micro-measurements@vishay.com
Crack Detection Patterns
Vishay Micro-Measurements
Document Number: 11520
Revision 10-Jan-03
www.vishaymg.com
96
Special Purpose Sensors - Crack Detection Patterns
CD-Series Crack Detection Gages are designed to provide a
convenient, economical method of indicating the presence of
a crack, or indicating when a crack has progressed to a
predetermined location on a test part or structure. By
employing several CD gages, it is also possible to monitor
the rate of crack growth; however, Crack Propagation Gages
would normally be selected for that purpose.
In some applications, thin copper wires bonded to the test
structure are used to provide a low-cost method of detecting
crack initiation or propagation. Because of the behavior of
copper wire, however, this method suffers from two
limitations: (a) the crack tip may progress considerably
beyond the wire without breaking the strand, and (b) in areas
of high cyclic strains, the wire may fail in fatigue without crack
initiation in the specimen. CD-Series Crack Detection Gages
are designed to overcome both of these limitations.
CD-Series gages consist of a single strand of high-
endurance alloy. A crack propagating beneath the gage will
induce local fracture of the sensing strand and open the
electrical circuit. When the CD gage is installed at critical
locations on a test part or structure and used as a sensing
element in a control system, the signal can serve to alter a
test sequence or to alert an operator to incipient component
failure.
CONSTRUCTION
Two gage constructions are currently available:
The CD-02 is made of beryllium copper alloy laminated to
polyimide, and offers a low-resistance sensing element.
Select the CD-02 type for maximum conformability to
irregular surfaces and ease of soldering, when greatest
fatigue life is not required.
The CD-23 type is constructed of isoelastic alloy laminated to
a glass-fiber-reinforced backing for applications where the
highest endurance is required. The superior fatigue life of
the isoelastic alloy allows the CD-23 to be used in high cyclic
strain fields without premature failure, while maintaining high
sensitivity to crack formation under the gage. This gage is
less conformable than the CD-02 and requires use of SS-
Flux for tinning of solder tabs for leadwire attachment.
Crack Detection Gages are available with various strand
lengths; from 0.4 in (10 mm) for applications where space is
limited, to 2.0 in (50 mm) for use where the direction of crack
propagation, or the point of crack initiation, is uncertain.
Resistance of the CD Series is nominally 0.05Ω/mm of active
strand length for beryllium copper and 1Ω/mm for isoelastic
gages.
The normal operating temperature range is –320° to +250°F
(–195° to +120°C).
ADHESIVES
Conventional strain gage adhesives are suitable for bonding
CD-Series gages. M-Bond 600, 610, or 43-B are preferred
for excellent performance over the widest operating
temperature range. However, M-Bond AE-10 and AE-15 are
also suitable where in-service temperatures will not exceed
+200°F (+95°C). M-Bond 200 is satisfactory for fast
installation, but should not be used for long-term testing.
PROTECTIVE COATINGS
Corrosion, which can cause premature filament failure, is
greatly accelerated in the presence of high cyclic strain
fields. For long-term use, it is essential to protect the crack
detection gage from atmospheric corrosion and other
contamination.
M-Bond 43-B is an excellent protective coating when the
bonding adhesive, leadwire insulation and solder can
tolerate the cure temperature. If lower cure temperatures are
necessary, M-Bond AE-10 and AE-15 are recommended.
When in-service environmental conditions are not extreme, a
softer coating may prove perfectly adequate. Either 3140
RTV or M-Coat D would be a good choice in these instances.
For repetitive use on identical structural shapes, special patterns may be designed to fit the expected crack formation area.
Contact our Applications Engineering Department for details.
micro-measurements@vishay.com
Crack Detection Patterns
Vishay Micro-Measurements
Document Number: 11520
Revision 10-Jan-03
www.vishaymg.com
97
Special Purpose Sensors - Crack Detection Patterns
CD-SERIES GAGE DESIGNATION SYSTEM
CD - 23 - 15 A
GEOMETRY (tabs as shown)
LENGTH OF ACTIVE LINE IN MILLIMETRES
MATERIAL AND CONSTRUCTION:
02 = 0.0005 in (0.013 mm) beryllium copper laminated to
0.001 in (0.025 mm) polyimide backing
23 = 0.00025 in (0.006 mm) isoelastic alloy laminated to
0.0008 in (0.020 mm) glass-fiber-reinforced backing
CRACK DETECTION GAGE (single strand)
DIMENSIONS
GAGE DESIGNATION
a b c Length Width
CD-02-10A 0.40 0.56 0.10 0.60 0.13
CD-23-10A 10.2 14.2 2.5 15.2 3.2
CD-02-15A 0.60 0.76 0.10 0.80 0.13
CD-23-15A 15.2 19.3 2.5 20.3 3.2
CD-02-20A 0.80 0.96 0.10 1.00 0.13
CD-23-20A 20.3 24.4 2.5 25.4 3.2
CD-02-25A 1.00 1.16 0.10 1.20 0.13
CD-23-25A 25.4 29.5 2.5 30.5 3.2
CD-02-50A 2.00 2.16 0.10 2.22 0.13
CD-23-50A 50.8 54.9 2.5 56.4 3.2
MATRIX
b
a
c
