Crack Detection

QUESTION:      What are Crack Propagation Sensors?


ANSWER:        Crack Propagation Sensors are bondable grid patterns that are useful for detecting the presence and propagation of a crack in a material under the active area of the sensor. Even cracks too small to be seen with the naked eye will propagate through the sensor backing and separate the foil strands directly above. This foil separation is detected as a resistance change in the sensor grid.


QUESTION:      How are Crack Propagation Sensors used?


ANSWER:        The most popular use of Crack Propagation Sensors is for metal fatigue testing, where either the onset of the initial crack or the rate of the crack propagation is of interest. Most often these sensors are bonded at several different locations suspected as crack origination points. For crack origination testing, the specimen is then cyclically loaded and the sensors monitored for the emergence of cracks. For crack propagation testing, Crack Sensors can be installed and monitored at the location of known cracks. The Crack Sensor will change resistance in proportion to the crack propagation.


QUESTION:      Are there any special techniques necessary for installing Crack Propagation Sensors?


ANSWER:        No. All Micro-Measurements strain sensors installation procedures are compatible with Crack Propagation Sensors; however, one note of caution. Un-encapsulated TK-Series sensors have very low foil peel strength. The foil is easily lifted from the backing. When installing Crack Propagation Sensors using adhesives that require tape for handling, the tape should be placed near the end of the backing, not over the grid. The sensor can then be pressed into position using a non-stick film before adding clamp pressure.


QUESTION:      What adhesive do you recommend for installing Crack Propagation Sensors?


ANSWER:        Any adhesive certified for strain gage installation will perform properly with crack sensors. However, for brittle fracture studies or for fatigue studies where the cracks are likely to be minute, the use of solvent-thinned adhesives, like

M-Bond 600 or 610, is usually recommended. This is only because 600 and 610 give the thinnest glue line of any of the strain sensor adhesives, which increases the sensing sensitivity of the Crack Sensor. M-Bond600 and 610 also provides the widest temperature range.


QUESTION:      How is the proper Crack Propagation Sensor selected?


ANSWER:        Basically, all Crack Propagation Sensors operate the same except for the Type CPD-01. The Type CPA, CPB and CPC series are selected based on the estimated crack growth length and also the available installation area. The CPD type Crack Propagation Sensor is used by connecting each strand individually into a separate circuit. Each of these individual circuits can then be used as inputs to controls which shut off motors, sound alarms, etc.  (See the Crack Propagation Sensor datasheet at


QUESTION:      Can a Crack Propagation Sensor erroneously indicate a crack, even when there is no crack underneath the grid?


ANSWER:        Yes. Just like conventional strain sensors (strain gage or gauge) , because of the metal foil used to produce them, Crack Propagation Sensors have a finite fatigue life (see fatigue data provided in the datasheet at cycles must be monitored closely in applications where the cyclic fatigue endurance of the test material exceeds that of the Crack Propagation Sensor. In such cases it is often possible to stop the test periodically to replace the Crack Propagation Sensor before exceeding its fatigue life. An alternate approach is to cycle the specimen a safe number of cycles before installing the gage.


QUESTION:      What is the smallest fatigue crack detectable using a Crack Propagation Sensor?


ANSWER:        This is not a simple question to answer. The minimum crack size detectable with the Crack Propagation Sensor is highly dependent on the type of crack being formed, and especially on the specimen material. Without going into great detail, it can be shown that Crack Propagation Sensors can, under certain circumstances, detect a crack that is completely invisible to the naked eye and even difficult to see using high-powered microscopes or nondestructive testing methods.



                        For fatigue testing of ductile materials, a zone of very high stress immediately precedes a propagating crack. These high cyclic stresses at the crack tip will actually sever the Crack Propagation Sensor strands before the crack reaches the strand. For all practical purposes, the strand fails at the crack tip as the crack is being formed.


                        For brittle materials, a combination of crack propagation speed and low ductility can allow the crack to pass underneath the grid strand before the strand actually breaks. Data on the crack size that caused the strand to fail is not well understood. We do know that it is a function of the sensor backing and bonding-adhesive layer thickness. A rough approximation for this crack size is between 1 and 5 times the installation thickness. Using TK-Series Crack Propagation Sensors and M-Bond 600 or 610 adhesive, the installation thickness would be approximately 0.040 mm (0.0016 inches).

bwatson's picture

Bob Watson

Director of Engineering