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Home > Stress (physics)

A simplified definition for stress is force divided by area (see also pressure (physics)). Force normal to the area causes normal stress (usually denoted by ), and force parallel to the area causes shear stress (usually denoted by ).

Stress can occur in liquids, gases and solids. Liquids and gasses support normal stress (pressure), but flow under shear stress (see viscosity). Solids support both shear and normal stress, with brittle materials failing under normal stress and plastic or ductile materials failing under shear stress.

1 Stress in one dimension

The idea of stress originates in two simple, but important, observations of the loading (in tension) of a one-dimensional body, for example, a steel wire.

1. When a wire is pulled tight, it streaches (undergoes strainStrain is also a biological term. Strain in any branch of science dealing with materials and their behaviour, is the geometrical expression of deformation caused by the action of stress on a physical body. Strain therefore expresses itself as a change in). The amount it streaches is proportionalThis article is about proportionality, the mathematical relation. For other uses of the term proportionality see proportionality (disambiguation). In mathematics, two related quantities x and y are called proportional (or directly proportional if there ex to the load divided by the cross-sectional areaThis article explains the meaning of area as a Physical quantity. Article area (geometry) is more mathematical. Area is a quantity expressing the size of a region of space. Surface area refers to the summation of the areas of the exposed sides of an objec of the wire: σ = F / A.

1. Failure occurs when the load exceeds a critical value for the material; the tensile strengthThe tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before it breaks. Tensile strength is an important concept in engineering, especially in the fields of material science, mechanical engineering and stru multiplied by the cross-sectional area of the wire, F_c = σ_t x A

These observations suggest that the fundamental characteristic that affects the deformation and failure of materials is stress, force divided by the area over which it is applied.

This definition of stress, σ = F / A, is sometimes called Engineering stress and is used for rating the strength of materials loaded in one dimension. The crossectional area is measured prior to applying strainStrain is also a biological term. Strain in any branch of science dealing with materials and their behaviour, is the geometrical expression of deformation caused by the action of stress on a physical body. Strain therefore expresses itself as a change in for testing. Poisson's ratioWhen a sample of material is stretched in one direction, it tends to get thinner in the other two directions. Poisson's ratio (ν) is a measure of this tendency. It is defined as the ratio of the strain in the direction of the applied load to the strain, however, reveals that any applied strain will produce a change in the area, A. Engineering stress neglects this change in area. Stress strain diagramsA stress-strain curve is a graph derived from measuring load ( stress) versus extension ( strain) for a sample of a material. The nature of the curve varies from material to material. The curve usually seen is for a ductile metal. This features an initial are usually presented as engineering stress, even though the sample may undergo a substantial change in crossectional area during testing.

True stress is a simplified definition of stress that includes the change in crossectional area. It should be noted, however, that True stress is still a simplified version of the stress tensor.

An example; a steel boltA bolt may be one of the following things: A type of threaded fastener. See screw . A rotating mechanism used in some firearms. See bolt action . A lightning bolt . The ammunition used in a crossbow. See quarrel . An item of door furniture: a bar that is of diameter 5 mm, has a cross-sectional area of 19.63 mm². A load of 50 N induces a stress (force distributed across the cross-section) of σ = 50/19.63 = 2.55 N/mm² ( MPa). This can be thought of as each m² of bolt supporting 2.55 MN of the total load. In another bolt with half the diameter, and hence a quarter the cross-sectional area, carrying the same 50 N load, the stress will be quadrupled (10.2 MPa).

The ultimate tensile strength is a property of a material loaded in one dimension. It allows the calculation of the load that would cause fracture. The compressive strength is a similar property for compressive loads. The yield tensile strength is the value of stress causing plastic deformation. These values are determined experimentally using the measurement procedure known as the tensile test .

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Topics: Stress (physics) Area Stresses Normal Shear Body Tensor Force...

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