DEVELOPMENT OF DamageCALC APPLICATION FOR AUTOMATIC CALCULATION OF THE DAMAGE INDICATOR

Transcription

1 Mechanical Testing an Diagnosis ISSN , 2012 (II), Volume 4, DEVELOPMENT OF DamageCALC APPLICATION FOR AUTOMATIC CALCULATION OF THE DAMAGE INDICATOR Valentina GOLUBOVIĆ-BUGARSKI, Branislav SREDANOVIĆ, Gorana GLOBOČKI-LAKIĆ University of Banja Luka, BOSNIA AND HERZEGOVINA ABSTRACT This paper presents the evelopment of DamageCALC application for automatic calculation of Damage Detection an Relative Quantification inicator - DRQ. DamageCALC is evelope using Visual Basic 6 an enables an automatic graphical interpretation of the amage inicator. The metho base on DRQ inicator uses the frequency response functions as the characteristics of the ynamic response of the mechanical system. The DamageCALC automatically reas the measurement results, previously obtaine from the moal testing of the beam structure, calculates the value of the corresponing coefficients an forms a chart for the graphical interpretation of the amage inicator values. Following the tren of the calculate DRQ inicators for several successive measurements of the structure, one can etermine the existence of a amage of the structure an its propagation. Keywors: amage inicator, FRF (frequency response function), moal testing, automatic calculation 1. INTRODUCTION In the most general terms, the structural amage can be efine as changes introuce into a system that aversely affect the current or future performance of that system. Implicitly, in this efinition, it is the concept that the amage is not meaningful without a comparison between two ifferent states of the system, one of which is assume to represent the initial an often unamage state [1]. The amage may be also efine as any eviation in the structure s original geometric or material properties that may cause unesirable stresses, isplacements or vibrations on the structure. These weaknesses an eviations may be ue to cracks, loose bolts, broken wels, corrosion, fatigue, etc. All of them shoul cause a ecrease in the structure s stiffness an some will also affect its mass an amping properties.

2 29 Therefore, at a sufficient level of severity, the structural amages shoul always cause a change in a structure s vibration behavior, escribe by the moal properties: natural frequencies, amping loss factor an moe shapes. Since the changes of the ynamic characteristics can be measure an stuie, it is possible to trace what structural changes have cause the ynamic characteristic to change, thus, ientifying the amage [2]. Sampaio an Maia [3] present some new evelopment of the Detection an Relative amage Quantification (DRQ) inicator metho, concerning the etection, the localization an the relative severity of the amage. This metho belongs to the class of methos using the change in the frequency response functions to etect, locate an relatively quantify the amage. The main avantages of the metho are: 1) it is not necessary to perform a moal ientification; 2) there is no nee for any analytical or numerical moel of the structure; 3) it uses all measure ata in the form of the frequency response functions, without further treatment. This metho is also suitable for an automatic calculation. Therefore, the DamageCALC application for an automatic calculation of DRQ inicator an the automatic graphical interpretation of the amage inicator was evelope using Visual Basic THEORETICAL DESCRIPTION OF DAMAGE DETECTION METHOD The Response Vector Assurance Criterion (RVAC) is efine in paper [4] as: RVAC 2 N i i i1 N N i i i i i1 i1 where ij () is the element of the system receptance matrix an correspons to an iniviual frequency response function, FRF. For only one applie force, the receptance matrix turns to be just a vector, so i () is a single FRF for i-th co-orinate or measuring point, an N is the total number of measuring points. The element i () correspons to the unamage structure, while the superscript stans for amage structure. The Detection an Relative Damage Quantification inicator is formulate in paper [3] as: where DRQ RVAC ( ) N N is the number of frequencies an, thus, DRQ varies between 0 an 1. The DRQ inicator is able to etect an relatively quantify the amage if the pattern of amage variation is recognize. 3. EXPERIMENTAL INVESTIGATION In the aim of obtaining the experimental FRFs of the unamage an amage structure, the steel beam having the imensions of 400 mm 10 mm 10 mm was moally teste [5]. A crack of 0.5 mm with was introuce by wire-cut. The beam was suspene with common strings to simulate free-free conitions (Fig. 1). An impact hammer (Enevco type 2230) generate the excitation on each of the 17 noes uniformly arrange along the beam. An accelerometer (B&K type 4507) was attache to noe 5 to capture the vibration signals. The signals were fe into Multi-channel Data Acquisition Unit Portable PULSE (B&K type 3560 C) an analyze in Labshop 9.0 (1) (2)

3 30 Pulse software, in the frequency range of Hz. The moal test was repeate for eight level of amage: =1, 2, 3, corresponing to unamage beam, =4, 5, 6, 7, 8, corresponing to a crack with the epth of 15 [mm]. The first three natural frequencies for the ifferent amage levels are given in Table 1. Figure 2 shows the overlai FRFs measure at the 5th measurement location for 8 amage levels. There is some frequency shift ue to the increase of the amage that is frequencies move to the left (ecrease) ue to the ecrease of the stiffness of the beam (when amage is increasing). After the calculation of the Response Vector Assurance Criterion (eq. 1) an the Detection an Relative amage Quantification inicator (eq. 2), the results are graphically interprete as follow in Fig. 3. It is obvious that DRQ inicator shows a ecreasing tren as the level of amage is increasing. Fig. 1. Freely suspene beam an the equipment use for moal testing Table 1. Moal frequencies for eight amage levels Moal frequencies (Hz) Level of amage "" f f f

4 31 [(m/s²)/n] Frequency Response H1(ogovor,pobua) - File (Magnitue) OVERLOAD D:\Valentina\DOKTORAT\G1\G1 bo1\frequency Response H1(ogovor,pobua) - Mark 10k 1k k 1.6k 2k 2.4k 2.8k 3.2k [Hz] Fig. 2. Overlappe FRFs G1 Test inikator 2:DRQ DRQ Level of amage Nivo oštećenja Fig. 3. DRQ recognizes the pattern of amage variation 4. DEVELOPMENT OF DamageCALC APPLICATION Structural health monitoring proceures base on the Detection an Relative amage Quantification (DRQ) inicator metho can be one using some automate proceure through the following steps: 1. Measure the FRF at a certain number of locations on the unamage structure by moal testing. 2. Loa FRFs as a text.file into the Damage CALC application. 3. Extract the FRF values for the selecte frequencies to calculate the RVAC an DRQ inicators. 4. Start the calculation of RVAC an DRQ inicators. 5. Graphically isplay the calculate value of the amage inicators. 6. Repeat the entire proceure for each measurement to assess the structural health. The Detection an Relative amage Quantification inicator metho is suitable for automatic calculation. The DamageCALC application for the automatic calculation of DRQ inicators an automatic graphical interpretation of results of calculation is evelope using Visual Basic 6 [6].

5 32 The application automatically reas the measurement results (FRFs) previously obtaine by moal testing of the beam structure. After reaing the results of measurements, the application calculates the values of the RVAC coefficients for the selecte frequency an calculates the DRQ amage inicator for each measurement. Finally, the application esigns the graph for a visual interpretation of the measurement results. Fig. 4. The evelope DamageCALC application For the evelopment of these applications, the stanar objects of Visual Basic6th are use. The rop-own menus for the frequency selection, a text fiels for input an correction of ata an objects for writing an selecting ata are place at the basic form of the application. The program coe of application an objects on the basic form enables the calculation for the ten levels of amage of the structure, for the three measuring points an for three natural frequencies. It is also possible to memorize the calculate ata an to raw iagrams on the basis of the previously memorize information from the amage structure. The frequency response functions measure in the experiment, as input ata for the application, nee to be preserve in the form of text ocuments with.txt extension. The program coe of the application fins path to the appropriate text ocuments an loas them into a working memory of a computer. Develope application DamageCALC is shown in Fig. 4. The application consists of several moules: 1. input frequency moule, 2. moule for the selection of ocuments with information on the measurement of the unamage structure,on about the measurement of the amage structure, 3. moule with the comman buttons for the ata calculation an memorizing, 4. moule for printing the results. The moule (iagram) for the graphical shows the results on a isplay.

6 Input Frequency Moule For one measurement, the FRF measure location contains 3200 ata that are iscrete values of the measure FRF in the range of Hz, with a resolution of 1 Hz (Fig. 5). For the calculation of RVAC coefficients only three certain frequencies, between 3200 values, are require. Therefore, the application must choose frequencies for which the measure FRF values are rea. This is one from the rop-own menu, with pre-selecting any ocument that contains the measure ata (Fig. 6), an the values of selecte frequency in [Hz] are automatically printe in the text fiels. Fig. 5. Text ocument with FRF ata Fig. 6. Drop-own menu for the frequency selection

7 Moules for Selectingof the Documents with Information on the Measurement of the Unamage an Damage Structure After selecting the frequency, the ocuments with FRFs ata measure on the unamage structure in three measurement locations are selecte. The path to the selecteocuments is automatically printe by pressing the button "Loapaths". Base on the printe path, the ata from a given ocument are liste by pressing the button "LoaData". Loaing can be one only if each of the three ocuments with the ata of measurements in three locations is selecte. Especially, a separate moule for the selection of ocuments containing measurement information from the unamage structure an a moule for the selection of ocuments containing measurement informing on the amage structures in the three points are separately given Fig. 7. Fig. 7. Fiels for selecting the ocuments containing the measurement ata Fig. 8.The comman moule of DamageCALC

8 Moule with the Comman Buttons for the ata Calculation an Memorization The comman part of the application consists of a few buttons, (Fig. 8). By pressing the button "Memorize the selecte ata", the application memorizes the ata that are rea from the liste ocuments, namely, those corresponing to the previously selecte frequency. After the ata are memorize, the coefficient RVAC for each of the three frequencies is calculate. DRQ inicator is calculate on the basis of the previously calculate RVAC values. The button "A DRQ" writes the calculate ata in the appropriate fiel Moules forprinting the Results an Graphical Display The calculate values of DQR inicator can be memorize in a separate ocument that can be re-loae into the application an use for a graphical analysis. The entire process of analysis for the next measurement (next level of amage) begins with pushing the button "new level of measurement", where currently memorize ata coul be elete an the user gets the possibility to select ata ocuments again. The output ata obtaine by calculation are printe in a separate moule, Fig. 9. Pressing the button "Diagram DRQ", the application shows the graph of the calculate value of the inicators DRQ. 5. CONCLUSION Fig. 9. Moule with output ata, graphically isplaye The DamageCALC application is evelope in a way that all operations are carrie out by a simple selection of ata an ocuments, which makes it easy to hanle. The application is simple for transferring an requires no installation. The ocuments containing the measurement ata are copie to the foler containing the application, or the application itself is copie into the foler with the corresponing ocuments. The structural health monitoring proceures base on the Detection an Relative Damage Quantification (DRQ) inicator metho can be one using Damage CALC application through the following steps: 1. Measure the FRF at three locations on the unamage structure by moal testing. 2. Loa FRFs as a text file into the Damage CALCapplication. 3. Extract the FRF values for the selecte frequencies to calculate the RVAC an DRQ inicators. 4. Start the calculation of RVAC an DRQ inicators. 5. Graphically isplay the calculate value of the amage inicators. 6. Repeat the entire proceure for each measurement to assess the structural health.

9 36 DRQ inicator is able to etect an relatively quantify the amage, if it recognize the pattern of amage variation. So, one can follow the tren of the change of DRQ inicator by graphically isplaying all the calculate DRQ for a series of measurements. REFERENCES 1. Farrar C.R., Doebling S.W., 1999, Damage etection an evaluation, Moal analysis an Testing, NATO Science series, pp Yan Y.J. et at., 2007, Development in vibration-base structural amage etection technique, Mechanical system an signal processing 21, pp Sampaio, R.P.C., Maia N.M.M., 2008, Strategies for an efficient inicator of structural amage, Mechanical System an Signal Processing, vol Heylen W., Lammens S., 1998, Moal Analysis Theory an Testing. K. U. Leuven-PMA, section A6. 5. Golubović-Bugarski V. et al., 2011, Detection of structural amage location using frequency response function ata, DEMI 2010 Conference, BanjaLuka, BiH, pp Golubović-Bugarski V., 2010, Moels of correlation between structural amages an ynamic response of mechanical system (Moeli korelacije strukturnih oštećenja sa inamičkim ogovorom mehaničkog sistema), PhD thesis, Faculty of mechanical Engineering, University of Banja Luka. 7. Maia N.M.M., Silva He J.M.M., Lieven N.A.J., Lin R.M., Skingle G.W., To W.-M., Urgueira A.P.V., 1997, Theoretical an experimental moal analysis, Research Stuies Press LTD, John-Wiley & Sons Inv. Ewins, D.J.. Ewins, D.