- Stress (physics)
Stress is a measure of the average amount of
forceexerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces. It was introduced into the theory of elasticity by Cauchyaround 1822. Stress is a concept that is based on the concept of continuum. In general, stress is expressed as
where : is the average stress, also called engineering or nominal stress, and : is the force acting over the area .
SIunit for stress is the pascal (symbol Pa), which is a shorthand name for one newton (Force) per square metre (Unit Area). The unit for stress is the same as that of pressure, which is also a measure of Force per unit area. Engineering quantities are usually measured in megapascals (MPa) or gigapascals (GPa). In Imperial units, stress is expressed in pounds-force per square inch (psi) or kilopounds-force per square inch (ksi).
As with force, stress cannot be measured directly but is usually inferred from measurements of strain and knowledge of elastic properties of the material. Devices capable of measuring stress indirectly in this way are
strain gauges and piezoresistors.
tress as a tensor
In its full form, linear stress is a rank-two
tensorquantity, and may be represented as a 3x3 matrix. A tensor may be seen as a linear vector operator - it takes a given vector and produces another vector as a result. In the case of the stress tensor , it takes the vector normal to any area element and yields the force (or "traction") acting on that area element. In matrix notation:
where are the components of the vector normal to a surface area element with a length equal to the area of the surface element, and are the components of the force vector (or traction vector) acting on that element. Using
index notation, we can eliminate the summation sign, since all sums will be the same over repeated indices. Thus:
Just as it is the case with a vector (which is actually a rank-one tensor), the matrix components of a tensor depend upon the particular coordinate system chosen. As with a vector, there are certain invariants associated with the stress tensor, whose value does not depend upon the coordinate system chosen (or the area element upon which the stress tensor operates). For a vector, there is only one invariant - the length. For a tensor, there are three - the eigenvalues of the stress tensor, which are called the principal stresses. It is important to note that the only physically significant parameters of the stress tensor are its invariants, since they are not dependent upon the choice of the coordinate system used to describe the tensor.
If we choose a particular surface area element, we may divide the force vector by the area (stress vector) and decompose it into two parts: a normal component acting normal to the stressed surface, and a shear component, acting parallel to the stressed surface. An axial stress is a normal stress produced when a force acts parallel to the major axis of a body, e.g. column. If the forces pull the body producing an elongation, the axial stress is termed tensile stress. If on the other hand the forces push the body reducing its length, the axial stress is termed compressive stress. Bending stresses, e.g. produced on a bent beam, are a combination of tensile and compressive stresses. Torsional stresses, e.g. produced on twisted shafts, are shearing stresses.
In the above description, little distinction is drawn between the "stress" and the "stress vector" since the body which is being stressed provides a particular coordinate system in which to discuss the effects of the stress. The distinction between "normal" and "shear" stresses is slightly different when considered independently of any coordinate system. The stress tensor yields a stress vector for a surface area element at any orientation, and this stress vector may be decomposed into normal and shear components. The normal part of the stress vector averaged over all orientations of the surface element yields an invariant value, and is known as the
hydrostatic pressure. Mathematically it is equal to the average value of the principal stresses (or, equivalently, the trace of the stress tensor divided by three). The normal stress tensor is then the product of the hydrostatic pressure and the unit tensor. Subtracting the normal stress tensor from the stress tensor gives what may be called the shear tensor. These two quantities are true tensors with physical significance, and their nature is independent of any coordinate system chosen to describe them. In fact, the extended Hooke's lawis basically the statement that each of these two tensors is proportional to its strain tensorcounterpart, and the two constants of proportionality ( elastic moduli) are independent of each other. Note that In rheology, the normal stress tensor is called extensional stress, and in acousticsis called longitudinal stress.
Solids, liquids and gases have stress fields. Static fluids support normal stress but will flow under shear stress. Moving viscous fluids can support shear stress (dynamic pressure). Solids can support both shear and normal stress, with
ductilematerials failing under shear and brittlematerials failing under normal stress. All materials have temperature dependent variations in stress related properties, and non-newtonian materials have rate-dependent variations.
Cauchy's stress principle
Cauchy's stress principle asserts that when a continuum body is acted on by forces, i.e. surface forces and
body forces, there are internal reactions (forces) throughout the body acting between the material points. Based on this principle, Cauchy demonstrated that the state of stress at a point in a body is completely defined by the nine components of a second-order Cartesian tensorcalled the Cauchy stress tensor, given by
where:, , and are the stress vectors associated with the planes perpendicular to the coordinate axis,:, , and are normal stresses, and :, , , , , and are shear stresses.
Voigt notationrepresentation of the Cauchy stress tensor takes advantage of the symmetry of the stress tensor to express the stress as a 6-dimensional vector of the form:The Voigt notation is used extensively in representing stress-strain relations in solid mechanics and for computational efficiency in numerical structural mechanics software.
At the same time, equilibrium requires that the summation of moments with respect to an arbitrary point is zero, which leads to the conclusion that the stress tensor is symmetric, i.e.
However, in the presence of couple-stresses, i.e. moments per unit volume, the stress tensor is non-symmetric. This also is the case when the
Knudsen numberis close to one, , e.g. Non-Newtonian fluid, which can lead to rotationally non-invariant fluids, such as polymers.
Principal stresses and stress invariants
The components of the stress tensor depend on the orientation of the coordinate system at the point under consideration. However, the stress tensor itself is a physical quantity and as such, it is independent of the coordinate system chosen to represent it. There are certain invariants associated with every tensor which are also independent of the coordinate system. For example, a vector is a simple tensor of rank one. In three dimensions, it has three components. The value of these components will depend on the coordinate system chosen to represent the vector, but the
lengthof the vector is a physical quantity (a scalar) and is independent of the coordinate system chosen to represent the vector. Similarly, every second rank tensor (such as the stress and the strain tensors) has three independent invariant quantities associated with it. One set of such invariants are the principal stresses of the stress tensor, which are just the eigenvaluesof the stress tensor. Their direction vectors are the principal directions or eigenvectors. When the coordinate system is chosen to coincide with the eigenvectors of the stress tensor, the stress tensor is represented by a diagonal matrix:
where , , and , are the principal stresses. These principal stresses may be combined to form three other commonly used invariants, , , and , which are the first, second and third stress invariants, respectively. The first and third invariant are the trace and determinant respectively, of the stress tensor. Thus, we have
Because of its simplicity, working and thinking in the principal coordinate system is often very useful when considering the state of the elastic medium at a particular point.
tress deviator tensor
The stress tensor can be expressed as the sum of two other stress tensors:
# a mean hydrostatic stress tensor or volumetric stress tensor or mean normal stress tensor, , which tends to change the volume of the stressed body; and
# a deviatoric component called the stress deviator tensor, , which tends to distort it.
where is the mean stress given by
The deviatoric stress tensor can be obtained by subtracting the hydrostatic stress tensor from the stress tensor:
Invariants of the stress deviator tensor
As it is a second order tensor, the stress deviator tensor also has a set of invariants, which can be obtained using the same procedure used to calculate the invariants of the stress tensor. It can be shown that the principal directions of the stress deviator tensor are the same as the principal directions of the stress tensor . Thus, the characteristic equation is
where , and are the first, second, and third deviatoric stress invariants, respectively. Their values are the same (invariant) regardless of the orientation of the coordinate system chosen. These deviatoric stress invariants can be expressed as a function of the components of or its principal values , , and , or alternatively, as a function of or its principal values , , and . Thus,::Because , the stress deviator tensor is in a state of pure shear.
A quantity called the equivalent stress or
von Mises stressis commonly used in solid mechanics. The equivalent stress is defined as:
Considering the principal directions as the coordinate axes, a plane which normal vector makes equal angles with each of the principal axes, i.e. having direction cosines equal to , is called an octahedral plane. There are a total of eight octahedral planes (Figure 6). The normal and shear components of the stress tensor on these planes are called octahedral normal stress and octahedral shear stress , respectively.
Knowing that the stress tensor of point O (Figure 6) in the principal axes is
the stress vector on an octahedral plane is then given by:
The normal component of the stress vector at point O associated with the octahedral plane is
which is the mean normal stress or hydrostatic stress. This value is the same in all eight octahedral planes.The shear stress on the octahedral plane is then
Analysis of stress
All real objects occupy a three-dimensional space. However, depending on the loading condition and viewpoint of the observer the same physical object can alternatively be assumed as one-dimensional or two-dimensional, thus simplifying the mathematical modelling of the object.
If two of the dimensions of the object are very large or very small compared to the others, the object may be modelled as one-dimensional. In this case the stress tensor has only one component and is indistinguishable from a scalar. One-dimensional objects include a piece of wire loaded at the ends and viewed from the side, and a metal sheet loaded on the face and viewed up close and through the cross section.
When a structural element is elongated or compressed, its cross-sectional area changes by an amount that depends on the
Poisson's ratioof the material. In engineering applications, structural members experience small deformations and the reduction in cross-sectional area is very small and can be neglected, i.e., the cross-sectional area is assumed constant during deformation. For this case, the stress is called engineering stress or nominal stress. In some other cases, e.g., elastomers and plastic materials, the change in cross-sectional area is significant, and the stress must be calculated assuming the current cross-sectional area instead of the initial cross-sectional area. This is termed true stress and is expressed as
where : is the nominal (engineering) strain, and : is nominal (engineering) stress.
The relationship between true strain and engineering strain is given by
In uniaxial tension, true stress is then greater than nominal stress. The converse holds in compression.
A state of plane stress exist when one of the principal stresses is zero, stresses with respect to the thin surface are zero. This usually occurs in structural elements where one dimension is very small compared to the other two, i.e. the element is flat or thin, and the stresses are negligible with respect to the smaller dimension as they are not able to develop within the material and are small compared to the in-plane stresses. Therefore, the face of the element is not acted by loads and the structural element can be analyzed as two-dimensional, e.g. thin-walled structures such as plates subject to in-plane loading or thin cylinders subject to pressure loading.The stress tensor can then be approximated by:
The corresponding strain tensor is:
in which the non-zero term arises from the Poisson's effect. This strain term can be temporarily removed from the stress analysis to leave only the in-plane terms, effectively reducing the analysis to two dimensions.
If one dimension is very large compared to the others, the principal strain in the direction of the longest dimension is constrained and can be assumed as zero, yielding a
plane straincondition. In this case, though all principal stresses are non-zero, the principal stress in the direction of the longest dimension can be disregarded for calculations. Thus, allowing a two dimensional analysis of stresses, e.g. a damanalyzed at a cross section loaded by the reservoir.
Mohr's circle for stresses
Mohr's circleis a graphical representation of any 2-D stress state and was named for Christian Otto Mohr. Mohr's circle may also be applied to three-dimensional stress. In this case, the diagram has three circles, two within a third.
Mohr's circle is used to find the principal stresses, maximum shear stresses, and principal planes. For example, if the material is
brittle, the engineer might use Mohr's circle to find the maximum component of normal stress (tension or compression); and for ductile materials, the engineer might look for the maximum shear stress.
Alternative measures of stress
The Cauchy stress is not the only measure of stress that is used in practice. Other measures of stress include the first and second Piola-Kirchhoff stress tensors, the Biot stress tensor, and the Kirchhoff stress tensor.
Piola-Kirchhoff stress tensor
In the case of finite deformations, the Piola-Kirchhoff stress tensors are used to express the stress relative to the reference configuration. This is in contrast to the
Cauchy stress tensorwhich expresses the stress relative to the present configuration. For infinitesimal deformations or rotations, the Cauchy and Piola-Kirchoff tensors are identical. These tensors take their names from Gabrio Piolaand Gustav Kirchhoff.
1st Piola-Kirchhoff stress tensor
Cauchy stress tensor, , relates forces in the present configuration to areas in the present configuration, the 1st Piola-Kirchhoff stress tensor, relates forces in the "present" configuration with areas in the "reference" ("material") configuration. is given by
where is the
Jacobian, and is the inverse of the deformation gradient.
Because it relates different coordinate systems, the 1st Piola-Kirchhoff stress is a
two-point tensor. In general, it is not symmetric. The 1st Piola-Kirchhoff stress is the 3D generalization of the 1D concept of engineering stress.
If the material rotates without a change in stress state (rigid rotation), the components of the 1st Piola-Kirchhoff stress tensor will vary with material orientation.
The 1st Piola-Kirchhoff stress is
energy conjugateto the deformation gradient.
2nd Piola-Kirchhoff stress tensor
Whereas the 1st Piola-Kirchhoff stress relates forces in the current configuration to areas in the reference configuration, the 2nd Piola-Kirchhoff stress tensor relates forces in the reference configuration to areas in the reference configuration. The force in the reference configuration is obtained via a mapping that preserves the relative relationship between the force direction and the area normal in the current configuration.
This tensor is symmetric.
If the material rotates without a change in stress state (rigid rotation), the components of the 2nd Piola-Kirchhoff stress tensor will remain constant, irrespective of material orientation.
The 2nd Piola-Kirchhoff stress tensor is energy conjugate to the Green-Lagrange finite strain tensor.
Von Mises stress
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