There are certain disadvantages to using round bars for plane strain fracture toughness testing. Specifically these are: (1) a large amount of material is required, for specimen fabrication, (2) a testing machine of great capacity is needed to fail such specimens, (3) there is considerable difficulty and expense involved in precracking the specimens. In attempt to Obtain a specimen free of these disadvantages, a hollow circumferentially notched round configuration has been suggested for plane strain fracture toughness testing. The effect of notch root radius and axial hole diameter were experimentally determined for hollow notched round aluminum bars of half inch outside diameter. From the results of the fracture toughness tests of these half inch specimens, it was concluded that a specimen of larger diameter was necessary in order to produce valid K_IC results. The equation used to calculate K_IC for hollow round bars was used to determine optimum dimensions for hollow round specimens. Several 7075-T651 aluminum. specimens of the analytically determined dimensions ' were failed and the corresponding apparent K_IC calculated. The resulting apparent K_IC values fell within an acceptable range of the true K_IC. It may be concluded that optimum dimensions may be analytically determined for hollow round plane strain fracture toughness specimens which will yield a reasonably accurate apparent K_IC. / Master of Science