Tensile Test of Metals

Universal testing Machine is used in This method .

In Tensile test, the test piece is first prepared by turning the same piece to the standard shapes specified. This may be either round or flat. 

Before commencing the test two gauge marks are made on the specimen longitudinally usually 50mm to 200mm apart according to the size of the test piece. The ends of the test piece are then gripped in the tensile testing machine and gradually increasing load is applied until failure is approached. The amount of elongation in the test piece caused by the load is measured accurately by a mechanical , electrical, or optical device called extensometer. As the loading of the test piece progresses, load and deformation readings are recorded simultaneously. The stress is calculated from the loads and the original dimension of the piece and this stress is plotted graphically with strain to show behavior of metal at different states. 

 

The parameters which are used to describe the stress-strain curve of a material are the elastic limit, proportional limit, yield strength or yield point, tensile or ultimate strength, per cent elongation or reduction of area. The first two indicate elasticity, the second and third are strength parameters, and the last two indicate ductility. In addition to these, resilience and toughness can also be computed from the curve.

As the load is applied the test piece tends to change or changes its dimensions, depending upon the magnitude of the load. When the load is removed it can be seen that the deformation disappears. This occurred up to a certain value of the strain called elastic limit. This is depicted by the straight line relationship and a small deviation thereafter, in the stress-strain curve.

Within the elastic range, the limiting value of the stress which the stress and strain are proportional, is called the limit of proportionality. In the actual plotting of the curve, the proportionality limit is obtained at a slightly lower value of a load than the elastic limit. This may be attributed to the time lag on the regaining of the original dimensions of the test piece. This effect is frequently noticed in some non-ferrous metals. Actually, the elastic limit is distinguishable from the proportionality limit more clearly depending upon the sensitivity of the measuring instrument.

When the load is increased beyond the elastic limit, plastic deformation starts. Simultaneously the specimen gets work hardened. A point is reached when the deformation starts to occur more rapidly than the increasing load. This point is called the yield point. The metal which was resisting the load till then, starts to deform somewhat rapidly , i.e., yield. 

The highest value of the stress after which sudden extension occurs is known as the upper yield point., whereas the lower yield point is the stress which produces a considerable extension or elongation. In routine testing, it is the lower yield point which is measured.

Some materials exhibit a definite yield point, in which the yield stress is simply the stress at this point.

However, most ductile materials do not have a clear-cut yield (sharp-kneed stress-strain diagram ) point. In this case, The curve passes smoothly from elastic action to the section corresponding to plastic deformation. For such materials a "proof-stress" at a specified strain is calculated. This is usually determined upon completion of the test by an "offset method". Although 0.2 percent is often employed for steels, and as much as 0.5 per cent for cast iron. However, from the point representing a strain of 0.2 per cent on the zero stress axis a line is drawn parallel to the elastic portion of stress- strain curve. This line will intercept the stress strain curve at the point at which the material being tested departs from elastic behavior by 0.2 percent. The stress at this point is the yield strength. 

As said before , the gauge length is marked on the standard specimen. After the specimen is broken , the two pieces are kept together as if the specimen is not broken at all, with the two fractured surfaces matching each other. The distance between the two gauge lengths is again measured. The elongation and the percent elongation are computed. 

Percentage reduction in area is the decrease in cross-sectional area of the specimen up to failure, expressed as a percentage of the original cross-sectional area. Both percentage reduction in area and elongation indicate ductility of a material.

The modulus of Resilience is represented graphically by the area under the stress-strain curve up to the proportional limit., It may be calculated other wise from data taken from the curve.

The modulus of toughness is assessed by the total area under the stress-strain curve. The total area under a stress-strain curve is greater from low and medium carbon steel and , therefore, they are tougher materials.

The Tensile or ultimate strength is the stress corresponding to the maximum load reached before rupturing the specimen.

Hot tensile test is carried out to determine mechanical material properties at high temperature. Hot elastic limit and hot tensile strength depends on the test time as well as temperature.