Stress corrosion cracking in aluminium alloy 7075-T651 by discrete crack jumps as indicated by fractography and acoustic emission

Martin, Philippe; Dickson, J. I.; Baïlon, J.-P.

doi:10.1016/0025-5416(85)90402-1

Cited by 13 publications

(7 citation statements)

References 9 publications

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“…(a) Martin et al [29] detected similar acoustic emission events occurring during crack propagation for AA7075-T651 in moist air and in 3.5 pct NaCl that are consistent with crack growth occurring by discrete jumps of approximately 200 to 400 nm, which correlated with the crack-arrest markings (striations) on the intergranular fracture surfaces produced in moist air and occasionally on the fracture surfaces generated in 3.5 pct NaCl. (b) Time-lapse video studies of crack initiation and growth from notched AA7150-T6 samples immersed in 0.6M NaCl, [40] which revealed that initial crack growth (K = 5 to 18 MNm À3/2 ) occurred as a series of discreet steps with instantaneous crack growth rates greater than 10 À6 m/s.…”

Section: Sustained-load Cracking In Moist Air and Distilled Watermentioning

confidence: 58%

“…[29] Based on the limited data available, the crack arrest marking spacings generated in saline environments at room temperature also appear to be insensitive to copper content, temper, and loading conditions. However, the spacings in saline solutions at room temperature,~400 to 500 nm, are significantly larger than those observed in moist air or distilled water,~150 nm.…”

Section: A Intergranular Crack Arrest Markingsmentioning

confidence: 97%

“…Since considerable experimental evidence suggests that intergranular sustained-load crack growth in moist air and distilled water under fixed-load conditions occurs by a discontinuous process, [17,23,[29][30][31][32][33][34][35][36][37][38][40][41][42]46] through a series of incremental crack jumps creating crack arrest markings, it is reasonable to assume that the dwell time between crack jumps will decrease as applied load increases and increase when an alloy is overaged. Apparent dwell times between increments of intergranular crack growth, from Table I, are given in Table VI, making the assumption crack growth is rapid.…”

Section: A Intergranular Crack Arrest Markingsmentioning

confidence: 99%

“…The evidence for chemical reactivity is provided by crack arrest markings (striations) remaining on the intergranular fracture faces for various Al-Zn-Mg and Al-Zn-Mg-Cu alloys after crack propagation in distilled water, inhibited acidified saline solutions or moist air, [5,23,[29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44] even at relative humidity as low as 20 pct. [5,20] The earliest observation of these ''crack arrest markings (striations)'' was probably made in 1967 by McEvily et al, [30] during electron microscopy studies conducted on replicas taken from intergranular stress corrosion fracture surfaces for an Al-5.5Zn-2.5Mg alloy tested in an aqueous 0.5M NaCl solution.…”

Section: Sustained-load Cracking In Moist Air and Distilled Watermentioning

confidence: 99%

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Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Holroyd¹,

Scamans

2011

Metall Mater Trans A

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Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water at temperatures in the range 283 K to 353 K (10°C to 80°C) has been reviewed in detail, paying particular attention to local processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions, but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing from~150 nm at room temperature to~400 nm at 313 K (40°C). A re-evaluation of published data reveals the apparent activation energy, E a for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently~35 kJ/mol for temperatures above~313 K (40°C), independent of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al 2 CuMg where E a is~80 kJ/mol. For temperatures below~313 K (40°C) E a is independent of copper content for stress intensity factors below~14 MNm À3/2 , with a value~80 kJ/mol but is sensitive to copper content for stress intensity factors abovẽ 14 MNm À3/2 , with E a , ranging from~35 kJ/mol for copper-free alloys to~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy for intergranular sustained-load crack initiation is consistently~110 kJ/mol for both notched and un-notched samples. Mechanistic implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH 3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces.

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Section: Sustained-load Cracking In Moist Air and Distilled Watermentioning

confidence: 58%

Section: A Intergranular Crack Arrest Markingsmentioning

confidence: 97%

Section: A Intergranular Crack Arrest Markingsmentioning

confidence: 99%

Section: Sustained-load Cracking In Moist Air and Distilled Watermentioning

confidence: 99%

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Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Holroyd¹,

Scamans

2011

Metall Mater Trans A

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“…(a) Crack arrest markings have been found on intergranular SCC fracture faces for high purity copper-free Al-5.5Zn-2.5Mg, [2,92] low-and high-copper-containing Al-Zn-Mg-Cu alloys in acidified-inhibited saline solutions (AA7018-T651, [93] AA7075-T7351 [34] ) and 0.6 M NaCl (AA7075-T651 [94] ), that closely resemble those seen on intergranular SCCs generated in moist air or distilled water. [95] (b) Time-lapse video studies of crack initiation and growth from notched AA7150-T6 samples immersed in 0.6 M NaCl, [96] have shown that initial crack growth (K = 5 to 18 MNm À3/2 ) occurs as a series of discreet steps with crack growth rates greater than 10 À6 m/s; (c) Crack propagation for AA7075-T651 in 0.6 M NaCl generated a greater number of electrochemical potential fluctuations (electrochemical noise) with a significant increase in the number of events at low frequencies, [97] which are consistent with the anticipated 10 through 20 second intervals between crack growth increments, based on the spacing between crack arrest marking spacing (200 through 400 nm [93,95] ) and the region 2 SCC growth rate (2 9 10 À8 m/s, Figure 1); and (d) Output from mini-load cells placed in bolts used during DCB SCC tests on AA7075-T651 totally immersed in 0.6 M NaCl showed perturbations in crack length-time measurements.…”

Section: B Mechanistic Implicationsmentioning

confidence: 99%

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Holroyd

Scamans

2012

Metall Mater Trans A

108

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Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20°C to 80°C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak-and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of precrack (''pop-in'' vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E a ) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20°C to 80°C) for under-, peak-, and over-aged low-copper-containing alloys (<0.2 wt pct) are typically ranging from 80 to 85 kJ/mol, whereas for high-copper-containing alloys (>~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under-and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak-and over-aged tempers, and for high-copper alloys in under-and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.

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On Separating the Effect of Corrosion on Inter-Lamellar Fatigue of Thin Sheet Aa7079-T6

Shah¹,

Fawaz²

2009

ICAF 2009, Bridging the Gap Between Theory and Operational Practice

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Stress corrosion cracking in aluminium alloy 7075-T651 by discrete crack jumps as indicated by fractography and acoustic emission

Cited by 13 publications

References 9 publications

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Crack Propagation During Sustained-Load Cracking of Al-Zn-Mg-Cu Aluminum Alloys Exposed to Moist Air or Distilled Water

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

On Separating the Effect of Corrosion on Inter-Lamellar Fatigue of Thin Sheet Aa7079-T6

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