Proof stress is the amount of stress that a material, usually metal or plastic, undergoes a level of deformation. Proof stress is important to monitor if the material being used in manufacturing is undergoing any deformation or what's commonly referred to as plastic deformation. Plastic deformation is when such an amount of stress has been applied to plastic or metal that it irreversibly changes its size, shape, or composition. Proof stress is used in metal manufacturing and is also particularly helpful for the maritime industry.
What Proof Stress Means
Proof stress also refers to the stress that will cause a specified small, permanent extension of a tensile test piece.
A tensile test is conducted when a piece of material (whose dimensions have been calculated) is placed between a pair of grips. Weight is applied to "the material gripped at one end while the other end is fixed," according to Michigan Technological University, a public research university located in Houghton, Michigan. According to Michigan Tech, "We keep increasing the weight (often called the load or force) and at the same time we measure the change in length of the sample."
In other words, proof stress is the point at which a particular degree of permanent deformation occurs in a test sample. Proof stress is also called offset yield stress. Typically, the stress needed to produce 0.2 percent of plastic deformation is considered proof stress. It's important to determine the value or level of stress that can be applied before a material "yields" or changes its composition, size, or shape.
Understanding Yield Stress
Yield stress refers to the level when a metal or other material is no longer elastic. This term can be used in reference to other materials as well. Another way to think of yield stress is when stress is no longer constant upon application of strain. When this takes place, it is known as the yield point. In other words, stress that's applied beyond the yield point can lead to significant permanent deformation or fracture of the material. For example, the steel beams used in the construction of a building can only handle a certain amount of stress before it yields.
The manufacturer Brookfield Amatek (the leader in viscosity, texture analysis, and powder flow instrumentation) describes yield stress, as it pertains to a wide range of different materials, like the following:
"A yield stress will often inhibit flow under the relatively low stresses induced by gravity; giving sag and slump resistance to products such as adhesives; plaster and stucco; thick-film inks; molten chocolate; paint, and fire-retardant coatings. With some products the presence of a yield stress is not so desirable, leading to, for example, dosing problems in gravity-feed systems or an excess of residue on the sides of inverted bottles."
Why Proof Stress Is Used
Whether the proof stress of steel, stucco or a condiment like ketchup is being assessed, the goal is to identify the point at which the substance will yield because many materials are used while they are still elastic—in other words, a while before their yield point. Proof stress tests the properties of materials, their microstructure, or metallurgical state.
Testing proof stress reportedly originated before the development of mathematical theories on stress and strain. Long ago, to test a sample's proof stress, people applied a load to the material they wanted to test to determine the resulting stress level. Following the application of the load, they removed the load. The stress was considered a failure if the level was more than the 0.2 percent mentioned above.
Detecting the proof stress of a material is said to be particularly helpful for the maritime industry because it shows how much force ships, or platform legs, can take before permanent damage occurs.