Estimation of environment-induced crack growth rate as a function of stress intensity factors generated during slow strain rate testing of aluminum alloys

N.J Henry Holroyd,John J Lewandowski,Timothy L Burnett,Benjamin C Palmer

doi:10.1515/corrrev-2019-0031

N.J Henry Holroyd, John J Lewandowski + Show 2 more

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https://doi.org/10.1515/corrrev-2019-0031

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Abstract

Abstract In this contribution, we introduce a simple approach to quickly estimate the environment-induced crack velocity (CV) as a function of the calculated applied stress intensity factor (K) developed during the slow strain rate testing of aluminum alloys exposed to aqueous or humid air-type environments. The CV-K behavior for a commercial aluminum-magnesium alloy, AA5083-H131, sensitized and pre-exposed to a 0.6 m NaCl solution has been estimated from slow strain rate test data. The predicted threshold K and crack velocities match recently published data for the same alloy in similarly sensitized conditions where the CV-K data were obtained using state-of-the-art fracture mechanics-based testing.

Highlights

Environment-induced cracking (EIC) in commercial higher strength aluminum alloys (Al-Zn-MgCu, 7xxx series and Al-Cu-Mg, 2000 series) has been assessed in mechanically loaded precracked test specimens exposed to various environments since the late 1960’s (Mulherin, 1967; McEvily et al, 1967; Hyatt, 1969)
Rising-displacement based tests methods, developed over the last 25 years or so, while potentially considerably shortening test times needed to generate threshold data have their own potential issues, including a) validity of output data is strongly dependent on the selection of an appropriate displacement rate (Dietzel, 2000; Dietzel & Mueller-Roos, 2001) which must be independently verified on a case-by-case basis, typically using fractographic evidence (Dietzel, 2000) and b) test methods require state-of-the-art fracture mechanics based testing equipment and thereby no longer offer a ‘low-cost’ test option
The method to estimate EIC depth and growth rates and the applied stress intensity factors generated during slow strain rate testing (SSRT) for tensile samples with a given sensitization/pre-exposure history, comprised of the following steps: 1) Calculation of nominal stress differentials as a function of the relative plastic strain for SSRT conducted in humid and dry air for the samples subjected to the same sensitization and preexposure treatments, 2) Assuming the stress differentials from 1) are directly relatable to an implied loss of loadbearing area attributable to EIC, calculate the EIC area developed during SSRT in humid air using the relationship: Area of EIC = Original cross-sectional area x [1 –(Stress in Humid Air)/Stress in Dry Air)] Equation 1 where Area is in mm2 and stress in MPa

Summary

Introduction

Environment-induced cracking (EIC) in commercial higher strength aluminum alloys (Al-Zn-MgCu, 7xxx series and Al-Cu-Mg, 2000 series) has been assessed in mechanically loaded precracked test specimens exposed to various environments since the late 1960’s (Mulherin, 1967; McEvily et al, 1967; Hyatt, 1969).

Results

Conclusion