One of the most important properties of metals is their elastic characteristics, defined as the allowable deformation in a given direction from which the metal can return to its original configuration, once the load is removed. Deforming a common aluminum alloy within the elastic region will illustrate this. Relevant formulas and charts will show how to use these metals safely in a variety of industrial applications.
The illustration below represents both the typical type of test and resulting effects on the specimen. Applying a uniaxial force “F” will result in a change in length of “ΔL”. The accompanying graph shows the results on the specimen as Stress (Y-Axis) versus Strain (X-Axis).
image courtesy of dg7ybn.de
We can verify this curve empirically by setting up a simple test using an MTII/Fullam SEMTester 100 Tensile Stage Unit with a standard “Dog Bone” style specimen made out of 6061 T6 aluminum, having a reduced center section (5 mm wide by 1.6 mm thick). This system is capable of applying 450 N (100 lbf) over 27 mm of travel.
Our evaluation will start with a simple, two-step load test of the specimen:
- a tensile load will be applied slowly until it reaches a maximum load of 450 N
- the system will then switch direction and gradually reduce the load until it reaches 0 N
While the system is tracking the load on the specimen it will also be monitoring the change in length. If the maximum load for this test remained within the elastic region of the Stress-Strain Chart, when the load is removed from the specimen it will return to its original length. If the load was too high and entered the plastic region, the position will not return to 0. Remaining positive position indicates there was some permanent deformation or elongation of the specimen. In the graph below, the position data correlates quite uniformly with both the loading and unloading of the specimen.
This shows that the deformation is linear, meaning it is within the elastic region. There is a small position shift at the end, indicating that when the load was fully released the specimen did not return to its original length. This shows us that the load of 450 N moved briefly into the plastic region and that this load exceeds the safe limit for this material in this cross-section.