Update 221012

Gauges and Instrumentation for Fracture Mechanics Testing

are no longer being produced.

Limited quantities of inventoried gauges will be sold.

Contact DonaldGeorgi@gmail.com

for further information.

GaugeLink™ is a combination of low resistance insulated gauges

 with matching gauge conditioner electronics, calibrator and

interface cables for reading crack length. 

The luxury of scientific theory precluding engineering experience is a wonderful happening, but unfortunately not the path of fracture mechanics.

Airframe failures of WW I aircraft drew attention to cracks such as that which the The Red Baron noted in the lower wing of his Albatross D.III, initiating its grounding and update. (see Albatross photo* to the left).  After the war, pioneers such as Griffith** with information from prior work done by Inglis, led to a quantitative connection between fracture stress and flaw size. This created the dawn of quantitative fracture mechanics which has contributed to the astonishing reliability of twenty-first century aircraft which, for example, safely ferried 18,274,647 passengers to and from the U.S.A. in the month of December 2016.***   As aircraft materials moved to composites, crack testing has continued to be of fundamental concern. Although the connection of fracture mechanics with aircraft remains strong to this day, the field is a basic consideration for many materials and structures used in medical, construction and other transportation.

Return to Top

Implementation of Crack Testing

Electrohydraulic and resonant test machines provided the precision loading to simulate real world fatigue

By the 1960's programmable, servo hydraulic and resonant electrical equipment allowed sophisticated laboratory testing to mate fracture mechanics theory with practice. The photo is of a Research Incorporated - MTS Division test machine of the 1960's. (The division was to become independent as MTS Systems Corporation in 1965.)

One of the test machine pioneers,Max Russenberger, invented and patented the Low Resistance Insulated Crack gauge (U.S. Patent No. 4,149,406) which was based on the technology of the then available foil strain gauge. Russenberger and an associate from Amsler days, Hans Rudolph "Rudy" Hartman, built a standard range of crack gauge sizes for sale in the U.S.A. Gauges were constructed by Russell Smith of Lawson Hall Laboratories through 2008. Hartman sold many sizes of KRAK GAGE and Fractomat Conditioner through his company,  Hartrun Corporation up to the early 2000's at which time he transferred exclusive ownership of Hartrun and the KRAK GAGE sales to Bob Churchill. In 2009, Teksym Corporation began production of the standard Krak Gages for Hartrun plus new gauge configurations designed for composite materials and structures testing. In 2014, Bob Churchill passed away but left no provision for continuation of the KRAK GAGE or Fractomat. Neither Churchill or other people associated with the manufacture or use of the Fractomat were interested in providing continuation of the Hartrun business or products. Teksym Corporation having used only internal resources for producing the gauges, decided to continue offering them directly  to the marketplace under the name Crack Gauge. ****

In the upper left of this photo is a strip of constantan foil, chosen for its  ability to be rolled to very thin sheets 0.0002 inches thick (0.005 mm.) Constantan exhibits very low resistance and dimensional variation with temperature.

To the right of the foil is  a backing consisting of a 'B stage' epoxy with or without embedded glass fibers. (B stage material is partially cured before final assembly and then with heat and pressure finally cured, bonding the foil to the epoxy in a brittle crystalline state.) Farther to the right is shown the material as it is photochemically etched to achieve repeatable gauge dimensions which provide repeatable gauge resistance change when subjected to a crack. The lower center left shows a completed crack gauge ready to be cemented to a specimen. The far lower left shows a specimen prepared with a crack gauge bonded to a specimen so that fatigue cycling will cause the crack to progress down the center of the gauge. A constant electrical current flows through the gauge which results in a longer path due to the increasing crack length. The resulting voltage, produced by the increasing resistance, is proportional to crack length.

After the foil is bonded to the base, it is glued to the specimen. Wires are attached to the gauge and then connected back to a Conditioner. The specimen is placed in a fatigue test machine and subjected to programmed cyclic loading. At the highest stress concentration point, a crack forms and grows longer with progressive cycles. The performance of the material under controlled stress and cycling allows numerical fracture toughness evaluation of the material.

The gauge is classified as a (1) Low resistance, (2) Insulated (3) crack (4) gauge because of its design. The foil is made a thin as possible first to not interfere with the cracking properties of the specimen and secondly to provide as much resistance change with change in crack length as possible. Despite the very thin thickness of constantan foil, the gauge resistance varies between less than 1 ohm to at best less than 20 ohms, and usually less than 2 ohms. The insulated feature comes from the bonding of the foil on the insulated epoxy base. This feature is desirable in the case of conductive metal specimens, and provides a convenient bonding method for either conductive or insulated specimens. There have been cases where the foil is directly bonded to a conductive specimen with special adhesive such as a very high temperature ceramic cement. In this case, sufficient adhesive must be uniformly placed to insure isolation of the gauge from the conductive specimen.  In cases where the specimen is non conductive such as composite, ceramic, or plastic a gauge could be directly bonded to the specimen. Even with composite gauges the standard crack gauge with

Simple explanation:Using the model above, Conditioner Constant Current I flows through the gauge resistances R4, R5 and R6. As the crack develops (Green Line), these resistances increase, with crack length. Tabs B and C sense that voltage across R4-R5 & R6. (V sig = I(R4+R5+R6)) which is amplified and adjusted for zero crack resistance. The unchanging values, R3 and R7 plus the initial values of R4, R5 and R6 provide the 'Zero crack resistance which must be subtracted from all readings. Resistances R4 and R6 linearly increase with crack length, while R5 increases exponentially  because its shrinking width is due to decreasing width. Keeping the final width of R5 large, minimizes end region nonlinearity. More detailed explanation: As the specimen develops a crack, the foil and insulating base of the gauge which is bonded to the specimen, experience a separation equal to that of the crack. The electrical current passes from Tab A, through R1, R3, R4,R5,R6,R7 and R9 to Tab D. The insulating base of the gauge which is bonded to the specimen, experiences a separation equal to that of the crack in the specimen. The foil, bonded to the base experiences the same crack. Current flowing to the center of the foil must traverse an increasingly longer path to get around the end of the crack. The path represented by R4, R5 and R6 will linearly increase as that path gets longer so that constant current I times those increasing resistances will produce an increasing voltage proportional to the crack length (E=IR.) At the intersections of R1 to R3 and R7 to R9, Tabs B and C allow sensing of the voltage developed across R3-R4-R5-R6 and R7. As the crack develops, R4 and R6 become longer, increasing their resistance linearly with crack length. R5 reduces in width, causing it to also increase resistance with crack length, but because the shrinking width affects the resistance in its denominator, it contributes an exponential rise in resistance, which is small as long as the final crack length is not too close to the gauge length. Tabs B and C are connected to the very high input impedance of an instrumentation amplifier, so the resistances R2 and R8 become inconsequential because the current through them is so small. The voltage at Tabs B to C becomes the signal proportional to crack length. R3 and R7  do not change as they are only due to the distance between the voltage tab and the active R4/R6 resistors. The crack sensing voltage produced from a constant current allows uncontrolled resistances of gauge elements, connecting wires and termination resistances to become "don't - care" components of the system.

What instrumentation accesses the crack resistance?

Rather than use constant voltage excitation as found in many passive transducer conditioners, the very low resistance of the crack gauge is best conditioned with a constant current. As the crack resistance increases the voltage linearly follows the crack length. Amplification allows for scaling of the voltage and of sufficient amplitude to be connected to a data acquisition system or test machine controller. The conditioner in the photo has provisions for simultaneously processing four gauges. Earlier conditioners relied on the precision of the dimensions of gauges to setup gain and offset voltages. As new gauge sizes were added the proper setups required had to be added by modifying the conditioners. The Teksym Power Interface has a continuous amplification range so that any present ad most future gauge configurations can be simply accommodated with a single setting of the channel span adjustment.

The GaugeLink Power interface has a continuously adjustable gain for any gauges which fall into the general range of sizes. This allows the use of all current gauges and new sizes which do not match other gauge sizes. A gauge calibrator for each gauge size is connected to the input of any of the four conditioner channels. The calibrator consists of a real gauge, with 0 mm crack, adjusted to the median value of all 30 mm gauges. Setting the range and span so that the display shows the factory preset number for a 30 mm gauge, the gain for the channel is properly set so that the displayed 'zero' voltage is the value to be sent to the Data Acquisition input, and 10.00 volts will be the 100% (30 mm crack length.) Calibration is now complete and the conditioner is connected to the test gauge. Setup for the gauge to be used for the test consists of:   1. Connecting the gauge   2. Set the range and span controls so that the display is the factory set value.   3. Set the Data Acquision enginering units so that 0 mm is the factory set value.   4. Set the Data Acqisition engineering units so that 100 % crack length.   5. Begin the test. As long as the Power Interface remains in calibration, the gague calibrator is not needed. Only the 5 step setup is needed. The 'zero'  conditioner voltage setting is a significant voltage generated with a new gauge with 0% crack because the gauge pattern has a significant initial value. No zero offset is needed for the output signal. It is handled completely by the data acquisition system.