1、BRITISH STANDARD BS 6888:1988 Methods for Calibration of bonded electrical resistance strain gauges UDC 620.17:681.2.089:621.317:620.17:53.08:531.781.2.718.2BS6888:1988 This British Standard, having been prepared under the directionof the Iron and Steel andthe Non-ferrous Metals Standards Committee,
2、 was published under the authority ofthe Board of BSI and comesintoeffect on 30September1988 BSI 01-2000 The following BSI references relate to the work on this standard: Committee reference ISM/NFM/4 Draft for comment 85/44240 DC ISBN 0 580 16186 2 Committees responsible for this BritishStandard Th
3、e preparation of this British Standard was entrusted by the Iron and Steel Standards Committee (ISM/-) to Technical Committee ISM/NFM/4 upon which the following bodies were represented: Aluminium Federation British Gas plc British Non-Ferrous Metals Federation British Railways Board British Steel In
4、dustry Copper Development Association Department of Trade and Industry (National Engineering Laboratory) Department of Trade and Industry (National Physical Laboratory) ERA Technology Ltd. GAMBICA (BEAMA Ltd.) Ministry of Defence Society of British Aerospace Companies Ltd. Welding Institute The foll
5、owing bodies were also represented in the drafting of the standard, through subcommittees and panels: British Society for Strain Measurements City University Institute of Physics Institution of Production Engineers Amendments issued since publication Amd. No. Date of issue CommentsBS6888:1988 BSI 01
6、-2000 i Contents Page Committees responsible Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 Performance characteristics of strain gauges 4 4 Strain gauge installation for calibration tests 4 5 Measuring equipment and strain gauge excitation 4 6 Calibration procedures 5 7 Calibration deta
7、ils supplied with packages of strain gauges 6 Appendix A Recommendations concerning testing equipment 8 Appendix B Production quality control 10 Appendix C Determination of optional strain gauge characteristics 19 Figure 1 Strain gauge outlines 10 Figure 2 Examples of rosette strain gauge types 12 F
8、igure 3 Strain gauge factor testing specimen 13 Figure 4 Transverse sensitivity calibration rig 14 Figure 5 Strain limit tensile testing specimen 15 Figure 6 Strain gauge factor calibration rig 16 Figure 7 Calibration rig for creep and strain gauge factor change with temperature 17 Figure 8 Calibrat
9、ion rig for determination of strain gauge factor for temperatures up to650 C 18 Table 1 Sampling rate 10 Publications referred to Inside back coverBS6888:1988 ii BSI 01-2000 Foreword This British Standard has been prepared under the direction of the Iron and Steel and the Non-ferrous Metals Standard
10、s Committees. It supersedes DD6:1972, which is withdrawn. The standard describes methods for calibration of bonded electrical resistance strain gauges in widespread use for a variety of applications in the mechanical testing of components and structures. In preparing this standard, consideration has
11、 been given to comments made on DD6:1972 and to corresponding documents prepared by the American Society for Testing Materials (ASTM), the Aerospace Industry Association of America Inc., and the International Committee for Legal Metrology (OIML) namely ASTM Designation E251-67 1)“Standard methods of
12、 test for performance characteristics of bonded resistance strain gauges”, NAS942 2) , “Strain gauges, bonded resistance”, and “Performance characteristics of metallic resistance strain gauges”. This standard is essentially concerned with establishing the performance characteristics of strain gauges
13、 during manufacture and also takes into account the bonding requirements which are necessary to ensure that the strain gauges when in service are permanently attached to the surface of the component or structure under test. However, this standard may also be applied by the user of strain gauges in o
14、rder to verify that performance characteristics are satisfactory for the purposes for which the strain gauge is to be employed and also to compare the performance of different types of strain gauge. Recommendations concerning testing equipment are given inAppendix A. Recommendations for production q
15、uality control are given inAppendix B. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Su
16、mmary of pages This document comprises a front cover, an inside front cover, pagesi andii, pages1 to20, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside fro
17、nt cover. 1) Available from the Library, BSI, Linford Wood, Milton Keynes MK146LE. 2) Available from the National Standards Association Inc.,5161River Road, Bethesda, MD20816, USA.BS6888:1988 BSI 01-2000 1 1 Scope This British Standard describes methods for calibration procedures to determine the pe
18、rformance characteristics of bonded electrical resistance strain gauges that have metallic elements. It defines the terms used, and states the accuracies required for instrumentation used for calibration purposes. The calibration of semi-conductor strain gauges is outside the scope of this standard.
19、 NOTE 1InAppendix A toAppendix C recommendations are given for design of suitable calibration rigs, sampling and quality control testing of production strain gauges and procedures to determine optional performance characteristics. NOTE 2The titles of the publications referred to in this standard are
20、 listed on the inside back cover. 2 Definitions For the purposes of this British Standard the following definitions apply. 2.1 Strain 2.1.1 strain change in length per unit length in a specified direction of a specimen of a material, i.e.dL/L, where L is the initial length in the specified direction
21、 and dL is the dimensional change in that direction (in mm) NOTEStrain is normally expressed in micrometres per metre (colloquially known as microstrain, and represented by the symbol, 4). 2.1.2 mechanical strain the strain in a testing specimen resulting from changes in mechanical forces 2.1.3 ther
22、mal strain the strain in a testing specimen that is free to move at all its boundaries resulting from a change in temperature, which is uniform throughout the material NOTEFor a homogeneous material, the thermal strain per unit change of temperature corresponds to the linear coefficient of thermal e
23、xpansion of the material of the testing specimen. 2.1.4 real strain the total strain in a testing specimen due to the summation of the mechanical and thermal strains 2.2 Strain gauge 2.2.1 bonded electrical resistance strain gauge a device, which when strained after bonding to a surface gives a chan
24、ge in electrical resistance proportional to strain in the surface NOTE 1Representative arrangements of foil and wire strain gauges are shown inFigure 1 which illustrates the definitions given in2.2.3 to2.2.10. NOTE 2In this standard the term “strain gauge” is used to denote the bonded electrical res
25、istance strain gauge. 2.2.2 rosette a rosette is a multiple strain gauge having two or more measurement axes associated with two or more sensing elements mounted on the same common matrix (seeFigure 2) 2.2.3 terminals small wires, tags or tabs by which lead wires are attached 2.2.4 lead wires the wi
26、res between the strain gauge terminals and the instrumentation used to measure the resistance change of the strain gauge assembly 2.2.5 active grid element the portion of the strain sensing material of a strain gauge which changes resistance when the strain gauge is subjected to strain 2.2.6 active
27、strain gauge length the length of the active grid element in the direction of the measurement axis. For configurations with end loops, the length is measured from the inside of the loops 2.2.7 active grid width the overall dimension of the active grid element in the direction of the transverse axis
28、2.2.8 matrix 3) an electrically non-conductive layer of material used to support a strain gauge grid NOTEThe two main functions of the matrix are to act as an aid to bonding the strain gauge to a structure and to provide electrical insulation from the structure. 3) Sometimes referred to as “backing”
29、.BS6888:1988 2 BSI 01-2000 2.2.9 measurement axis the axis of the strain gauge along which it is designed to have a maximum strain sensitivity 2.2.10 transverse axis the axis of the strain gauge, in the plane of the strain gauge, at90 to the measurement axis along which the strain gauge is designed
30、to have minimum strain sensitivity 2.2.11 testing specimen a specimen, of specified material and dimensions, on which a strain gauge is bonded for the purpose of establishing the characteristics of the strain gauge (seeFigure 3 to Figure 5) 2.2.12 strain gauge installation a strain gauge, bonded to
31、the surface of a testing specimen with lead wires attached 2.2.13 adhesive cure the setting of the adhesive by chemical reaction or solvent evaporation 2.2.14 active strain gauge a strain gauge used to effect strain measurement 2.2.15 temperature compensating dummy strain gauge a strain gauge not su
32、bject to mechanical strain installed to experience the same environmental conditions as an active strain gauge and used to cancel out environmental effects on the active strain gauge 2.2.16 strain gauge family strain gauges that have similar structural features and performance characteristics, but a
33、re not necessarily identical in size or geometry 2.2.17 strain gauge type strain gauges of the same family having no physical, dimensional or manufacturing differences but whose elements are not necessarily from the same melt of alloy 2.2.18 strain gauge lot a group of strain gauges, not necessarily
34、 of the same type, with the active grid element (see2.2.5) manufactured from the same melt and which during manufacture, have been subjected together to the same mechanical and thermal processes 2.2.19 strain gauge batch a group of strain gauges of the same type having the active grid element (see2.
35、2.5) manufactured from the same lot and which, during manufacture, have been subjected, together, to the same mechanical and thermal processes 2.3 Strain gauge characteristics 2.3.1 characteristic a property or feature of a strain gauge that can be given a numerical value by measurement 2.3.2 nomina
36、l characteristic a nominal value of the characteristic ascribed to a type or family of strain gauges by the manufacturer 2.3.3 average characteristic the average value of a specific characteristic determined on several strain gauges of the same batch 2.3.4 tolerance the maximum difference between va
37、lues determined for a specific characteristic of individual strain gauges and the average value of that characteristic usually expressed as a percentage of the value of the average characteristic 2.3.5 initial strain gauge resistance the electrical resistance at a stated temperature of a strain gaug
38、e, normally flat, prior to its application to a specimen NOTECommon nominal values of strain gauge resistance are60,120,350,600 and10007. 2.3.6 installed strain gauge resistance the electrical resistance at a stated temperature of a strain gauge bonded to an unstrained testing specimenBS6888:1988 BS
39、I 01-2000 3 2.3.7 insulation the electrical resistance between one of the strain gauge terminals and the testing specimen or structure on which the strain gauge is bonded 2.3.8 strain limit the maximum mechanical strain that may be imposed on a strain gauge installation on a specified material at a
40、specified temperature without causing strain gauge failure or a change of characteristic outside a stated tolerance 2.3.9 strain gauge drift the change of strain gauge resistance with time when the strain gauge is bonded to a strain-free stable specimen and held at a constant temperature 2.3.10 stra
41、in gauge creep the change in strain gauge resistance with time, determined on a drift free strain gauge installation on a stable specimen held at a constant temperature, when the surface of the specimen is strained and held at a constant value of strain for a set period 2.3.11 operating temperature
42、limits the limiting temperatures for predictable performance NOTEAn individual type of strain gauge may be assigned more than one set of operating temperature limits for different levels of performance. 2.3.12 fatigue life the number of mechanical strain cycles that may be imposed on a strain gauge
43、installation without mechanical or electrical breakdown occurring, and without the strain gauge resistance at any part of the cycle varying more than a specified amount from the strain gauge resistance value at the corresponding point in the first strain cycle. The minimum and maximum mechanical str
44、ain levels are to be specified 2.3.13 strain gauge factor the ratio of the proportional change of resistance of a strain gauge installation (%R/R o ) to the strain(%L/L o ) on the surface to which it is bonded when the strain is induced by uniaxial stress applied in the direction of the measurement
45、axis, all other variables remaining constant, i.e.strain gauge factor where NOTEThe strain gauge factor determined by application of a uniaxial stress is dependent on the Poissons ratio and possibly other mechanical properties of the material on which the strain gauge is installed. 2.3.14 temperatur
46、e coefficient of strain gauge factor the incremental or decremental change of strain gauge factor per unit change in temperature NOTEThis value is not constant over the entire operating range of most strain gauges but a linear increase between two set temperatures is usually an adequate assumption.
47、2.3.15 apparent strain the combined reversible effects of the change in resistance and thermoelectric output resulting from a change in temperature of a strain gauge installation on a specimen free from constraint NOTEApparent strain (sometimes referred to as thermal output) is different for similar
48、 strain gauges installed on different materials or for different strain gauges installed on the same material. 2.3.16 transverse sensitivity the response of a strain gauge to a uniaxial strain in the direction of the transverse axis expressed as a percentage of the strain gauge factor 2.4 Strain gau
49、ge excitation 2.4.1 excitation the electrical supply (a.c.ord.c., constant current or constant voltage) connected to the strain gauge to effect measurement of strain R o is the strain gauge resistance at zero or reference strain (in 4); L o is the strain gauge length at zero or reference strain (in 4). K RR o LL o - =BS6888:1988 4 BSI 01-2000 2.4.2 safe excitation limit the highest continuous excitation at which the strain gauge characteristics can be maintained. The limit is dependent upon both the strain gauge and its environment
