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JUL 1 2 1971 NBS TECHNICAL NOTE 578

Determination of Dynamic Loads

in a High-Frequenoy

Direct-Stress Fatigue Machine
NATIONAL BUREAU OF STANDARDS

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NBS TECHNICAL NOTE 578

ISSUED JUNE 1971

Nat. Bur. Stand. (U.S.), Tech. Note 578, 24 pages (June 1971) CODEN: NBTNA

Determination of Dynamic Loads

in a High-Frequency

Direct-Stress Fatigue Machine

Donald C. Robinson

Engineering Mechanics Section

Mechanics Division

Institute for Basic Standards

National Bureau of Standards

Washington, D.C. 20234

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402.

(Order by SD Catalog No. C 13.46:578). Price 35 cents.

Stock Number 0303-0869
Contents

Page

1.- Introduction 1

2. Fatigue Machine Characteristics 2

3. Test Bar Design 3

4. Instrumentation 3

5. Measurement Procedure

5.1. Static Machine. 4

5.2. Fatigue Machine 5

6. Results 6

7. Discussion 7

8. References 9
Determination of Dynamic Loads in a High-Frequency

Direct-Stress Fatigue Machine

Donald C. Robinson

A refined test procedure is given for

accurately recording the dynamic signals from

instrumented test bars used to measure loads

during simulated fatigue tests. Loads indicated by the load transducer of a direct-stress

fatigue machine are compared with those determined from these instrumented test bar specimens

which have been calibrated under static conditions. An application of this procedure is

described for a resonant, electromagnetic, directstress fatigue machine having an optical dynamometer for indicating programmed loads. The

variables investigated were the operating frequency,

cyclic load and specimen stiffness. In addition,

the influence of test fixture mass on the load

measurements was determined at one test frequency.

The mean loads indicated by the instrumented test

bars are compared with those indicated by the

dynamometer to determine the errors in load measurements under various test conditions.

Key words: Dynamic loads; dynamometer; fatigue

machine.

1. Introduction

In the fatigue testing of structural and mechanical members, it is

necessary to have some means of verifying the loads indicated by the

machine, since the fatigue life of a specimen depends critically on the

applied loads [1] . To evaluate the performance of a fatigue-machine

load-indicating device a method is to compare the intended, or programmed, loads with those indicated by a test specimen over the machine load

and frequency operating ranges [2] . From this information, it is possible to determine corrections to apply to the static calibration of the

machine to account for differences in the programmed and indicated loads

due to undesired inertial effects, which vary for different types of

equipment and test conditions.

In this investigation, instrumented test bars, calibrated statically, were used as dummy fatigue test specimens to determine the loads

in a resonant, direct-stress fatigue machine. A new recording procedure

is described for measuring the cyclic loads applied to the test bars.

The equipment includes a comparator type differential amplifier with an

4e Figures in brackets indicate the literature references at the end of

this paper.
adjustable offset voltage control for comparison of the alternating

signal from the test bars with a reference voltage signal. Due to the

wide dynamic range of this unit, it was possible to expand the effective

screen height of a conventional type oscilloscope by several orders of

magnitude to permit voltage measurements to be made with high resolution.

2. Fatigue Machine Characteristics

it The machine used in this investigation is a 15 ton capacity, electromagnetic, direct-stress fatigue machine which operates at the resonant

frequency of the vibrating parts. A schematic of the test machine is

shown in figure 1. The principal oscillating members consist of a main

moving mass which oscillates on the specimen and dynamometer connected in

series. The output from the impulse generator, fixed between the specimen and dynamometer, is amplified and fed back to the driving magnet to

maintain operation of the machine at the resonant frequency. The test

frequency is varied by manually altering the moving mass.

The deflection of the dynamometer was measured visually by a beam of

light which indicated the static and dynamic loads directly on a scale.

The static loads were indicated by a sharp line of light and the dynamic

loads were indicated by a band of light. The mean load, which appeared

as a narrow line when the machine did not oscillate, corresponded to the

average of the extreme values of the light band during operation of the

machine. The load range was controlled automatically by means of a photo-electric regulating device actuated by the light beam of the dynamometer and thereby controlling the power input to the electrical drive.

When a fatigue machine is in operation, sources of error in load measurements appear due to unrecognized inertial forces. These forces are caused by the flexibility of elastic members and the complicated distribution of masses within a machine. According to the manufacturer's operating manual for the machine investigated, transverse and torsional

parasitic oscillations may occur in addition to the desired longitudinal

oscillation. By control of the geometry and location of the main moving

mass, it is usually possible to keep such parasitic frequencies away from

the actual test frequency. The longitudinal frequency of all the oscillating elements of the dynamometer is well above the operating frequency

range of the machine. The highest operating frequency is governed by

critical frequencies of the upper crosshead which makes control of the

machine difficult at a test frequency above 200 Hz. An investigation of

the dynamic load indication of the dynamometer is given in reference 3.

In this study it was found that the differences between the loads

U. S. customary units are used throughout this paper since these are

the units most frequently used in fatigue testing work in the U. S. Conversion to SI units can be made by use of the relationships found in ASTM

Standard Metric Practice Guide, ASTM Designation: E 380-70 (available

from American Society for Testing and Materials, 1916 Race St., Philadelphia, Pa. 19103).
indicated by an instrumented specimen and the dynamometer at high test

frequencies was caused by the mass of the dynamometer and the gripping

head carried at its end.

3. Test Bar Design

Three test bars were used to determine the dynamic loads in the

fatigue machine for a range of specimen stiffnesses. The diameters of

the reduced section of the bars were 0.375 in, 0.500 in and 0.750 in and

their capacities were 4,500 lbf, 6,500 lbf and 15,000 lbf, respectively.

The bars were fabricated from type AISI 4150 steel and their load capacity was based on a design stress of 41,000 lbf/in 2

. The geometry of the

test bars is given in figure 2a.

Strain gages were attached to the bars in a manner which permitted

determining the bending due to misalinement as well as measuring the

axial loads while the bars were installed in the fatigue machine. The

gage circuit designed for this purpose is shown in figure 2b. This circuit permitted measuring the output of each of the axial gages separately

to determine bending in the bar or, by connecting 5 to 6 and 7 to 8, determining the combined strain of the four active gages due to applied

axial loads. Connectors with silver plated contacts and shielded cables

having the shield grounded were used to simplify the measurement process

and minimize noise signals.

4. Instrumentation

A portable strain indicator and a switching and balancing unit were used to determine the amount of bending in the test bars due to eccentric

loading while they were installed in the machine. For measuring the

average axial strain, the power for the strain gage bridge was provided

by a regulated d-c power supply. The recording equipment for the cyclic

load measurements consisted of a d-c amplifier, a low-pass filter to suppress noise components, and a conventional oscilloscope with a plug-in

unit having modes as a differential comparator and as a differential preamplifier [4] .

When used as a differential comparator, the plug-in unit has an adjustable d-c offset voltage control for comparison of an input signal

with respect to a reference signal established within the unit. Principal requirements for this equipment were (1) negligible drift of the

reference signal, and (2) capacity for measuring the lowest signals of

interest, in the order of 100 millivolts. The unit employed in this investigation had a recently improved solid state source follower stage for

achieving stability of the reference signal. The measured drift after

warm up was within 400 microvolts per hour at room temperature. Similar

amplifiers having greater trace stability were not compatible with the

recording oscilloscope used. A schematic of the measurement system is

shown in figure 3.
The comparison voltage amplitude could be read to four significant

figures on a graduated dial. The accuracy of the comparison voltage

reading of the comparator amplifier was evaluated by correlating its output with that of a four digit precision voltmeter as a constant voltage

was applied. The manufacturer's stated accuracy for the comparison voltage was ±0.15 percent of the indicated value plus 0.05 percent of the

full scale range. The full scale range used during the load measurements

for the fatigue machine was ±1.1 volts. The indicated voltage on the

comparator unit was found, on the average, to occur within 0.03 percent

of the digital voltmeter value within this voltage range.

5. Measurement Procedure

5.1. Static Machine

The test bars were calibrated statically by determining their re- sponse to a series of axial loads in a 20,000 lbf capacity universal

testing machine. The errors in static calibration for the latter machine were less than 1 percent, and it complied with the requirements of

ASTM Method E4 [5]. After inserting the test bar in the machine, the

bending in the bar due to eccentric loading was determined by measuring

the axial strain using the strain indicator and switching and balancing

unit. The bar and threaded members connecting it to the machine fixtures

were oriented and alined until the strain indicated by any one of the

axial gages did not vary by more than 4 percent from the mean strain indicated by all the gages.

The response of a bar to static loads was determined in the orientation where the bending was found to be minimum. Loads were applied in

5 or 6 increments up to the capacity of the bar. Several series of readings were taken using different sensitivity ranges of the differential

comparator unit and different values for amplification and filter cutoff

frequency. The values of these test parameters which would be used during dynamic load measurements could not be determined in advance.

The output voltage from the gaged test bars was measured using a "slide-back" feature of the differential comparator unit. For this measurement, the display switch of the unit was placed in the A-V

c mode,

where A is the input and V is an adjustable d-c comparison voltage

which is added differentially to the input signal. A reference line for

this measurement was established before the test bar was loaded by positioning the trace on the oscilloscope at the center of the screen for

the highest sensitivity range which could be conveniently used. For the

static tests this range was 5 to 10 mV/cm. After the load was applied

to the bar, the comparison voltage was adjusted to slide the signal from

the test bar to the reference line. Curves were then plotted of the re- lationship between the static load and test bar electrical output.

These curves were used to determine the dynamic loads during cyclic

loading tests.
5.2. Fatigue Machine

The alinement and voltage measurements in the fatigue machine were

made in essentially the same manner as during the static tests. Generally, the test bars were installed directly into the machine fixtures

by the use of appropriate bushings. For this arrangement, it was possible to orient and aline a bar until the strain indicated by any one

of the axial gages did not vary by more than 5 percent from the mean

strain indicated by all the gages. For one test arrangement, additional

fixtures (connectors) were used to install a test bar into the machine

in order to determine the influence of the additional fixture mass placed between the specimen and machine dynamometer. Due to the added

play introduced between mating parts when using these connectors, it was possible to aline the bar so that the strain indicated by any one of the

axial gages was within 3 percent of the mean strain indicated by all of

the gages. Schematics showing the two test arrangements are given in

figure 4.

In the determination of the loads indicated by the test bar under

cyclic loading, the comparison voltage was adjusted to slide the upper

and lower peaks of the oscillating waveform to the reference line. The

offset voltage values were recorded and the corresponding loads were

determined by use of curves plotted for the output voltage versus the

static load. The polarities for the upper and lower portions of the

waveform were established by a range switch in the comparator unit.

In order to obtain an adequate sensitivity range for the comparison voltage measurement, it was necessary during some of the tests to

adjust the reference line on the oscilloscope to some voltage greater

than zero. Generally, it was possible to use the 10 mV/cm range of the

comparator unit. Although the noise level was high while the machine

was running, spurious signals due to the noise were suppressed by the

low pass filter whose cutoff frequency was set at 300 Hz, well above the

test frequency range.

The test bars were generally installed directly into the fatigue

machine using bushings supplied with the machine. In this arrangement,

the lower end of the specimen was in the immediate proximity of the upper end of the machine dynamometer and the bushing mass was negligible.

For some fatigue tests, however, it is convenient to employ the connectors to facilitate rapid installation and removal of specimens. These

connectors are sometimes large and since it was known that such a mass introduced between the specimen and dynamometer can influence the load

measurement, one set of tests was made using the connectors for the

0.375 in diameter test bar.
6. Results

Because of the different capacities of the test bars, the load

range varied for each bar. For the 0.750 in diameter bar, the portions

of the load range investigated were 300 - 3000 lbf, 600 - 6000 lbf, 900 - 9000 lbf, 1200 - 12,000 lbf, and 1500-15,000 lbf. For the 0.500 in diameter bar, the load ranges investigated were 300 - 3000 ltf, 450 - 4500

lbf, and 600 - 6000 lbf. For the 0.375 in diameter bar, load ranges of

150 - 1500 lbf, 300 - 3000 lbf, and 450 - 4500 lbf were investigated.

The ratio of the minimum to maximum load of 1:10 is often used in fatigue testing of airframe fasteners. The minimum loads were selected in

multiples of 150 or 300 lbf which were found to be convenient increments

for the 15 ton capacity dynamometer used.

A comparison of the programmed and indicated mean loads determined

using the 0.750 in diameter test bar is given in table 1 for test frequencies of 150, 183 and 200 Hz. The data for frequencies of 150 and

183 Hz are plotted in figures 5 and 6, respectively, to show the relation between the programmed mean loads and those indicated by the machine

dynamometer and test bars. A solid 45 degree line is plotted in these

figures showing the locus of points where the programmed and indicated

loads would be exactly correlated. No data were recorded at the highest

load range at 200 Hz due to difficulty in operating the machine at this

frequency.

Test frequencies of 142, 162 and 195 Hz were obtained when using

the 0.500 in diameter bar. A comparison of the programmed and indicated

mean loads using this test bar are given in table 2.

The test frequencies when using the 0.375 in diameter test bar were

125, 137 and 167 Hz. A comparison of the programmed and indicated mean

loads when using this bar are given in table 3. This data and the previously tabulated results were taken when the test bars were installed

directly into the machine fatigue fixtures. The tests run at 137 Hz

were repeated with the test bar inserted in the connectors. The difference in the loads measured for both installations are listed in table 4 and are plotted in figure 7.

In figure 8, the mean load error is plotted against the test frequency for the three size test bars at a programmed mean load of 1650

lbf. This error is defined as

Mean Load (Programmed Mean Load) - (Indicated Mean Load) „ Error (percent) Indicated Mean Load

The average of the mean load error of the test bars at various test frequencies is plotted against the programmed mean load in figure 9.
7. Discussion

A test procedure is presented which can be used to provide more

accurate measurement of load data during fatigue tests. By accurately

recording the dynamic signals from instrumented test bars simulating

tt t specimens and comparing these signals with the output of the machine dynamometer, it is possible to provide corrections to account for

such variables as specimen stiffness and changes in the test fixture

mass. A specialized component of the recording system is a differential

comparator amplifier used to increase the resolving power of an oscilloscope and to provide a reference voltage with which to compare the electrical output of the test bars. Using the maximum sensitivity which may

be achieved, the displayed signal amplitude is magnified by several

orders of magnitude, thus increasing the effective oscilloscope screen

height. To implement this procedure, it is necessary to ensure that the

voltage reference has adequate stability for the environment in which

the equipment is used.

When reading the optical dynamometer while the fatigue machine was operating, it was sometimes difficult for the operator to determine the

loads in a consistantly reliable manner. Reading errors could perhaps

be reduced by photographing the light beam projected on the scale and

then examining a magnified portion of the photograph to determine the

load. Since the dynamometer readings when using the 0.375 in and 0.750

in diameter bars were taken by one machine operator and the data for the

0.500 in diameter bar by a second operator, some of the difference in

the loads determined when using these bars may reflect the operator's

ability to read the dynamometer. The use of a lower capacity dynamometer would perhaps have enabled the operator to determine the loads indicated on the light scale with somewhat better resolution than the 15

ton capacity dynamometer employed.

The dynamometer and the optical scale for the machine described in

this paper are calibrated under static loading by inserting a measuring

spring designed by the manufacturer in place of a specimen. It is rec- ommended that the dynamometer be sent to the manufacturer for checking

if its readings deviate by more than 1.5 percent from the indications of

the spring. The overall uncertainty in the calibration of the instrumented test bars is estimated to be not more than 1.2 percent. The com- parison between the two load measuring devices was complicated somewhat

by the difficulty in reading the machine dynamometer and controlling the

load. The comparison would have been more direct if both devices were

strain gage based transducers.

The mean loads indicated by the dynamometer were larger than those

indicated by the test bar. This has been found to be characteristic of

the type machine investigated [6]. The load errors, due to the difference in the loads indicated by the dynamometer and test bar, increased

with the test speed. Further investigation would be required to verify

the apparent tendency for the load errors to increase with the cyclic

load range. The load error was found to be greater when the 0.375 in
diameter test bar was inserted in the connectors than when it was installed directly in the machine fixtures by use of small bushings. Due

to the relatively smaller difference between the loads indicated by the

dynamometer and the largest two bars, the load errors determined for the

two conditions of fixturing might have been less for the larger bars.

The manufacturer of the fatigue machine supplies correction terms

for each dynamometer used with a particular machine. As the results of

this investigation and related studies have shown, it is necessary to

account for the influence of any mass introduced between the dynamometer

and the specimen in applying corrections to load measurements due to un- desired inertial effects [6,7].

The results of this investigation indicate that the difference between the loads indicated by the machine dynamometer and the test bars

varied with the size, and therefore the stiffness, of the bar. As noted

in reference 6, changes in the specimen stiffness cause the dynamic

properties of the entire vibrating system to be altered. Furthermore,

the influence of the specimen stiffness on the load errors may depend

on the position of the specimen in the mass-elastic arrangement of the

machine. It is therefore generally necessary to determine a range of

test bar sizes, which may be considered representative of the fatigue

specimens to be tested, during the calibration of a particular fatigue

machine [2]

The author is indebted to Mr. G. Meyer of Standard Pressed Steel

Company for permission to conduct tests on fatigue equipment at their

plant in Jenkintown, Pennsylvania. The valuable assistance of Mr. R.

E. Snyder in the construction of the test bars and Mr. G. D. Boswell in

conducting the tests is gratefully acknowledged.
8. References

[1] Weibull, W. , Fatigue Testing and Analysis of Results (Pergamon

Press, 1961).

[2] International Organization for Standardization, Draft proposal

for dynamic force calibration of direct stress fatigue testing

machines, ISO/TC 17/WG 1 (Secretariat-165) 324 (May 1969).

[3] Russenberger, M. and Foldes, G. , High-speed universal testing

machine, verification of statically calibrated mechanical-optical

dynamometer, Experimental Stress Analysis, 1_2, No. 2 (1955).

[4] Middlebrook, R. D. , Differential Amplifiers, John Wiley & Sons

(1963).

[5] ASTM Designation: E 4 - 64 , Standard methods of verification of

testing machines, ASTM Book of Standards, Part 31 (1967).

[6] Serensen, S. V., Garf, M. E. and Kuz'menko, V. A., The Dynamics

of a Fatigue-Testing Machine (1970) . Translated from Russian

(1967 publication). TT 70-50033. Available from National

Technical Information Service, Springfield, Va. 22151.

[7] Schloss, F. , Recent advances in mechanical impedance instrumentation and applications, David Taylor Model Basin Report 1960

(Feb. 1965).
Table 1 - Comparison of Programmed and Indicated Mean Loads

During Cyclic Loading Using the 0.750 in Diameter

Test Bar

Test Pre)grammecI load

Mean

Indicated mean load

frequency Range Dynamometer Test bar

Hz lbf lbf lbf lb~f

150 300 - 3,000 1650 1760 1740

600 - 6,000 3300 3500 3460

900 - 9,000 4950 5125 5070

1,200 - 12,000 6600 6730 6630

1,500 - 15,000 8250 8520 8380

183 300 - 3,000 1650 1800 1780

600 - 6,000 3300 3350 3310

900 - 9,000 4950 5120 5050

1,200 - 12,000 6600 6775 6670

1,500 - 15,000 8250 8350 8115

200 300 - 3,000 1650 1390 1372

600 - 6,000 3300 3325 3275

900 - 9,000 4950 - -

1,200 - 12,000 6600 6825 6710

Mean load

1

2

r P

max

+ P . mm

*

10
Table 2 - Comparison of

During Cyclic

Test Bar

Prog

Load

rammed and

ing Using

Indicatec

the 0.500

Mean Loads

in Diameter

Test Programmed

Range

load

Mean

Indicator mean load

frequency Dynamometer Test bar

Hz lbf lbf lbf lbf

142 300 - 3,000 1650 1765 1750

450 - 4,500 2475 2550 2535

600 - 6,000 3300 3410 3385

162 300 - 3,000 1650 1755 1735

450 - 4,500 2475 2550 2510

600 - 6,000 3300 3390 3340

195 300 - 3,000 1650 1750 1725

450 - 4,500 2475 2570 2535

600 - 6,000 3300 3400 3345

11
Table 3 - Comparison of Programmed and Indicated Mean Loads

During Cyclic Loading Using 0.375 in Diameter Test

Bar

Test

frequency

Programmed load Indicated mean load

Rang e Mean Dynamometer Test bar

Hz lbf lbf lbf lbf

125

137

167

150 - 1,500 825 1180 1165

300 - 3,000 1650 1740 1722

450 - 4,500 2475 2600 2555

150 - 1,500 825 920 910

300 - 3,000 1650 1735 1710

450 - 4,500 2475 2590 2545

150 - 1,500 825 935 920

300 - 3,000 1650 - -

450 - 4,500 2475 2550 2492

12
Table 4 - Comparison of Indicated Mean Loads Using 0.375 in

Diameter Test Bar With and Without Connectors at

137 Hz

Programmed

Range

load

Mean

Indica ted mean load

Fixturing Dynamometer Test bar

lbf lbf lbf lbf

With

connectors 150 - 1,500 825 900 888

300 - 3,000 1650 1720 1688

450 - 4,500 2475 2570 2510

Without

connectors 150 - 1,500 825 920 910

300 - 3,000 1650 1735 1710

450 - 4,500 2475 2590 2545

13
20

a

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V6

W7,

Driving Magnet

Preload Spring

Vibrating Mass

Specimen

Dynamometer

Test Stand

Figure 1 - Schematic of Electromagnetic Direct-Stress Fatigue Machine

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22
Latest developments in the subject area of this publication, as well as

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1. PUBLICATION OR REPORT NO. NBS-TN-578

2. Gov't Accession No.

4. TITLE AND SUBTITLE

Determination of Dynamic Loads in a High-Frequency Direct- Stress

Fatigue Machine

3. Recipient's Accession No.

5. Publication Date June 1971

6. Performing Organization Code

7. AUTHOR(S)

Donald C. Robinson

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NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) A refined test procedure is given for accurately recording the dynamic signals from

instrumented test bars used to measure loads during simulated fatigue tests. Loads indicated by the load transducer of a direct-stress fatigue machine are compared with thos^

determined from these instrumented test bar specimens which have been calibrated under

static conditions* An application of this procedure is described for a resonant, electromagnetic, direct-stress fatigue machine having an optical dynamometer for indicating

programmed loads. The variables investigated were the operating speed, cyclic load and

specimen stiffness. In addition, the influence of test fixture mass on the load meas- urements was determined at one test frequency. The mean loads indicated by the instrumented test bars are compared with those indicated by the dynamometer to determine the errors in load measurements under various test conditions.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Dynamic loads; dynamometer; fatigue machine

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