Figure 3-68. Fluorescent Magnetic Particle Indications of Cracks in Crankshaft
of Small Aircraft Engine Damaged in Plane Accident.
b. Stress Corrosion Cracks. Parts that are under either residual or applied tensile stress and are exposed to
a corrosive environment may develop stress corrosion cracking. The primary role of corrosion in this
cracking mode is to produce hydrogen. The hydrogen migrates to the tip of a stress corrosion crack
where its presence increases the stresses at the tip, thus driving the crack even deeper. When corrosion
is added to a fatigue-producing service condition, this type of service failure is called corrosion fatigue.
c. Overstressing. Parts that are stressed beyond the level for which they were designed can crack or break.
Such over-stressing 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 with regard to the fractured part. But
other parts of the assembly that 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.
Other Sources of Discontinuities.
In this chapter an attempt has been made to familiarize the reader with most of the common sources of discontinuities
that can occur in iron and steel. Actually the list given here is incomplete. But the inspector working with magnetic
particle testing will encounter these discontinuities which have been described more frequently than those from less
common conditions. He will often have the metallurgical laboratory of a support organization available for
consultation, and the metallurgist will usually be able to assign a cause to an indicated discontinuity and assess its
Nature and Type.
It is easier to distinguish between relevant and nonrelevant indications when
fluorescent rather than visible magnetic particles are used.
It is possible to magnetize parts of certain shapes in such a way that magnetic leakage fields are created even though
there is no discontinuity in the metal at that point. Such indications are sometimes called erroneous indications or false
indications. They should be called "non-relevant indications" since they are actually caused by distortion of the