The Effects of Loading Frequency, Sensitization, and Crack Length on Corrosion Fatigue of AA5456-H116

Abstract

The effects of fatigue loading frequency (f), sensitization, and crack length on corrosion fatigue crack growth rates (da/dN) were investigated for AA5456-H116 under full immersion in 3.5 wt% NaCl. Results from fracture mechanics-based experiments conducted at a constant stress-intensity range (ΔK) and load ratio (R) suggest that highly sensitized AA5456-H116 microstructures (ASTM G67 nitric acid mass loss tests [NAMLT] value of 24 mg/cm2 and higher) exhibit increased da/dN over microstructures in the as-received condition (NAMLT 5 mg/cm2) and the onset of an inverse f-dependence. For a single, high level of sensitization (70 mg/cm2), da/dN increased 3.5× as f decreased from 10 Hz to 0.03 Hz. At a singular low loading f of 0.03 Hz, high sensitization levels (65 mg/cm2) accelerated da/dN fivefold over da/dN in the as-received condition. The da/dN of microstructures below a critical NAMLT value of 24 mg/cm2 were f-independent. Specifically, in microstructures with a low sensitization level (ASTM G67 NAMLT value less than 24 mg/cm2), there was no increase in da/dN as f decreased from 1 Hz to 0.03 Hz. At any singular f, sensitization up to 24 mg/cm2 did not accelerate da/dN over the rate in an as-received microstructure. Additional testing established that the inverse f-dependence of da/dN observed in highly sensitized microstructures cannot be attributed to crack length effects. Two hypotheses are discussed that may explain the observed inverse relationship between f and da/dN in microstructures at or above the critical NAMLT value of 24 mg/cm2.