The effect of section size on the room-temperature notch strength of H-11 steel, 2014-T6 and 7075-T6 aluminum alloys, and Plexiglas was investigated in three different series of notched cylindrical tensile specimens and notched rectangular bend specimens. The three test series permit the separation of the influences of the geometrical parameters, which determine the stress field near the base of the notch, on the notch strength as the size of the specimen increases. The notch strength decreases with increasing size for all series investigated. The decrease in strength is most pronounced for the sharp machine-notched and fatigue-cracked specimens but it is also observed, to a much smaller degree, in the other series of notched specimens for all materials. An analysis of the results of this study indicates that the loss in strength with increasing section size is due to at least two factors: An increase in stress concentration factor and an increase in critically stressed volume with increasing size. With a superposition of stress concentration and volumetric effects, a loss in strength with increasing size greater than that predicted by existing fracture concepts for a brittle material may be anticipated and was actually observed for sharply notched H-11 steel specimens. Insight into the size effect and experimental scatter observed in notched specimens of real materials is gained from a consideration of the behavior of a model of an inhomogeneous material in the presence of external notches. The constant-fracture stress concept applied to this model yields a size-effect prediction on notch strength and an expression for experimental scatter in terms of the interflow spacing and the notch root radius.

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