The slow fatigue crack growth and threshold behaviour of a medium carbon alloy steel in air and vacuum

Oda

2011

JMMP

In order to investigate the mechanism for the intergranular fatigue crack propagation of a low carbon steel in a low pressure hydrogen gas environment, two kinds of examinations were carried out. One examined the effect of the cyclic pre-strain on the crack growth behavior. The other was the in-situ observation of intergranular fatigue crack propagation in a hydrogen gas environment. The main results are as follows. (1) SEM observation of the specimen surface morphology of a plain specimen fatigued in hydrogen gas showed that a gap at the grain boundary was induced by slip behavior, but not in nitrogen gas. (2) A specimen cyclic-prestrained in hydrogen gas showed slight influences on the increase in the crack propagation rate. (3) Mating intergranular facets on the fatigue fracture surfaces showed the matching of a striation-like pattern in the manner anticipated from the damage mechanism of (1). (4) The environment-change test from hydrogen to nitrogen during the fatigue test showed that intergranular facets appeared even in nitrogen. Results (3) and (4) suggests that the damaging process of (1) is valid in an actual fatigue crack. (5) In-situ observation of the crack propagation behavior in hydrogen gas showed that a crack propagated faster along the grain boundary. However, no visible discontinuous crack advances appeared as a large number of load repetitions was required. Phenomena considered to be a damage process (1) appeared ahead of a crack tip. Based on these results, a convincing mechanism for the intergranular fatigue crack propagation process is as follows. Grain boundaries just ahead of a crack tip are damaged due to the large number of hydrogen-enhanced slip repetitions, therefore, the fatigue crack becomes easier to propagate along the grain boundary in hydrogen.

Section: Discussionmentioning

confidence: 99%

Investigation of Mechanism for Intergranular Fatigue Crack Propagation of Low Carbon Steel JIS S10C in Hydrogen Gas Environment

Oda

2011

JMMP

Fatigue Fract Eng Mat Struct

“…However, at low growth rates where variables such as R ratio [2-123, microstructure [6,[13][14][15][16][17][18], mechanical properties [4,8], stress history effects [S] and environment become significant, the power law cannot be used and it is observed that fatigue crack growth rates decrease asymptotically to a value of A K f at which no growth can be detected. Several models [ 19,201 have been suggested which take into account a threshold condition and in particular Klesnil …”

mentioning

confidence: 99%

Slow Fatigue Crack Growth and Threshold Behaviour in Air and Vacuum of Commercial Aluminium Alloys

Kirby

Beevers

1979

114

Abstract-Fatigue crack growth and threshold behaviour have been examined in three commercial aluminium alloys in both air and vacuum environments. It was observed that, in air, the threshold stress intensity range AK,,, varied linearly with the R ratio. In contrast AK, in vacuum was found to be independent of R. Over the whole growth rate range examined fatigue crack growth in vacuum was AK controlled and failure occurred by a dimple and ductile striation mechanism. This also applied to failure in the intermediate growth rate ranges in air. However, at slow growth rates in air, fatigue crack growth was structure sensitive and crystallographic facets were formed during the crack propagation process.

Environmentally Assisted Cracking: Predictive Methods for Risk Assessment and Evaluation of Materials, Equipment, and Structure

“…Similar behavior was reported for underaged, peak-aged, and overaged 7075 aluminum alloys tested in moist air of 95% relative humidity [1], 2024-T351 aluminum alloy in dry and wet argon [2], Al-3% Mg alloy in dry nitrogen at 298°K and in liquid nitrogen at 77°K [3], 7091 aluminum alloy in air of humidity greater than 90% [4], and 2024-T3 aluminum alloy in laboratory air at ambient temperature [5]. Such observations have also been made for low and medium carbon, medium carbon alloy, 2 l A Cr-1 Mo, rotor, 4340, JIS SB42, and AerMet 100 steels [6][7][8][9][10][11][12][13][14][15].…”

Section: Fatigue Crack Growth Ratementioning

confidence: 96%

Environmentally Influenced Near-Threshold Fatigue Crack Growth in 7075-T651 Aluminum Alloy

Lee

Sanders

George

et al. 2000

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, SPONSOR/MONITOR'S ACRONYM(S) SPONSOR/MONITOR'S REPORT NUMBER(S)Approved for public release; distribution is unlimited. SUPPLEMENTARY NOTES ABSTRACTThe near-threshold fatigue crack growth behavior of a 7075-T651 aluminum alloy was studied in laboratory air, vacuum, and an aqueous 3.5% NaCl solution. Results obtained indicate that raising the stress ratio R enhanced the near-threshold fatigue crack growth with a greater crack growth rate da/dN and smaller threshold stress intensity range AK" in laboratory air and an aqueous 3.5% NaCl solution. However, the reverse was observed in vacuum. The near-threshold fatigue crack growth was most sluggish with the smallest da/dN and greatest AK^, values in vacuum, intermediate with an intermediate da/dN and AK^, in the aqueous 3.5% NaCl solution, and fastest with the greatest da/dN and smallest AK," values in laboratory air. In laboratory air and aqueous 3.5% NaCl solution AK", initially decreased with increasing R until a value of 0.5 was reached, and then leveled off or decreased slightly. The AK ft values for these two environments appear to converge at a higher R. On the other hand, in vacuum, the AKth increased linearly with increasing R. In addition, at lower R, a greater resistance to nearthreshold fatigue crack growth was detected in the aqueous 3.5% NaCl solution than in laboratory air. This is attributable to crack closure, induced by corrosion product at the crack tip. 15. SUBJECT TERMS fatigue, near-threshold fatigue crack growth, stress ratio, vacuum, laboratory air, 3.5% NaCl solution, threshold stress intensity range, maximum stress intensity 16. Abstract:The near-threshold fatigue crack growth behavior of a 7075-T651 aluminum alloy was studied in laboratory air, vacuum, and an aqueous 3.5% NaCl solution. Results obtained indicate that raising the stress ratio R enhanced the near-threshold fatigue crack growth with a greater crack growth rate da/dN and smaller threshold stress intensity range AK t h in laboratory air and an aqueous 3.5% NaCl solution. However, the reverse was observed in vacuum. The near-threshold fatigue crack growth was most sluggish with the smallest da/dN and greatest AK t h values in vacuum, intermediate with an intermediate da/dN and AK t h in the aqueous 3.5% NaCl solution, and fastest with the greatest da/dN and smallest AK t h values in laboratory air. In laboratory air and aqueous 3.5% NaCl solution AK t h initially decreased with increasing R until a value of 0.5 was reached, and then leveled off or decreased slightly. The AK t h values for these two environments appear to converge at a higher R. On the...