Rapid Determination of the Fracture Toughness of Metallic Materials Using Circumferentially Notched Bars Ali Bayram, Agah Uguz, and Ali Durmus (Submitted 14 November 2001) Three types of material whose fracture toughness tests were previously performed by using circumferen- tially notched bars, namely (1) a dual-phase steel with three different morphologies; (2) an Al-Zn-Mg-Cu- wrought alloy; and (3) Al-Si-cast alloys with three different Si contents, were investigated in terms of accuracy and reliability of the testing method. Also, the advantages of using circumferentially notched bars for fracture toughness determination of metallic materials were discussed. With the help of stress con- centration factors, which are associated with the bluntness of the notch, correction factors for the fracture toughness calculations are derived. The corrected fracture toughness values are found to be close to the uncorrected ones, implying that the testing procedure is reliable. Keywords Al-Si-cast alloys, Al-Zn-Mg-Cu-wrought alloy, dual- phase steel, fracture toughness 1. Introduction There are a number of fracture-toughness measurement techniques of metallic materials, which are standardized by different institutions. For example, a standard test method for plane-strain fracture toughness (K IC ) of metallic materials is given by the American Society for Testing and Materials (ASTM) (designation E399), [1] and the crack opening displace- ment fracture toughness measurement method is standardized both by the British Standards Institution (BSI) (BS 5762) [2] using bend specimens, and by ASTM (E1290) [3] using either a three-point bend or compact tension specimens. The ASTM Standard Test Method is argued to be one of the most accurate ways to measure K IC of low-ductility, high-strength alloys. [4] These methods, however, are difficult and exhausting to per- form, and the specimen preparation procedure is tedious. Par- ticularly, the fatigue precracking must be done with utmost care, and if the precrack is not appropriate, the whole fracture toughness test is invalid. Ule et al. [5] argue that the measured fracture toughness was affected by the eccentricity of the fa- tigued area. They measured the eccentricity of the fatigue pre- cracked region and found an underestimation of fracture tough- ness with increasing eccentricity. A rapid and at the same time reliable technique has always been sought, and the most familiar and easy to machine speci- men type, that is, a cylindrical specimen with a circumferential notch, has attracted much attention for this purpose. [4,6-9] Notched round specimens have been widely used for the de- termination of mechanical properties of materials. [10-14] The advantages of using circumferentially notched bars for fracture toughness testing can be summarized as follows: • The plane strain condition can be obtained because the circumferential crack has no end in the plane stress region compared with the standard specimen geometries [9] ; • Because of the radial symmetry of heat transfer, the mi- crostructure of the material along the circumferential area is completely uniform [5] ; • The specimens are easy to machine; • The fracture toughness test is easy to perform. It has been argued that a circumferentially notched cylindrical specimen without a fatigue precrack can be readily used for a rapid determination of the fracture toughness of a metallic material. In the previous studies, [6-8] fracture toughness of three metallic materials were measured by using circumferen- tially notched cylindrical specimens and in this investiga- tion, the validity and the accuracy of the technique is dis- cussed. 2. Experimental Materials and Methods The materials inspected in this study are of three different types whose mechanical properties were determined previ- ously. [6-8] The compositions and the heat-treatment procedures of the alloys are as follows: • A low-carbon steel [6] with 0.097% C, 0.49% Mn, and very low Si, S, and P contents. This alloy was heat treated to obtain three different dual-phase steels with different mi- crostructures and named as MSA, MSB, and MSC; • An Al-Zn-Mg-Cu-wrought alloy [7] having a composition of 5.68% Zn, 2.56% Mg, and 1.72% Cu with a relatively low impurity level. The alloy was examined under peak- aged condition; • Al-Si casting alloys, [8] which were cast in metal molds in the University Laboratories. The Si content of the alloy A5 is 5%, A8 is 8%, and A11 is 11% by weight. Other ele- ments are approximately the same in all the alloys as fol- lows: 1.1% Cu, 0.9% Mg, 0.9% Ni, 0.5% Fe, 0.2% Zn. All the compositions were examined in (1) as-cast condition Ali Bayram, Agah Uguz, and Ali Durmus, Uludag University, Fac- ulty of Engineering and Architecture, Gorukle, 16059, Bursa, Turkey. Contact e-mail: uguz@uludag.edu.tr. JMEPEG (2002) 11:571–576 ©ASM International Journal of Materials Engineering and Performance Volume 11(5) October 2002—571