What are In-Service Discontinuities?

One of the most common classifications of discontinuities, service discontinuities comprise discontinuities which are formed or produced after all fabrication has been completed and the part has gone into service. The objective of magnetic particle testing to locate and eliminate discontinuities during fabrication to put the part into service free from defects. However, even when this is fully accomplished, failures in service still occur as a result of service conditions.

Fatigue Cracks

As a source of discontinuities, the phenomenon of fatigue is a prolific one. Metals which are subjected to alternating or fluctuating stresses above a certain critical level (the fatigue strength) will eventually dew lop cracks, and finally fracture. Fatigue cracks, even very shallow ones, can readily be found with magnetic particles, and the part often can be salvaged.

Corrosion

Parts which are under tension stress in service and are at the same time exposed to corrosion from whatever cause may develop cracking at the surface, referred to as stress corrosion cracking. Such cracks, under continuing corrosion and stress (whether reversing. or fluctuating or not) will progress through the section until failure occurs. When corrosion is added to fatigue-producing service conditions, this level of service failure is called corrosion fatigue.

Overstressing

Parts of an assembly in service that are stressed beyond the level for which they were designed are very likely to crack or break. Such overstressing may occur as the result of an accident; or a part may become overloaded due to some unusual or emergency condition not anticipated by the designer; or a part may be loaded beyond Its strength because of the failure of some related member of the structure. After complete failure has occurred magnetic particle testing obviously has no application as regards the fractured part. But other parts of the assembly which may appear undamaged may have been overstressed during the accident or overloading from other causes. Examination by magnetic particle testing is usually carried out in such cases to determine whether any cracks have actually been formed. With this precaution salvage of good parts after an assembly failure is often possible.

Want to learn more about discontinuities and defects? Read our detailed blog about the difference between the two.

Want to discuss the discontinuities in your manufacturing process and how to find them? Connect with our expert team.

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