A hollow enclosure is formed between two infinitely long concentric cylinders of radii 1 m and 2 m respectively, Radiative neat exchange takes place between the inner surface of the larger cylinder (surface-2) and the outer surface of the smaller cylinder (surface-1). The radiating surfaces are diffuse and the medium in the enclosure is non- participating. The fraction of the thermal radiation leaving the larger surface and striking itself is Direct shear stress (in MPa) in the most heavily loaded rivet is