Problem understanding the proof of the principle of virtual work I'm having some trouble understanding the proof of the Principle of Virtual Work for deformable bodies. I'll give below the proof that I've read, and, next, I'll remark what I'm not understanding. I asked this question on Physics and Engineering Stack Exchange, but the answers given didn't convince me. 
The first thing to remember before going through the proof is that the virtual work done by a system of virtual forces in equilibrium as a rigid body undergoes a small, compatible displacement is zero.
PROOF:
Suppose that a deformable body is in static equilibrium under the external loads of a virtual $Q$-force system.
Since the body as a whole is in equilibrium, any particular particle can be isolated and will be in equilibrium under the internal virtual $Q$ stresses developed by the external virtual $Q$ forces.
Now suppose that the body is subjected to a small change in shape caused by some other source than the virtual $Q$-force system. Owing to this change in shape, any particle might be deformed as well as translated and rotated as a rigid particle. Hence, the boundaries of such a particle would move and hence do virtual work. Let the virtual work done by the $Q$ stresses on the boundaries of the differential particle be designated by $\text{d}W_s$. Part of this virtual work will be done because of the movements of the boundaries of the particle caused by the deformation of the particle itself; this part will be called $\text{d}W_d$. The remaining part of $\text{d}W_s$ will be the virtual work done by the $Q$ stresses during the remaining part of the displacement of the boundaries and will be equal to $\text{d}W_s-\text{d}W_d$. However, this remaining is caused by the translation and rotation of the particle as a rigid body, and, as reminded above, the virtual work done in such a case is equal to zero. Hence
$$\text{d}W_s=\text{d}W_d$$
If the virtual work done by the $Q$ stresses on all particles of the body is now added, this equation becomes
$$W_s=W_d$$
To evaluate first $W_s$, we recognize that this term represents the total virtual work done by the virtual $Q$ stresses on all the boundaries of all the particles. However, for every internal boundary of a particle there is an adjoining particle whose adjacent boundary is actual the same line on the body as whole, and therefore these adjacent boundaries are displaced exactly the same amount. Since the forces acting on the two adjacent internal boundaries are numerically equal but opposite in direction, the total virtual work done on the pair of adjoining internal boundaries is zero. Hence, since all internal boundaries occur in pairs, there is no net virtual work done by the forces on all the internal boundaries. $W_s$ therefore consists only of the work done by the external $Q$ forces on the external boundaries.
$W_d$, on the other hand, was obtained by integrating the virtual work associated with deformation of the element. This work includes the effects of all forces on the element, both stress resultants and external forces. However, when an element deforms, only the stress resultants perform any work. Thus, $W_d$ represents the virtual work done by the stress resultants alone.
END OF PROOF
I understood the part concerning $W_s$. 
Next, it is said that "$W_d$ represents the virtual work done by the stress resultants alone". But, according to the paragraph pertaining to $W_s$, this work should be zero, since all particle's internal boundaries occur in pairs and that one side of an internal boundary of a particle undergoes the same displacement as the common side of a neighboring particle and such two sides are subjected to equal and opposite stresses, respectively.
If we were to apply this principle according to my (flawed) conclusion, all points of the deformable body would undergo no displacement at all: a complete nonsense.
Where's the flaw in my reasoning?
 A: I realized that people around here weren't really interested by my question, but I'll give an answer to it anyway, as I finally understood the proof. Here it goes:
The original proof given above is, as it should be expected, correct. But it can be made clearer if broken down in additional steps.
Consider, again, a small particle of the deformable body. The total work done by the virtual forces, $\text{d}W_t,$  system is such that $$\text{d}W_t=\text{d}W_r+\text{d}W_d$$where $\text{d}W_r$ is the work done due to the displacement of the particle as a rigid body, and $\text{d}W_d$ is the work done due to the displacement of the points on the boundaries of the particle as they deform. As said before, $\text{d}W_r=0$, and so $$\text{d}W_t=\text{d}W_d$$.
Integration of the last equation over the entire volume gives $$W_t=W_d.$$
To evaluate $W_t$ let's consider one of the internal boundaries of the particle in question. Call it boundary $B$. The total work done at such boundary by the internal virtual stresses, $\text{d}W_t^B,$ is such that  $$\text{d}W_t^B=\text{d}W_r^B+\text{d}W_d^B,$$ where $\text{d}W_r^B$ is the work done by the internal virtual stresses due to the displacement of the points of boundary $B$ as the particle undergoes a displacement as a rigid body, and $\text{d}W_d^B$ is the work done by the internal virtual stresses as boundary $B$ deforms. It should be noted that both both $\text{d}W_r^B$, $\text{d}W_d^B$ and $\text{d}W_r^B+\text{d}W_d^B$ are generally not equal to zero. 
Now, for every internal boundary of a particle there is an adjoining particle whose adjacent boundary is actual the same line on the body as whole, and therefore these adjacent boundaries are displaced exactly the same amount. Since the forces acting on the two adjacent internal boundaries are numerically equal but opposite in direction, the total virtual work done on the pair of adjoining internal boundaries is zero. Hence, since all internal boundaries occur in pairs, there is no net virtual work done by the forces on all the internal boundaries. Hence, since all internal boundaries occur in pairs, there is no net virtual work done by the forces on all the internal boundaries. $W_t$ therefore consists only of the work done by the external 
forces on the external boundaries.
So, what should be noted in the proof is that it is the total work done at boundary $B$ at the original particle plus the total work done at boundary $B$ of the adjoining particle that adds up to zero. If we were to consider the sum $\text{d}W_d^B + \text[{d}W_d^B]'$ ($\text[{d}W_d^B]'$ being the work done by the virtual stresses on the adjoining particle as its boundary $B$ deforms), then, for the reasons pointed by Brian Moths in his answer, such sum would not be zero. 
