  Effect of constraint on fracture parameters of piping materials N. Narasaiah 1 , S. Sivaprasad 1 , J. Chattopadhyay 2 , H. S. Kushwaha 2 , E. Roos 3 and S. Tarafder 1 1 National Metallurgical Laboratory, CSIR, Jamshedpur–831 007, India. 2 Bhabha Atomic Research Centre, Mumbai - 400 085, India. 3 Materials Testing Institute (MPA), University of Stuttgart, Pfaffenwaldring 32, Stuttgart D-70569, Germany. E-mail: narasaiahn@yahoo.co.in Received 6, August, 2008 Revised 15 January, 2009 Accepted 15 January, 2009 Online at www.springerlink.com © 2010 TIIM, India Abstract Fracture behaviour under quasi-static tearing load has been studied through fracture toughness tests on SA333 Grade 6 and 20MnMoNi55 steels using single-edge notched bend (SENB) specimens. To understand the response of variation of constraint, J-R curves were obtained from specimens precracked to a/W ratio in the range of 0.25 to 0.75, in steps of approximately 0.1. Stretch zone widths (SZW) were measured on the fractured surfaces of broken specimens. The stretch zone dimensions that were determined have been used in conjunction with the experimentally derived J-R curve to obtain a value of the ductile fracture toughness parameter J SZW . The initiation toughness, J i , obtained at the intersection of the blunting line and the power-law fit to the J-R curve, and the critical toughness, J c , determined following the procedure of the ASTM standard, were estimated. Comparisons of J SZW with J i and J c have been made for 20MnMoNi55 and SA333 steels as a function of the a/W ratio. The J i and J c values are higher at lower a/W, i.e. under low constraint, and fall with increasing a/W. A loss of constraint thus enhances these parameters. It was observed that J SZW does not effect by a/W, and that its value is lower bound to J i and J c values for both the materials investigated. 1. Introduction Determination of initiation toughness in ductile materials is not straightforward, unlike in brittle materials, where the point of crack initiation is easily detectable due to sharp changes in the load carrying capacity of specimens being tested. The matter may be further complicated due to variation of constraint attending crack tips. Fracture behaviour of ductile materials is usually characterized by elastic-plastic fracture parameters such as the J-integral, stretch zone width, crack tip opening displacement etc. The variations in these parameters with variation in constraint are often difficult to rationalize from the premise of their suitability to represent material behaviour. An attempt has been made to understand the nature of variation of fracture resistance parameters with change in crack length (variation of constraint) in two pressure vessel piping materials. The suitability of the parameters to represent the fracture toughness of materials irrespective of the constraint condition, to which cracks in them are subjected, is discussed. It is known that the fracture resistance of materials obtained from testing standard specimens is often not applicable to the fracture of components made of that material due to the difference in the constraint or triaxiality conditions at the tip of the crack in the two cases [1]. In order to obtain an appreciation of the nature of variation of fracture resistance of 20MnMoNi55 and SA333 Grade 6 steels with variation of constraint, a study has been conducted employing specimens with various initial crack lengths so as to impose a systematic variation of constraint. Keywords: fracture resistance; toughness; JR curves 2. Material and experimental details The materials selected for this investigation were 20MnMoNi55 and SA333 Grade 6 steels, supplied by the Bhabha Atomic Research Centre, Mumbai. The 20MnMoNi55 steel was in the form of blanks and the SA333 was in the form of pipe. The nominal composition of both the materials is given in Table 1. Figure 1 shows the typical microstructures of the investigated steels. The SA333 steel consists banded structure of pearlite in a ferritic matrix whereas in the 20MnMoNi55 steel pearlite is distributed uniformly in ferritic matrix. Table 2 lists the mechanical properties of the materials obtained from standard tensile tests at room temperature. The tensile flow curves of both the steels exhibited prominent yield-point effect. Single-edge notched bend (SENB) specimens, of 20mm thickness (B) and 50mm width (W), were employed for carrying out monotonic single specimen J – R tests at room temperature. For studying constraint effects on ductile fracture behaviour, SENB specimen geometry was preferred to avoid the distortion that occur at the loading holes of the compact tension (CT) geometry at lower initial crack lengths. All specimens were equipped with integral knife-edges on their front face for compliance based crack length measurement, and were side-grooved after fatigue pre- cracking. Specimens were fatigue pre-cracked under decreasing ΔK envelopes in servohydraulic testing systems interfaced to computers for test control and data acquisition. The single-specimen technique was employed for generating J – R curves as per the procedures laid down in Transactions of The Indian Institute of Metals        