Importance of Properties of Materials which no one wants you to know!

Machine elements are fairly often made of one in each of the metals or metal alloys like steel, PROPERTIES OF aluminum, cast iron, zinc, titanium, or bronze. This section describes the important materials as they affect mechanical design.

Strength, elastic, and ductility properties for metals, plastics, and other sorts of materials are usually determined from a tensile test within which a sample of the fabric, typically within the sort of a round or flat bar, is clamped between jaws and pulled slowly until it breaks in tension. The magnitude of the force on the bar and therefore the corresponding change length (strain) are monitored and recorded continuously during the test. Because the strain within the bar is capable of the applied force divided by the realm, stress is proportional to the applied force. the information from such tensile tests is often shown on stress-strain diagrams.

TENSILE STRENGTH

The peak of the stress-strain curve is taken into account by the last word enduringness. sometimes called the final word strength or just enduringness.

At this time during the test, the best apparent stress on a test bar of the fabric is measured.

The apparent stress is computed by dividing the load by the first cross-sectional area of the test bar.

After the height of the curve is reached, there’s a pronounced decrease within the bar’s diameter, refined to neck down.

Thus, the load acts over a smaller area, and therefore the actual stress continues to extend until failure. it’s very difficult to scale back in diameter during the necking-down process, so it’s become customary to use the height of the curve because of the strength, although it’s a more conservative value.

YIELD STRENGTH

That portion of the stress-strain diagram where there’s an oversized increase in strain with little or no increase in stress is termed yield strength.

This property indicates that the fabric has, in fact, yielded or elongated plastically, permanently, and to an oversized degree.

PROPORTIONAL LIMIT

That points on the stress-strain curve where it deviates from a line is named the proportional limit.

That is, at or above that stress value, stress isn’t any longer proportional to strain. Below the proportional limit.
ELASTIC LIMIT

At some point, called the elastic limit, a cloth experiences some amount of plastic strain and thus won’t return to its original shape after the release of the load.

Below that level, the fabric behaves completely elastically.

The proportional limit and therefore the elastic limit lie quite near the yield strength. Because they’re difficult to see, they’re rarely reported.

MODULUS OF ELASTICITY IN TENSION

For a part of the stress-strain diagram that’s straight, stress is proportional to strain, and also the value of £, the modulus of elasticity, is the constant of proportionality.

DUCTILITY AND PERCENT ELONGATION

Ductility is the degree to which a fabric will deform and ultimately fracture.

The opposite of ductility is brittleness.

When ductile materials are utilized in machine members, impending failure is detected easily, and sudden failure is unlikely.

POISSON’S RATIO

When a fabric is subjected to a tensile strain, there’s a simultaneous shortening of the cross-sectional dimensions perpendicular to the direction of the tensile strain.

The ratio of the shielding strain to the tensile strain is named Poisson’s ratio, usually denoted by v.

FLEXURAL MODULUS

Another stiffness measure often reported, particularly for plastics, is named the flexural modulus, or modulus of elasticity in flexure.

As the name implies, a specimen of the fabric is loaded as a beam in flexure (bending) with data taken and plotted for load versus deflection.

From these data and from knowledge of the geometry of the specimen, stress, and strain are computed. The ratio of stress to strain may be a measure of the flexural modulus.

ASTM standard D 790′ defines the entire method.

Note that the values are significantly different from the tensile modulus because the strain pattern within the specimen could be a combination of tension and compression. the info is useful for comparing the stiffness of various materials when a load-carrying part is subjected to bending in commission.

HARDNESS

The resistance of a fabric to indentation by a penetrator is a sign of its hardness.

Several forms of devices, procedures, and penetrators measure hardness; the Brinell hardness tester and also the Rockwell hardness tester is most often used for machine elements.

For steel, the Brinell hardness tester employs a hardened steel ball 10 mm in diameter because the penetrator is under a load of 3000 kg force.

The load causes a permanent indentation within the test material, and also the diameter of the indentation is expounded to the Brinell hardness number, which is abbreviated BHN or HB.

The actual quantity being measured is the load divided by the contact area of the indentation.

For steels, the worth of HB ranges from approximately 100 for an annealed, mild steel to quite 700 for high-strength, high-alloy steels within the as-quenched condition.

In the high ranges, above HB 500, the penetrator is typically fabricated from tungsten carbide instead of steel.

For softer metals, a 500-kg load is employed. The Rockwell hardness tester uses a hardened steel ball with a 1/16-inch diameter under a load of 100 kg force for softer metals, and therefore the resulting hardness is listed as Rockwell B, Rg, or HRB.

For harder metals, like heat-treated alloy steels, the Rockwell C scale is employed. A load of 150 kg force is placed on a diamond penetrator (a brake penetrator) made with a sphere-conical shape. Rockwell C hardness is usually remarked as Rc or HRC.

Many other Rockwell scales are used.

The Brinell and Rockwell methods are supported by different parameters and result in quite different numbers.