NDT Notes for MT

1. Chapter 8
Evaluation -
Interpretation

2. PURPOSE
The purpose of this chapter is to introduce the student to the proper way to
evaluate and interpret relevant and non-relevant discontinuities.
Subjects covered include the following:
Evaluating Indications
Interpretation of Indications
Non-relevant Indications
Producing Indications
Steps of an Inspection

3. Steps of Inspection
In order to perform an acceptable magnetic particle test, it is important to
follow a few basic steps.
These are:
1. Producing an indication on the part.
2. Interpretation of the indication.
3. Evaluating the indication to the appropriate specification.
Producing Indications
In the previous chapters you have learned how to produce longitudinal and
circular magnetic fields and the type of equipment used to produce those fields.
It is important to understand the applications of the equipment you are using and
their limitations. Using prods may easily damage a part due to arcing or localized
heating. A contour probe may be more applicable but will it produce a strong
enough magnetic field? You may need to use a portable or mobile magnetic
particle machine.
It's also important that the technician understand the principles of magnetism,
the type of material under inspection, and the technique under consideration.

4. Interpretation
After an indication has been identified, the technician will have to evaluate and
interpret that indication.
Knowing the history of the part and the manufacturing process helps the
technician evaluate the indication and determine if it is relevant or non-relevant.
An indication on the surface of the part allows the magnetic particles to be held
tightly together resulting in a sharp fine accumulation of particles. In most cases
the discontinuity can be seen visually or with a magnifying glass.
An indication below the surface of the
part produces a wide shallow
accumulation of particles. This looks
fuzzy and is usually a difficult
discontinuity to interpret. An
inexperienced technician should obtain a
second opinion before rejecting a part
with a subsurface discontinuity.

5. Evaluation
After an indication has been found, it must be interpreted and than
evaluated. This is determined by the accept/reject criteria of the
specification.
If the part is rejected, it may be reworked or repaired and then
reinspected. Some considerations that can be used to evaluate an
indication are listed below.
1. The location of a discontinuity. Is the discontinuity in an area of high stress
and what type of stress is placed in the area? Is it placed under a cyclic
or static load?
2. Discontinuities such as grinding cracks or fatigue cracks are more
harmful than rounded discontinuities created by porosity. If a part is
repaired, the technician should perform periodic testing during the
grinding or arc gouging process in order to ensure the discontinuity is
completely removed. Testing should also take place during the repair
of the discontinuity.
3. A discontinuity detected on the surface of the part is more harmful than
a discontinuity of the same size located below the surface of the part.

6. Nonrelevant Indications
Remember, not all discontinuities are rejectable. Magnetic particles will attract
where there is flux leakage. This could be a crack, or any sharp edge on the part.
Sources of non-relevant indications are:
1. Localized heat treating.
2. A force fit between two metals.
3. Materials that have two types of metals with different permeabilities.
4. Joining two ferromagnetic materials by brazing.
5. Joining a ferromagnetic material with a non-magnetic material.
6. Sharp edges or corners.

7. This picture shows two parts that are about to be inspected with a central
conductor.
The arrows show a source of a non-
relevant indication. This is a subsurface
discontinuity.
Notice that the subsurface indication is
very consistent throughout the part.

8. A relevant indications with dry magnetic particles.