For a while I've been working on applying the finite difference method to various problems and am now trying to get a model of stress and strain working. I wrote a sim a few weeks ago and I got one example to run and thought it was good. Now, I've tried it on another example and I see that something is seriously wrong with the sim and my understanding of stress and strain. The problem is that if I model a 2D square with a uniform force pressing down on the top side, the thing compresses like a spring but doesn't expand laterally, even though the Poisson's ratio of the material is non-zero. Letting u and v be the displacement functions, the computed solution is that v is linear, like the compression of a spring, from the bottom to the top edge. And, u is 0 everywhere. The thing is, that these functions satisfy the differential equation model, in which all the derivatives are 2nd order and all the 2nd order derivatives of u and v are 0, so, it satisfies the model, but it ain't right, as there is no lateral displacement. I have a web page I created with the details ! www.berkeleyscience.com/stress.htm [Broken] The thing is that the solution has x-direction stress as Hooke's law says that that x stress = A*xstrain + Av*ystrain, and ystrain is not 0, but there is no x displacement associated with the xstress, so ..... I think this is where the problem is .. The beauty of FDM is that it's a cookbook thing more or less, and the error isn't there as the sim solution satisfies the differential equation model, so the error has to be in the differential equations, which seem OK, or the boundary conditions which also seem OK. Any insights into the matter will be appreciated.