Brittle Behavior • Let us look at some pictures of failure in metal. Here they are. • Notes on Brittle • Gross deformation is not great • Failure is by cleavage mechanism • Fracture surface is faceted and shiny • Notes on Ductile • Gross deformation is visible • Failure is by microvoid coalescence • Fracture surface is dimpled and dull

  4. Test: What type of fracture is this?

  5. Difference between Stiffness and Strength • Material A has E = 10x106 psi and UTS = 50 ksi • Material B has E = 30x106 psi and UTS = 35 ksi WHICH IS STRONGER? WHICH IS STIFFER?

  6. Summary • Stress and strain: These are size-independent measures of load and displacement, respectively. • Elastic behavior: This reversible behavior often shows a linear relation between stress and strain. To minimize deformation, select a material with a large elastic modulus (E or G). • Plastic behavior: This permanent deformation behavior occurs when the tensile (or compressive) uniaxial stress reaches sy. • Toughness: The energy needed to break a unit volume of material. • Ductility: The plastic strain at failure.

  7. Hardness and Hardness Testing • What’s the problem with the tension test? • Expensive & complex • Destructive • Has to be done in the lab • Takes a while • This leads to a more simple characterization of material behavior, through the test for hardness. Hardness is resistance to indentation by a hard object like a needle or ball.

  8. apply known force measure size e.g., of indent after 10 mm sphere removing load Smaller indents d D mean larger hardness. most brasses easy to machine cutting nitrided plastics Al alloys steels file hard tools steels diamond increasing hardness Hardness • Resistance to permanently indenting the surface. • Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties.

  9. Hardness Testing Methods • Rockwell. 1) Apply preload. 2) Add main load and measure depth. 3) Remove main load and measure depth. Difference in depths is read out as a hardness.

  10. Machine forTesting Hardness (specimen placed On circular stage)

  11. Hardness: Resistance to Penetration/ Indentation/ Scratching

  12. Kinds of Hardness Tests • Rockwell A, B, and C. B is soft metals, C is for hard metals. Eg. Steel knife blade RC about 60. Crankshaft RC about 45. Values below 20 and above 100 RC are not reliable. Also superficial. • Brinell Hardness. Oldest measure. A steel (HBS) or tungsten (HBW) balls are used as indenters. You measure the diameter of the indentation and use a formula to get HB. UTS (psi) = 500 x BHN • Vickers. A diamond indenter is used. A very wide ranging scale. • Here is a comparison or more

  13. Hardness: Measurement Table 6.5

  14. Hardness vs. Strength Correlate depending on metal UTS Cast Iron Steel: UTS = 500 BHN BHN

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