What Is the Definition of Proof Stress?

How This Method Is Used in Metal Manufacturing

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Proof stress is a method that is used in metal manufacturing and is also particularly helpful for the maritime industry.

What Proof Stress Means

Simply put, proof stress is a term used in general engineering to describe 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. Then, 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."

A simpler way to think of proof stress is to imagine the point at which a particular degree of permanent deformation occurs in a test sample. Commonly, the stress needed to produce 0.2 percent extension is quoted in N/mm2 for steel. This value approximates to the yield stress in materials not exhibiting a definite yield point. 

Understanding Yield Stress

If you're unfamiliar with what yield stress is, think of it this way. Stress is the term used in general physics to describe the level when a metal 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.  

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, as 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 simply applied a load to what they wanted to test to determine that the correct stress level resulted. After they did that, they took the load away. 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 results in one knowing how much force ships, or platform legs, can take before permanent damage occurs.