FALL 2021

158
THE DISTRIBUTOR’S LINK
ROB LaPOINTE MAGNETIC PARTICLE TESTING VS. PENETRANT TESTING – WHICH TEST IS BEST? from page 100
FIGURE 5 - MAGNETIC PARTICLE TESTING SHOWING FLUORESCENT
FLUID FLOWING OVER TEST SAMPLE IN A HEAD SHOT ORIENTATION
This is an easy way to visualize the relationship
and to understand their directional connection. The
magnetic particle test method makes use of this physical
relationship. To perform the magnetic particle test, a
current is conducted through the test sample (a fastener
for our discussion) and a corresponding magnetic field is
produced at and just below the surface of the test sample.
In Figure 5, the current is traveling through the bolt from
right to left so by using the right-hand rule, the magnetic
field would be encircling the bolt in a clockwise direction
as seen in the photo. This is the same orientation shown
diagrammatically in Figure 6. As the magnetic field is
created, a light oil filled with iron particles which have
been coated with fluorescent material are drizzled over the
test sample (Figure 5). If the magnetic field encounters a
discontinuity or break in the material, the field jumps out
of the material in a small loop above the discontinuity.
This is known as the leakage flux and shown in Figure 6.
The iron particles in the oil being drizzled over the test
sample interact and are caught within this leakage flux
and the fluorescent coating creates a luminous signature
along the discontinuity.
Using this method, only discontinuities that have
component at a right angle (90°) to the magnetic field
are detectable. To catch all possible discontinuities in
any direction, the test sample must also be observed by
creating a magnetic field that is directed along the part.
This is done by placing the part inside a conductive coil
which creates a magnetic field in the direction of the
long axis of the bolt. The part of the test that detects
discontinuities oriented along the long axis of the bolt
is called the “Head Shot” and the part that detects
discontinuities oriented perpendicular to the long axis of
the bolt is called the “Coil Shot.” With these two shots
together (head and coil), we can detect all possible
surface discontinuities in a part. Figure 7 shows a
quench crack caused by thermal stress during heat
treatment running the length of the bolt and wrapping
around the head.
Penetrant testing makes use of an oily fluid that has
very low viscosity and surface tension. Viscosity is a
measure of a fluid’s internal friction which controls the
fluid’s ability to flow. Fluids with a low viscosity flow
very easily. Surface tension is a measure of a fluid’s
cohesion (stickiness) to itself. Low viscosity combined
with low surface tension allow penetrant to flow into very
small cracks in the material.
FIGURE 6 - THIS DIAGRAM SHOWS THE COMPONENTS OF
THE MAGNETIC PARTICLE TEST THIS TEST ORIENTATION
IS CALLED THE HEAD SHOT
FIGURE 7 - QUENCH CRACK VISIBLE BY FLUORESCENT PARTICLES
HELD IN MAGNETIC FIELD LOOP
CONTINUED ON PAGE 172