Results are presented from a systematic investigation of the influence of crack length and specimen thickness on the fracture properties of 4340 steel bend specimens heat treated to yield strength levels between 180 and 213 ksi. It is shown that KIc values determined in accordance with the present ASTM Tenative Method of Test for Plane Strain Fracture Toughness of Metallic Materials (E 399-70T) can vary moderately within the specimen size and geometric limitations imposed by the Test Method. The magnitude of these variations will depend on the material properties and would be increased by relaxation of the size requirements.
The possibility of employing subsized specimens for screening materials regarding their plane strain fracture toughness was explored as well as several methods for relating KIc values to uniaxial tensile data. The results indicated that use of subsized specimens with the ASTM Committee E-24 Test Method does not constitute a useful screening procedure. For steels subject to single aging or tempering reactions, it appears that the relation KIc ∼ σYS-m may be useful for estimating KIc values. However, on the basis of present knowledge, it is not possible to calculate KIc with useful accuracy from uniaxial tensile data alone.