Materials Engineering – Day 2 Deriving Information from Stress Strain curve of a ductile metal. We will do an exercise. Review ductility and brittleness.

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.

Exercise

• The exercise is a sample quiz question. Please plan to participate.

• We will be comparing answers as we go.

Ductile vs. Brittle Behavior• Let us look at some pictures of failure in metal.Here they are.• Notes on Brittle1. Gross deformation is not great2. Failure is by cleavage mechanism3. Fracture surface is faceted and shiny• Notes on Ductile1. Gross deformation is visible2. Failure is by microvoid coalescence3. Fracture surface is dimpled and dull

Test: What type of fracture is this?

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?

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• 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).

• Toughness: The energy needed to break a unit volume of material.

• Ductility: The plastic strain at failure.

Summary

• Plastic behavior: This permanent deformation behavior occurs when the tensile (or compressive) uniaxial stress reaches y.

Hardness and Hardness Testing• What’s the problem with the tension test?1. Expensive & complex2. Destructive3. Has to be done in the lab4. 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.

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Hardness• Resistance to permanently indenting the surface.• Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties.

e.g., 10 mm sphere

apply known force measure size of indent after removing load

dDSmaller indents mean larger hardness.

increasing hardness

most plastics

brasses Al alloys

easy to machine steels file hard

cutting tools

nitrided steels diamond

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.

Machine forTesting Hardness

(specimen placedOn circular stage)

Advantages Disadvantages

can predict strength, wear resistance

inexpensive, easy relatively nondestructive common designer specification

strength prediction only quantitative for hard steel

predictions are qualitative (no design numbers)

Hardness: Resistance to Penetration/ Indentation/ Scratching

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

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Hardness: MeasurementTable 6.5

Hardness vs. Strength

Correlate depending on metal

BHN

UTS

Steel: UTS = 500 BHN

Cast Iron