PSI - Issue 60

M.K. Sahu et al. / Procedia Structural Integrity 60 (2024) 390–401 M. K. Sahu et.al./ Structural Integrity Procedia 00 (2019) 000 – 000

392

3

pipe specimen. The load ratio (ratio of minimum load to maximum load in a cycle) was 0.1 and the frequency was maintained in the range of 1.0 Hz-4.0 Hz. In the case of 219 mm OD pipe specimens, fatigue pre-cracking was carried out till crack growth in the circumferential direction reached approximately 2 mm to 5 mm at both the notch tips. Similarly, in the case of 324 mm OD pipe specimens, fatigue pre-cracking was carried out till crack growth in the circumferential direction reached approximately 20 mm to 25 mm at both the notch tips for through-wall notch. 2.2. Heating procedure and temperature management Heating of local region of the pipe near the crack location (200 mm both side) is done using wrap type resistance heater which is mounted externally on 8-inch pipes while internally and externally on 12-inch pipes due to higher thickness. The necessary electrical and thermal insulation were provided. The region near crack was heated and temperature was maintained to constant 300 o C till completion of the test. Total four thermocouples were fitted around crack tip region and temperature was maintained with deviation of ±5 o C during entire testing period. 2.3. Measurements for test results All the tests were extensively instrumented in order to reliably measure the parameters of interest. These parameters were measured during entire period of test at appropriate intervals. The Load-Line Displacement (LLD) was measured by linear variable differential transformer (LVDT). Total applied load was measured using strain gauge-based load cell of loading machine. Crack Mouth Opening Displacement (CMOD) was measured at the center of the crack using pre-calibrated clip gauges. The initiation of crack and amount of stable crack growth thereafter was measured. Imaging based methods was employed for through-wall cracked pipes for crack initiation and crack growth for continuous measurements during the tests. In view of high temperature and insulation warping, this method was more challenging than the room temperature tests. The region near crack tip was painted by white paint for better visibility of crack tips during different stages of crack growth during entire test period. Further dotted grids of 0.5mm×0.5mm were made ahead of crack tip in the probable crack extension region, with the help of dotted semi-transparent tracing paper. During fracture test, the two images were taken instantaneously of both crack tips and, a single image of load and position value from the computer-monitor connected with data logger and cameras. Thus, all three images are related to same time-point of the experiment. The crack growth value is predicted by simple image processing technique (IPT) by interpolating the instantaneous position of crack tip. The average crack growth value is calculated by taking average of crack growth from both the crack tips. 3. Monotonic fracture and cyclic tearing experiments Total four numbers of 8-inch pipes of identical geometries of carbon steel pipes tested which includes one monotonic fracture test and remaining three under cyclic loadings of different values of constant load amplitudes. Similarly, two numbers of 12-inch Similar Metal Welded (SMW) stainless steel pipes with identical geometries have also been tested. One pipe is tested under monotonic loading for base line data while second pipe is tested under cyclic loadings. 3.1. Monotonic fracture testing Total two pipes one 8-inch pipe 8-QMSP-60-TWC-CSB-M1 and one 12-inch pipe 12-QMSP-60-TWC-SSW-M2 have been tested under monotonic displacement-controlled loadings. Fig. 1 (a) and (b) show respectively the schematic and actual image of the test setup for monotonic fracture test. Fig. 1 (c) shows the temperature values near crack tip, which shows that the both crack tips are maintained around 300 o C. Table 1 gives the details of the pipes tested under monotonic loading. In this table, all relevant geometric parameters like outer diameter ( ), thickness ( ), crack size ( 2 ), inner span ( ), outer span ( ) are tabulated.