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5{h 11Ili!/"I1(1/iollal COI/Jerel/cc on Mechanics alld Material!; ill D,'sigllREF: AO] PU) 11 8RESIDUAL STRESS ASSESSMENT USING OPTICAL TECHNIQUES.J ouo Riheirol", Juimc Montcll'o ~, 1\1. A. P. V31..\ HCl'nfllli Lopes!, and POllio Piloto lI Instit ute Pe litccnico de Bragan9D. Portugal11nstiluto de Engenhuria MecfUlica e Gcstiio Industrial, Lel;a do Balio, Portugal)Fac uldndc de En!!cn harin dn Universidndc do Porto, PortuaalEm:li l' (·Ijrih('iro i~l ipb.p l -I1 ~I1)SYNOPSISThe goal of this work is the development or experimental techniques to measure residualstresses as alternative [Q the hole-drilling method. The techniques here proposed are based onthe use of Moire Interferometry and Electronic Speckle Pallern Interfero111etTY (ES P!). Theyare fi eld techniques that allow in-plane displacements measurement without contact and v,dthhigh resolution. Grating replication techniques were developed to record high qualitydiffracli on gratings onto the specimen's surfnces. The developed an optical set-up to genemtethe master gratings (virtual) is also discussed. In this case an in-plane interferometer tomeasure displacements in two directions was impl c:mented as well as a dri ll ing equipmclll toprovoke the stress relaxation in the specimens. During residual stresses measurements theobtained fringe pattel115 (1vloirc and speckle) are video recorded. Image processing techniquesare used to assess the in-plane strain fie ld. A fin ite clemenls code (ANSYS®) is used tosimulate the process or relaxation of stresses to compare with experimental results and tocalculate the hole-drilling calibrLltion constants.)INTRODUCTIONIn this work, we developed rnl experimental process to measure residual stresses with twoopticol techniques (in-plone ESPI and Moire ul terferometry) combined with the hole-drillingmethod. A ring and plug specimen with known residual stress distribution was used tovalidate the techniques. An aluminium ring and a plug with 0.08 mm of diametricalinterrerence between them were assembled for that puropose. A nu merical simulation wi thfinit e elements, llsing the ANSYS® programme, \I./as developed to compute the calibrationcoefficients needed to obtain the stress values.The first test was performed using the in-plane ESPI set-up. Seveml holes with di fferentdiameters and depths were drill ed in both ring and plug. Image processing algorithms wereused to obtain the displacement fi eld from the interferometric recordings. The second test wusdone with Moire lnterferometry. For that a 1200 Jlnun grating (x and y crosses gratings) wasreplicated on the specimen surface. An optical set up to measure in two directions was used toassess the radial and circumferential stress released by several holes drilled in different radialdistances from the centre of the specimen.RESULTSFor the ESPI technique the phase maps of the interfero metric patterns were calculated usingtemporal phase shift. \Vhe n drills are made a strain relaxation occurs and the ESPI systemmeasures the difference between tlle two stages, berore and after the drilling. The obtained,I,C/;apw/, I: Experim ~1I1al Mechanics and Tc.'sriIJg ill Desigll 53
POrTo/Porlllg(l/. 24-26 Ju~v ](JU6phase maps nre simi lar to the one presented In Figure I (n). In Figure I (b) it is possible to seelhe displacement field afler image processing. In this case the stress values were codified in afalse colour table.)-;-.. ,(b).. " .... ,~ . ~ .- .~ .. " ~',(a)f ig. I Example of a Phase map obtained afler stress relaxation (a) find displacement field ofstress relaxation after image processi ng. (b).The results obtained after a set of measurements in the specimen used to validate thetechnique ~lI'e presented in Figure 2-,_--,~, - "-, ......IU"l:~""" "' '' ''''''· I ' ~ ; '_ ·'_·_ · ' v '-, .......... ....... ~ ."'""~ 1'1 _ ....., .."",.. . - - -; - ..• •• , l>< .. "~ ,, .... ,'", ........ ~-.... ...... - ........... """IFig.1 Comparison helwecn analYl'lca l calculnti ons and experimental measurements with ESP!.)CONCLUSIONSThe experimental results that were measured with ESP! have a good agreement with theanalytical calculations. This optical technique is il very interesting alternative to the traditionalhole-drilling method with strain gages and with the advantage that is possible to measuredisplncement fie lds instead of the discrete measurements obtained with the traditionalmethods based on the use of strain gages.REFERENCESLu_ J: James, M. R. and others " Handbook of Measurement of Residual Stresses", Society forExperimental, Inc .. Edited by Jian Lu, 1996.Post, D.; Han, B. ; Ifju , P. " High Sensitivily Moire: Experimenta l Analysis for Mechanics andMaterials" Springer Verlag, 1997.Nelson, D. V. and McCrickerd . .l. T., Residual-stress delennin;} tion through combined use ofholographic interferometry and blind hul e drilling, Experimental Mechanics, 26, pg. 371 -378.1986.t'I,Ii\';, r·t .,-I1I1"' !54 £ditol's: J.F. Silm GWllltS & Shaker A. Meguid
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