Threshold Stress Intensity Factor for Corrosion

Kjsq- Threshold stress-intensity factor for stress-corrosion cracking. The critical plane-strain stress intensity at the onset of stress-corrosion cracking under specified conditions. 

The chapter builds on our critical reviews on Mg corrosion [1-4] and Mg SCC [5,6]. SCC [5-8] involves (1) a stress, (2) a susceptible alloy and (3) an environment. SCC is related to hydrogen embrittlement (HE). HE is SCC that is caused by hydrogen (H), which can be gaseous, can come from corrosion, or can be internal from prior processing. HE is often postulated as the SCC mechanism. SCC can be extremely dangerous. Under safe loading conditions, SCC causes slow crack growth. Fast fracture occurs when the crack reaches a critical size. SCC, for any alloy + environment combination, can be characterised by [7,8] the threshold stress, ctscc> threshold stress intensity factor, iscc> the stress corrosion crack velocity. 

The best tools for weighing the various aspects are quantitative expressions of properties and performance data valid under various conditions, such as corrosion rate and distribution, lifetime in eorrosion fatigue, mechanical or electrochemical threshold values (Kiscc, Kthi Ep etc.), compared with corresponding quantified requirements or service conditions, i.e. specified lifetime, actual stress intensity factors and functions, and corrosion potential. 

Part (b) of the Figure 11.48 shows the superposition of the behaviors of mechanical fatigue and stress corrosion cracking. In general, the influence of the latter shows up mostly at low strain rates (d /dy = 10 - 10 s ). In the graph that represents Aa/AN as a function of log K, a step appears at the point where the stress intensity factor that corresponds to the maximum value of the applied cyclic stress, reaches the value iscc. the threshold stress intensity for stress corrosion cracking. 

For an edge crack, the crack length is a, for a center crack the crack length is 2 a. Most of the past research of stress corrosion cracking utilizes the fracture mechanics approach in which the relation between the initially applied stress, stress intensity factor Ki and the time to failure of pre-cracked specimen is observed. The time to failure increases as Ki decreases from the level required for the fracture to occur (Kic, fracture toughness) to a threshold level (Kiscc) below which SCC does not occur. 

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