Tensile Strength of Materials

Tensile Strength of Materials.

Introduction.

The strength of a material is important when selecting materials for specific applications. If the material selected cannot withstand the stress applied to it during service, it will fracture as the tensile strength is exceeded. In this laboratory, the tensile strength of two aluminum alloys and a polymer will be measured. The strain rate sensitivity of polymers will also be investigated by straining at different rates.

Equipment.

An Instron tensile test machine will be used to load samples until failure. The maximum load capability of this particular machine is 10,000 lbs. A load cell measures the load on the sample. The load cell capability should match the load frame and the sample being measured. Usually the load cell will operate in percentages of its range, and today with computer technology, this range is automatically updated to provide maximum sensitivity. The machine is a screw driven machine. On either side of the machine are long screw rods, and on these screw rods is mounted the “Crosshead”. As the rods rotate the crosshead moves either up or down depending on the direction of driver motor in the base of the machine which is computer controlled. The load cell is mounted in the center of the crosshead and one end of the load train connects to the load cell, the other connects to the fixed base of the machine. The computer controls the rate of rotation of the screw rods and therefore the rate of cross head movement, which in turns imparts a displacement rate to the sample.

The tensile sample.

Usually the tensile sample has a middle section which has a reduced cross sectional area compared to the ends. Samples can be either rods or flat sheets. The central section of the sample, called the “gage length” has a parallel region and usually extends over some distance. ASTM standards?

The Grips

The sample has to be held and there are several ways of achieving this. The grips for this experiment are self tightening wedge grips, other types are flat grips. Another way is to machine the sample so that it is threaded on the end or has a button where the end diameter is larger than the midsection.

The Test.

Prior to any test several items of data are required. The sample should be numbered on both ends, as after the test two pieces will be present. The dimensions of the gage length should be measured, for example for round bar samples the diameter, for flat sheet samples the width and thickness. These measurements are important for later data analysis. The test conditions to be applied by the tensile test machine should be entered and recorded, for example, the test number, the displacement rate, how the data will be displayed etc.

After the sample has been mounted in the grips, the load and distance should be zeroed. In this way, the weight of the grips is removed from the load applied to the sample. The test can now start. Note that this test is a displacement controlled test, in that the sample is extended and the load required for this extension is measured.

Data.

For metals, normally, the early stages of the load extension curve is non linear as any slack in the loading system is taken up. After this early section, the data becomes linear for a while. At a particular load level, the data again becomes non-linear with the load still increasing, but with a decreasing rate, with increasing extension. Another point arrives where instead of the load increasing with extension, the load starts to decrease with further extension. After a period of this behavior, the sample will fail.

For polymers, the first three stages are sometimes followed by a region of increasing load with increasing extension toward the final stages of extension.

Discussion.

1. What deformation processes are occurring in metals during the tensile test? Elastic deformation, plastic deformation, bond stretching, dislocations, reduction in area, ductility, elongation to failure, yield stress, ultimate tensile stress, Hooke’s Law?
2. What deformation processes are occurring in polymers during the tensile test? Chain sliding, bond stretching, strain rate effects?
3. Failure process, cup and cone failure?
4. Effect of second phases, age hardening compared to strain hardening?
5. Other material behavior, brittle behavior, elastomers, composites?