I have a conceptual misunderstanding it seems. Poisson's ratio is the ratio of elastic strain deformation of the transverse and longitudinal components. That being said, if I were to induce thermal stress (heating up) to a rod by keeping its ends (longitudinal component) rigid, would there be a mechanical contribution to the transverse strain on top of the thermal one? My textbook solutions manual seems to think so however I don't understand how you can have a transverse strain when you have no longitudinal one (no change in length since the rod is maintained rigid throughout the thermal stress exposure). Any insight on this would be greatly appreciated.