Why permissible shear stress is taken as 0.4Fy and in bending 0.66Fy?

Answer 1

There are several failure theories to answer this question. ASME did lot of work and established these things. For a convincing explanation, "FAILURE OF MATERIALS IN MECHANICAL DESIGN by J.A. COLLINS can also be read together with international design codes and similar books and research papers on the subject. I am working on this subject since three decades to provide a most appropriate and convincing formulae for calculation of permissible stresses for a given material. I am confident that my research in this direction gives immense and permanent relief to the worldwide community of mechanical design engineers involved in the design process ranging from safety-pin to spacecraft. Most importantly, my research work in this direction is aimed at putting the world on a common platform since the permissible stresses or allowable stresses are different for different international design codes. Since all my research works are self-assigned, self-guided and most painfully - self-financed, paucity of funds often forces me to go slow. However, come what may, I will complete my research before my death or inability to benefit our rapidly changing technological world and to cause rapid growth of science, engineering and technology essentially meant for the overall well-being of human life, animal life and plant life. Nori VSN Murthy, Independent Researcher, A temporary visitor on earth working for a common cause.