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There are situations when fatigue precracking of fracture toughness test specimens is not feasible. A case in point is when mixed mode fracture toughness is to be evaluated [1]. It, then, becomes necessary to determine the influence of the notch root radius on the fracture toughness of a material. It is well established that notch root radius has a significant influence on the apparent fracture toughness of a material [2][3][4][5]." Usually a critical notch root radius, below which fracture toughness is independent of notch root radius, exists and is measured. We present in this communication the results of an investigation of the effect of notch-root radius on mode I fracture toughness (J~o) of Armco iron and a procedure to correct for the same to arrive at the true Jic value.Armco iron, containing by wt.%, 0.008C, <0.02Si, <0.03Mn, <0.003S and 0.004P, has been used. To eliminate the effect of grain size [6], a constant grain size of -381.tm was maintained in one set and 781zm in another set of specimens. Compact tension specimens of 25mm thickness and 50mm width were used to evaluate mode I fracture toughness, Jxo. Notches with different notch root radii were introduced employing a wire cut EDM (electro-discharge machine) using 0.10, 0.15, 0.20 and 0.25mm diameter wires. Notch root radius measurements were made with the help of a profile projector. A set of specimens was also fatigue precracked. Single specimen J~o technique employing partial unloading compliance could not be utilized as the predictions of the crack length were erratic [7-9]. Fracture toughness J~ was evaluated by measuring the critical stretch zone width (SZWo) and employing the relation [ 10] J~c = 4%,SZWc(1)where o, is the average of yield and ultimate tensile strengths. This equation was found to be applicable for estimating J~c of Armco iron [6,7]. Fracture toughness tests were carried out under displacement control at a ramp rate of 1 mm/min. To delineate the stretch zone, post-fatigue cracking was carried out. The fracture Int Journ of Fracture 58 (1992) R16 specimens were then examined under ISI-100A scanning electron microscope (SEM) with the fracture surfaces tilted by an angle of 45 ° with respect to the incident beam about an axis parallel to the initial machined notch.

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