The Breakdown of Tensile Testing

Monday, January 27, 2014

Just What is Tensile Testing?
Tensile testing is an experiment to test the strength or mechanical behavior of a certain material. The basic idea of the test is to place a tensile specimen, (commonly referred to as a “dog bone” due to its shape) between two grips which hold the specimen in place. The known starting dimensions of the specimen such as length and cross-sectional area are noted. A load is applied to the specimen at one end, while the other end is held in a fixed position. The load or force is continually increased until the part begins to stretch and eventually breaks. Changes in the length of the specimen are noted along the way.

What do the results look like?
The results of a tensile test are a graph, known as the “stress-strain curve”, showing the ratio of force (amount of load) to displacement (amount stretched).

How do you compare it to other materials?
Comparison to other materials can be very challenging, but is important when designing for structural applications.

Since all components come in different sizes and shapes, we must compare the strength of the material independent of size. To do so requires the machining of the dog bone specimen. You must divide the load applied to the specimen by the initial cross sectional area which gives you the stress parameter. You must also divide the displacement by the initial length of the material giving you the strain parameter. With these two parameters you can compare the strength of different materials without their size being an issue.

Standard Cross-sectional Area Equation: A = πd2/4 where d is the initial diameter
Stress = load/cross-sectional area or σ = F/A
Strain = displacement/initial length or ε = ΔL/L

Below is a standard tensile “stress-strain curve” thanks to Michigan Tech University’s Department of Science and Engineering. This will go more in-depth into how to read the results of a tensile test.

  1. Elastic Slope: This is the elastic region of the material meaning that until Point 2 is reached the material stretches in an elastic or reversible manner. This also means that the specimen should return to its original length if the load is removed.
  2. 0.2% Offset Yield Strength: This is where the curve has begun to bend over and marks where the specimen begins to permanently change length. This tells us the amount of stress that can be applied to the material before it starts to yield or permanently deforms.  For this specimen the yield stress (or strength) value is 88 ksi and it is important that when this material is used that the amount of stress applied never exceeds this value.
  3. Ultimate Tensile Strength (UTS): This is the maximum amount of stress that the material can withstand. After this point the stress begins to trend downward. This is marked by necking of the specimen where significant localized reduction of the diameter occurs.
  4. Failure or Fracture: This is the point at which the specimen breaks or fails. The total amount of change in length enables us to figure the ductility of the material also known as the elongation.

Want to do a Tensile Test at Home?
Follow the steps at to make your own at home experiment.

No comments:

Post a Comment

There was an error in this gadget