Effect of cyclic frequency on fracture mode transitions during corrosion fatigue cracking of an Al-Zn-Mg-Cu alloy

Abstract

There are complex issues remaining to be resolved before environment-assisted cracking models can be included in structural mechanics programs that are currently used to analyze mechanical fatigue crack growth. Considered here is the effect of cyclic frequency on fracture mode transitions that occur during corrosion fatigue of high-strength Al-Zn-Mg-Cu alloys. These alloys are used in civilian and military aircraft applications where they are exposed to detrimental aqueous saline environments. A previously developed “critical hydrogen at a critical distance” crack growth model is used to rationalize the observed transitions in crack-path fracture-modes, from intergranular (IG), to brittle transgranular (BTG), to ductile transgranular (DTG), as the alternating stress intensity factor and cyclic frequency increase beyond critical values. Corrosion fatigue crack growth rate data obtained on Al-Zn-Mg-Cu alloy 7017-T651 base metal and heat-affected “white zone” metal tested in aqueous NaCl solutions over a frequency range of 0.01–70 Hz are analyzed. For the white zone metal, dependence of the critical crack velocity on the critical frequency, at which the IG-BTG transition occurs, undergoes an abrupt reversal as the critical frequency increases above about 0.1 Hz. Mechanisms potentially responsible for this change in frequency dependency are discussed in the context of the critical hydrogen model. The transition from low to intermediate frequency behavior is speculated to be due to a change in the critical distance from microstructural control, for frequencies at or below 0.1 Hz, to control by the critical hydrogen criterion at higher frequencies. The low frequency behavior is discussed relative to the transition from static load stress corrosion cracking to low frequency corrosion fatigue, which occurs as cyclic frequency increases above zero.

Synergism between fatigue and cyclic‐stress corrosion cracking: Electrochemically active surface area

“…However, previous studies of high strength aluminum alloys show that transitions in fracture modes, 13and adjusting the value of k to get the best fit to the data from brittle intergranular to brittle transgranular to ductile transgranular, occur as the independent-variables frequency and ΔK are each increased above critical levels. 27,53 The data analyzed here show that the cracking mechanism is unaffected by environment for K MAX greater than about 44.5 MPa√m (t R = 22.5 s) and 48 MPa√m (t R = 55 s) where corrosion fatigue (without hold-time) and the dry air data trends converge (typically ductile transgranular cracking mode). For K MAX less than this, the time dependence of the data shows that the cracking mechanism is environmentally affected.…”

“…Hall [24][25][26][27] expanded on the hydrogen embrittlement step by developing a diffusion-limited critical-hydrogen model of crack advance. This model assumes that HE embrittlement occurs in an additional seven steps: embrittlement occurs when hydrogen is diffusively segregated to and trapped at prospective fracture interfaces, cracks advance when hydrogen coverage of a fracture interface equals a critical value at a critical distance, cracks advance during the up-load half cycle when K ¼ K MAX , the critical-hydrogen coverage is a decreasing function of increasing applied K, crack advance is ratelimited by hydrogen diffusion, diffusible and trapped H are in thermodynamic equilibrium, and cracks advance steadily and quasi-continuously.…”

“…An EAC model that assumes crack growth is rate limited by hydrogen diffusion was developed and used to consider the effect of cyclic frequency and cyclic crack opening displacement on corrosion-fatigue crack velocity and fracture mode transition in AA 7017. 26,27 These two papers provide a discussion of the model development and issues for application of HE models that are rate limited by hydrogen diffusion.…”