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In physics, stress is the internal distribution of forces within a body that balance and react the loads applied to it. Stress is a nine-dimension tensor that can be fully described with six dimensions. Simplifying assumptions are often used to represent stress as a vector for engineering calculations.
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 <math> \boldsymbol{\sigma} <math>), and force parallel to the area causes shear stress (usually denoted by <math> \boldsymbol{\tau} <math>).
Engineers use this technique to break down stress calculations to simpler elements. The actual distribution of stresses is much more complicated (even for simply loaded members).
By definition, brittle materials fail under normal stress, and plastic or ductile materials fail under shear stress.
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.
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.
For instance, a steel bolt 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 and, for any particular geometry, 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.
Augustin Louis Cauchy enunciated the principle that, within a body, the forces that an enclosed volume imposes on the remainder of the material must be in equilibrium with the forces upon it from the rest of the body.
This intuition provides a route to characterizing and calculating complicated patterns of stress. To be exact, the stress at a point may be determined by taking the limit of the load being carried by a particular cross section, divided by that cross section, as the area of the cross section approaches zero. In general the stress may vary from point to point, but for simple cases, such as circular cylinders with pure axial loading, the stress is constant and equal to the cross-sectional area divided by the applied load.We can consider a small element of the body that has an area ΔA, over which a force ΔP acts. By making the element indefinitely small and taking the limit:
\sigma = \lim_{\Delta A \to 0} \frac {\Delta P} {\Delta A} <math>
- where σ is the stress. Because the force is a vector, the stress has two components, one in the plane of the area, A, the shear stress, and one perpendicular, the normal stress.
Plane stress is a two-dimensional state of stress in a body. This is a good model when a flat thin body is loaded in the plane of the body. A small volume element in equilibrium experiences forces in balance (Figure 1).
By specifying some co-ordinates, the forces P and Q can be resolved normal and perpendicular to the faces of the volume element (Figure 2).
The stresses on the element are (Figure 3):
Cauchy was the first to demonstrate that it is always possible to transform the co-ordinates on the body into a set in which the shear stress vanishes. The remaining normal stresses are called the principal stresses.
A graphical method for analyzing plane stress was proposed by Otto Mohr in 1882.
1. Construct an orthogonal pair of axes where the horizontal represents normal stress and the vertical, shear stress (clockwise shears are represented as positive).
2. For any pair of normal stresses (σx, σy) measured orthogonally, mark their magnitudes on the horizontal axes.
3. Mark the mid-point of the two normal stresses, O. (Figure 4)
4. Draw a perpendicular from each marked normal stress with magnitude equal to the corresponding shear stress (τxy) measured in the same co-ordinate system and call its end-point A (Figure 5)
5. Draw a circle with radius OA, centered at O.
6. The points where the circle crosses the horizontal axis represent the magnitudes of the principal stresses. (Figure 6)
Mohrs circle may also be applied to three-dimension stress. In this case the diagram has three circles, two within a third.
Engineers use Mohrs circle to fully examine the state of stress in a loaded structural element. For example, if the material is Brittle the engineer might use Mohrs circle to find the maximum component of normal stress (tension or compression), and for Ductile materials the engineer might look for the maximum shear component of stress.
The considerations above can be generalised to three dimensions.
Because the behavior of a body does not depend on the co-ordinates used to measure it, stress can be described by a tensor. The stress tensor is symmetric and can always be resolved into the sum of two symmetric tensors:
In the case of a fluid, Pascal's Law shows that the hydrostatic stress is the same in all directions, at least to a first approximation, so can be captured by the scalar quantity pressure.
As with force, stress cannot be measured directly but is usually inferred from measurements of strain and knowledge of elastic properties of the material.
The SI unit for stress is the pascal (symbol Pa); in US Customary units, stress is expressed in pounds per square inch (PSI). See also: Pressure (physics)
Residual stress occures in un-loaded structurs for a variety of reasons. Heat from welding may cause localized expansion, when the finished weldment cools, some areas cool and contract more than others, leaving residual stresses. Another example are certian types of gun barrles, which are made with two tubes forced together. The inner tube is compressed and the outer tube streched. This has the effect of better stress distribution when the gun is fired.
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