Topic 4- Mechanical properties of materials Flashcards by Tavia Lewis

What are the British/European standards for test methods?

ISO

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What American group set standard test methods?

American Society for Testing and Materials (ASTM)

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How can we measure strain and deformation as a material is stretched?

Using a strain gauge and clamps.

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Why is it important to use a strain gauge when testing materials?

Without the strain gauge we would end up measuring the displacement of everything, not just the sample, e.g. grips.

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How do we measure stress?

Most commonly using a tensile test with a uniform static load applied.

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What are the units for stress?

N/m^2 or N/mm^2

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What is the equation for stress? What do each of the symbols stand for?

Ft/Ao.
Ft = tensile force.
Ao = original area before loading.

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What is the difference between tensile strain and lateral strain?

Tensile strain is applied, lateral strain is resulting.

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What is the equation for tensile strain?

Extension / original length

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What is the equation for lateral strain?

extension x original length / original width

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What is the gauge length?

The length of the material that will be subject to stretching.

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Is elastic deformation reversible or non-reversible?

Reversible

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What type of materials have a high E (Young’s Modulus)?

Ceramics

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Which materials have a low E?

Polymers

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How does Young’s Modulus relate to stiffness?

Higher E = more stiff.

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What does the slope of a stress-strain graph depend on? (Slope proportional to elastic modulus)

Bond strength of material.

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What is E for Aluminium (Al)?

70 GPa

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What is E for steel?

210 GPa

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What is E for aluminium oxide (Al2O3)? (alumina ceramic)

380 GPa

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What is E for Red Oak wood (anisotropic) along the grain and across the grain?

Along the grain: 12 GPa.

Across the grain: 0.6 GPa.

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What is Poisson’s ratio, v?

The ratio of lateral to axial strains.

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What is Poisson’s ratio for metal?

0.33

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What is Poisson’s ratio for ceramics?

0.25

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What is Poisson’s ratio for polymers?

0.4

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What is Poisson's ratio for cork? What does this mean?

0 | It doesn't expand as you push down, hence you can get cork in a wine bottle.

What are auxetic materials? Give two applications.

Materials that have a negative poisson's ratio when stretched. Explosion curtains and comfortable beds.

What does plywood allow for?

Get around problem of low GPa across the grain.

When does plastic deformation occur?

If we exceed the limits of elasticity.

Until what strain do metals elastically deform?

Strain of 0.5%

What is plastic deformation?

Permanent, non-recoverable deformation.

What happens during plastic deformation?

Breaking bonds, reforming bonds with new neighbours and large numbers of atoms move relative to one another.

What plays a key role in plastic deformation?

Dislocations

What law to materials obey in the elastic region?

Hooke's Law.

What three features occur during the plastic region?

1. Hardening occurs. 2. Dislocation motion and generation. 3. Strain is uniform across material.

What happens to the sample beyond the tensile stress?

Necking occurs, deformation in non-uniform plastic.

What is 'necking'? How does it affect a stress plot?

When cross-section gets smaller and smaller, stress plots aren't the true stress. Not due to Poisson's ratio.

What does proof stress eliminate?

Misreading of yield point.

What is proof stress?

Stress at which noticeable plastic deformation has occurred. Measured at strain = 0.002.

What is yield strength?

The point at which stress is no longer proportional to strain.

What occurs beyond yield strength?

Permanent deformation.

What is yield strength a measure of?

Resistance to plastic deformation (onset of dislocation movement in most metals).

What is tensile strength?

Maximum stress that a structure can sustain.

Where is tensile strength measure on a stress-strain curve?

Maximum stress.

When does tensile strength occur in metals?

When noticeable necking starts.

When does tensile strength occur in polymers?

When polymer backbone chains are aligned and about to break.

Why do we engineer to yield strength not tensile strength?

Want to avoid dangerous region of permanent deformation and necking.

What is ductility?

The ability of a material to be stretched. It is the degree of plastic deformation before failure.

How do we measure ductility?

Plastic tensile strain at failure.

What is the equation for ductility?

%EL = (Lf - Lo)/Lo x 100

What are the two definitions of toughness?

1. Measure of the ability of a material to absorb energy up to fracture. 2. Materials resistance to fracture when a crack is present (fracture toughness).

How can we easily approximate toughness?

Area under stress-strain curve.

How do we measure toughness?

Energy to break a unit volume of material.

In general are ductile or brittle materials tough?

Ductile materials are usually tough (e.g. aluminium), brittle materials usually have a low toughness (e.g. Al2O3).

Which materials have high tensile strength but small toughness?

Ceramics.

Which materials have medium tensile strength but large toughness?

Metals.

Which materials have low tensile strength and very small toughness?

Unreinforced polymers.

What is resilience?

Capability of a material to absorb energy during elastic deformation and then on unloading recover the energy.

What do resilient materials have?

High yield strengths and low elastic modulus.

Give two examples of resilient materials?

Spring materials, diving boards.

What is the modulus of resilience, Ur?

Strain energy per unit volume required to stress a material to the yield point.

How can we measure modulus of resilience?

Area under stress-strain curve. Easier if we assume linear stress-strain curve.

Who devised a scale of hardness by arranging 10 minerals so that one mineral could only scratch those below it?

Friedrich Mohrs, 1812.

What is Mohrs scale of hardness (1-10)?

1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase 7. Quartz 8. Topaz 9. Corundum 10. Diamond

What is hardness?

Resistance of materials to localised plastic deformation (dents or scratches).

What is hardness related to?

Yield strength and Young's Modulus.

How do we measure hardness?

Quantitative analysis - small indenter forced into the surface and depth of indent is a measure of hardness.

What is an arbitary property?

Hardness.

Give two example of hardness tests and the indenter used.

Brinell - 10mm sphere of steel or tungsten carbide. | Vickers microhardness - Diamond pyramid

How do we calculate safety factor, N?

Working stress = yield stress / N

Topic 4- Mechanical properties of materials Flashcards

(69 cards)