Tensile Strength: Yield or Break

Most of us in the plastic materials industry have some familiarity with the various ASTM and ISO test methods that are reported on data sheets and certificates of analysis. However, if you do a deep dive into any of the test methods, and by deep dive I mean read from stem to stern, you will find all kinds of things that you might not have thought of. Having set up and run an accredited testing laboratory in the past, I performed deep dives into many of the ASTM and ISO procedures.

Today, I want to explore one small part of the ASTM D 638 procedure for tensile properties. Before you say that you would rather drink battery acid, let me assure you that I will not dive too deep and I will leave you with some practical information and you will be able to point out a mistake on a lot of data sheets and certificates of analysis in the future.

When a tensile strength (or stress) test is performed, a test specimen is clamped into two jaws; the top jaw is attached to a movable cross-head with a load transducer that measures the load being exerted on the specimen. The machine also measures how far the specimen is stretching. From these two measurements, the machine can generate what is called a stress/strain chart. The Y axis (vertical) is the load (stress) and the X axis (horizontal) is the elongation (strain).

Now that you understand that, take a look at this graph that is buried deep in the ASTM D 638 procedure.

The graph shows three different types of stress/strain curves that can happen when testing plastics.

The curve with the A at the end (farthest left) is the type of curve that you get if you test a very rigid material like glass fiber filled materials, or mineral filled materials (other than TPO). The stress and strain increase in a linear fashion until a break happens.

The curve with the B and C at the end (middle) is similar to what you get with something like unfilled polypropylene or nylon or even an unfilled styrenic. The load increases until it reaches a peak and then decreases until the specimen breaks.

The curve with the D and E at the end (farthest right) is quite different. The stress increases, has a small flat spot and then continues to increase and then breaks at a higher load. You would get a curve like this when testing polyethylene or TPE.

Here is the interesting part. Only the second and third curves result in a yield point. The 2^nd curve yield point is at the B but the third curve yield point is at D, the flat spot in the curve or plateau. The first curve does not give you a yield point; it only gives you a break point.

The bottom line here is that for glass or mineral filled material with the exception of some TPEs, you should only report a tensile strength at break on your data sheets and certificates of analysis. These materials usually do not give a tensile strength at yield.

For materials that give a curve similar to the second one, the point labeled B is typically reported as the yield point and while you could report a tensile strength at break, it would be a bit unusual. The material has gone well past any elasticity that it had when it reaches this point so the value is not considered important.

Materials that give a curve similar to the third one have to be measured very carefully. In this case, the tensile strength at break will be higher than the tensile strength at yield. This tensile strength at break is not typically reported for materials that exhibit a curve like this for reasons similar to why it is not reported on the second curve in the illustration. On a side note, the third curve shown in the illustration is a bit more exaggerated than what you get in real life. The plateau is not usually that pronounced and is often not completely horizontal. These can be a bit tricky to measure.

If you have one of the new fangled fancy machines, the computer software kind of measures all of this behind the scenes and then it spits a number out at you but it pays to understand what is going on and how different materials behave.

So to recap, higher elongation unfilled materials will exhibit a stress/strain curve similar to the center and farthest right curves. For these materials, tensile strength at yield should be reported and tensile strength at break should not be reported (measured at points B and D respectively). Low elongation materials like highly reinforced materials will give curves similar to the farthest left curve. For these materials, tensile strength at break should be reported because they do not typically exhibit a yield point.

If you look through data sheets for glass filled materials, you will find a fair amount of examples of this being incorrectly reported.