Ludwig-Davidenkov-Orowan hypothesis

Many aspects of the brittle ductile transition in metals, including the effect of notching, which we will discuss separately, have been discussed in terms of the Ludwig-Davidenkov-Orowan hypothesis that brittle fracture occurs when the yield stress exceeds a critical value [95], as illustrated in Figure 12.29(a). It is assumed that brittle fracture and plastic flow are independent processes, giving separate characteristic curves for the brittle fracture stress and the yield stress 0 ... 

For line A the ratio Hb /hy 2 rather than unity, but the difference may be accounted for by the measurement of Hb at very low temperatures and possibly by the measurement of aB in flexure rather than in tension. (The latter may reduce the possibility of fracture at serious flaws in the surface.) It is encouraging that even an approximate relationship holds along the lines of the Ludwig-Davidenkov-Orowan hypothesis. Even more encouraging is the fact that line B has a slope Hb/hy 6, which is three times that of A, as expected on the basis of the plastic constraint theory. 

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