Standard Practice for Inspection of Transparent Parts by Prism

SIGNIFICANCE AND USE
Prisms are useful tools for viewing areas otherwise obstructed or hidden from normal viewing. For example, transparency bolt holes, voids, and delaminations that are covered by edge strips or frames can be seen through a prism. The prism, once put against the transparency, with a coupling agent to wet the surfaces, allows viewing of the area below where the prism is placed.
SCOPE
1.1 Aerospace transparencies undergo high stresses induced by flight, environmental, or other factors. The transparency attachment points are especially subject to fatigue. These areas of fatigue are often obstructed or hidden from normal inspection. An inspector, following the techniques described in this practice, shall use a prism to view damage located near transparency bolt holes, voids, and delamination that are hidden by edge strips or frames.
1.2 The purpose of this practice is to provide acceptable methods for performing prism inspections of transparent materials with specific emphasis on aircraft windscreens and canopies. Caveats and lessons learned from experience are included to assist authors in writing tailored inspection instructions for specific applications.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

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Publication Date
30-Nov-2011
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ASTM F2108-06(2011) - Standard Practice for Inspection of Transparent Parts by Prism
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: F2108 − 06 (Reapproved 2011)
Standard Practice for
Inspection of Transparent Parts by Prism
This standard is issued under the fixed designation F2108; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope refraction or internal reflection or both to change the direction
of the propagation of light.
1.1 Aerospace transparencies undergo high stresses induced
by flight, environmental, or other factors. The transparency
3. Summary of Practice
attachment points are especially subject to fatigue. These areas
of fatigue are often obstructed or hidden from normal inspec- 3.1 Prism inspections shall be performed with the aid of a
tion. An inspector, following the techniques described in this prism and a wetting agent. The wetting agent is between the
practice, shall use a prism to view damage located near prism and transparency to ensure good visibility.The examiner
transparency bolt holes, voids, and delamination that are looks through the prism to see the area of the transparency
hidden by edge strips or frames. below the prism. The exam is best done without bright
sunlight, which causes glare. The examiner changes his angle
1.2 The purpose of this practice is to provide acceptable
of view and inspects the area seen in the prism. A trained eye
methods for performing prism inspections of transparent ma-
can detect anomalies such as cracks, delaminations, and voids.
terials with specific emphasis on aircraft windscreens and
canopies. Caveats and lessons learned from experience are
4. Significance and Use
included to assist authors in writing tailored inspection instruc-
4.1 Prisms are useful tools for viewing areas otherwise
tions for specific applications.
obstructed or hidden from normal viewing. For example,
1.3 This standard does not purport to address all of the
transparency bolt holes, voids, and delaminations that are
safety concerns, if any, associated with its use. It is the
covered by edge strips or frames can be seen through a prism.
responsibility of the user of this standard to establish appro-
The prism, once put against the transparency, with a coupling
priate safety and health practices and determine the applica-
agent to wet the surfaces, allows viewing of the area below
bility of regulatory limitations prior to use.
where the prism is placed.
2. Terminology
5. Apparatus
2.1 Definitions of Terms Specific to This Standard:
5.1 Prism—Material shall be of the same hardness as or
2.1.1 coupler, n—a liquid refractive index-matching agent
softer than the transparency to avoid scratches. The angles of
or wetting agent (for example, glycerol) used to create optical
the prism should be set so as to maximize the viewing area. In
contact (coupling) between the prism and transparency.
the case of a complex shape, the position of the inspector’s
2.1.2 delamination, n—the separation of a laminate into its
head and potential interference problems shall be considered to
constituent parts. An edge delamination is open to the lami-
ensure the inspector can look through the prism at the proper
nate’s free edge. An internal delamination is contained within
angles. A typical prism is a 30º–60º–90º triangle, although the
the laminate.
prism does not need to be triangular; it could be a simple
2.1.3 edge attachment, n—the means of fastening the side
rectagon shape.
edges of a transparency to the aircraft structure. Edge attach-
5.2 Flashlight—A high output, halogen bulb works best.
ments also include expansion joints and any other connection
between the transparency and the aircraft structure.
5.3 Cover—To allow the inspector to shade the inspection
area if the inspection is done in bright sunlight.
2.1.4 prism, n—a transparent optical element with polished
plane faces for the entrance and exit sides. Prisms use
6. Reagents
6.1 Use glycerol (USPGrade) as a coupler or wetting agent.
This practice is under the jurisdiction of ASTM Committee F07 on Aerospace
6.1.1 The wetting agent shall not harm the transparency (for
andAircraft and is the direct responsibility of Subcommittee F07.08 on Transparent
Enclosures and Materials. example, cause crazing), shall be clear, and shall have a high
Current edition approved Dec. 1, 2011. Published May 2012. Originally
enough viscosity to stay in place reasonably well.
approved in 2001. Last previous edition approved in 2006 as F2108 - 06. DOI:
10.1520/F2108-06R11. 6.2 Use distilled water.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2108 − 06 (2011)
7. Procedure
7.1 Use methods shall be approved by the procuring agency
or parts manufacturer. One example is to use a water rinse to
clean the area of the transparency thoroughly where the prism
will be placed. Assure the prism is also clean. Any debris will
cause scratches to the delicate optical surfaces.
7.1.1 Clean the prism with standard optical cleaning solu-
tions appropriate for the material from which it is made. See
Note 1.
7.2 If using glycerol, store it in a cool dry place, such as a
refrigerator designated for chemicals, so it will be thicker.
Apply several drops of coupler to the side of the prism that
goes against the transparency. Ensure there is enough coupling
between the prism and transparency to allow a good viewing
area. (Warning—Do not allow the prism to slide along the
transparency with no wetting agent between them or scratches
will occur.)
7.3 The prism is placed on the transparency and the viewing
FIG. 2 Standard Nomenclature
area location as shown in Fig. 1.When moving the prism along
the transparency use light pressure. Do not allow the glycerol
to get on the viewing surface of the prism, as it will smear the
viewing area.
edges of the prism and use gentle pressure. (Warning—If the
7.4 Acoupler is required to provide an optical path between
coupler gets onto the viewing face of the prism, the area of
theprismandcanopy.Placingsmallamounts(severaldrops)of
interest will be blurred. Keep the prism’s viewing face clean.)
coupler on both the canopy and the prism tends to work best.
7.6 A light source, such as a flashlight aids in optical
Glycerol is a good coupler. The coupler material shall be
clarification. This inspection technique does not work well in
compatible with the transparency material, the clearer and the
bright sunlight. Bright sunlight causes glare that clouds the
more viscous the better. Gentle pressure between the prism and
viewing area. The inspection works best at night while using a
canopy keeps air out of the coupler and allows the coupler to
flashlight.
spread farther. See Note 2.
NOTE 1—Any debris will cause scratches to the delicate optical
7.5 Gently press the prism (of appropriate size and shape, as
surfaces.
described below) to the transparency.Alter the pressure on the
NOTE 2—Using too much coupler will make cleanup more difficult and
prism corners to remove any trapped air. To keep the prism
make it difficult to keep the prism viewing area clean.
clean, the least amount of glycerol shall be used. Hold only the
7.7 Look through the top of the prism to view the area of
interest. The prism shall be moved up or down to get the
viewing area to the proper depth. When looking through the
prism, change the angle of view (left to right). Begin to move
the prism along the area of interest. Continue looking through
the prism, altering angle of view. If an air bubble gets trapped
between the prism and transparency, ge
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