When is proof stress used

The stress corresponding to the proof point is called as proof stress. Proof stress hence forms design stress for some of the metals. It tests the properties of the materials, the metallurgical state or their microstructure. Join Join TheConstructor to ask questions, answer questions, write articles, and connect with other people.

Have an account? Log in Now. Join for free or log in to continue reading Remember Me! Don't have account, Join Here. Forgot Password Lost your password? Ask A Question. What is Proof Stress? Importance and Determination. Importance and Features of Proof Stress The role of proof stress comes into play during the material selection process.

Determination of Proof Stress The stress-strain curve of certain ductile materials from their tension test does not provide a specific value for yield stress and yield point. This type of stress is typically used in the manufacturing industry to ensure that a material is not stressed far beyond its elastic limit.

That is, a safe load is applied such that the object is not stressed to the point where it irreversibly changes in size or shape. If a material is allowed to exceed its proof stress, it can deform permanently, and failure or fracture can ultimately occur. Proof stress is particularly useful in the marine and offshore industries. The proof stress of ships and platform legs plays an important role in determining how much force these structures can take before they become permanently deformed.

Proof stress is also known as offset stress because it is the stress required to make the material yield. Proof stress is usually calculated for metals that do not have a properly defined yield strength. For example, consider the stress-strain graph below for low carbon steel. The graph has a distinct shape where the yield stress is easily identifiable.

In other words, beyond the yield point , if the force is released from the object it will experience permanent deformation and no longer return to its original shape.

Therefore, the yield point is an approximate estimate of the elastic limit of the material. In this case, we simply project a horizontal line from the yield point to the y-axis of the stress-strain curve to get the value of the yield stress. However, in some other steels determining the yield stress graphically is not as straightforward. For example, non-low-carbon steels and some non-ferrous steels such as copper, brass and aluminum have no definitive yield point.

Rather, the straight-line portion of the graph the elastic zone gradually transitions to a non-linear curve. This blurred transition from the elastic zone to the plastic zone makes identifying the yield stress less difficult. Since the yield point or yield stress is not obvious, proof stress is used. This gives a stress value that will produce a very small amount of known plastic deformation.

By keeping a material below its proof stress, we can ensure that parts and components can be used safely in their application even though the exact yield value may not be known.

Proof stress is determined using a tensile test. Also known as nominal strain. Engineering stress is the applied load divided by the original cross-sectional area of a material. Also known as nominal stress. Engineering strain is the amount that a material deforms per unit length in a tensile test…. If the true stress, based on the actual cross-sectional area of the specimen, is used, it is found that the stress-strain curve increases continuously up to fracture.

If the strain measurement is also based on instantaneous measurements, the curve, which is obtained, is known as a true-stress-true-strain curve. What is Strain? According to the strain definition, it is defined as the amount of deformation experienced by the body in the direction of force applied, divided by initial dimensions of the body. The relation for deformation in terms of length of a solid is given below. The stress-strain curve is a graph that shows the change in stress as strain increases.

It is a widely used reference graph for metals in material science and manufacturing. So, the proportional limit is defined as the highest stress at which the stress-strain curve is a straight line. Below the proportional limit, there is no permanent deformation in a structure, that is, the object returns to its original position after the removal of applied force.

The curve based on the original cross-section and gauge length is called the engineering stress-strain curve, while the curve based on the instantaneous cross-section area and length is called the true stress-strain curve. Unless stated otherwise, engineering stress-strain is generally used. However, if you perform uniaxial compression, the instantaneous cross section area actually increases and the value of true stress will be lower than the engineering stress.

True strain is however always larger than engineering strain! That is because most materials have a elastic strain limit close to 0. For linear elastic, Hookean materials, it is generally the case strain at the elastic limit is very small. Yield stress, marking the transition from elastic to plastic behaviour, is the minimum stress at which a solid will undergo permanent deformation or plastic flow without a significant increase in the load or external force.

Yield strength can be explained, in engineering and materials science, as the stress at which a material begins to plastically deform. The value of yield strength is important in the construction of structures, such that the structures are able to perform in the elastic region under normal servicing conditions.

To find yield strength, the predetermined amount of permanent strain is set along the strain axis of the graph, to the right of the origin zero. It is indicated in Figure 5 as Point D.