Monday, February 21, 2011

Tensile Strength Test

One of material testing for steel is tensile strength. Among other values, the tensile strength Rm, the elastic limit Rst, or the yield strength Rp, and the breaking elongation A, are specified in the DIN norms for strength and deforming characteristics. Example for steel C35 in DIN 17200. To measure tensile strength of steel use special tool that can show the power when the steel begin to broke.

Rm = 600 N/mm2, A = 20%. These characteristics value for materials are determined by means sol a tension test.

Force and extension.
The test sample increases in length from the original value Lo to the final value L under the effect of a tensile force F. The elongation ΔL is the difference between the final length L and the initial length Lo.

ΔL = L – Lo, example: ΔL = 204 mm – 200 = 4 mm

Tension and extension.
Tension are generated within the material of the test piece. If the tensile force F is divided by the initial cross section So, the value for tension is obtained.

σ = F/So,  σ in N/mm2, example: σ = 5000 N/ 50 mm2 = 100 N/mm2

The extension є of the test piece is obtained by dividing the elongation ΔL by the initial length Lo. This is specified as a percentage value.

Є = ΔL/Lo x 100,  Є in % example: Є = 4 mm/200 mm x 100 = 2%

Elongation, tension and extension on increase with increasing tensile force. These relationships are expressed by a curve in the tension extension diagram.

Elastic, disproportional and permanent extension

In the first zone of elastic extension (Єo), the material always returns to its initial length Lo once the load has been completely removed. When the test rod is elongated beyond this range, the extension, increases disproportionally to the increase in tensile, i.e., the material passes over from the elastic to the plastic state. A permanent extension (Єr) takes place in the region of disproportionate extension (Єp). The test rod does not return to its original length Lo when the load is removed, the elongation ΔL, remains.

Elastic Limit and Yield Strength
For some materials, the transition from the elastic to the plastic range is discontinuous; e.g. cast iron, alloyed steel non-ferrous metals. In ordinary structural steel, the atomic lattice is set into motion (flow region), the rod elongates as the upper elastic limit Rst, is therefore that at which the required tensile force first begins to fall off with increasing extension in the flow region.

Example: RsH = 360N/mm2; ReL = 300 N/mm2. When the transition is continuous, the boundary between the elastic and plastic regions cannot be precisely specified.

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