a.
The area to be analyzed is coated with a brittle lacquer coating
(see Reference 21, Holister, Experimental Stress Analysis, Principles and
Methods).
b.
The pressure in the vessel is raised very slowly in small
increments until a crack pattern just starts to develop in the coating. (The
crack will form perpendicular to the direction of the principal tensile
stress at that point.) The pressurization is halted and the crack is marked,
lengthwise along the crack, with grease pencil and numbered No. 1.
c.
The pressurization procedure is continued until a definitive crack
pattern is established.
d.
The crack pattern will give a comprehensive picture of the
directions of the principal stresses in the area. Further, by the
observation and sequential numbering procedure described above, coupled with
the relative density of cracks in the area, the location of the maximum
stress can be determined.
e.
At the point of maximum stress, and at other points of interest,
scratch marks are made in the form of a cross (along the crack and perpen-
dicular to the crack), thus indicating where gages are to be located.
f.
The pressure in the vessel is lowered to zero and the coating is
removed, leaving the scratch pattern.
g.
Bi-axial, electrical resistance, strain gages are then mounted at
and in line with the cross scratches.
h. The pressure in the vessel is increased incrementally while the
strains in the gages are read by a suitable instrument, such as a strain
indicator.
CAUTION:
The experimenter is cautioned that brittle lacquers are
extremely sensitive to temperature and humidity effects.
Further, great care and patience must be exercised in
observing and marking the crack pattern as it develops. If
this is not done it is possible that the maximum stress point
may be missed. Finally, he is cautioned not to read the
crack pattern in a quantitative sense in an attempt to
determine the stresses directly without the use of the strain
gages. Such a procedure is theoretically possible but is
definitely not recommended.
Whatever method of experimental analysis is employed, the designer/
experimenter is urged to maintain clear and definitive records for
incorporation in the Design Report.
NOTE:
The designer should note that Paragraph 6-102, "When
Reevaluation is Not Required," clearly states that if
detailed experimental data exist for the exact or a very
similar configuration, then a new experimental evaluation
need not be carried out. The burden of proof for the
justification of the use of such data, again, lies completely
with the designer.
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