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printing as a positive as in photography. This is because, in printing, some of the detail is
always lost and no useful purpose is served.
Before commencing a radiographic examination, it is always advisable to examine the
component with one's own eyes, to eliminate any possible external defects. If the surface of a
weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely
to be limited to those cases in which the surface irregularities (which will be visible on the
radiograph) may make detecting internal defects difficult.
After this visual examination, the operator will have a clear idea of the possibilities of
access to the two faces of the weld, which is important both for the setting up of the
equipment and for the choice of the most appropriate technique.
Defects such as delaminations and planar cracks are difficult to detect using radiography,
which is why ultrasonics is the preferred method for detecting this type of discontinuity.
Indication
Indications are classified as true or false. False indications are those caused by factors not
related to the principles of the testing method or by improper implementation of the method,
like film damage in radiography, electrical interference in ultrasonic testing etc. True
indications are further classified as relevant and non relevant. Relevant indications are those
caused by flaws. Non relevant indications are those caused by known features of the tested
object, like gaps, threads, case hardening etc. For example, in electromagnetic testing,
indications from metal loss are considered flaws because they should usually be investigated,
but indications due to variations in the material properties may be harmless and no relevant.
Flaw. A type of discontinuity that must be investigated to see if it is rejectable. For example,
porosity in a weld or metal loss.
Evaluation. Determining if a flaw is rejectable. For example, is porosity in a weld larger than
acceptable by code?
Defect A flaw that is rejectable — i.e. does not meet acceptance criteria. Defects are generally
removed or repaired.
Penetrant testing. Non-destructive test typically comprising a penetrant, a method of excess
removal and a developer to produce a visible indication of surface-breaking discontinuities.
In NDT, the structure undergoes a dynamic input, such as the tap of a hammer or a
controlled impulse. Key properties, such as displacement or acceleration at different points of
the structure, are measured as the corresponding output. This output is recorded and compared
to the corresponding output given by the transfer function and the known input. Differences
may indicate an inappropriate model (which may alert engineers to unpredicted instabilities or
performance outside of tolerances), failed components, or an inadequate control system. As a
system, the human body is difficult to model as a complete transfer function. Elements of the
body, however, such as bones or molecules, have a known response to certain radiographic
inputs, such as x-rays or magnetic resonance. Coupled with the controlled introduction of a
known element, such as digested barium, radiography can be used to image parts or functions
of the body by measuring and interpreting the response to the radiographic input. In this
manner, many bone fractures and diseases may be detected and localized in preparation for
treatment. X-rays may also be used to examine the interior of mechanical systems in
manufacturing using NDT techniques, as well.
NDT is used in a variety of settings that covers a wide range of industrial activity, with
new NDT methods and applications, being continuously developed. NDT services are not
only integrated with Asset Integrity Management (AIM) solutions, but also with Material
Testing laboratories and seamlessly fit into Supply Chain services.
NDT is divided into various methods of nondestructive testing, each based on a
particular scientific principle. These methods may be further subdivided into various
techniques. The various methods and techniques, due to their particular natures, may lend

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