Issue 7

S. K. Kudari et alii, Frattura ed Integrità Strutturale, 7 (2009) 57-64 ; DOI: 10.3221/IGF-ESIS.07.04 58 material in two specimen geometry having varied a/W ratio to substantiate the theoretical analysis of Kudari et al. [5]. A comparative assessment of the experimental measurements of PZS with the results computed by elastic-plastic finite element analysis (FEA) and earlier analytical formulation s [7, 8, 9] h as been also carried out. E XPERIMENTAL commercial interstitial free (IF) steel has been selected for experimental determination of plastic zones in the present investigation. The IF steel has been obtained in the form of 3 mm thick sheets in the cold rolled state as courtesy of Tata Iron and Steel Co. Ltd., Jamshedpur, India. The chemical composition of the material is given in Tab. 1. In the present experimental investigation, micro-hardness technique [6] is used for estimation of plastic zone size. Hence, it was necessary to relieve the surface stresses in the as received material due to cold rolling, and material is given a heat treatment. All specimens of the interstitial free steel for various investigations were subjected to stress-relief annealing. The stress relief annealing consisted of soaking the specimens at 700 ± 2°C for 1 hour in a resistance heating furnace followed by air-cooling. The microstructure of the steel was found to exhibit equiaxed polygonal ferritic structure having average size of 27.1 ± 2.2 µ m, as determined by linear intercept method. The single-phase structures of this material constitute the selection basis because this makes application of the microhardness technique amenable for determining plastic zone size. Element Weight percentage C 0.003 Mn 0.110 Si 0.009 Cr 0.027 Ni 0.017 Mo 0.002 S 0.007 P 0.012 Al 0.040 Cu 0.006 Nb 0.001 Ti 0.055 V 0.001 N 30 ppm Fe Bal Table 1 : Chemical composition of the interstitial free steel used in the analysis Tensile tests following ASTM procedure, ASTM E8-00 [10] were carried out on flat specimens of gauge length 25 mm and width 10 mm with the help of a 50 kN capacity screw driven Shimadzu universal testing machine (model: AG5000G) at ambient temperature (30°C) using a cross head speed of 0.5 mm/min. The crosshead speed corresponds to nominal strain rate of 3.33x10 -4 s -1 . Two tensile tests were carried out to obtain the average tensile properties of the selected steel. The micro-hardness of the material was determined with the help of a micro-hardness tester (LECO, model: DM400) using a load of 10 gm f for 15 sec duration. The average micro-hardness value was estimated from 25 randomly taken readings. Two types of specimens were fabricated for the measurement of plastic zone size. These are: (a) single edge notched tensile (SENT) and (b) compact tension (CT) specimens. All the SENT specimens were fabricated with width (W)=25 mm and height (H) = 100 mm. The value of a/W considered in this study are 0.25 and 0.50. The CT specimens were made following the ASTM standard [1] with W = 25 mm and a/W = 0.5. The specimen surfaces were grinded (before heat treatment) to facilitate a fine polishing, which is needed for measuring microhardness. Because of surface grinding, the final thickness of all the specimens was reduced to 2.7 mm. Typical configurations of SENT and CT specimens used in this analyses are shown in Fig.1. A