Hgh Temperature Stran Measurements usng Welded Stran Gages Emerson G. Rabello, Paulo de Tarso V. Gomes and Tanus R. Mansur Center of Development of the Nuclear Technology (CDTNCNEN), P.O. Box 941, Belo Horzonte, MG, Brazl ABSTRACT Ths work presents stran measurements n A 516-grade 65 carbon steel and AISI 304 austentc stanless steel, usng HTW russan stran gages. The measurement was done usng a half Wheatstone brdge wth two dstnct confguratons: a- two fxed stran gages, b - one fxed and other not rgdly attached to the specmen. The expermental results were compared wth that obtaned theoretcally for two dstnct loads. The temperature nfluence on the results was also verfed. INTRODUCTION The expermental stress analyss usng electrc stran gages n hgh temperature atmospheres requests the prevous knowledge of the stran gage nstallaton behavor, manly n what t concerns the stran gage electrc resstance varaton. In such case, the electrc stran gages resstance R must be consdered as functon of the temperature T, tme t and stran s,.e., R = f (T, t, s) [ 1 ]. Many problems n the hgh temperature stran gage measurement are generated by envronmental condton lke eroson and oxdaton. Humdty and chemcal agents wll attack the stran gage nstallaton more severely n hgh temperature. After fxed on adapted specmen one or more stran gages from the same lot are submtted to known loadng under known envronmental condtons. Startng from smultaneous measurements of temperature and stran s possble to construct curves that characterze the stran gages behavor n the test condtons. The objectve of ths work s to analyze the stran n two dfferent stran gage nstallatons under loadng and temperature varaton. METODOLOGY It was used two constant-stress beams bult n A 516-grade 65 carbon steel and AISI 304 austentc stanless steel. Fg. 1 shows a constant-stress cantlever beam drawng wth the dmensons and the stran gages locaton. 0 0 O= I---I ------ o --------- - o,==ol o=o= I"-'1 14 L ~1 "C k ~-{-- Beam Carbon steel Stanless steel Dmenson (mm] L b h 100 50 4.22 100 50 4.40 Fg. 1 Constant-stress cantlever beam Each beam was prepared wth stran gages of Russan producton and thermocouples K, to obtan smultaneous temperature and stran readngs. The fxed stran gages were welded on the beam by capactve dscharge process and connected n two dfferent electrc confguraton (Fg. 2):
Two fxed stran gages connected n half Wheatstone brdge- the connecton named "nstallaton A" (Fg. 2-a) s consttuted of two welded stran gages (fxed) wth ther longtudnal axes makng an angle of 90 to each other. A fxed stran gage and a passve stran gage connected n half Wheatstone brdge- the connecton named "nstallaton B" (Fg. 2-b) s consttuted of one welded stran gage (fxed) and one non welded stran gage (passve). Ths passve stran gage s postoned only n contact wth the beam surface. As t s not welded, t wll not accompany the beam deformaton and t wll just measure the temperature varaton. I ~ fxed N strar "- I I + N fxed strar I ~raf ~ g~e R a Y~ b R Fg. 2 Scheme of stran gages connecton To verfy the stran gages nstallatons performance, tests were made at room temperature wth loads of 5 kgf, 10 kgf e 15 kgf. Then, the stran gages nstallatons were submtted to the temperature varaton between 20 C e 420 C to apparent stran determnaton. At last, tests were made under load and temperature varaton. THEORETICAL CALCULATIONS The stresses n the constant-stress cantlever beams were obtaned from the strength of materals theory, usng the followng equatons [2]' cr. M.. 6FL. (1) W bh 2 cr = Ec :=> c. cr.. 6FL. (2) E Ebh 2 Where: F s the appled force; L s the dstance from the force to the consdered transverse secton; E s the modulus of elastcty of the beam materal, E = 210000 MPa; B s the wdth of the beam transverse secton; h s the heght of the beam transverse secton; cy s the normal stress; s the normal stran. The stran gages technque calculatons were made wth the followngs equatons [3]" a) ½ Wheatstone brdge connecton wth one fxed stran gage and one passve stran gage',5' = 4AV VK x 10 3 (3) b) ½ Wheatstone brdge connecton wth two fxed stran gages"
E' = 4AV VK(1 +.z) x 10 3 (4) Where: s the measured stran; la s the Posson's coeffcent of the beam materal; AV s the Wheatstone brdge output tenson n mv; V s the Wheatstone brdge nput tenson n V. MATERIALS AND EQUIPMENTS The furnace used for the constant-stress cantlever beam heatng has the followng characterstcs: Maxmum temperature: 1200 C; Heatng control: 25 C; Heatng rate : 3.8 Cmn. The stran gage used has the followng characterstcs: Russan stran gage specfcaton: HTW; Nomnal resstance: 100 + 1 f2; Gage factor (K): 2.0; Maxmum servce temperature: 500 C. The measurement equpment were: Source of constant tenson: 0 a 5 Volt; Dgtal voltmeter of 4 ½ dgt; Channels selector to 10 Wheatstone brdges. EXPERIMENTAL RESULTS Table 1 shows the calculated stran values for both beams under loads of 4 kgf e 10 kgf, from Eq. (2). The dfferences of the calculated values for the same load are related to the dfferent dmensons of the beams. Table 1. Theoretcal values of stran under loads of 4 kgf e 10 kgf Constant-stress Stran (p mm) cantlever beam Load: 4 kgf Load: 10 kgf Carbon steel 128 320 Stanless steel 118 296 Fg. 3 shows both nstallatons A and B behavors, under loadng of 5 kgf, 10 kgf, and 15 kgf at room temperature. 500 - Installaton A and B behavor, under loadng, at ambent temperature 450-400 - 350-300 - 1 250-200 - 150 - " [.. " ~ value - carbon steel.. " ~ l a t o n A - carbon steel A-stanless steel " ~ t o n :'.~:"... Installaton B - carbon steel - - Installaton B - stanless steel ; ' '~ ' ;o ~'~ ' ~'~ ' load (kgf) Fg. 3 Installatons A and B behavors, under dfferent loads, at room temperature
Fg. 4 and 5 show the apparent stran n both nstallatons A and B, for the carbon steel and stanless steel beams, respectvely. 200-0- Apparent stran presented by both A and B nstallatons n the carbon steel beam -200 - -400 - k -600 - k -800 - -1000-1200 - " I' Instalaton A 1400 - I - - Instalaton B Zero stran 1600 0 100 200 3;0 400 50O ' Temperature ( C) Fg. 4 Apparent stran presented by nstallatons A and B n the carbon steel beam 140o Apparent stran presented by both A and B nstallatons n the stanless steel beam 1200 1000 I Installaton A - - - Installaton B zero stran 800 600 f 400 I" 200 0, -200,, 100 200 300 400 500 Temperature ( C) Fg. 5 Apparent stran presented by nstallatons A and B n the stanless steel beam Fg. 6 and 7 show both nstallatons A and B behavors, n the carbon steel beam, under loadng of 4 kgf and l0 kgf, respectvely. 200 - Theoretcal and expermental stran values to the carbon steel beam under loadng of 40 N 0- -200 - ~. -400 - "" -600 - -800-1000- 1200 4 4 1400 - Theoretcal value --Installaton A - - Installaton B ; ~oo ~;o 3oo 4oo ~;o Fg. 6 Theoretcal and expermental values for the carbon steel beam under load of 4 kgf
. " Theoretcal - - - - - - 400 Theoretcal and expermental stran values to the carbon steel beam under loadng of 100 N 2o0 0-200 -400-600. -800. -1000-1200 Theoretcal value Installaton A Installaton B 0 100 200 300 400,00 Fg. 7 Theoretcal and expermental values for the carbon steel beam under load of 10 kgf Fg. 8 and 9 show both nstallatons A and B behavors, of the stanless steel beam, under loadng of 4 kgf and l0 kgf, respectvely. 1400- Theoretcal and expermental stran values to the stanless steel beam under loadng of 40 N,# 1200-1000- theoretcal value Installaton A Installaton A I" "E" 800-600- 400 - z ~.. 0 100 2(30 3C)0 400 500 Fg. 8 Theoretcal and expermental values for the carbon steel beam under load of 4 kgf Theoretcal and expermental stran values to the stanless steel beam under loadng of 100 N 160o - _ value 1400 - Installaton A., Installaton B, " F 1200 - ~" looo-. - " 800 - " 600-400 - z z 0 100 200 300 400 500 Fg. 9 Theoretcal and expermental values for the stanless steel beam under load of 10 kgf
RESULTS DISCUSSION Accordng to Fg. 3 both nstallaton A and B show good behavor for stran measurement at room temperature. The apparent stran values obtaned wth the nstallaton A show ts effcency to elmnate the temperature effect over the stran gage nstallaton. Between 40 C and 420 C, the apparent stran decrease (absolute values) presented by the carbon steel beam was between 95 and 98. For the stanless steel beam t was between 90 and 98. The dfference observed between calculated and expermental values presented by the nstallaton B, s due, probably, to the great stffness of the passve stran gage fxaton system that restrcted ts moblty beyond the expected. The results obtaned wth nstallaton A were very close to the calculated values for both stanless steel and carbon steel beams. It s mportant to observe that the dfference remaned constant to each load durng the temperature varaton. CONCLUSIONS Although the theory shows the vablty of the use of movable stran gages, we should search for methods that guarantee the total moblty of them. The nstallaton A n the carbon steel beam presented better behavor for stran measurement relatve to the load and temperature. REFERENCES 1. Dally, ames W. & Rley, Wllam F., "Expermental Stress analyss", 2ed. New York, NY, McGrall-Hll, 1978. 2. Beer, F.P., ohnston, E.R., "Resst~nca dos Materas", S~o Paulo, SP, McGrall-Hll, 1982. 3. Hannah, R.L., Reed, S.E., "Stran Gage User's Handbook", Betherl, USA, Elsever Scence Publshers Ltda and Socety of Expermental Mechancs, 1992. 4. Sergeev, A., Hursudov, G.H., Senn, V.S., Mjlev,.U.K., Maslov, S.V., Comt~ Governamental da Rfssa para Normalzayfo. "C~lculos e testes em Resst~nca Mecfnca", Centro de Investgay~es Centfcas da URSS para Normalzayfo e Estruturay~o de Mfqunas, Moscou, 1989. 5. "Une Soluton h L6xtensom6tre Haute Temperature: Le proced6 Rokde", Schenk, France: Mesures, 1972.