The shear stress in this case, with the force parallel to the plane, is just F/A. But what would be the area in this case? Either the thickness*width or length*width of the sample.
I agree with Muhammad Irfan , lap are will be overlap length * overlap width in the case of adhesive. To see laps shear strength in the welding you can use
Article Laser transmission welding of thermoplastic with beam wobbli...