A BRIEF HISTORY OF B O N D E D RESISTANCE STRAIN GAGES FROM CONCEPTION

TO COMMERCIALIZATION

Peter K. Stein, President Stein Engineering Services, Inc.

Phoenix, Arizona

The bonded resistance strain gage became a practical measurement device roughly fifty years ago. It was im- mediately applied to measure all sorts of static and dynamic quantities, including strain, force, acceleration, torque, and pressure.

A strain gage consists of a “filament” which today may be wire or foil, metallic or semiconductor, a bulk material or deposited film. The filament is bonded to the surface of a structure of interest, and any change in dimension of the sur- face beneath the filament will be sensed.

The impact of the strain gage on the engineering profession as it existed in 1939 is difficult to exaggerate. Upon its introduction, the strain gage allowed engineers to measure, for the first time, the static and dynamic loadings experi- enced by structures during service. From that point on, the strain gage has been an indispensable tool during the development of high performance vehicles including aircraft, spacecraft, and automobiles. Today the use of strain gages is no longer solely limited to the design and development of high performance structures. Strain gages are an integral part of common items we all use (or depend on) everyday including bathroom, butcher, or postal scales.

During 1988-89, I organized several Golden Jubilee Cele- brations held throughout the world marking the fiftieth anniversary of the invention of the bonded resistance strain gage. As a part of my preparation for these events I conduct- ed scores of interviews with people directly responsible for the development of the strain gage, wrote to dozens of peo- ple, and unearthed mounds of technical literature. This paper is a brief synopsis of my reconstruction of these events. The discussion includes various technical developments and achievements which led up to the conception of the bonded gage and on to the point of successful commercialization. 1827. Georg Simon Ohm discovered the laws of the elec-

tric circuit - the relationship between current, resistance and voltage. Today his name represents the unit of electrical resistance. 1833. Samuel Hunter Christie, M.A., F.R.S., M.C.P.S.,

published his paper, “Experimental Determination of the Laws of Magneto-Electric Induction,” in the Philosophical Tkansactions. In it he anticipated “Wheatstone’s” bridge. Of it, Wheatstone later wrote that Christie “has described a dif-

A 1200 Magnification Huggenberger Tensometer. Invented by Dr. Arnold U. Huggenberger of Switzerland in the 1920s, it was sold byBaldwin Southwark in the United States before 1930. It was clamped to the test specimen by a variety of ways - mechanical, magnetic, electro-magnetic. Perhaps this was the mostpopular strain gage prior to the arrival of the bonded resis- tance gage.

Experimental Techniques 18

ferential arrangement of which the principle is the same as that on which the instrument described in this section (of his own paper), has been devised. To Mr. Christie must therefore be attributed the first idea of this useful and accurate method of measuring resistance.”

1843. Sir Charles Wheatstone, D.C.L., F.R.S., presented the Bakerian Lecture to the Royal Society entitled, “On New Processes for Determining the Constants of a Voltaic Cir- cuit.” In the section, “A Differential Resistance Measurer,” now known as the Wheatstone Bridge, he noted, “Slight dif- ferences in the lengths and even in the tensions of the wires are sufficient to disturb the equilibrium (of my circuit).”

1856. William Thomson (later Lord Kelvin), M.A., F.R.S., presented the Bakerian Lecture to the Royal Society entitled, “On the Electro-dynamic Qualities of Metals. He really set out to investigate and document the effect of mechanical strain on the thermoelectric properties of various metals. He found, quite incidentally, that copper and iron wires also change their electrical resistance when mechanically strained. He also documented that there is not only a tem- porary, recoverable effect, but that when wires were deformed sufficiently, there was a permanent resistance change.

1923. Burton McCullom and O.S. Peters of the National Bureau of Standards developed and published a paper on, “A New Electrical Telemeter.” It is based on unbonded carbon gages - two piles of 50 carbon rings each, were mounted pre- loaded, in a substantial frame with a tongue between the two pre-loaded stacks. Displacement of this tongue increased the compression in one stack, decreasing it in the other. In a Wheatstone bridge circuit, the bridge was unbalanced in pro- portion to whatever caused the motion of the tongue - dis- placement, force, pressure, etc.

When Frank Tatnall saw this development, he immediately convinced his employer, Baldwin Southwark, to include this Telemeter in their line of products as a strain measuring transducer.

Francis G. “Frank” Tatnall was a technical representative/ salesman for Baldwin Southwark‘s testing machines. He traveled widely and knew everyone in the field. He repeated this new approach with a large number of mechanical strain- measurement-related inventions made by the many re- searchers he visited on his trips. It is believed that in most cases Frank helped the inventors set up their little businesses and had an interest in them.

1924. In Switzerland, Dr. Arnold U. Huggenberger inven- ted the compound-lever mechanical strain gage which became known around the world as the Huggenberger Ten- someter. I t remained perhaps the most widely-used strain gage until the arrival of the bonded-wire resistance gage.

1934. A.V. de Forest formed Magnaflux Corporation with Wilfrid L. “Bill” Walsh to commercialize the magnetic- particle crack-detection system. He joined forces with Hoke and profited from the development and application patents.

De Forest had extensive contacts in industry and was a well-known consultant. His ability to commercialize his inventions and to overcome set-backs made him the most important catalyst in the stories of the bonded-wire resis- tance strain gage and of “Stresscoat” brittle coatings. Roy

Carlson filed his “Telemetric Device” patent using unbonded strain-sensitive wire as the sensor. Carlson became an inter- nationally renowned expert on concrete and construction of dams. He had used carbon pile-based strain meters on the Experimental Arch Dam on Stevenson Creek in 1925, and wanted a more stable transducer. His was based on fine car- bon steel wires, pre-tensioned between supports in pairs of windings. Carlson gages to measure displacement and pres- sure were among the first to be commercially manufactured by Carlson Instruments even before the patent was filed in its final form.

A Carlson Unbonded- Wire nansducer. Invented by Roy W. Carlson in the 1930s. It was filled with castor oil when assembled. Displacement and pressure transducers used by the thousands were embedded in dams. A force applied to the left end increases the tension in one pair of windings while decreas- ing the tension in a second pair.

1935.Roy Carlson, arrived a t Massachusetts Institute of Technology (MI”) to develop a stress gage for his doctoral dissertation. He was Associate Professor of Civil Engineer- ing through 1943. His office and laboratory facilities were in the basement of Building 1, where he worked with his unbonded-wire transducers, next door to those of Arthur C. Ruge.

Ruge had arrived a t MIT in 1932, received his Master’s Degree in 1933 and was Instructor, Research Associate and then Assistant Professor in Civil Engineering during the period described here. He was interested in engineering seis- mology, vibrations and experimental stress analysis. He, too, was studying for his doctorate, and involved in research spon- sored by an insurance company on elevated water tanks sub- jected to earthquakes. His dissertation was “Development of a Precise Speed Control Equipment for Seismographic Recording Drums.”

1936. E.H. Hull of General Electric Company developed “Aquadag,” a graphite emulsion in water which could be painted on surfaces as strain gages, usually mixed with a sul- fur compound. It was, in practice, impossible to reproduce thickness, gage length and resistance. Its use, however, became wide-spread. It was used for dynamic strain measurement, where nothing else worked. Charles M. Kearns, Jr., at Hamilton Standard, filed down a radio carbon resistor to create a flat surface which he bonded to an aluminum specimen. He checked the resistance change of

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this first bonded-carbon strain gage against a Huggenberger Tensometer and developed the bonded-carbon gage for prac- tical application. Within a short period, his carbon gages and those copied and modified by his competitors, had spread all over the US., to England and Italy, where they were also manufactured.

To Kearns, this invention was a means to an end - the end of in-flight propeller failures, which were reduced from two to three dozen per year to six in 1939 and none in 1940. He retired in 1975 as Staff Vice-president of United Technologies, the parent company of Hamilton Standard and Pratt & Whitney.

Edward E. Simmons, Jr., was asked by Dr. Donald S. Clark, Assistant Professor, and Dr. Gottfried Datwyler, Research Fellow in Aeronautics, to help them with an industry-sponsored project on impact properties of mate- rials in the Impact Research Lab a t California Institute of Technology in Pasadena. They wanted to measure the actual dynamic impact forces applied to their test specimens since the use of Aquadag had been unsatisfactory. Simmons was at the time constructing electrical equipment for the project on a part-time basis. He suggested the use of a metallic wire bonded to the surface of a prismatic bar as the force- measuring element. Datwyler bonded 14 feet (4 1/4+ meters) of No. 40 AWG constantan wire on the four faces of a 7/8 in. x 3/4 in. x 4-5/8 in. long steel bar (24 1/4 mm x 19 mm x 117 1/2 mm). He and Simmons thought nothing of it. They had, in

fact, conceived the world’s first bonded-wire strain-gage load cell and successfully used it for dynamic force measurement during impact testing. The use of bonded wires for strain measurement remained confined to the Impact Research Project a t Cal Tech. The circuit and measuring system which Ed Simmons designed would be considered quite up-to- date today.

It is to be noted that Simmons did not bond those wires, or any other strain gage, ever. He provided the concept and the idea. That alone entitled him to the patent granted six years later when the significance of his invention had finally been recognized.

The principle of the Burton McCullom-Orville S . Peters carbon pile “Electric Telemeter” from Baldwin-Southwark Corpora- tion. Bulletin #132, July 1936. There are from 20 to 55 rings under about 180 psi depending on the application.

Amelia Earhart and Zbm ’IFplett, May 3, 1937. Zbm mplet t used X-ray to inspect Amelia Earhart’s plane. His expertise was X-ray inspection of light alloy castings. Mass production X-ray inspection was a new method in 1937 and the firm of 7kiplett & Barton was the first in that business.

The Impact Apparatus at Cal Tech. The impact apparatus for which Ed Simmons suggested the first bonded-resistance wire strain-gage load cell.

Experimental Technique. l5

1937. A. V. de Forest, as consultant to Hamilton Standard and to Pratt & Whitney, had become familiar with the early Kearns gage. He and Wilfrid L. “Bill” Walsh, in the Mechani- cal Engineering Department at MIT, developed an improved version which they named the “ESS-Strip” - Electrical Strain Sensitive Strip. A.V. induced Arthur D. Little to develop the graphite and phenolic impregnated papers from which the ESS-Strip was made, and large numbers were manufactured and sold by Bill Walsh. The original patent application of December 15,1937 was abandoned in 1938 with the advent, at MIT and in A.V.’s lab, of the obviously superior bonded-wire strain gage developed by Prof. Arthur C . Ruge in the basement of the same building in the Civil Engin- eering Department.

to green celluloid bars. They lacked a galvanometer for read- ing the Wheatstone bridge output. Hans Meier borrowed one from Prof. A.V. de Forest’s lab, who joined them to witness the test. The linear and substantially creep-free behavior of the gages was encouraging. The beam was deflection-loaded so that the creep in the green celluloid was not a factor. The material was used because it had been extensively charac- terized for use in civil engineering model structures. The tests showed great promise.

Ruge immediately realized what he had discovered and pursued the bonded resistance strain gage to the exclusion of almost everything else, but he did complete his doctoral dis- sertation. De Forest also realized what had happened and launched his own research project in mechanical engineering

Letter to the Editor November 27, 1989

Dear Sir: In December, 1936, a short report was made of the dcvelop-

ment of strain-sensitive materials as applied to strain measure- ments in airplane propellers. Since that time a great deal of progress has been made in improving the resistance element in the direction of making it suitable both for static and dynamic

The previous resistor elements were composed of aggregates of carbon or graphite in which the strain sensitivity depends on contact areas of the carbon aggregate. While suitable for dy- namic strains, an undesirable hysteresis between strain and elec- tric resistance appeared under static conditions which, together with a considerable temperature coefficient of resistance, made the prcvious strips uncertain for static work.

Since that time a large amount of work has been done by Prof. A C. Ruge and the writer at the Massachusetts Institute of Technology using metallic conductors as the strained element and it has been found possible to reduce both the temperature dif- ficulty and the hysteresis so that strain may be dependably measured to better than one millionth of an inch on a one-inch gage length.

The difficulty of cementing the strained conductor to a metal surface has been overcome to a point where the gage is sufficiently free of creep so that measurements can be made over a period of

use.

months Tests of the change of resistance a t frequencies up to 40,000 cycles per second have not indicated any upper limit to the response and we believe that the resistance change occurs as rapidly as a change of strain can occur in the metal to which the wire is bonded. While the strain sensitivity is somewhat less than that of the carbon elements, it is well within the limits of portable galvanometers for static use and no difficulty has been encountered in utilizing present-day high-fidelity amplifiers and oscillographs for dynamic conditions.

The change in electrical resistance of wire has previously been described by Dr. Carlson who has long made use of it in his strain gages. An important modification of this method is due to Dr. D. S. Clark and his associates of the California Institute of Tech- nology, who described a wire-wound dynamometer in conncction with his impact machine in a papcr for the American Society for Testing Materials entitled, ”Stress-Strain Relations under Ten- sion Impact Loading” (by D. S Clark and G. Ditwyler, A.S.T.M. Proceedings, Vol. 38, Part 11, 1938). The problem of temperature compensation and a possible method of forming the gage, together with performance tests, is to be published as a report of the N.A.C.A. It is hoped that this development will contribute to the growing knowledge of stress distribution in such complicated structures as modern airplanes.

A. V. DI; FOREST Massachusetts Institute of Technology

The First Publication on the Bonded Wire Resistance Strain Gage. It was rushed intoprint as a Letter to the Editor to estab- lish MIT’s claim to invention priority.

De Forest’s paper, “Measurement of Propeller Stresses in Flight,” described the use of Aquadag and ESS-Strip gages. In his, “Measurement of Impact Strains,” de Forest also showed ESS-Strip applications.

1938. Charles Kearns produced a “flexible” carbon gage, now made in quantities for internal use and for sale by Hamilton Standard. It was an improvement over both his original design and the ESS-Strip.

Professor Arthur C. Ruge conceived of bonding a fine wire to the surface of a specimen for strain measurement. He and J. Hans Meier, his Swiss Graduate Assistant, made several specimens, bonding 5 in. (12 1/4 cm) of 0.001 in. (0.25 mm) diameter Elinvar wire (52Fe, 36Ni, l2Cr plus other ingredients)

on the third floor while Ruge, in civil engineering, worked in the basement. Hans Meier adapted the new invention to measuring strain on the model of the .elevated water tank, exited by an earth quake-simulating machine. The original topic of studying the stresses in the tank was quickly mod- ified into an exhaustive, detailed, and superbly crafted study and characterization of the bonded resistance strain gage. An analysis of the tank was also included in the thesis.

Frank Hines, a transfer student to MIT in 1936, ran out of courses to take and had to wait a semester to get “in phase” with the course offerings. He worked for Prof. Ruge during that time, maintaining his notoriously trouble-prone earth- quake simulating machine. He was immediately drafted into

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strain-gage development and completed his bachelor’s thesis on strains in a pulley, building the first strain-gage-based torquemeter, while Hans Meier built an accelerometer for his vibrating elevated water-tank project. Frank’s manual dex- terity, ingenuity and inventiveness were prized additions to the team - he could find the end of a 0.001 in. diameter wire on a spool which others could not see. He remained in the strain gage and temperature sensor field the rest of his life and was the source of the early load cell designs and numerous other developments and inventions.

Hans Meier, who came to MIT with his Dip1.-ing. degree from Switzerland, got his MS a t MIT under Dr. William M. Murray in 1938 and continued his doctorate under Prof. Ruge. Meier’s doctoral dissertation was the original fun- damental study of all aspects of bonded resistance strain gages, developing concepts and procedures which anticipated the future. He made miniature strain rosettes for the small model tank, developed self-temperature-compensated gages and noted the effect of heat treatment and cold work on the temperature coefficient of resistance of the wires.

Drs. Clark and Datwyler, from Cal Tech presented a paper, “Stress-Strain Relations Under Impact Loading,” at the ASTM Symposium on Impact Testing, Atlantic City, NJ in June 1938. Casually and without fanfare, they described the bonded-resistance strain-gage-based dynamometer con- ceived by Ed Simmons in September 1936. The paper credited Ed Simmons with developing the measuring system and circuitry for the test. News of this development did not reach the MIT team until Hans Meier’s thesis was almost completed. “I just had time to include the reference!” he says.

When the news reached Prof. Ruge, he was “devastated and downcast, ready to drop the whole thing,” remembers Ralph Conner, who sold the first “Metalectric Strain Gage,” as Baldwin-Southwark called the new invention. A.V. de Forest, having been through the same experience in founding Magnaflux encouraged him to continue and to rely on improvement and development patents for commercial success.

The two joined forces as Ruge - de Forest Associates, Inc. and de Forest convinced Baldwin-Southwark to exploit the bonded wire gage commercially. Frank Tatnall, after some initial hesitation, went along, and later became an enthusias- tic supporter. Ed Simmons and Donald Clark were brought

The de Forest Scratch Gage. A self-recording mechanical strain gage, with two-in. (5 cm)gage length invented by A. V. de Forest. Wilfrid L. “Bill” Walsh made the first and all subsequent units until production was moved to Baldwin Southwark in the 1940s.

The Impact Dynamometer of E d Simmons, J r . Wires are shown bonded to a circular-section force-measuring transducer. The original design had a square section. From U.S. Patent #2,292,549filed February 23,1940 a t the initiative of Baldwin Southwark who were able to get the Cal Tech and MIT inven- tors together. Simmons got the basis patent. Ruge got the development patents.

into the picture. Baldwin-Southwark prepared the basic patent on behalf of Ed Simmons. The development and improvement patents bear Prof. Ruge’s name.

The trademark for the new strain gage was “SR-4” com- memorating Simmons and Ruge as the S and the R, and the fact that four individuals were involved in the final develop- ment and commercialization: Don Clark and A.V. de Forest, making the -4. 1939. Prof. Ruge received a letter from Edward L.

Moreland, Chairman, Patent Committee, Office of the Dean of Engineering, MIT, in reply to his disclosure of his dis- covery of the bonded-wire strain gage. It contained the following statements: “It is the general policy of the Commit- tee, however, to pay attention primarily to matters which may prove to be of major importance, and while this develop- ment is interesting, the Committee does not feel that the commercial use is likely to be of major importance. Accor- dingly, the Committee has voted that any rights which the Institute may have in this invention should be waived in your favor. This leaves you free to treat the invention entirely as a personal matter.” Only six months later, the Committee might well have decided differently!

Within months, the new strain measurement technology had spread along the West Coast from San Diego to Seattle to The Boeing Co. Triplett introduced it to Douglas Aircraft, and they to others. By early 1940 everyone was making their own strain gages, to Baldwin-Southwark‘s horror and desperation.

It was the first time that engineers in the air frame industry were able to actually measure the flight loads imposed on their product. It was also the first time they were able to impose such measured flight loads on structures on the ground and measured the strain distributions statically and

Experimental Tecbniques 17

publicity photographs and other customary formalities. Sim- mons did not show up so I just explained that he was embarrassed, all the while knowing that he wasn’t a t all; he just did not care. Simmons was still absent when we dressed for dinner, Medalists in white tie and tails, others in black tie. When the Medalists filed in for their designated places at the head table, I had not the nerve to look to see if my boy was a t his place a t the table. When I did get up nerve to look, my heart jumped with joy. There he was, sitting between the great chemist Dr. Leo Baekelund, tall, mustached and impressive, inventor of Bakelite, and on the other side, Dr. Harlow Shapley, the famous astronomer and Director of the Harvard Observatory, both in white tie and tails. Ed Sim- mons was looking very small, very young, and very bored, and he was in his familiar tennis clothes, or a slightly enhanced version of them.”

An Early “Meta1ectric”Strain Gage. An earlyproduction bond- ed resistance wire strain gage by Baldwin Southwark, taken from the April 1940, Bulletin #153. The brass bar is for stability during shipping, handling and installation. It is removed before use.

dynamically. It was the first time that a product could truly be designed for known service conditions. The MIT Patent Committee’s viewpoint might indeed have been different only six months after their letter cited above.

Because of the war effort a t the time, Baldwin-Southwark gave everyone a royalty-free license until the end of the war. But that begins another story. The bonded resistance strain gage had now been f i i l y launched on its successful career.

Postscripts:

1942. The Longstreth Medal of the Franklin Institute was awarded to Charles M. Kearns for his development of the bonded-carbon-resistance strain gage.

1944. The Longstreth Medal of the Franklin Institute was awarded to Edward E. Simmons, Jr., for his invention of the bonded-resistance-wire strain gage only a month after Kearns’ carbon gage. That ceremony, as chronicled by Frank Tatnall in his, “Tatnall on Testing,” deserves to be included in this brief history.

“Medal day begins with a formal luncheon at noon, followed by meetings with the Medalists, press conferences,

Bonded Carbon and Wire Resistance Strain Gages Used by General Motors Aeroproducts Division in Dayton, OH in 1944.

II) September/Ocbber 1-

Edward E. Simmons, Jr. and Peter K. Stein at the Southern California Section meeting of the Society for Experimental Mechanics, February 15, 1989.

Frank F. Hines at the 1988 Jubilee Strain Gage Celebration in Portland, Oregon.

Charles M. Kearns, Jr. Bonded Carbon Resistance Strain Gage Inventor

Ed Simmons was then, and is now, a “character.” In the early days, his standard attire was a tennis outfit. In the last few decades it has been a Renaissance costume, in which he appears wherever he goes. After his initial conception of the gage, he had nothing more to do with its development. He does stay abreast of the field and still attends meetings of the local section of the Society for Experimental Mechanics, and seminars at Cal Tech.

Also in 1944, bonded carbon and wire strain gages were being used side by side. The carbon gage has persisted this long against all expectations, especially in studies on rotating propellers and dynamic applications.

But Was It An Invention?

Prof. Ruge tells of being totally distraught and in despair about how to measure the strains on the elevated water tank model on which his assistant, Hans Meier, was working. None of the existing mechanical strain gages were small or light enough and none of the carbon-based gages they had tried were stable enough or small enough. And yet he had to fulfill his research commitments and his commitments to Hans Meier to provide a viable doctoral dissertation. Roy Carlson recalls suggesting to Ruge to wind unbonded wires between insulating pins attached to the tank. Ruge realized, of course, that this would not give him the surface membrane and bend- ing strains he wanted. The “Eureka” effect which he experi- enced that Sunday morning in April 1938 was coalescing into a new invention of all his experiences applied to this problem. It had not occurred to any of the other researchers around him, or, for that matter, in the field. Remember now, he was totally unaware of Simmons’ work a t the other end of the country.

Was the idea of bonding a wire to the surface of a specimen such an original and earth-shaking contribution?The judge in the law suit which was bitterly contested in “Baldwin-Lime- Hamilton Corporation and Edward E. Simmons, Jr., vs. Tat- nall Measuring Systems Company,” (Civil Action No. 23595, U.S. District Court for the Eastern District of Pennsylvania) in his decision, December 1, 1958, thought so.

He pointed to the scientists, researchers and engineers who led the state-of-the-art in those days, such as A.V. de Forest, Charles Kearns, Dr. Roy Carlson, and all the others who have paraded before you in this chronicle, and who never thought of that apparently simple idea. It was a stroke of genius! The Simmons patent was vindicated.

ErperImentd lbehniquea 19