Materials Characteristics Questions??

Question

What information does the notched-bar Impact Test provide that the tensile test does not?

What is the difference between notch toughness and tensile toughness? Is toughness equivalent to ductility?

I there a relationship between hardness and strength?

Assuming that the densities of steel, aluminium and acrylic are 7.9, 2.7 and 1.2 g cm-3 respectively, which material has the best strength–to-weight ratio?

Answer 1

>>What information does the notched-bar Impact Test provide that the tensile test does not?<<

By "notched-bar Impact Test" I'm assuming you mean the Charpy impact test.

Both a tensile test and a Charpy impact test attempt to measure how a material behaves under an applied loading. A tensile test typically generates a curve that compares applied load to elongation (or stress to strain). This allows the engineer to see at what applied load (or stress) the material will fail at.

A Charpy impact test also is used to see how the material fails. A pendulum is released which rotates down and strikes the notched sample. Because the engineer knows from what height the pendulum was released, an approximate idea of the energy necessary to cause fracture can be gained. A tensile test says nothing of the energy necessary to create failure.

>>What is the difference between notch toughness and tensile toughness?<<

Notch toughness is a measure of the energy necessary to cause fracture of a material with a stress concentration under shear loading. Tensile toughness is a measure of the energy necessary to cause failure under an applied axial load.

>>Is toughness equivalent to ductility?<<

Toughness is a measure of the energy necessary to cause complete failure of a material. It is equal to the area under the stress-strain curve

Ductility is a measure of a materials ability to  undergo plastic deformation before fracture.

They are not the same.

>>I there a relationship between hardness and strength?<<

Typically the harder a material is, the greater it's tensile strength. For many steels 500 * Brinell Hardness = Tensile Strength (in psi).

>>Assuming that the densities of steel, aluminium and acrylic are 7.9, 2.7 and 1.2 g cm-3 respectively, which material has the best > strength–to-weight ratio?<<

Steel

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For A36 (commonly used in builing construction)

500 MPa / 7.9 g/cm^3 = 63 kN·m/kg

For Maraging Steels (used in aerospace and nuclear industry)

3500 MPa / 7.9 g/cm^3 = 443 kN·m/kg

Aluminum

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For 6061 (annealed)

124 MPa / 2.7 = 46 kN·m/kg

For 7068 T6,T6511 temper (used in aerospace)

710 MPa / 2.7 g/cm^3 = 263 kN·m/kg

Acrylic

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PMMA (trade name acrylic/plexiglass)

73 Mpa / 1.2 g/cm^3 = 61 kN·m/kg

As you can see, the "strength–to-weight ratio" depends  on what alloy/ temper of the material you choose. Out of all the materials, the maraging steel alloy has the highest ratio of 443 kN·m/kg

Answer 2

Notched-bar impact tests are used to determine the tendency of a material to behave in a brittle manner. It detects differences between materials which are not observable in a tension test. The results from notched-bar tests are not easily expressed in design requirements, since it is not possible to measure the components of the triaxial stress condition at the notch. Also, there is no general agreement on the interpretation or significance of results obtained with this type of test.

Tensile strength measures the amount of stress applied to a material at its breaking point or the point at which it fails. The tensile strength of a material is the point at which a material, under the stress of an applied force, snaps, breaks or can no longer maintain its structural integrity. It is, in other words, the amount of force the material can withstand without breaking.

Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically without fracture. Ductility is the capacity of a metal to be hammered into a thin sheet or drawn into a fine wire. Ductility is the most important parameter to consider in metal forming operations such as rolling, extrusion, and drawing. Examples of highly ductile metals are silver, gold, copper, and aluminium.

hardness is the characteristic of a solid material expressing its resistance to permanent deformation.

The specific strength is a material strength divided by its density. It is expressed in newton metres per kilogram, and is used for tensile strength as for compressive strength. It is sometimes known as the strength-to-weight ratio.

You cannot have a 'best' strength–to-weight ratio any more than you can have a 'best' car or school or computer. It depends at least on use and expectations.