Laboratory

TENSILE TESTING

1. Objective:

The objective of this test is to determine how different materials will react to forces being applied in tension.

2. Tensile testing theory:

A tensile test, also known as tension test, is probably the most fundamental type of mechanical test you can perform on material. Tensile tests are simple, relatively inexpensive. By pulling on a material, it will very quickly react to forces being applied in tension.

A complete tensile profile ( curve) will result showing how the material reacted to the forces being applied. Typical profiles for different materials are shown in Figure .1 Each material has a characteristic pattern of stress and strain.  Important material data can be read from the stress-strain diagram.

σ It can be noticed that in the initial portion of the test, the relationship between the applied force, or load, and the elongation the specimen exhibits is linear. In this linear region, the line obeys the relationship defined as “Hooke’s Law” where the ratio of stress to strain is a constant,

E is the slope of the line in this region where stress (σ) is proportional to strain (ε) and is called the “Modulus of Elasticity” or “Young’s Modulus“.

Stress

The Stress  is defined as the ratio of the force applied F (Newton N) to the cross sectional area A (mm²) of the material being tested

Strain

It is the ratio of the change in length to the original length L_o

Yield Strength

A value called “yield strength” of a material is defined as the stress applied to the material at which plastic deformation starts to occur while the material is loaded.

Offset Method

For some materials (e.g., metals and plastics), the departure from the linear elastic region cannot be easily identified. Therefore, an offset method to determine the yield strength of the material tested is allowed. An offset is specified as a % of strain (for metals, usually 0.2% and sometimes for plastics a value of 2% is used)..

Ultimate Tensile Strength

One of the properties that can be determined about a material is its ultimate tensile strength (UTS). This is the maximum load the specimen sustains during the test. The UTS may or may not equate to the strength at break. This all depends on what type of material you are testing. . .brittle, ductile, or a substance that even exhibits both properties.

3.Experimental Apparatus

Basically the specimen is placed between the gripping heads (6). The hand wheel is turned clockwise . Any increase in the load is read and recorded from the Load gage (3) . At the same time any increase in the material elongation read and recorded from the gauge for deformation displacement (5).

 

Samples                                                    Copper, steel, Aluminum, brass

All samples have the same dimension

4-Procedure:

• Measure and note down both the test length Lo and the diameter of the sample.
• Screw the sample by hand into the lower gripping head as far as the end stop
• Screw the sample into the upper gripping head as far as the end stop, by rotating the gripping head itself.
• Tighten the nut on the upper gripping head by hand until the gripping head is seated without slack in the upper cross-member.
• Set the maximum pointer on the display to zero.
• Make sure all the wiring connection to the PC (10) and the measuring amplifier (11) are properly done.
• Adjust properly the electronic position sensor (12)
• Slowly load the sample by rotating the hand wheel (2).
• Avoid sudden force application.
• Record both the Force F and the Elongation ΔL  from the measuring amplifier(13)
• Keep loading the sample and recording F and ΔL until the sample breaks.
• Repeat steps 1 to 11 for other samples.

Questions

• Show the Stress – Strain relation of all the materials on one common diagram or chart.
• Develop a table that summarizes the main important properties that can be learned from a tensile test.
• Discuss your results in terms of ductility, brittleness, strength, elongation, yield strength and break point strength.