Corrosion fatigue of a 2024-T3 aluminum alloy in the short crack domain

Abstract

It is well accepted that the steady-state fatigue crack growth rates of long cracks depend uniquely on AK for a fixed load ratio R and test environment. Anomalous growth behavior of short cracks in either inert or deleterious environments that has been reported [1][2][3][4][5][6][7], however, calls into question the validity of using only long-crack results in evaluating the service life of a structural component and argues for the need for considering the effects of crack size. Crack-size effects have been extensively studied from the perspectives of mechanical, metallurgical and chemical principles. Microstructurally and mechanically short cracks [1][2][3] are associated with the influences of fine-scale microstructure, the limitation of continuum mechanics (or LEFM-linear elastic fracture mechanics), or crack closure. Chemically short cracks [4][5][6][7], on the other hand, normally extend to longer lengths and are attributed to the differences in local crack-tip chemistry (e.g., pH, [Oz], potential) between the long and short cracks and also, perhaps, from the bulk solution. The objective of this study is to explore the effect of crack size in various environments and to identify the key variables that affect crack growth behavior. The crack size investigated herein was chemically short but microstructurally and mechanically long.A 1.6 mm thick 2024-T3 (bare) aluminum alloy was used and its chemical composition (in wt%) is as follows: 4.24 Cu, 1.26 Mg, 0.65 Mn, 0.15 Fe, 0.08 Zn, 0.06 Si, 0.031 Ti, <0.01 Cr and balance A1. The elongation, 0.2% offset yield strength and tensile strength are 17.0%, 355 MPa (51 ksi) and 480 MPa (70 ksi), respectively. The specimens were cut in the L-T orientation as dog-bone shaped, center-pin-loaded, single-edge-cracked specimens with a 31.75 mm wide and 76.2 mm long (final dimension) mid-section. A 1.6 mm long electro-discharge machined (EDM) starter notch was introduced at the center of one edge of each specimen. The specimens were precracked by fatigue in air to a crack length of 3.42 mm (including the notch). Extra material (2.92 mm wide) was included on the notched side, and was removed by EDM to produce the final specimen, with a symmetrical test section and an approximaely 0.5 mm long edge crack. The following K calibration equation was used:P is the applied tensile load; B is the specimen thickness; W is the specimen width and a is the crack length. Equation (2) is an empirical formula obtained by Tada [8] and (3) is the correction accounting for the bending effect due to rotation at the loading pins [9]. A PC-controlled closed-loop electrohydraulic testing machine was used for the crack growth tests. Crack lengths were monitored by an a.c. potential system [10]. The specimens were loaded under pseudo-constant AK condition by automatically shedding the load range after each crack length determination (about every 10 ~m). The load ratio, frequency and temperature for all tests were R=0.1, f=10 Hz and T = 295 K, respectively. Crack growth rates at two AK levels (5 and 10 ...

Fatigue Crack Propagation in Austenitic Stainless Steel Under Low Frequency Loading and Salt Water Conditions

“…The short-crack effect is usually observed for cracks shorter than 0.5-1 mm for tests in air, and in some tests in corrosive environments [9,36], although sometimes also for cracks up to 2 mm deep [37]. If an alloy is covered with a passive film in a given environment (this is the present case), the short crack effect is observed for deeper cracks than for alloys in an active state [38,39], up to 5 mm [38].…”

“…The oxygen concentration in an electrolyte within a crack enclave drops as the crack depth increases [40,41]. In a well-aerated NaCl solution, the FCGR for chemically short cracks decreased with an increase in crack length, while in a deaerated solution, they were low and independent of the crack length [38]. This suggests that the corrosion process inside short cracks goes on with oxygen depolarization, and the oxygen concentration controls the corrosion FCGR.…”