Testing Plastics and Measuring Performance
In order to better understand the various discussions about loads, stresses and the expected performance of plastics, it helps to have an understanding of the fundamental terms used in those discussions. The following paragraphs give a very brief background on those terms. Please note that this is a very basic explanation of subjects, which some engineers spend their entire careers studying.
Stress and Strain
When a system of opposing forces acts on a component, the material is subject to some form of stress. Normal stress is the ratio of applied load to the original cross sectional area: load divided by area. Stress is a value, which describes the amount of load carried by each unit of cross sectional area of a component.
For example, suppose a shaft 1cm² holds the block shown in the picture to the left, and say the load applied is 10Kg. If you double the cross section of the shaft, you half the stress on it – the same weight supported by twice the area. Its just the same as you would expect in an office – if there is the same load on the office, you would be under a lot of stress if you were on your own, but half that if there were two of you, and so on.
Since the load shown in the picture is trying to lengthen the shaft, it is called tensile stress. If the shaft was supporting the block from underneath, the block would be trying to shorten the shaft, and the stress would be called compressive stress.
Loads, which produce tensile and compressive stresses, are acting at right angles to the areas on which they act. Tensile and compressive stresses are often referred to as normal stresses. Since most plastics are usually subjected to tensile stress, this is the value most quoted. A good example is polyethylene bags – the handles of these seemingly innocuous items are the subject of much research – manufacturers are constantly trying to reduce the thickness of the bags to reduce costs, but the handles have to have the same resistance to stress – we have all had experience of what happens to a bag when they fail!
Forces which act parallel to the areas resisting them are known as shear forces, and produce shear stress in the elements which carry those loads.
For example, the forces in the picture to the right are trying to pull the clevises apart, and in so doing, they apply a shear force (parallel to the cross-sectional area of the bolt) on the bolt holding the parts together. The shear force is trying to cut the bolt in half across its diameter in two places, where the two outer faces of the lower clevis meet the two inner faces of the upper clevis. This instance is known as "double shear" because there are two separate areas of the bolt exposed to the shear force.
Bending Stress occurs when a component is loaded by forces which, instead of trying to stretch or shrink the component, are trying to bend it. Those bending forces generate a combination of tensile and compressive stress in the load-carrying components, known as bending stress.
Bending Stress Illustration 1
An example of that type of loading is shown in this picture, where the tubular shaft is resting on two triangular supports. The two vertical arrows represent downward forces applied to the tube. The tube deflects, or bends, downward under the influence of those forces, as illustrated. This is a much more complex force, as if you imagine cutting across the tube, the area on the inside of the curve will be under compression, and the area on the outside will be under tension – two opposing stresses on the same sample!
Strain is the measure of how much a material deforms when a load is applied to it.
Suppose you measure a specimen which has no load applied to it. Then you apply a load to the specimen, then release the load and measure the specimen again. If you find that it has returned to its original length, then the specimen experienced elastic deformation when it stretched under the load.
Most materials are elastic. That is, if you apply a load to the material, it will deform in some way, by an amount which is proportional to the load. When you remove the load, the material will return to its original shape, as long as the load wasn't too large. The deformation might be too small to measure, but it still occurs.
If, after the load is released, you can measure some permanent deformation in the specimen, (the specimen does not return to its original length), then the material has been stressed beyond its elastic limit and has experienced plastic deformation. Using our carrier bag as an example, the bags with the bread in will look similar after use to the ones with the tins in, but there will be a difference in their lengths due to the extra weight applied.
To summarise, using people as an example…
STRESS is an external force – this is the pressure applied to you and increases if there are fewer of you. If you can resist the stress, you can bounce back for the next day. If, however, the load on the office exceeds your limits, you feel STRAIN, and cannot recover fully for the following day. If the same pressure is applied tomorrow, you will feel more strain, and will eventually break! – Not a nice example, but I hope it helped explain it more clearly.