A111D2 TiaOTT

NBS

PUBLICATIONS

NATL INST OF STANDARDS & TECH R.I.C.

JT OF COMMERCE

f Standards

NBS Technical Note 1233

w. .. .

Requirements for the Calibration of

Mechanical Shock Transducers

D. C. Robinson

NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

iS NBS NBS NBS NBS NBS NBS NBS NBS NBS N£

NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

[S NBS NBS NBS NBS NBS NBS NBS NBS NBS Nh

f^fBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

is NBS NBS NBS NBS NBS NBS NBS NBS NBS NE

!BS NBS NBS NBS NBS NBS NBS NBS . ^ ,0

\NBS NBS NBS NBS NBS NBS NBS NBS NBS NL

BS NBS NBS NBS NBS NBS NBS NBS V/?

[NBS NBS NBS NBS NBS NBS NBS NBS NBS Nl

BS NBS NBS National Bureau ofStandards NBy

[S NBS NBS NBS NBS NBS NBS NBS NBS NBS M

NBS NBS NBS NB"^ V»^ '''^"^ NBS NBS , > '^S

fS NBS NBS NBS NBS NBS NBS NBS iMi^i 1

s'BS NBS NBS NBS NBS NBS NBS NBS NB.

S NBS NBS NBS NBS NBS NBS NBS NBS 1
rheNational Bureau of Standards^ was established by an act of Congress on March 3, 1901 The

Bur^au's^er^l god is to strengthen and advance the Nation's science and techno ogy and facditate their Bureau s overau goai

^^ i" 6 Bureau conducts research to assure international competitivr^^^dtTdrh^^Tu ^ '^^di'tr^^^cit'^^^^^^^^ work involves development and transfer of r^emen^ stS'^fan^^^^^^ sd^nce and technology, in support of continually ^^P^^T.^yi.S work if SS?qSlit^ and reliability, innovation and underlying science and engineering The Bureau s techmcal work is rrfomeT^y^National M;asurement Laboratory, the National Engineermg Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials Science and Engineermg.

The National Measurement Laboratory

Provides the national system of physical and chemical measurement;

coordinates the system with measurement systems of other nations and furnishes essential services leading to accurate and umform

physical and chemical measurement throughout the Nation's scientific, community, industry, and commerce; provides advisory and research

services to other Government agencies; conducts physical and chemical

research; develops, produces, and distributes Standard Reference

Materials; provides calibration services; and manages the National

Standard Reference Data System. The Laboratory consists of the following centers: The National Engineering Laboratory

Provides technology and technical services to the public and private

sectors to address national needs and to solve national problems;

conducts research in engineering and applied science in support of these

efforts; builds and maintains competence in the necessary disciplines

required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measure- ment traceability services; develops test methods and proposes engineering standards and code changes; develops and proposes new

engineering practices; and develops and improves mechanisms to

transfer results of its research to the ultimate user. The Laboratory

consists of the following centers: The Institute for Computer Sciences and Technology

Conducts research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of

computer technology to improve effectiveness and economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this nussion

by managing the Federal Information Processing Standards Program,

developing Federal ADP standards guidelines, and managing Federal

participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal

agencies; and provides the technical foundation for computer-related

policies of the Federal Government The Institute consists of the following divisions: The Institute for Materials Science and Engineering

Conducts research and provides measurements, data, standards, refer- ence materials, quantitative understanding and other technical information fundamental to the processing, structure, properties and perfor- mance of materials; addresses the scientific basis for new advanced

materials technologies; plans research around cross-cutting scientific themes such as nondestructive evaluation and phase diagram development; oversees Bureau-wide technical programs in nuclear reactor

radiation research and nondestructive evaluation; and broadly disseminates generic technical information resulting firom its programs. The

Institute consists of the following divisions:

'Hcadquartcni and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address

GaithcTjburg, MD 20899. %omc divisions within the center arc located at Boulder, CO 80303.

3 1 rsn>«<wH at Oniirtttt cci with aoms elements at Gaithersburg, MD

Basic Standards^

Radiation Research

Chemical Physics

Analytical Chemistry

Applied Mathematics

Electronics and Electrical Engineering^

Manufacturing Engineering

Building Technology

Fire Research

Chemical Engineering^

Information Systems

Engineering

Systems and Software

Technology

Computer Security

Systems and Network

Architecture

Advanced Systems

• Ceramics

• Fracture and Deformation^

• Polymers

• Metallurgy

• Reactor Radiation

^OmC UJVISIOUS WllOUa lUC \i^Ul^L ai^ iwai^^ w. A^vvuMwi, WW w«.^ —

• Lncated at Boutder, CO, with some elemerrts at Gaithersburg, MD
NaiionaJ Bureau of Standards

Gaithersburg, ftiaryiaad 20899

NBS Technical Note 1233

Requirements for the Calibration of

Mechanical Shock Transducers

D. C. Robinson

Acoustic Measurements Group

Automated Production Technology Division

National Bureau of Standards

Gaithersburg, MD 20899

Prepared for Department of Defense

Calibration Coordination Group

June 1987

>tv-V

U.S. Department of Commerce

Malcolm Baldrige, Secretary

National Bureau of Standards

Ernest Ambler, Director
National Bureau of Standards U.S. Government Printing Office For sale by the Superintendent

Technical Note 1233 Washington: 1987 of Documents,

Natl. Bur. Stand. (U.S.), U.S. Government Printing Office,

Tech. Note 1233 Washington, DC 20402

20 pages (June 1987)

CODEN: NBTNAE
TABLE OF CONTENTS

Overview

1.1 Introduction 2

1.2 Questionnaire for Shock Accelerometer Users 2

2. Results of Shock User Survey 3

3Future Needs "

4. Discussion and Conclusions

5

Recommendations 8

6 Acknowledgements 8

7. References

Appendix A. General Shock Survey Information

9

10

Appendix B. Nonlinear Effects in Accelerometer Designs and Materials 13
1 . Overview

1.1 Introduction

The Engineering Working Group of the Tri -Service ' s Calibration Coordination

Group (CCG) has prepared a 5-year plan designed to provide solutions to DOD's

current calibration problems and to address technologies which require metrology

and calibration support essential to the operation of modern, sophisticated

military hardware. This Metrology and Calibration (METCAL) Research,

Development and Engineering (RD&E) Plan was first prepared in 1983 and has

subsequently been revised and updated [1].

As noted in the 1985 METCAL RD&E Plan, the use of obsolete, inadequate, or nonexistent calibration standards during weapons system development and acquisition makes both the DOD and its contractors unable to validate system

performance legally during testing, evaluation and production acceptance [1].

DOD METCAL organizations are unable to maintain confidence in their own test

results when it is impossible to compare their critical measurements to higher

accuracy national standards. To correct current weaknesses and to meet the DOD

calibration requirements of the immediate future, in areas of physical and

mechanical metrology such as mechanical shock, new calibration standards must be

developed and existing standards must be improved.

In order to establish a valid basis for the improvement of existing calibration

standards for mechanical shock, a telephone survey was made to define current

needs for the calibration of accelerometers in shock measurement applications.

The results of this survey of various government agencies and government con- tractors who are actively engaged in the measurement of shock motions of 100 g or higher is described in this report (the acceleration amplitude is described

as a dimensionless multiple of the gravitational constant g) . Among the or- ganizations contacted for this survey were: (1) major test laboratories of the

three Services, (2) laboratories within DOE, NASA, and government contractors

engaged in the measurement of high amplitude shock motion, and (3) several

manufacturers of accelerometers and associated dynamic instrumentation.

As part of the effort to establish current needs for the calibration of ac- celerometers, a study was made of the accuracy requirements in various

engineering applications for the measurement of shock motions up to 200,000 g, and an evaluation was made of the accuracy required to calibrate such

transducers for various ranges of acceleration. The principal results of these

investigations are given in the following sections of this report, with some

detailed information about the survey being provided in the appendices.

1.2 Questionnaire for Shock Accelerometer Users

The objective in formulating the questionnaire for shock accelerometer users was

to identify problem areas in shock measurements and inadequacies in calibration

standards as a basis to begin developing those improved shock calibration serv- ices felt to be most urgent by the survey respondents. A telephone survey of

four transducer manufacturers and approximately 45 users of shock ac- celerometers was made to determine the accuracy requirements in various

engineering applications and to evaluate the accuracy required to calibrate

accelerometers for several ranges of acceleration up to 200,000 g. The data
from this survey are biased toward high- level -shock users, where the most urgent

calibration needs have become evident. The user respondents were selected to

represent government agencies, government contractors, and independent test

laboratories who were likely to be performing a wide variety of shock tests.*

The content of the questionnaire form is shown in the following section. In

addition to the prepared questions, the respondents were encouraged to provide

relevant information useful in assessing the accuracy actually being achieved in

the measurement and calibration of shock accelerometers . User identification

and general survey information is given in Appendix A. The subject of nonlinear

effects in acceleration measurements was discussed with several users and

transducer manufacturers to determine where assumptions are made concerning

accelerometer linearity in the calibration process. This information is sum- marized in Appendix B of this report.

1.2 SHOCK USER QUESTIONNAIRE FORM

1. User ID

2. User Type Army Navy AF DOE NASA

Industrial User of Shock Measurements

Transducer Manufacturer

3. Organization

4. Address

5. City

6. State

7. Zip Code

8Name of Contact

9

Telephone Number

10. Reason for/description of shock measurement application

11. Lowest amplitude of interest (g)

12. Highest amplitude of interest (g)

13. Lowest frequency of interest (Hz)

14. Highest frequency of interest (Hz)

15. Uncertainty needed (%)

16. Uncertainty being achieved (%)

17. How calibration is now achieved

18. Comments

Suggestions for other contacts

2. Results of Shock User Survey

The principal results of the telephone survey are given in this section. User

identification information, represented by the first nine items in the questionnaire form, is given in Appendix A. The shock measurement application

^Certain commercial companies and their products are identified in this report

to adequately specify the results of the survey. Such identification does not

imply recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the products mentioned are necessarily the best available for

the purpose or that NBS accepts the performance and accuracy claims made by

these firms
and maximum shock amplitudes and maximum frequencies of interest, items 10, 12

and 14, respectively, are given in Table 1. The lowest amplitudes of interest,

item 11, were usually 10 g or less and the lowest frequency of interest, item

13, was generally down to DC; these responses are not tabulated. The response

to items 12 and 14, the maximum acceleration amplitudes and frequencies of

interest, respectively, are also summarized in Figure 1.

The response to items 15 through 17 are given in Table 1 and in the following

text. The respondents did not always provide clear quantitative information

regarding the uncertainties and methods of calibration, and they frequently

provided information on general measurement problems instead. This latter

information is briefly summarized in Appendix A.
Table 1 Tabulation of User Survey Responses by Accuracy Requirement

MAXIMUM MAXIMUM CALIBR. CLAIMED ACHIEVED

USER** APPLICATIONS AMPLITUDES FREQUENCY METHOD*** ACCURACY ACCURACY

rkilo-g) (kHz) (%) (%)

GEN.DYN. MIL. SPEC. 1.5 10 V. 5 5 H.D.L. MIL. SPEC. 2 * V. 5 * HUGHES MIL. SPEC. 2.5 D.B. 5

HUGHES PYRO 2.5 D.B. 5

HUGHES BALLISTIC 5 D.B. 5

N.R.L. UNDERWATER 3 M. 5

PATUXENT MIL. SPEC. 1 S.T. 5

U.E.R.D. UNDERWATER 2 D.T. 5

WSTGHSE MIL. SPEC. 1 V. 5

WH. SANDS PYRO.M.SPC. 20 D.T. 5 FT . MONMTH MIL. SPEC. 0.5 V. ( 5-10) 5 KIRTLAND BLAST 50 100 D.B. ( 5-10) ( 5-10)

BENDIX , NJ MIL. SPEC. 1.2 V. 10

BENDIX.NJ PYRO 2 V. 10

B.R.L. BALLISTIC 5 5 V. 10 10

CAI OPT. CUST.SPEC. 0.25 M. 10

CAI OPT. MIL. SPEC. 0.4 M. 10 —

CAI OPT. PYRO 100(F) M. 10 —

D . T . BROWN MIL. SPEC. 2.5 M. 10

D . T . BROWN PYRO 30(F) M. 10

H.D.L. BALLISTIC 100(F) D.B. 10 5 IND . HEAD MIL. SPEC. 0.3 10 V. 5 5

LOCKHEED PYRO 30 10 D.T. ,V. 10 10

LOS ALMOS WTR. IMPACT 50 70 D.B. 10 5

LTV DAL. LRU 5(F) — V. 10

LTV DAL. SUBSYST. 10(F) — V. 10

MRSHL SFC PYRO 100 10 D.B. 10 (10-20)

MRTN MAR. PYRO 10 10 D.T. 10 ( 5-10)

MCDAC CAL. PYRO 100 D.B. ,V. 10

MCDAC HNT. PYRO 70(F) 80 D.B. 10

MOTOROLA PYRO 30 — M. 10

MOTOROLA BALLISTIC 100 — M. 10

NOSC S.D. MIL. SPEC. 0.1 — V. 10

NOSC S.D. UNDERWATER 3 V. 10

NOSC S.D. MIL 901 5 — V. 10

NTS MFCS. VARIETY 20 — M. 10

PICATINNY MIL. SPEC. 20 — D.B. 10 10

ROCKTDYNE BASTR.CMP. 20(F) D.B. 10 —

SANDIA VARIOUS 30(F) 30 D.B. 10 ( 6-8)

S. W.R.I. BALLISTIC 30 50 D.T. ( 5-15) ( 5-20)

TRW , RDNDO PYRO 25(F) D.B. 10

WPAFB MIL. SPEC. 1 — V. 10 —

HOLLOMAN IMPACT 100 10 D.B. (10-15) (15-20)

N.S.W.C. BALLISTIC 50 20 S.M. 10 (15-20)

CHINA LK. PYRO 40 40 D.B. 20 (20-30)

S- CUBED BLAST 250 350 H.B. 10 (20-30)

ENG.WTRWY. BLAST 1000 100 H.B. 20 (20-30)

ABERDEEN BALLISTIC 1000 — H.B. 50 —

* Indicates that no information was provided by the user. ** The full names of the shock accelerometer users are given in Appendix A. *** V. - Vibration exciter, D.B. - Drop ball, M. - Manufacturer, S.T. - Shock tube, D.T.

- Drop test, S.M. - Shock machine, H.B. - Hopkinson bar

(F) - Anticipated future maximum shock amplitudes for user are shown in Figure 2.
For low level shock measurements up to 10,000 g, users stated accuracy requirements of ± 5 to 10 percent. Shock accelerometers for MIL-STD-810D must meet the

following specifications [2]: (1) transverse sensitivity, 5 percent maximum,

(2) amplitude linearity, ± 10 percent over the range from 5 percent to 100

percent of the peak acceleration amplitude, and (3) frequency response, ± 10

percent from 5 to 2000 Hz, or to the highest frequency specified for the

pyrotechnic shock spectrum. Few other military or civilian specifications

mention accelerometer accuracy. Major manufacturers claim maximum probable

errors of 1 to 5 percent. All the manufacturers surveyed claim NBS traceable

capability up to a range of 10,000 g, several different calibration methods

being employed. The respondents were uniformly satisfied with their perception

that uncertainties less than 5 percent are achievable in this range.

Users expressed a desire for uncertainties of ± 10 percent in the range between

10.000 g and 100,000 g. Because manufacturers certify such accuracies of ac- celerometers rated in this range, most users believe they are getting such

accuracies. A few of the users, however, are concerned about traceability of

the calibrations in this range. Most shock tests and measurements in this range

are specified by response spectra tolerances of ± 3 or ± 6 dB. Accelerometer

uncertainties of ± 10 percent are generally judged to be acceptable for these

measurements. Endevco offers a commercially available calibration service

between 10,000 g and 100,000 g, with an estimated uncertainty less than 6 percent. Several government facilities also have similar capabilities, and claim

an uncertainty of 6 percent. The accelerometer sensitivity information provided

by the manufacturers in this range is extrapolated from lower level measurements .

3Future Needs

Requirements for high shock measurements are increasing due to such factors as increasing use of electronics in projectiles, use of pyrotechnic devices in

satellite and missile designs, and increasingly severe survivability requirements for military equipment and vehicles. Many of the tests performed at high

levels are to assure compliance to contractual requirements for shock sur- vivability. Nine of the respondents provided data concerning their anticipated

future shock acceleration levels. These users are identified by the symbol (F)

in Table 1. A plot of the present shock levels for all of the respondents is

shown in Figure 2 . A comparison of the present versus future shock levels for

the accelerometer users who provided such data is given in Figure 3. These

respondents also foresee an increase in the number of shock tests required, but

this information was not quantified.

4. Discussion and Conclusions

The questionnaire for shock accelerometer users was designed to determine the

accuracies needed and being achieved in both shock measurements and calibration

of the accelerometers. The principal survey results are summarized in the

following sections. Measurement uncertainty requirements at low shock levels

(less than 10,000 g) , if specified, are generally ± 10 or 15 percent.

Measurement accuracies at higher levels are usually specified in terms of

tolerances on shock response spectra. A few contractual requirements specify

uncertainties of ± 10 percent.
Users stated accelerometer uncertainty requirements of ± 5 percent for low shock

levels. Military specifications do not specify accelerometer uncertainties,

although MIL-STD-810D specifies ± 10 percent [2]. At shock levels between

10,000 g and 100,000 g, uncertainties claimed by manufacturers of ± 10 percent

are acceptable to the users surveyed. Very few users are making shock measurements above 100,000 g (the number of measurements per month for various

acceleration levels reported by the respondents is shown in Figure 4, with the

uncertainties also being noted) . Those users are most concerned about survival

of their instrioments . However, they are concerned about the lack of unquestionable traceability at shock levels above 10,000 g.

For shock levels below 10,000 g, present specifications and calibration methods

seem to be adequate. Major transducer manufacturers claim maximum probable

errors of from 1 to 5 percent. Each of the 4 manufacturers surveyed claim NBS

traceable capability in this range, several different calibration methods being

used.

It should be noted that NBS does not presently offer a shock calibration service

which is "traceable" either to sinewave calibrations above 10 g nor to any

absolute shock measurements. The existing NBS shock facility provides a com- parison calibration of accelerometers by subjecting them to half -sinewave pulses

with peak amplitudes of 50 to 5000 g and pulse widths from 0.2 to 40 milliseconds. (There have been no requests from transducer manufacturers for this

service.) The standard accelerometer used for this service has been calibrated

at lower amplitudes, but its linearity has been checked only over a range of

acceleration amplitudes up to 5000 g, using other accelerometers in their linear

range

.

In conclusion, the available services for -- and the current state of -- shock

accelerometer calibrations will be briefly summarized. Most measurements and

accelerometers are employed at shock amplitudes less than 10,000 g. Shock

accelerometers are commonly calibrated in this range by a drop ball or by a gravimetric technique [3]; these low-level calibrations are directly traceable

to NBS calibrated instruments. Endevco has sold approximately 100 drop ball

shock calibration systems for shock levels below 10,000 g. Many independent

calibration laboratories use the drop ball method. Sandia Laboratories and

others have modified the drop ball calibration for higher acceleration levels.

In order to overcome the limitations in the short fixture used in many com- parison techniques when the shock pulse -wavelength" approaches the

fixture/accelerometer dimensions, calibrations between 10,000 g and 100,000 g are limited to the use of some variation of a Hopkinson bar. The latter fixture

is a long and slender elastic cylindrical bar which is longer than the

wavelength of the shock pulse such that the entire pulse becomes embodied as a compression-wave which travels toward the test accelerometer mounted on the end.

Very few calibration facilities maintain this capability, among them being

Sandia Laboratories, Aberdeen Proving Ground, and S-Cubed (Systems, Science and

Software). Above 15,000 g, Endevco uses a specially modified Hopkinson bar with

the test accelerometer mounted on one end and struck by a projectile on the

other end [4,5]. The bar is calibrated with a transfer standard which has been

calibrated on the drop ball calibrator. Endevco estimates an uncertainty of 6.5

percent for this "test", which is not used on standard accelerometers and is

available as an optional extra cost service. No other manufacturer offers

calibration service above 15,000 g, and Endevco does not offer such service

above 100,000 g.
There is no commercially available calibration service for shock levels above

100,000 g. The few users who are making shock measurements in this range are

mostly concerned about survival of their instruments. The respondents who have

current needs and others who foresee the need in the near future for shock

measurements greater than 100,000 g would like to have a traceable, certifiable

accuracy specification for shock levels in this range. In this regard, it

should be noted that many of the respondents are performing tests which are contractually required and intended to resolve contractual compliance questions.

Above 10,000 g to 100,000 g, " traceability" requires extrapolation from lower

level calibrations. The extrapolations, in turn, require assumptions of the

nature of linearity and frequency response of the standard accelerometers used.

Several respondents expressed concern for the effects of transverse motion at

the test frequencies in addition to the lack of assurances of linearity.

Some of the questions raised by this survey concerning the current state of

shock transducer calibrations are: (1) Are the accelerometers used at ranges

extending to over 100,000 g really linear between their usage level and their

calibration level of less than 10,000 g? , and (2) Would the assumed accuracy

stand up in court, if necessary?

5. Recommendations

Based on the results of this survey, the following recommendations are made for

improving the state of calibration standards for mechanical shock ac- celerometers :

1. Conduct an NBS intercomparison of a variety of commercially available shock

accelerometers in order to compare their sensitivities under standard

sinewave calibration and shock calibration methods. The purpose of this is

to determine the amplitude linearity of the selected accelerometers over as large an acceleration range as possible with existing calibration

facilities.

2. Conduct a technical feasibility study to determine the technical approach

and estimated program cost to develop NBS comparison shock calibration

capability for shock amplitudes up to 30,000 g.

3. Conduct a technical feasibility study to determine the optimum technical

approach and estimated cost for a future program to develop an absolute

shock calibration capability for shock amplitudes up to 10,000 g.

4. Conduct a technical feasibility study to determine the optimum technical

approach and estimated program cost to develop NBS shock calibration

capability (absolute and comparison) for shock amplitudes up to 100,000 g.

6Acknowledgments

The assistance of D. R. Flynn, M. R. Serbyn and B. F. Payne in developing the

shock user questionnaire and in conducting the survey of shock accelerometer

users is gratefully acknowledged. Useful information was obtained from Jon S. Wilson concerning the current state of shock accelerometer calibrations.
7. References

1. Tri- Service Metrology Research, Development, and Engineering Plan, Vol.1,

Program Overview, CCG Engineering Working Group, Sept. 30, 1985.

2. MIL-STD- 810D , Environmental Test Methods and Engineering Guidelines,

Department of Defense, 1983.

3. Kobayashi, A.S., (Ed.), "Handbook on Experimental Mechanics" (Prentice

Hall, 1987).

4. Sill, R.D. , "Shock Calibration of Accelerometer at Amplitudes to 100,000 g

using Compression Waves", Endevco Technical Paper 283, Endevco, Rancho

Viejo Road, San Juan Capistrano, California, 1984.

5. Sill, R.D., "Testing Techniques Involved With the Development of High

Acceleration Sensors", Endevco Technical Paper 284, Endevco, Rancho Viejo

Road, San Juan Capistrano, California, 1985.

6. Wilson, J.S., " Accelerometers for Pyroshock Measurements", Proceeedings of

the lES Pyrotechnic Shock Tutorial Session, May, 1985.

7. Powers, D.R., "Pyro Shock Test Simulation Methods", Proceedings of lES

Pyrotechnic Shock Tutorial Session, May, 1985.

8. van Randeraat, J. and Setter ington , R.E., (Eds.), "Piezoelectric

Ceramics", (Mullard Limited, 1974).

9. Personal Communication with Jon S. Wilson, 1986.

10. Bouche, R.R. , "Calibration of Shock and Vibration Measuring Transducers",

Shock and Vibration Monograph Series SVM-11, The Shock and Vibration

Information Center, Naval Research Laboratory, Washington, DC 1979.
Appendix A. General Shock Survey Information

Four manufacturers of accelerometers whose transducers are frequently employed

by shock measurement users were surveyed: Dytran, Endevco , Kistler, and PCB.

All four manufacture, calibrate, and recalibrate accelerometers with ratings up

to 100,000 g. Endevco is the only manufacturer among these which makes both

piezoelectric and piezoresistive accelerometers, one of the latter being rated

higher than 100,000 g. Manufacturers' data sheets seldom state calibration

uncertainty. Accuracies of calibration are stated in supplementary catalog

information, or in listings of available calibration services. All four

manufacturers perform vibration calibrations that are directly traceable to NBS

;

however, the traceability of their shock calibrations is more circuitous.

Forty five users of accelerometers for shock measurements were surveyed to

determine usage, problems, and needs relative to shock calibrations. Among the

government respondents there were nine Army, ten Navy, four Air Force, one NASA

and three DOE laboratories . There were eighteen respondents from industrial

contractors. There were several additional laboratories who were surveyed but

did not provide enough useful information to be included in the tabulated

results. Table A.l gives a listing of all the various organizations who were

contacted to determine the the current needs for shock measurements and shock

accelerometer calibration requirements.

There were seven measurement problem areas identified from the telephone survey

of shock accelerometer users. The three dominant problem areas identified among

those laboratories engaged in measuring the largest shock motions were: (1)

transducer survivability, (2) zero shift, and (3) insufficiently high resonance

frequency of the accelerometer. These problem areas and their interactions have

been discussed in a recent paper by Wilson [6].

The next two most frequently mentioned areas of concern were: (4) the ac- celerometer transverse sensitivity and (5) base or case strain sensitivity.

Powers [7] has suggested that for some shock environments, an accelerometer may

be subjected to simultaneous motion in and around all three orthogonal axes of

the transducer. One of the survey respondents noted the Hopkinson bar was not an appropriate means for the calibration of accelerometers which are subjected

to oblique shock motions of large amplitude.

The two problem areas least noted by most of the shock accelerometer users were:

(6) cable noise and (7) temperature effects. Only one respondent suggested that

he would find it useful to calibrate his accelerometers at the temperature

extremes required by MIL STD 901C.

Ballistic and pyroshock tests are the applications with the highest acceleration

level requirements . Several of the survey respondents who were most concerned

with the fragility of accelerometers in pyroshock and similar measurements had

dissected accelerometers which had failed in service, but these investigations

were not conclusive in finding a dominant failure mechanism.

10
Table A.l Tabulation of Shock Accelerometer Users Surveyed

I . Army Laboratories

1. Aberdeen Proving Ground (ABERDEEN)

2. Ballistic Research Laboratory (B.R.L.)

3. Engineer Waterways Experiment Station (ENG.WTRWY.)

4. Army Electronic R&D Command Fort Monmouth (FT.MONMTH.)

5. Harry Diamond Laboratories (H.D.L.)

6. Army Armament R&D Command- Picatinny Arsenal (PICATINNY)

7. White Sands Missile Range (WH. SANDS)

8Army Tank Command

9

Yuma Proving Ground

II. Navy Laboratories

1. Naval Weapons Center- China Lake (CHINA LK.)

2. Naval Ocean Systems Command (NOSC S.D.)

3. Naval Research Laboratory (N.R.L.)

4. Naval Surface Weapons Center- White Oak (N.S.W.C.)

5. Naval Ordnance Station- Indian Head (IND.HEAD)

6. Naval Air Test Center- Patuxent (PATUXENT)

7. Underwater Explosions Research Development (U.E.R.D.)

8. Navy Primary Standards Laboratory

9. Naval Surface Weapons Center- Dahlgren

10. Naval Underwater Systems Center

III. Air Force Laboratories

1. Holloman AFB (HOLLOMAN)

2. Kirtland AFB (KIRTLAND)

3. Wright Patterson AFB (WPAFB)

4. Eglin AFB

IV. DOE Laboratories

1. Los Alamos (LOS ALMOS)

2. Sandia Laboratories (SANDIA)

3Lawrence Livermore Laboratory

V. NASA Laboratory

1. Marshall Space Flight Center (MRSHL SFC)

VI. Industrial Contractors

1. Bendix, New Jersey (BENDIX,NJ)

2. CAI Optical (CAI OPT.)

3. Dayton T. Brown (D.T.BROWN)

4. Epoch Engineering, Inc.

5. General Dynamics (GEN.DYN.)

11
6. Hughes (HUGHES)

7. Lockheed (LOCKHEED)

8. LTV Dallas (LTV DAL.)

9. Martin Marietta (MRTN MAR.)

10, McDonnell Douglas, California (MCDAC CAL.)

11, McDonnell Douglas, Huntsville (MCDAC HNT.)

12, NKF Engineering Associates

13, NTS Massachusetts (NTS MASS.)

14, Rocketdyne (ROCKTDYNE)

15

,

Rosemount

16, Systems, Science & Software (S-CUBED)

17, TRW, Redondo (TRW.RDNDO)

18, Westinghouse (WSTGHSE)

12
Appendix B Nonlinear Effects in Accelerometer Designs and Materials

Nonlinearity is the change in sensitivity over the amplitude range.

Accelerometer design and sensing material affect nonlinearity. Ceramic

"crystals" inherently exhibit greater nonlinearity than quartz or other natural

crystals. This is thought to be caused by partial depolarization of the artifically polarized materials as internal stress increases [8]. However, ceramics

also provide greater charge sensitivity, so that nonlinearity of the crystal

material within the usable range is often insignificant.

In general, the sensitivities of piezoelectric accelerometers increase linearly

with acceleration. Nonlinearity of four or five percent is usually accepted for

shock measurements. Nonlinearity specifications are usually based on worst-case

production variations of a particular design, and acceleration ranges are some- times established based on an acceptable nonlinearity for particular

applications

.

Undamped piezoresistive accelerometers generally exhibit very good linearity up

to their fracture point. Damped piezoresistive designs may exhibit complex

nonlinearity relationships caused by variations in damping. Wilson has noted

that both piezoelectric and piezoresistive accelerometer designs may have non- linearities caused by poor design or assembly practices. Adhesive

characteristics, insufficiently torqued threaded fasteners, and defective crystal materials are some of the possible causes [9] . Nonlinearity may vary from

unit to unit but it is not measured on a routine basis.

The principal issue raised regarding the above noted limitations in accelerometer materials and designs is whether accelerometers used at levels one

or several orders of magnitude greater than their calibration level are really

linear. Extrapolations require assumptions of the nature of linearity which may

be unverified. One of the principal reasons for extending the range of existing

standards for shock calibration to meet current measurement needs would be to

verify the linearity of accelerometers over a larger range of shock amplitudes

[10].

13
(O

0) o>

CO C *— Q.

m in u

CO UJ ^ o Q.

(0

0)

\- E 4^

o

z > c o

o

o

• ®

CO 1-

c

JI

(0 O

•*- 2 o O) CO o

>» CO 3 k. Q. (0 O <

I 1 1 i «—«. .^^ ,^^ ^m^ z >

^~ 3

.» ,»

»« « Q. (0 o <

LO

o

(0

CO c

k. o

0) 4-« 0) (0

o

o

N < Q.

Q.

r <

(0 4^

>* s c

O

c

0)

>

E

o >* 3 3 o (/) CO

LL

k.

^

o

u. o X

(0 X

(0

(0

0)

3

O)

CO

CO m CO

6 'uo!)eje|eo3v 'xe|/\|

14
O)

Io

c

o

2

o

o

o

<

o>

(0

1

1 0)

(0

o ^ ^" 0)

o E

o

T" 0)

1 c 0)

o

in o o ^^ <

(0 ^o

0) o (0

in 0)

1 o >

CO o 0)

CM < ^

o

in

CM

X

CO

o

CO

c 1 0) o (0

0)

CM

o>

in CO CM TAouenbajj aAjieiau

15
c

o c ^^ o

CO ^-» V. CO

0) k.

o

0)

o o

o o

< o

<

c 0)

<D ^

(0 3 O *^ ^ 3

0. UKat^

:^

^

sx

sx

si

s

o>

00

CO

0)

(0

o

in £

o

o

o

<

CO

CM

o

o _ E

CO

CO

E =>

^ o

W 0) CO

2 2

CO 5

© 5

Q- <

CO

d)

II.

CM 00 (O Csl

6-oi!)| 'uojiejaieoov

16
(O

TSSS>

00 CD CM

i||U0|Ai iad s)uauiainseaiAi

o >

in

O)

o o

^

"D

cr CO

CO

0)O

o

<

X

CO

CM

in O <o

1- lO

iO I S

Dm

« S o ^

c -'

cS CO

i E ^

o

(0 (0

E

^

^ O —o<

a>

CO

z ^

0)

17
NBS-n4A IREV. 2-8C1 U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS/TN-1233

2. Performing Organ. Report No 3. Publication Date

June 1987

4. TITLE AND SUBTITLE

Requirements for the Calibration of Mechanical Shock Transducers

5. AUTHOR(S)

D. C. Robinson

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

Same as item 6.

10. SUPPLEMENTARY NOTES

QJ Document describes a computer program; SF-185, FlPS Software Summary, is attached.

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

bibliography or literature survey, mention it here) . - ., . ^ j j j • , „„„„

The use of obsolete, inadequate, or nonexistent calibration standards during weapons

system development and acquisition make DoD and its contractors unable to validate

system performance legally during testing, evaluation and production acceptance. To

correct current weakness and to meet the DoD calibration requirements of the immediate

future in the area of mechanical shock new accelerometer calibration standards must be

produced and existing standards must be improved. This report describes the results of

a survey of various government agencies and government contractors who are actively

engaged in the measurement of shock motions of 100 g or higher. As part of the effort

to establish current needs for the calibration of accelerometeij, a study was made of the

accuracy requirements in various engineering applications for the measurement of shock

motions up to 200 000 g, and an evaluation was made of the accuracy required to calibratje

such transducers for various ranges of accelerations. Based on this study, several

recommendations are made for improving the state of calibration standards for mechanical

shock accelerometers

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Accelerometer calibration standards; calibration requirements; mechanical shock transducers; military specifications; nonlinear effects; pyrotechnic shock; shock accelerometer users; transducer manufacturers

13. AVAILABILITY

[X] Unlimited

I I For Official Distribution. Do Not Release to NTIS

[X] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. Q^ Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

20

15. Price

USCOMM-DC 6043-P80
NBSTechnical Publications

Periodical

Jounial of Research—The Journal of Research of the National Bureau of Standards reports NBS research and development in those disciplines of the physical and engineering sciences in which the Bureau is active These include physics, chemistry, engineering, mathematics, and computer sciences. Papers cover a broad

range of subjects, with major emphasis on measurement methodology and the basic technology underlying

standardization. Also included from time to time are survey articles on topics closely related to the Bureau's

technical and scientific programs. Issued six times a year.

Nonperiodicals

Monographs—Major contributions to the technical Uterature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with interested industries, professional organizations, and regulatory bodies.

Special PubUcations—Include proceedings of conferences sponsored by NBS, NBS annual reports and other

special pubbcations appropriate to this grouping such as wall charts, pocket cards, and bibliographies.

AppUed Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and

technical work.

National Standard Reference Data Series-Provides quantitative data on the physical and chemical properties of matenals, compiled from the world's literature and criticaUy evaluated. Developed under a worldwidVorogram coordinated by NBS under the authority of the National Standard Data Act (Public Law 90-3%) NOTE: The Journal of Physical and Chemical Reference Data (JPCRD) is published quarterly for NBS by

the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions reprints and supplements are available from ACS, 1155 Sixteenth St., NW, Washington, DC 20056. BuUding Science Series—Disseminates technical information developed at the Bureau on building materials components, systems, and whole structures. The series presents research results, test methods and perfor-

' mance cnteria related to the structural and environmental functions and the durability and safety

characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a

subjecL Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject

area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce in Part 10, Title 15, of the Code of Federal Regulations. The standards establish nationally recognized re- quirements for products, and provide all concerned interests with a basis for common understanding of the charactenstics of the products. NBS administers this program as a supplement to the activities of the private

sector standardizing organizations.

Consumer Information Series—Practical information, based on NBS research and experience, covering areas

ot interest to the consumer. Easily understandable language and iUustrations provide useful background

knowledge for shopping in today's technological marketplace.

Order the above NBS publications from: Superintendent of Documents, Government Printing Office Washington, DC 20402. ' ' Order the following NBS publications—FIPS and NBSIR's-from the National Technical Information Service, Springfield, VA 22161.

Federal Information Processing Standards PubUcations (FIPS PUB)—Publications in this series collectivelv constitute the Federal Information Processing Standards Register. The Register serves as the official source of

intormation in the Federal Government regarding standards issued by NBS pursuant to the Federal Propcrtv and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 1 1717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS tor outside sponsors (both government and non-government). In general, initial distribution is handled b\ the sponsor; public distribution is by the National Technical Information Service, Springfield. VA 22161 in paner copy or microfiche form. ' » j
m

U.S. Department of Commerce

National Bureau of Standards

Gaithersburg, MD 20899

Official Business

Penalty for Private Use $300