AMMTIAC Quarterly, Vol. 2, No. 2 - Advanced Materials ...

techsolutions 5
Brett J. Ingold
AMMTIAC
Rome, NY
A Selecting Brief Introduction a Nondestructive to Precious Testing Metals Method, Part IV: Radiography
This edition of TechSolutions is the fourth installment in a series dedicated to the subject of nondestructive testing.
TechSolutions 1, published in Vol. 1, No. 2 of the AMMTIAC Quarterly, introduced the concept of nondestructive
testing and provided brief descriptions of the various techniques currently available. TechSolutions 2 and 3, published
in subsequent issues of the AMMTIAC Quarterly, focused on visual inspection and eddy current testing. The current
article continues the series and provides a general and informative overview of the radiography nondestructive testing
method. In addition, this article will highlight some of the physical principles, inspection requirements, and implementation
considerations involved in an effective radiographic inspection process. 1 Once the series on nondestructive testing
methods is complete, we will combine all of the articles into a valuable desk reference on nondestructive testing and place
it on our website. – Editor
INTRODUCTION
After visual and optical testing (VT), the next method of nondestructive
testing (NDT) most commonly employed in industry is
radiographic testing. Also simply referred to as radiography, it is
perhaps the most versatile of the nondestructive testing methods.[1]
The basic radiographic process in use today is in large part
still the same as it was when it was introduced in the late 1800s.
Radiography uses radiation energy to penetrate solid objects in
order to assess variations in thickness or density. The second part
of the process involves capturing a shadow image of the component
being inspected on film using procedures similar to those that
technicians used when the technology was first
developed. Identifying density differences on an
X-ray, which indicate flaws or cracks, is still the
foundation of radiographic analysis.
Beam
PHYSICAL PRINCIPLES
Radiography basically involves the projection
and penetration of radiation energy through the
sample being inspected. The radiation energy is
absorbed uniformly by the material or component
being inspected except where variations in
thickness or density occur. The energy not
absorbed is passed through to a sensing medium
that captures an image of the radiation pattern.
The uniform absorption and any deviations in
uniformity are subsequently captured on the
sensing material and indicate the potential presence
of a discontinuity.
Image Capturing Media
In simple terms, a radiograph is a photographic record produced by
the passage of X-rays or gamma rays through an object onto a film
or other recording medium (see Figure 1). The developing, fixing
and washing of the film after exposure can be performed manually
or by automated processing equipment. The development
process begins after the film is exposed to the radiation and an
invisible change called a latent image develops on the film emulsion.
These exposed areas become dark when the film is placed in
Flaw
Radiation Source
Test Piece
(Object)
Medium for
Converting
the Radiation
Image of Flaw
Figure 1. Diagram of Typical
Radiography Test Setup.[2]
a developing solution. The degree of darkening that occurs during
this process depends on the amount of exposure that occurred. The
next step is to place the film into a special bath and rinse it to stop
the development process. Lastly, the film is put into a fixing bath
and then washed to remove the fixer solution. At this point the
film is fully developed, the process is complete and the radiograph
is ready to be handled and analyzed.[1]
As the digital world has evolved, a quicker and much more
efficient alternative to the meticulous film development process
has also emerged to benefit the radiography NDT community.
Computed radiography, which is described in
the related article entitled “Computed Radiography
in the Pacific Northwest: Benefits, Drawbacks
and Requirements”, makes use of an
alternative image capturing media and development
process.
Electromagnetic Radiation
Two types of electromagnetic radiation are used
to perform radiographic inspection: X-rays and
gamma rays (see Figure 2). The primary distinguishing
characteristic between these two types
of radiation is the different wavelengths of the
electromagnetic energy. Compared to other
types of radiation both X-rays and gamma rays
have relatively short wavelengths which allows
them to penetrate opaque materials. This
inherent capability is what enables their use
for nondestructive testing, as they can reveal
flaws embedded in visually non-transparent materials. The advent
of radiography came quickly after the discovery of X-rays because
of the penetration properties of this electromagnetic energy.[3]
Types of Discontinuities
A number of different types of discontinuities can be detected
with radiographic NDT. Table 1 lists the suitability of traditional
radiographic NDT methods for identifying various types of
discontinuities in several applications.
http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 2, Number 2
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