Issue 11

D. Taylor, Frattura ed Integrità Strutturale, 11 (2009) 3-9 ; DOI: 10.3221/IGF-ESIS.11.01 7 leading to an erroneously low prediction of the fracture strength of the specimen, an error which can apparently be corrected by letting L increase. Figure 4 : Stress as a function of distance from notch root, calculated using the approximate formula (Eq. 5), compared to an accurate result obtained from FEA. Choice of Test Specimen Fig. 5a shows stress/distance curves calculated for the specimens tested by Whitney and Nuismer (as reproduced here in Fig. 2), for loading conditions corresponding to fracture of the specimen in each case. If the Point Method is exactly correct then all of these curves should pass through a single point, at which r = L/2 and the stress is equal to the UTS (represented here by a horizontal dashed line). Based on this data one would conclude that there is a tendency for L to increase with increasing hole radius, by about a factor of 2. However, the situation changes drastically if we add data from specimens containing sharp notches (see Fig. 5b) . The sharp-notch data shows no such trend, and the fracture strength of all the specimens can be predicted using a constant value of L, with a prediction error of no more than 10%. Figure 5a : Stress-distance curves at failure for specimens containing holes as shown in Fig.2. The symbols R1-R6 indicate increasing hole radius. Figure 5b : The same data as in Fig.5a, plus lines representing the stresses in sharply-notched specimens of the same material. The symbols N1-N4 represent increasing notch depth. Many workers have based their conclusions solely on data from circular holes. This is a mistake because the stress gradients in these specimens are quite shallow, so it is difficult to obtain an accurate value of L in any case, since the estimate relies on finding the point at which the stress/distance curve crosses the horizontal line representing the UTS.