Influence of atmospheric moisture on slow fatigue crack growth at ultrasonic frequency in aluminium and magnesium alloys

Abstract

The deleterious influence of atmospheric moisture on the fatigue properties of an aluminium wrought alloy AlZnMgCu1.5‐T6, an aluminium cast alloy AlSi9Cu3 and magnesium cast alloys AM60 hp, AZ91 hp and AS21 hp has been studied at a cycling frequency of 20 kHz. Atmospheric moisture accelerates fatigue crack growth and decreases the threshold stress intensities to 55–75% of the respective values in vacuum. In ambient air, fatigue crack growth rates were up to two decades higher than those in vacuum. Accelerated crack growth was found at propagation rates below about 2 × 10−9 m cycle−1 in aluminium alloys and below about 3 × 10−8 m cycle−1 in magnesium alloys. As the threshold regime is approached, fatigue cracks in ambient air either propagate at a minimum mean growth rate on average of approximately one lattice spacing per cycle or they stop propagating, whereas mean growth rates of 10−12 m cycle−1 were found in vacuum. Crack initiation and slow fatigue crack growth mainly determine lifetimes in the high cycle regime, and endurance data obtained at ultrasonic frequency in ambient air of 40–60% relative humidity are similar to lifetimes measured at conventional frequencies.

Near‐threshold fatigue crack growth properties of wrought magnesium alloy AZ61 in ambient air, dry air, and vacuum

“…Most obvious effect is the significantly decreased threshold stress intensity factor in ambient air, which is about 60% of the threshold value measured in vacuum. Previous investigations on three cast magnesium alloys (AZ91, AM60, and AS21) by Papakyriacou et al delivered similar results of threshold stress intensity factors in ambient air, which were between 55% and 65% of the respective threshold value measured in vacuum. Adams et al investigated the propagation of short cracks in wrought WE43 alloy and found that growth rates in vacuum are nearly two orders of magnitude lower than in laboratory air.…”

“…Investigations of fatigue crack growth in three cast magnesium alloys (AZ91, AM60, and AS21) in ambient air and vacuum reveal a strong influence of the environment on slowly propagating fatigue cracks. Threshold stress intensity factor amplitudes are in the range of 1.25 to 1.55 MPam 1/2 in ambient air but 2.1 to 2.7 MPam 1/2 in the inert environment . In differently heat‐treated wrought magnesium alloy WE43, fatigue crack growth of short cracks strongly decelerates or even stops, when the testing environment is changed from ambient air to vacuum …”

“…Threshold stress intensity factor amplitudes are in the range of 1.25 to 1.55 MPam 1/2 in ambient air but 2.1 to 2.7 MPam 1/2 in the inert environment. 9 In differently heat-treated wrought magnesium alloy WE43, fatigue crack growth of short cracks strongly decelerates or even stops, when the testing environment is changed from ambient air to vacuum. 10 Atmospheric corrosion of magnesium alloys is highly dependent on the availability of water.…”

Elevated temperature low cycle fatigue of a gravity casting Al–Si–Cu alloy used for engine cylinder heads

“…In addition, Al-Si-Cu alloys have also shown fatigue properties, dependent on other factors, such as solidification rate [10,11], strontium modification [12,13], and content of elements (i. e. Fe, Mn, Ti, Zr and V) [5,14]. Furthermore, studies on the effect of heat treatments [11] and environment [15,16] have provided substantial information on fatigue behaviors of Al-Si-Cu alloys.…”

Effect of Frequency, Environment, and Temperature on Fatigue Behavior of E319 Cast-Aluminum Alloy: Small-Crack Propagation

“…It has been observed that the fatigue-crack growth threshold decreased and fatigue-crack growth rate increased in the presence of water vapor in atmospheric air for aluminum alloys. [10][11][12][13][14][15][16][17] Because the duration of crack-tip opening under ultrasonic-frequency loading at 20 kHz is an order of magnitude shorter for each cycle than for conventional fatigue experiments, any environmentally assisted increase in fatigue-crack growth rate is generally presumed to be less pronounced at 20 kHz, leading to lower fatigue-crack growth rate at this frequency. Holper et al [9] studied the influence of frequency on fatiguecrack growth of aluminum alloys and reported that fatigue cracks propagated at lower growth rates at 20 kHz than at 20 Hz in ambient air if cycled above threshold; however, the test frequency had no influence on the fatigue-crack growth threshold itself.…”