Virtual work on a system is the work resulting from either virtual forces acting through a real displacement or real forces acting through a virtual displacement. In this discussion, the term displacement may refer to a translation or a rotation, and the term force to a force or a moment. When the virtual quantities are independent variables, they are also arbitrary. Being arbitrary is an essential characteristic that enables one to draw important conclusions from mathematical relations. For example, in the matrix relation
if is an arbitrary vector, then one can conclude that . In this way, the arbitrary quantities disappear from the final useful results.
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Consider a system of particles, i, in static equilibrium. The total force on each particle is^{[1]}^{:263}
Summing the work exerted by the force on each particle that acts through an arbitrary virtual displacement, , of the system leads to an expression for the virtual work that must be zero since the forces are zero:^{[1]}^{:263}
At this point it should be noted that the original vector equation could be recovered by recognizing that the work expression must hold for arbitrary displacements. Separating the forces into applied forces, , and constraint forces, , yields^{[1]}^{:263}
If arbitrary virtual displacements are assumed to be in directions that are orthogonal to the constraint forces, the constraint forces do no work. Such displacements are said to be consistent with the constraints.^{[2]} This leads to the formulation of the principle of virtual work for applied forces, which states that forces applied to a static system do no virtual work:^{[1]}^{:263}
There is also a corresponding principle for accelerating systems called D'Alembert's principle, which forms a theoretical basis for Lagrangian mechanics.
If the principle of virtual work for applied forces is used on individual particles of a rigid body, the principle can be generalized for a rigid body: When a rigid body that is in equilibrium is subject to virtual compatible displacements, the total virtual work of all external forces is zero; and conversely, if the total virtual work of all external forces acting on a rigid body is zero then the body is in equilibrium.
The expression compatible displacements means that the particles remain in contact and displace together so that the work done by pairs of action/reaction interparticle forces cancel out. Various forms of this principle have been credited to Johann (Jean) Bernoulli (16671748) and Daniel Bernoulli (17001782).
Consider now the free body diagram of a deformable body, which is composed of an infinite number of differential cubes as shown in the figure. Let's define two unrelated states for the body:
The superscript * emphasizes that the two states are unrelated. Other than the above stated conditions, there is no need to specify if any of the states are real or virtual.
Imagine now that the forces and stresses in the State undergo the displacements and deformations in the State: We can compute the total virtual (imaginary) work done by all forces acting on the faces of all cubes in two different ways:
Equating the two results leads to the principle of virtual work for a deformable body:
where the total external virtual work is done by T and f. Thus,
The righthandside of (d,e) is often called the internal virtual work. The principle of virtual work then states: External virtual work is equal to internal virtual work when equilibrated forces and stresses undergo unrelated but consistent displacements and strains. It includes the principle of virtual work for rigid bodies as a special case where the internal virtual work is zero.
For practical applications:
These two general scenarios give rise to two often stated variational principles. They are valid irrespective of material behaviour.
Depending on the purpose, we may specialize the virtual work equation. For example, to derive the principle of virtual displacements in variational notations for supported bodies, we specify:
The virtual work equation then becomes the principle of virtual displacements:
This relation is equivalent to the set of equilibrium equations written for a differential element in the deformable body as well as of the stress boundary conditions on the part S_{t} of the surface. Conversely, (f) can be reached, albeit in a nontrivial manner, by starting with the differential equilibrium equations and the stress boundary conditions on S_{t}, and proceeding in the manner similar to (a) and (b).
Since virtual displacements are automatically compatible when they are expressed in terms of continuous, singlevalued functions, we often mention only the need for consistency between strains and displacements. The virtual work principle is also valid for large real displacements; however, Eq.(f) would then be written using more complex measures of stresses and strains.
Here, we specify:
The virtual work equation becomes the principle of virtual forces:
This relation is equivalent to the set of straincompatibility equations as well as of the displacement boundary conditions on the part S_{u}. It has another name: the principle of complementary virtual work.
A specialization of the principle of virtual forces is the unit dummy force method, which is very useful for computing displacements in structural systems. According to D'Alembert's principle, inclusion of inertial forces as additional body forces will give the virtual work equation applicable to dynamical systems. More generalized principles can be derived by:
These are described in some of the references.
Among the many energy principles in structural mechanics, the virtual work principle deserves a special place due to its generality that leads to powerful applications in structural analysis, solid mechanics, and finite element method in structural mechanics.
