Previous chapters have dealt with either a pure normal, or direct, stress (i.e. tension or compression), or a pure shear stress. In many instances, however, both direct and shear stresses are brought into play, and the resultant stress across any section will be neither normal nor tangential to the plane. If σ r is the resultant stress, making an angle ϕ with the normal to the plane on which it acts (Fig. 3.1), it is usually more convenient to calculate the normal and tangential components σ and τ, then, by equilibrium
and, from Fig. 3.2
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© G. H. Ryder 1969