Papers by Keyword: DCPD Method

Abstract: Non-destructive inspection of an operated section of DN 300 high pressure gas pipeline (internal diameter 300 mm) indicated a quite frequent occurrence of longitudinal cracks of depths up to 50 % of the pipe wall thickness. As such cracks would be very dangerous from the viewpoint of further pipeline operation, selected sections of the pipeline were taken from the ground for an experimental programme aimed at evaluation of residual fatigue life. The first part of the programme concerned (i) experimental calibration of direct current potential drop (DCPD) method of crack depth estimation, complicated by the longitudinal position of the cracks, (ii) estimation of actual depths of selected, most distinct cracks and (iii) metallographical evaluation of profiles and depths of selected cracks. The DCPD calibration confirmed that for the specific configuration of electrodes used and characteristic crack profile, analytical calibration using Johnson´s formula corresponds to the actual crack depth quite exactly provided that the cracks are of radial direction. However, metallographical analyses showed that analysed actual cracks, though quite long, even more than 2 mm in some cases, were quite inclined, in some cases almost parallel to the pipe surface. These cracks occurred obviously in production during rolling process, which was a good sign for a relative safety of the affected pipeline. Further theoretical considerations were made to enable estimations of the inclined crack depths using the DCPD method, necessary for crack growth measurement during internal pressure fatigue tests, which followed.

Abstract: The paper contains results of a study aimed at exploring possibilities of use of direct current potential drop (DCPD) method for evaluation of depth and profile of cracks occurred under or near press fitted hubs in a full-scale axle during severe rotating bending loading. DCPD method was applied on section of an axle after fatigue by a specific way and potentials were measured in several circumferential areas of the axle section, when direct current passed longitudinally. Results are analysed and discussed. It was indicated that unlike previous use of the method for crack measurement of different components, even large and complicated, described in the literature, when potentials corresponded to ratio of cracked and uncracked area, in this case of approximately circumferential crack, measured potentials correspond better to relative crack depth in the relevant point of the circumference.

Abstract: Corrosion damage (pit) is a stress raiser that can have deleterious effects on the fatigue life of airframe structural components. A better understanding and method for modeling the corrosion pit to fatigue crack transition would advance the fidelity of aircraft structural integrity estimates and fleet management decision making. Here, the focus is on developing a standardized fatigue test method for investigating the transition of a corrosion pit to fatigue crack in aluminum alloy AA 7075-T651 specimens. The standardized test method requires the development and validation of two sub-protocols (1) a pitting protocol to create a corrosion pit less than 200 μm diameter at the intersection of the central hole bore and planar surface of sheet and (2) a spot welding protocol for attaching the direct current potential drop (dcPD) probes on either side of the corrosion pit for fatigue crack growth measurement. A dcPD fatigue test method coupled with a unique 10-4-6 marker load sequence is used to measure the fatigue crack growth. The crack shape evolution and crack growth life are predicted using AFGROW.

Abstract: In order to perform elastic-plastic fracture mechanical analyses, fracture resistance curves for concerned materials are required. A standard CT specimen was used to obtain fracture resistance curves. However the fracture resistance curve by the standard CT specimen was very conservative to evaluate the integrity of the structure. Also the fracture resistance curve was affected by the specimen geometry, crack plane orientation, reverse cyclic loading and dynamic strain aging. The objective of this paper is to be certain the conservativeness of the fracture resistance curve by the standard CT specimen. For these purpose, fracture tests using the real-scale pipe specimen and standard CT specimen test were performed. A 4-point bending jig was manufactured for the pipe test and the direct current potential drop method was used to measure the crack extension and the length for the pipe test. From the result of the pipe and the standard CT specimen tests, it was observed that the fracture resistance curve of the standard CT specimen test was conservative compare to that of the pipe specimen test.