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 TB 55-1900-201-45/1
CHAPTER II. GUIDE TO INSPECTION SURVEY
SECTION I. Inspection of Steel Hull Vessels
2-1. Types of Inspections.
a. Deterioration of the metal structure is probably the most common, single defect in steel vessels. It can be due
to a number of different causes or combinations thereof, including age, inadequate maintenance, working of the hull
structure, chemical or erosive action, electrolysis, local wear, some improper feature of design, etc. In some cases, such
as deep pitting and external grooving, deterioration is easy to detect. In other cases, such as the general erosion of age
or incipient joint or member failure, it is impossible to ascertain the deterioration without actually gaging or very careful
examination. In any case, judgment is required to evaluate and to determine, if any, to what extent repairs are neces-
sary.
b. The only practical way of determining the degree of deterioration is to measure the thickness of the member in
question and compare it with the original thickness. This comparison is usually expressed in terms of percentage of
wastage from the original scant- ling. This determination usually requires drilling of the hull and should not be under-
taken unless there is a reasonable basis for doubt as to the condition of the present scantling.
c. Thickness measurements can be made by drilling and gaging or by ultrasonic measurement. The term "gaging"
is meant to include both methods of measurement; however, the acceptance of ultrasonic measurement in lieu of drilling
and gaging is subject to the approval of the U. S. Army Troop Support and Aviation Materiel Readiness Command,
ATTN: DRSTS-MMM.
d. Ultrasonic testing can be defined as "The testing and inspection of materials through the application of ultra-
sonic vibrations and observations of the resulting reflection of the vibrations from the material". The ultrasonic waves are
usually sound waves of a frequency well above the audible range. These waves are reflected within the material either by
its opposite side or by a flow or discontinuity, so that the thickness of most parts can be measured. Ultrasonic thickness
indicators are intended to eliminate gaging of wall thickness by drilling and measuring the thickness by mechanical
means. However, where metal thickness may be calipered without drilling, the ultrasonic device should not be used in
lieu of the former method of gaging. It should be recognized that there are definite limitations in the use of ultrasonic de-
vices for measuring thickness. When this method is used to comply with a specific requirement for gaging, the surveyor
should choose the locations and witness the gaging. Also, he should satisfy himself as to the accuracy and adjustment of
the instrument. Prior to acceptance of thickness measurements, it must be demonstrated to the U. S. Army surveyor that
the operator and instrument are capable of obtaining an accuracy of 4%.
2-1
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