MEASUREMENT OF STRETCH ZONE HEIGHT AND ITS RELATIONSHIP TO CRACK TIP OPENING DISPLACEMENT AND INITIATION J‐VALUE IN AN AISI 316 STAINLESS STEEL

Abstract

An accurate method using SEM for the measurement of stretch zone height (SZH) on fracture surfaces has been established This method does not make any assumption regarding crack blunting angle S, and for 316 stainless steel in the present investigation 6 was in the range of 50 to 67", contrary to the common assumption of 45". The semi-empirical equations, SZH(T) = 2.5 SZW (stretch zone width) and SZW=89(J/fi), reported in the literature were found to give very conservative predictions of initiation toughness Ji for an AISI 316 stainless steel. Lowerbound values of the coefficient m in the equation, J = ma,& relating J to CTOD, 6, is found to be 1.5-1.7 for the 316 SS used in this investigation; the upperbound value for m is found to be 2.6. The plastic CTOD values (6,)corresponding to the maximum load-point on the instrumented impact test traces are not sensitive to the aging conditions and as they incorporate significant crack growth effects they cannot be used for predicting Ji by a procedure similar to that discussed above. The ratio tiltT, where ti is the time to crack initiation and tT is the time to total fracture, in precracked Charpy tests of unaged and aged 316 SS used in this investigation (a/"= 0.55 to 0.8) corresponds to 0.15 to 0.17.Keywords-Stretch zone height; Crack tip opening displacement; J-integral; AISI 316 stainless steel NOMENCLATURE a, a, = crack length and initial length b, b, = remaining ligament ( W-a) and initial ligament ( W -%) B = specimen thickness CVN = Charpy V-notch CTOD = crack tip opening displacement d,, = factor relating CTOD to 3-integral; d: = Ea,/d,, E = Youngs' modulus Ei = energy under the load(P) -time(t) trace up to crack initiation ET = total impact energy h = SZH on one half of the fracture surface Ji = value of J integral at the point of crack initiation J,, = critical value of mode-I 3-integral satisfying ASTM E813 validity criteria m = J/ay 6 n = work-hardening index P, = load on the instrumented impact test load-time trace corresponding to crack LPD = load-point displacement PGy = general yield load on the instrumented impact test load-time trace initiation PmaX = maximum load on the instrumented test load-time trace r = rotation constant used in the expression relating CTOD to LPD for three-pointbend specimens SZD, SZH, SZW = stretch zone depth, height and width SZH(T) = sum of the SZH values (h) from the two halves of the specimen (= CTOD) ti = time to crack initiation (from the load-time trace) t,, = time to maximum load (from the load-time trace) t, = time to total (complete) fracture 855 856 P. R. SREENIVASAN et al. u = Poisson's ratio W = specimen width a = angle of rotation of the specimen in the SEM with respect to the electron beam 6 = CTOD Ssl, 6, = elastic and plastic parts of 6 diva, Bi,pl = elastic and plastic parts of 6, 6, = CTOD at maximum load Anb = blunting crack extensions evaluated by different procedures E,, = uniform true strain in the tensile test B = crack blunting angle uo = reference stress in the power law fit to the true stress-true str...

Role of Chloride in the Corrosion and Fracture Behavior of Micro-Alloyed Steel in E80 Simulated Fuel Grade Ethanol Environment

“…The stretch zone is essentially described as an imprint of the initiation fracture regime of ductile materials, and thus has a correlation with the initiation fracture toughness of a material [ 30 , 31 ]. Numerous attempts have been made to measure stretch zone dimensions and acquire a suitable correlation with ductile fracture toughness [ 30 , 31 , 32 , 33 ]. Two stretch zone components are typically associated with extremely ductile materials, namely stretch zone width ( SZW ) and stretch zone depth ( SZD ).…”

Modelling ductile fracture behaviour from deformation parameters in HSLA steels

“…However, there is also a general opinion that SZD is a better parameter than SZW for ductile fracture characterization 29–31 . Determination of SZD is tedious and no standard procedure is available as in the case of SZW.…”

Charpy energy–lateral expansion relations for a wide range of steels

“…The steels covered are: AISI 316 SSs in the solution treated (base metal-BM) and various thermal-aged/cold-worked (CW) conditions (including some old unpublished data) [4][5][6][7], AISI 308 SS weld [8,6], CF3 castings [7], AISI 403 SS [9], ASTM A533B type steels [10], 9Cr-1Mo steels [11] and welds [12] and their simulated heat-affected zones (HAZ) [13,14] and a service-exposed 2.25Cr-1Mo steel [15]. The material designations used for AISI 316 SS are solution-treated (ST-TS or BM: 1323 K/0.5 h plus water quench), STC1073 K/ 50 h aged (H5), STC1073 K/1008 h aged (H8), STC20% cold-work (CW) and CWCdouble aged conditions (GT).…”