In the three-point bending of a beam of width, b, height, h, and length, l, subject to a central load, P, the tensile stress, s, at distance, x, from the nearest end point and depth, y, from the central axis is 4axy s = (1) lh where 0 ~ x ~ 1/2, 0 <. y <~ h/2, 0 ~ z ~ b, and o. = 3Pl/2bh 2 is the maximum stress attained. For h/2 <~ y < 0, the stress is compressive, so failure is unlikely to occur. Using the Weibull model, the probability of fracture of the beam at maximum stress, o-, is given by F(a) = 1 -exp --jv d (2) where V = lbh/2, ao is a scaling parameter and m is the Weibull modulus. Oh and Finnie [1] and Schultrich and Fahrmann [2] have shown that the joint probability density of failure at maximum stress, a, and position (x, y, z) is f ( a , x , y , zl = [1 -F ( a ) ]~ (3) where 0 [1 ( s '~ m ]is the probability that failure occurs in d V as the maximum stress is increased from o-to a + do-and [1 -F(a)] is the probability that in the remaining volume no failure occurs before a to first order in dV. On substituting for s from Equation 1 and evaluating the integral in Equation 2