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Structural test labs often need to test structures to failure, stopping the applied load once a surface crack has formed in the part. In some applications, thin copper wires (commonly 44 AWG) 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, and the general size relative to strain-gage-type rolled foil, 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 thin 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.
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 Propagation Gages provide a convenient method for indicating rate of crack propagation in a test part or structure. The CPA, CPB, and CPC patterns consist of a number of resistor strands connected in parallel. When bonded to a structure, progression of a surface crack through the gage pattern causes successive open-circuiting of the strands, resulting in an increase in total resistance. The CPA pattern incorporates 20 resistor strands; the CPB, with the same basic configuration, incorporates ten. Both series produce stepped increases in resistance with successive open-circuiting. In applications where space permits, the CPC pattern may be preferred because of greater uniformity of increases in total resistance with successive strand fractures.
Crack Detection Sensors and Crack Propagation Gages are both constructed from micro-thin (<0.013 mm) foil and carrier systems to optimize crack initiation through the sensor from the specimen surface. Convenient and practical, both types offer outstanding value to the stress analyst.
Learn more about surface crack measurements at http://www.vishaypg.com/micro-measurements/stress-analysis-strain-gages/crack/
Article by Bob Watson, Director of Engineering, Micro-Measurements®, a Vishay Precision Group, Inc. (VPG) brand
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