Corrosion fatigue crack detection

Corrosion-fatigue cracks can be detected by nondestructive testing techniques such as magnetic particle inspection, radiography, ultrasonics, and dye penetrant. Corrosion-fatigue cracks may occur in numerous tubes simultaneously. Nondestructive testing of tubes at locations similar to those in which cracks are observed can be useftil. 

Fig. 9.61 Schematic drawing of approach to detection of corrosion fatigue cracks [226].

E. Tada, Detection of corrosion fatigue cracking through current responses induced by cyclic stressing, Corros. Sci. 49 (2007) 248—254. 

D ye penetration inspection. This is a simple technique, requiring a minimum of operator training. In the hands of a skilled operator, it is capable of detecting fine cracks such as chloride stress corrosion cracks in austenitic stainless steels and fatigue cracks. 

Axle shafts and journals can develop fatigue cracks. Crack initiation can start from a scratch or corrosion. Axle related derailments ate generally associated with the fracture of the axle under a vehicle, usually under conditions of high dynamic loading. These types of failures can be detected in the workshop by NDT inspection, e.g. ultrasound. There is potential for automated technologies dealing with axle defects. 

Acoustic emission Leaks, collapse of cavitation, bubbles, vibration level in equipment. Cracks by detection of the sound emitted during their propagation A new technique capable of detecting leaks, cavitation, corrosion fatigue pitting and stress corrosion cracking in vessels and lines Infrequent... 

A technique capable of detecting leaks, cavitation. Infrequent corrosion fatigue, pitting, and stress-corrosion cracking in vessels and Unes. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

Liquid penetrants Cracks, gouges, porosity, laps, and seams open to a surface Castings, forgings, weldments, and components subject to fatigue or stress-corrosion cracking Inexpensive easy to apply portable easily interpreted Flaw must be open to an accessible surface, level of detectability operator-dependent... 

Cracking has occurred in PWR piping systems as a result of stress corrosion, vibratory and thermal fatigue and dynamic loadings. All incidents of cracking have been detected and corrective actions taken prior to any catastrophic failures. (Pipe thinning due to erosion has caused catastrophic failure, but that mechanism is considered separate from cracking.)... 

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