present state of experimental stress analysis by strain gages in japan

E / / M s Report Periodic reviews of the state of the art--techniques, instrumentation, theory--in areas of experimental mechanics. Report No. 2:

Present State of Experimental Stress Analysis by Strain Gages in Japan

by Masao Naruoka

ABSTRACT--This paper describes the present state of strain gages, measuring devices and stress-cycle counters in Japan. Details are given to the following items: (1) electric-resistance-wire strain gages, (2) vibrating-string gages, (3) magnetostriction gages, and (4) stress-cycle counters. I tems (1) and (2) manufactured in Japan are comparable to those manufactured in other advanced countries. I tem (3) was specially developed in Japan. Research on item (4) is now in progress.

Introd uction Before W o r l d W a r l I , the re were ve ry few s t ra in gages in J a p a n and expe r imen ta l s t ress ana lys i s was no t wide ly app l ied to the var ious fields of engineer ing. However , a f te r the war, s t imu la t ed b y the r e ma rk - able a d v a n c e m e n t of the e lec t r ic- res is tance-wire s t ra in gage m a d e in the USA, wire s t r a in gages have come in to wide use for expe r imen ta l s t ress ana lys i s in engineer ing research. Th is t e n d e n c y has en- couraged g rea t ly the use of h o m e - m a d e wire s t r a in gages and measur ing devices. T o d a y , wire s t r a in gages a re wide ly used in expe r imen ta l s t ress -ana lys i s researches.

V ib ra t ing - s t r ing gages have been used in J a p a n for long-dura t ion measurement s . A t first, M a i h a k gages were impor ted ; now, super ior J a p a n e s e p rod- uc ts a re in genera l use.

S t r a in m e a s u r e m e n t b y the pr inc ip le of magne- tos t r i c t ion has been inves t iga ted in J a p a n b y K e n j i r o Azumi. Because of i ts super io r i ty , th is m e t h o d is now app l i ed ex tens ive ly to m a n y engineer ing p rob - lems which canno t be solved b y o r d i n a r y s t r a in gages.

Masao Naruoka is Professor of Civil Engineering, Kyoto University, Kyoto, Japan. This "'Field Report" is based on a paper presented, by title only, at the 1960 S E S A Spring Meeting held in Indianapolis, Ind. , on M a y 18 20.

I II I III

Stress-cycle coun te r has been investigated actively in J apan , f rom t h e p o i n t of v iew that the stress cycle has the mos t s ignif icant bear ing on the strength of materials a n d s t ruc tures .

Th i s research work has been done m a i n l y b y the staffs of t he T r a n s p o r t a t i o n Techn ica l Resea rch I n s t i t u t e and R a i l w a y Techn ica l Resea rch I n s t i t u t e of J apan . Th i s a r t i c le deals w i th the ou t look of the inves t iga t ions which were m a d e m a i n l y a f te r W o r l d W a r I I .

Electric-resistance-wire Strain Gage

General

Elec t r i c s t r a in gages can be classified into: (a) m a g n e t o s t r i c t i o n s t r a in gage, (b) e lec t r ic - res i s tance- wire s t r a in gage, (c) e l ec t r i c -capac i t ance s t r a in gage, a n d (d) i nduc t a nc e s t r a in gage. I n J a p a n , t h e elec- t r ic - res i s tance-wire s t r a in gage is the t y p e mos t wide ly used. Th is s t r a in gage has come into wide use n o t on ly for e x p e r i m e n t a l s t ress ana lys i s in m a n y engineer ing problems, b u t also in var ious medica l - science researches since W o r l d W a r I I . T h e r e are severa l m a n u f a c t u r e r s of wire s t r a in gages in J a p a n . T h e y m a n u f a c t u r e no t on ly wire s t r a in gages a n d measu r ing devices , b u t also s t r a in -gage a p p l i c a t i o n a p p a r a t u s .

Electric-resistance-wire Strain Gages

T a b l e 1 shows the wire s t r a in gages which a re genera l ly ava i l ab l e on the m a r k e t in J a p a n . T h e m i n i m u m gage l eng th is 3.5 mm.

S u b c o m m i t t e e 4.2 of the J a p a n Assoc ia t ion for N o n d e s t r u c t i v e In spec t ion ( J A N D I ) r ecen t ly inves t iga t ed the accu racy of gage res i s tance a n d gage

II I II][11[[11 IIIIIIIIII [I- [[ I1[I I

Experimental Mechanics I l l A

I I ..... ['

TABLE 1--TYPES OF ELECTRIC-RESISTANCE-WIRE STRAIN GAGES IN JAPAN

Grid-type gage

Wound-~pe gage

Foil gage

rWith thick felt /Paper-base gage ~Thin-paper gage X L Post-yield gage

'Single-axis gage / Bakelite-base dual gage Polyester-base gage

{H gh-ternperature gage

(Paper-base rosette gage " / with thick felt

l lWith thin felt Multi-axes gage / Bakelite-base gage

L Polyester-base gage ~'Paper-base gage

Single-axis gage ~Bakelite-base gage LPolyester-base gage

f Bakelite-base gage M ulti-axes gage ~ Polyester-base gage

[Grid type Single-axis gage ~Special-chart type

/Rosette type Multi-axes gage / For diaphragm

LFor torque

factor for gages selected from the market . As the result of this investigation, the accuracy of gage resistance and gage factor was found to be remarkably improved. I t was also found tha t the gage resistance and gage factor can be guaranteed to an accuracy of 1 percent.

Also, a subcommit tee on water-proofing procedures for gages was organized. Thus, a t emporary con- clusion on effective water-proofing procedures and measuring method in pressurized water was obtained.

There has been no act ive research on the high- tempera ture s train gage in Japan. J A N D I is now planning to organize subcommittees on high-tempera ture s train gages and also on the s tabi l i ty of wire s t ra in gage on long-durat ion measurement.

Strain-measuring Devices

STATIC S T R A I N - M E A S U R I N G DEVICES. Japanese man- ufacturers are now making stat ic strain-measuring devices. Their capacities are close to those produced by the Baldwin-Lima-Hamil ton Corp. Some recent phases of this field in Japan are as follows: (1) range-switching type; (2) new type, of which the zero and dial indicators are located in the same window; and (3) transistorized type which is con- venient to carry or for field measurement. DYNAMIC S T R A I N - M E A S U R I N G DEVICES. Those devices, which use the carrier of 5 kc and are capable of measuring the dynamic strain within the range of 0-500 c//s, are generally used. They are almost all of 6-12 elements. Several types of por table elec- t romagnet ic oscillographs are on the market . They are conveniently used in field and labora tory experi-

ments. Such devices have recently become t ran- sistorized. H I G H - S P E E D S T R A I N - M E A S U R I N G DEVICES. As a measuring device of high-speed strain within the range of 0-50 kc, a six-elements Brown Tube is now being manufactured and is in wide use in Japan. Also, the responsiveness of the electric-resistance- wire strain gage to impact s train has been investigated theoretically. As a result, it was found tha t ordinary electric-wire strain gage can be used to measure the impact strain up to 50 kc. A 50-mm camera, with a large caliber F 1.2 lens, is used as the recording device. A U T O M A T I C R E C O R D I N G D E V I C E S . Automat ic recording devices of the balancing-motor type have been used in Japan, but the moving-coil type have been developed only recently. The responsiveness is 2 c /s in 12 mm full scale.

Transducer for Engineering and Industrial Uses

Many transducers for engineering and industrial uses are now being manufactured in Japan by using resistance-wire s t ra in gages. The transducers can be classified into three types: bonded type, unbonded type and Carlson type. Certain types manufac tured in Japan are shown in Table 2.

Vibrating-string Gage Because of its superior s tabi l i ty on long-duration

measurements, the vibrat ing-str ing strain gage has been used in those structural-engineering researches where long-durat ion measurements are necessary,

]2A I June 1962

such as researches on the behavior of concrete gravity-and-arch dams, of earth dams, of simple and continuous composite girders, and of prestressed- concrete beams.

The Maihak gage was first used in the investigation of the Maruyama Dam (concrete gravity dam) for the purpose of measuring the static strain and temperature in the interior of the concrete dam. They were installed together with Carlson-type strain gages for the purpose of comparison. As the result of the measurements taken, the vibrating-string gage proved itself superior to the Carlson-type string gage.

Because of its superiority, the vibrating-string gage has been used in several concrete gravity-and- arch dams which were constructed after the com- pletion of the Maruyama Dam.

The creep of reinforced and prestressed concrete is the most important problem in the design of (a) dead- and live-loads composite, simple girders, and (b) continuous-composite girders. In order to check whether such a girder behaves in the same way as the design or not, the vibrating-string gage is used in the strain measurement of concrete and steel. Such measurements were successfully achieved in Soganoya Bridge (dead- and live-load composite, simple-girder bridge), Tomobuchi Bridge and Kema Bridge (continuous composite-girder bridge).

The vibrating-string gages now used in Japan are shown in Table 3.

Magnetostriction Gage

General

The magnetism of ordinary, strongly magnetic materials varies considerably with the magnitude of stress. The magnitude of this variation is the same as that due to the variation of a magnetic field. For example, the permeability of a sensi- tive magnetic material at the stress of 8 k g / m m 2 is twenty times that of 0 k g / m m 2. A strongly magnetic material which has such a property is known as a magnetostriction material. Some materials such as steel, nickel, Fe-Ni alloys and Fe-A1 alloys are, therefore, used to advantage.

There are many kinds of magnetostriction effects between the direction of strain applied to the material and the direction of the magnetic field. Three magnetostriction effects which are generally used in Japan are as follows:

(a) Villari effect: This effect is the phenomenon, whereby, if a rod of strongly magnetic material is subjected to tension or compression in the axial direction, the magnetizing curve in the axial direction varies.

(b) Inverse Wiedemann effect: This effect is the

TABLE 2--TYPES OF TRANSDUCERS IN JAPAN

Measurable Accuracy, Kind range %

1. Pressure gage --1 to +1 kg/cm ~ 0.5-1 2. Pressure gage 0.05-1,000 kg/cm = 0.5-1 3. Differential 10% of initial pres- 0.5-1

Pressure gage sure 5-150 kg/cm 2

4. Engine pressure Indicator 50-500 kg/cm 2 1

5. Earth-pressure gage 0.03-100 kg/cm 2 0.5-1 6. Tension meter 2-5,000 kg 0.5-1 7. Load cell 2-200,000 kg 0.5-1 8. Torque meter 0.1-750 kg-m 0:5-1 9. Accelerometer

U-al type 5-100 g 0.5-1 U-a2 type 1-10 g 0.5-1 U-a3 type 0.05-1 g 0.5-1 B-a type 1-200 g 0.5-1

10. Deflection meter, Weight meter

0-0.04 mm 0.5 1 U-T type ~ 100-2,000 gram 0.5-1

t 0-0.4 mm 0.5-1 U-L type 2-100 gram 0.5-1

phenomenon, whereby, if a rod, magnetized by a cir- cumferential magnetic field around the rod axis, is twisted, the rod is magnetized in the axial direction.

(c) Wertheim effect: This effect is the phenomenon, whereby, if a rod, magnetized in the axial direction, is twisted, it is magnetized in the cir- cumferential direction.

By using the proper magnetostriction effect of suitabm materials, transducers which change a mechanical quanti ty to an electrical quant i ty can be made. By utilizing various transducers, pressure, force and strain can be measured.

Generally speaking, mechanical quant i ty is usually measured by means of transducers; however, if a material has magnetostriction properties, the material itself can be used as a magnetostriction gage.

TABLE 3--TYPES OF VIBRATING-STRING GAGES IN JAPAN

Gage length, Measurable

mm range Sensitivity Remarks

100 1,250 X 10 -6 2.5 X 10 6 t 50 1,000X10 ̀ 6 2 .0X10 6~ Knife edge

100 1,250 X 10 ̀ 6 2.5 X 10 6 t 50 50X10 _6 2 .5X10 ~ Bolt

100 1,250X10 _6 2 .5X10 6 Embedded type 50 --2~ ~ C 0.2 ~ C Temperature

Experimental Mechanics I 13A

Measurement of Residual Strain

I f ac of a constant frequency is applied to a round steel bar cut by a lathe in the axial direction, a magnetic field is produced in the bar in the circumferen- tial direction of the applied current. I f a residual torque is left in a round steel bar, a magnetic flux is produced in the axial direction by the inverse Wiedemann effect. I f the round bar is put through a narrow search coil and if the search coil is moved along the axial direction of the round bar, the output voltage of the search coil varies. Thus, the distribution of residual torsional strain in the axial direction can be recorded. Moreover, if the frequency of applied ac becomes high, the above magnetic flux can be concentrated in the neighbor- hood of the surface by skin effects. Thus, the distribution of strain on the surface can be made clear. Using these methods, one can accurately pick up the strain and determine its magnitude with regard to the following: (1) variation of residual strain due to working conditions; (2) residual strain-time curve; (3) relaxation of residual strain due to an- nealing; (4) variation of strain due to propagation of fatigue, if this method is applied to torsional fatigue-test specimens.

Stress Tester

The stress tester is made of many thin sheets of magnetostriction materials punched in form of U, cemented by adhesives between laminations, and then wound by magnetizing coil in the middle part. I f the top of both legs of the U-shaped stress tester is pressed against the material to be tested, such as steel wire, steel rope or steel plate, the tension or compression force existing between both legs of the materials tested can be determined. Since the material tested has magnetostriction properties, the permeability due to tension or compression changes with the Villari effect. Therefore, the impedance of the magnetizing coil changes and the amount of change is measured by the bridge method.

In the design of a stress tester, the selection of material is important. For measuring tension or compression of steel wire and steel rope, this method is comparatively simple and can be achieved with considerable accuracy. In the case of structural steel, this method becomes a little difficult, but approximate procedures have now been worked out.

The characteristic advantage of stress tester lies in the fact that it is capable of measuring the initial stress, or dead-load stress, which cannot be measured by ordinary strain gages.

Measurement of Torque

By means of magnetostriction effects, the torque

of rotating shafts can be measured without attaching anything mechanically to the rotating shaft, such as a brush in the case of measurement of torque by wire strain gage bonded to rotating shafts. Three methods have been developed and used effectively in Japan. (The details of these methods are omitted.)

Magnetostriction Transducer

The characteristic of a magnetostriction transducer is its remarkably high strain sensitivity. Let us consider the steel-rod transducer bonded by wire strain gage (for example, tension gage). I f it is assumed that the gage factor is 2.1 and the applied stress is 1 k g / m m ~, the change of resistance per unit resistance is 0.01 percent, wherever, for the same strain, the change of impedance per unit impedance of magnetostriction tube is 10-20 percent. This large strain sensitivity of the magnetostriction material, if used as a transducer, produces the following characteristics:

1. The strain or displacement produced in the transducer does not need to be large.

2. Measuring devices of high natural frequency can be made, using the characteristic of very small displacement which is necessary to produce the same strain compared with a wire strain gage.

3. The output, even if very small, can be measured by a simple circuit and, if large, can be measured without amplifier.

The problems of hysterisis and nonlinearity of the magnetostriction materials, which are considered as defects by engineers, have almost been solved by the extensive investigations of these materials and the development of suitable methods for utilizing them. The effects due to the magnitude of magnetic field and the change of temperature can be eliminated by using the automatic control circuit or automatic compensating circuit.

The kinds of transducers generally used in Japan are as follows: (1) strain gage; (2) load-measuring devices, such as (a) load cell, (b) snow-slide-impact load cell, (c) tension meter, and (d) wind-tunnel balance; (3) pressure gages, such as (a) vibrating earth-pressure gage, (b) two- and three-dimensional earth-pressure gage, (c) vibrating water-pressure gage, (d) differential water-pressure gage, (e) wave- pressure gage, (f) wave-force gage, and (g) rock- pressure gage.

Stress,cycle Counter Stress-cycle measurement is now used on railway

bridges, highway bridges, ship structures, automobiles, airplanes and railway cars in Japan. I n

14A I June 1962

I I I - I I I . . . . I l l | III II I I I I I I I I I I I I I I . . . . [

ship structures, the speed of stress change is usually slowest among all of the above structures and, thus, the combination of mechanical strain gage and electric cycle counter is sufficient to measure the stress cycle. The stress change in railway bridges is faster than that in ship structures, but the combination of the mechanical strain gage (which responds with greater speed to the stress change) with the electric cycle counter, is sufficiently responsive to permit the measurement of the stress cycle in railway bridges. The same can be said for the stress-cycle measurement of highway bridges.

To the contrary, stresses change very fast in automobiles and railway cars, and therefore, mechanical strain gages cannot be used as the main part of the stress-cycle counter. Consequently, the combination of the electric-resistance-wire strain gage and the electronically controlled counting device is used for these cars.

The stress-cycle counter, which counts the frequency of overstressing above the previously determined stress level, called the maximum stress- cycle counter, was manufactured first in Japan. A stress-cycle counter, capable of counting the frequency of stress amplitude between the maximum and minimum stresses, was manufactured next. I t is noted that the stress amplitude is an important factor in the fluctuating and alternating stresses, considering the influence of stress on the strength of material and structure.

However, as is well known, the average stress has significant importance as does the stress amplitude in the fluctuating and alternating stresses. There- fore, the perfect stress-cycle counter must be the type of stress-cycle counter which can (1) pick up both average stress and stress amplitude and (2) count the frequency. Such a measuring device, capable of counting electronically two-dimensional stress magnitude into the counter, has been manufactured and used effectively in Japan.

In field measurement, there are many known or unknown obstacles. For example, thermal stresses due to temperature or sunshine are produced in outdoor structures. Such a thermal stress changes the basic stress level and brings a great deal of difficulty in analyzing the measured results.

There are now available many precise measuring devices and much field-measurement experience has been obtained. However, in order to apply this knowledge of the stress-cycle counter and this experience effectively, there must be an exchange of information on practical experiences. With this in mind, a symposium on stress-cycle counters was held on November 13, 1959, in Tokyo, Japan. The subjects covered in the papers read at the symposium

were as follows: (a) some points on stress-cycle measurements; (b) fatigue strength due to varying load; (c) present state of load-cycle counters for airplanes; (d) an example of measuring the accelera- tion frequency of vertical vibration in automobiles; (e) an example of stress-cycle counter by electric- resistance-wire strain gage; (f) electric~resistance- wire strain-gage type stress-cycle counter; (g) amplitude-type stress-cycle counter for ship structures, and (h) automatic data-processing-device of vehicle vibrations.

Conclusions As indicated by its title, this paper deals only

with experimental stress analysis by means of strain gages. I t does not discuss experimental stress analysis by X-rays, brittle coatings and photoelasticity. The few lines of description on the magnetostriction gages that appear in the "Handbook of Experi- mental Stress Analysis," edited by M. Hetenyi, include a brief statement to the effect tha t "very little use has been made of this method." This is not true for Japanese magnetostriction gages, as described herein. Also, the vibrating-string gages are actively used in the field of civil engineering, as can be seen in England, France and Germany.

Acknowledgment

The author gratefully acknowledges that the following parts of this paper were prepared by these authors: "Electric-resistance-wire Strain Gages," by Ichiro Ishiyama, Transportat ion Technical Re- search Institute; "Vibrating-string Gage," by Ichiro Ishiyama and Masao Naruoka; "Magnetostriction Gage," by Kenjiro Azumi, Transportat ion Technical Research Institute; and "Stress-cycle Counter," by Yoshiaki Tada, Railway Transportat ion Research Institute.

Bibliography l. Re: Accuracy of Wire Strain Gage (in English): Yoshiaki Tada and

Fumihito Itoh: Scattering in Gage Factor of Domestic Resistance Wire Strain Gages, Proc. of the Second Japan Congress on Testing Materials, 1959, pp. 231 233 (Published by J S T M ) .

2. Magnetostriction Strain Gage (in Japanese) : K . Azumi and Tsuchiya, Report of Transportation Technical Research Institute, 1.1 (1951) K . Azumi, J . Iwayanagi, and Y. Ando ibid., 1.2 (1951). K . Azumi, J . Iwayanagi and Y. Ando, ibid., 1 . ~ 8 (1951). K . Azumi and Y. Ando, ibid., 1 . ~ 8 (1951). K . Azumi and H. Maeda, ibid., 2.5 (1952). K . Azumi, and Y. Hori, ibid., 4.11 (1954). K . Azumi, J . Iwayanagi, Y. Hori, Y. Fujii, J. Terao, and H Maeda, ibid., 4 . l l (1954). K . Azumi, M. Ichihara, Y. Hori and T. Otsuru, ibid., 9.6 (1959). K . Azumi, Journal of the Japanese Society for Non-Destrur'tive Inspection, 3.4 (1954). K . Azumi, ibid., 8.4 (1959).

3. Re: Papers presented at First International Congress on Experimental Mechanics, New York, N. Y., Nov. 1-3, 1961: T. Yamaguchi and J . Imamasa, Experimental Analysis on the Strength of Cylindrical Shell under External Pressure. K . Oi, Friction-type Wire Resistance Strain Gage.

4. Re: Papers presented at International Symposium on Photoelasticity, Chicago, Ill., Oct. 29-31, 1961: Z. Zuji: Recent Activity in Photoelasticity in Japan. N . Nishida, New Photoelastic Methods for Torsion Problem. K . Kawata, Elast-plastic Stress Analysis and the Determination of Flow Limit by Means of Photoelastic-coating Method,

I '1111' IIII I I I I

Experimentcl Mechanics I 15A

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