Metal Tensile Test

1. The purpose:

① Measure the yield limit, the ultimate strength, the elongation fracture of low carbon steel.

② Measure the ultimate strength of cast-iron.

③ Observe the phenomena when low carbon steel and cast-iron are pulling, work out the curve that the deformation changing with the outer force(F-△L curve).

2. The equipments and the instruments.

Universal material testing machine, screw micrometer.

3. Sample.

In this experiment, we choose “ten times sample”, which means Lo=10do.

4. The theory in this experiment.

① The experiment of pulling low carbon steel.

There are 4 stages in the deformation of low carbon steel: elastic stage, yield stage, intensive stage, local deformation stage. Ps is the minimum load in the second stage. And Pb is the load after which the sample comes to the 4th stage.

② The experiment of pulling cast-iron.

There is no yield stage in the test of cast-iron, when the load comes to the ultimate strength Pb, cast-iron will broken. And

we do not test the elongation of cast-iron.

5. Procedure.

① Measure the size of low carbon steel.

Measure the diameter this sample, calculate the original sectional area of this sample. Ao=πdo2/4. And Lo=100mm.

② Measure the size of cast-iron.

Measure the diameter “do” of this sample, and then calculate the original sectional area Ao.

③ Install samples.

④ Set up parameter.

⑤ Do the experiment.

Do the test about low carbon steel first, with the help of computer get the pulling curve, get the ultimate load and the yield load. When comes to the experiment about cast-iron, get the ultimate load only.

Get the diameters of the broken two samples.

⑥ End the experiment.

6. Final data processing.

All the data have three significant figures.

The mechanical properties of low carbon steel:

yield limit σs=Ps/Ao=0.296GPa

ultimate strength σb=Pb/Ao=0.426GPa

elongation δ=(L1-Lo)/Lo×100%=31.8%

sectional shrinkage ?=(Ao-A1)/Ao×100%=67.8%

Ao=πdo2/4; A1=πd12/4;

The mechanical properties of cast-iron:

ultimate strength σb=Pb/Ao =0.174GPa Ao=πdo2/4.

Questions:

1. The fracture of low carbon steel is uneven with gray color. However the fracture of cast iron is flatter than low carbon steel, and there is shinning crystals at the fracture.

The essential reason is that the crystal structure of these two materials is different, hence it is plastic deformation on low carbon steel and as cast iron is brittle material there is no plastic deformation on it.

2. The σs , σb for low carbon steel is the same as the true stress at yield stage. However when the steel rupture, the σs , σb is smaller than the true ones.

3. Low carbon steel is ductile material and cast iron is brittle material.

4. The percent elongation decreases when the length increases. This is because the elongation mainly occurs at the necking place. And the sectional shrinkage has nothing to do with gage length.