How to Measure Tensile Strength, Elastic Modulus, and Ductility
Tensile strength, or ultimate tensile strength, measures a metal’s resistance to breaking or pulling apart in two pieces. To find out how strong a wire is, one could just hang heavier and heavier weights on it until it breaks. The weight of the load, or number of pounds, it finally took to break that wire is its breaking strength, in pounds. You will see both working load and breaking loads shown on packages of rope from the hardware, for example.
Obviously a bigger wire takes more pounds to break than does a smaller one. If we want to know about the metal itself, not just how big is the wire, we measure stress. Stress is simply the number of pounds pulling on that wire, divided by the cross sectional area, in square inches. The result is “psi”, or pounds per square inch.
Consider a 1x1” square bar of RA330 that broke after pulling a load of 80,000 pounds. The tensile strength of that RA330 bar would be 80,000 pounds, divided by one square inch, or 80,000 pounds/square inch. If a bar of the same metal were only 1/2x1/2”, its cross sectional area would be 1/2’ times 1/2’ = 1/4 square inch. That smaller bar of RA330 would break after pulling on it with only 20,000 pounds, but its tensile strength (20,000# divided by 1/4 inch2 ) would still be 80,000psi.
Stress is the amount of pounds pulling on the specimen, divided by the cross sectional area in square inches. So the result is “psi”, pounds per square inch.
Before that bar of RA330 actually broke, it would have stretched out, a bit like taffy, to a lot longer than its original size. We call that “% elongation”. The tensile specimen has two marks on it, usually 2” apart. After the metal breaks, the two halves are fit back together and the distance between those two marks measured. It would be common for that 2” of metal to have stretched out to about 2-3/4” before the metal broke. 3/4” of stretch divided by that original 2” is 0.375, so the tensile test specimen elongated 37-1/2%. That is the %EL shown on a Mill Test Report.
This is a basic measure of how stiff the metal is, not something you will find on an MTR.
When you start pulling on the tensile test specimen, it begins to stretching like a rubber band. If you stopped the test, when the load was removed the specimen would spring back to its original length. This is the “elastic” portion of the tensile test.
If those of you who are mathematically inclined would graph the stress on one axis, versus strain (how much it stretched), you would get a straight line. The slope of that straight line is called the Elastic Modulus, also called Young’s Modulus, with the symbol “E”. We publish this Modulus in our data sheets, as it is important to those designing parts near room temperature.
Above perhaps 1000°F metals no longer behave like a rubber band or a spring, and the elastic modulus has no meaning or use.
At some point during the tensile test, usually well before the specimen breaks, it takes a set, or a permanent stretch. This is called the “Yield Strength” (or Proof Strength). For austenitic alloys this point is a little vague, the curve just slowly bends over. So engineers have made a definition of yield strength by drawing a line parallel to the elastic part, just offset a bit. This is usually recorded on the mill test report as the 0.2% Offset Yield Strength.
Before the specimen breaks it has stretched out a great deal, and has necked down in the area where it breaks. The amount it had stretched when it broke is the “% Elongation”, and the amount it necked down is the “% Reduction of Area”. Both are measures of ductility. The fabricator needs ductility so he can form the metal. In service the metal needs ductility so that it will bend a little, if something goes wrong, rather than shatter.
When designing a machine part, obviously the design stress has to be below the tensile strength of the metal, or the thing would break in two. But the machine would also be useless if its parts bent, or yielded, so the designer must keep the stress somewhere below the yield strength of the metal.
For heat resistant alloys, yield and tensile strength may be used for design up to about 1000°F. Above this temperature, the life of the part will be limited by the metal’s creep-rupture properties, and not by its tensile properties. “Creep-rupture” means that, even though the load may not be enough to yield the metal, it will very, very slowly stretch (or creep) as time goes on.