Aerofoils - Stress Distribution and Strain Calculation

Unfortunately the field of aero/thermodynamics isn't completely calculable due to chaos inherent in turbulent flow, surface variations and the likes, but if you have a smoke stream that you can use a high speed camera to observe (small styrene balls or something introduced into the airline) you may be able to collect enough data to develop a result, from the wind speed at different points accross the wing.

Using bernouli's equation, taking a pressure and wind speed measurement at a point in the flow stream and assuming minimal change in density(unlikely) you can calculate the static pressure which will give you the perpendicular stress to the surface. Then you need to calculate the shear which is a function of velocity squared ( I assume you have something to measure the drag on the aerofoil as a whole), this will be the hardest part as you will need to integrate the drag equation over the entire geometry of the part and reverse engineer the drag coefficient for the surface to result in your resultant drag. Once you have done this you should end up with a drag force for each point on the wing which can be transformed into a shear stress on each elemental area that you have the velocity for. This will give you the two principle stresses on the surface which if you create a finite element model of the wing profile (including internal geometry) you should be able to determine the stress in the wing.

However this is not a simple process, and for the effort it would take to go through this exercise i would recommend use of a finite flow model using computational flow dynamics.