AN APPROACH TO DAMAGE DETECTION IN A BEAM LIKE STRUCTURES FROM EXPERIMENTALLY MEASURED FRF DATA

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1 A APPROACH TO DAMAGE DETECTIO I A BEAM LIKE STRUCTURES FROM EXPERIMETALLY MEASURED FRF DATA Valentina GOLUBOVIĆ-BUGARSKI, Drago BLAGOJEVIĆ 2, 2 University of Bana Luka, Faculty of Mechanical Engineering, BOSIA & HERZEGOVIA ABSTRACT: The metho base on amage etection an relative quantification inicator is investigate in this paper, in orer to etect, locate an quantify the amage of the beam like structure. This metho uses frequency response functions as the characteristics of ynamics response of the mechanical system, from which the amage etection an relative quantification inicators are calculate. The goal of investigation is to etermine effectiveness of the metho base on amage etection an relative quantification inicator to etect, locate an quantify amage of the beam, from the stanpoint of inepenency of the metho to the previously built FEM or analytical moel an for ifferent bounary conitions. KEY WORDS: Damage etection, experimental FRF ata, moal testing.. ITRODUCTIO Structural amage can be efine as changes introuce into a system that aversely affect the current or future performance of that system. If one is focuse on the stuy of amage ientification in structural an mechanical systems, than amage may also be 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. Therefore, structural amages shoul always, at a sufficient level of severity, cause a change in a structure s vibration behaviour, escribe by moal properties: natural frequencies, amping loss factor an moe shapes. Since the changes on 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 amage. It is obvious that concept of amage etection is base on comparison between two ifferent states of the system, one of which is assume to represent the initial an often unamage state. The structural amage etection can be ivie into four levels, base on the amount of information provie regaring the amage state. Accoring to Rytter [7] these four levels are: ) ientification of amage existence in a structure; 2) ientification of amage existence in a structure an location of amage; 3) ientification of amage existence in a structure, location of amage an quantification of amage severity; 4) ientification of amage existence in a structure, location of amage an quantification of amage severity an preiction of the remaining service life of the structure. The interest in the ability to monitor a structure an etect amage at the earliest possible stage is pervasive throughout the civil, mechanical an aerospace engineering communities uring last three ecaes. At present, using non-estructive techniques to etect amage status of engineering structures is put into a focus of engineering interests. on-estructive techniques are the means by which structures may be inspecte without isruption or impairment of serviceability. Generally, structural amage etection methos can be classifie as local amage etection methos an global amage etection methos []. Local amage etection techniques refer to non-estructive testing such as acoustic an ultrasonic methos, magnetic fiel methos, raiography, ey-current methos, etc., is mainly use to etect local amage in structure. All these methos require that the vicinity of the amage is known an that the portion of the structure being inspecte is accessible, an can etect amage on or near the surface of the structure. Local amage etection methos utilise only ata obtaine from the amage structure. Baseline ata an theoretical moels of the unamage structure are not use. These are the main avantages of local amage etection, which is very effective for small an regular structures, such as pressure vessels. 296 Tome VIII (year 200), Fascicule 3, (ISS )

2 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) However, for the large an complex structures, for the structures in invisible or close environment, it is very ifficult to etect amage using local amage etection methos, because local amage etection methoology can only be use to inspect some special an accessible components of a structure. In orer to etect amage throughout the whole structure, especially some large, complicate structure, a methoology calle global vibration base structural amage etection has been evelope. Its basic principle is explaine previously, an in a nutshell, when some amage emerges in the structure then structural parameters will change an frequency response functions (FRF) an moal parameters of the structural system will also change. So, the change of the structural FRF s an moal parameters can be taken as the signal of early amage occurrence in the structural system. Such an approach has been introuce for several years in fiels like automotive, aeronautical, mechanical an civil engineering. 2. THE BRIEF SURVEY OF VIBRATIO-BASED DAMAGE DETECTIO APPROACH Structural amage etection using measure vibration ata has been a subect of much practical interests an research efforts an receive consierable attention in the literature. First very etaile survey of the technical literature to etermine the state of the art of the amage-etection fiel using moal analysis proceures was presente by Richarson [6]. The survey focuse on structural integrity monitoring for nuclear power plants, large structures, rotating machinery, an offshore platforms, with by far the largest amount of literature associate with rotating machinery. The author state that while monitoring of overall vibration levels for rotating machinery ha become commonplace, attempts at relating structural amage to measure moal changes was still in its primitive stages. ext survey of the structural health monitoring stuies that have appeare in the technical literature until 996 was presente by Doebling, et al []. That report first categorizes the methos accoring to require measure ata an analysis technique. The analysis categories inclue changes in moal frequencies, changes in measure moe shapes (an their erivatives), an changes in measure flexibility coefficients. Methos that use property (stiffness, mass, amping) matrix upating, etection of nonlinear response, an amage etection via neural networks are also summarize. The applications of the various methos to ifferent types of engineering problems are categorize by type of structure an are summarize. The types of structures inclue beams, trusses, plates, shells, briges, offshore platforms, other large civil structures, aerospace structures, an composite structures. The report escribes the evelopment of the amage-ientification methos an applications an summarizes the current state-of-the-art of the technology. Sohn, et al [9] presents etaile report which is an upate version of the previous literature review report by Doebling, et al. That report contains new technical evelopments publishe between 996. an 200. in the iscipline of structural health monitoring. The authors have organize reviewe articles following the statistical pattern recognition paraigm reporte in [2]. This paraigm can be escribe as a four-part process: () Operational Evaluation, (2) Data Acquisition, Fusion, an Cleansing, (3) Feature Extraction an Information Conensation, an (4) Statistical Moel Development for Feature Discrimination. The reviewe articles are then categorize by type of applications, which inclue beams, truss, plates, briges, aerospace structures, an composite structures. Yan, et al. [] presents general summary an review of state-of-the-art an evelopment of vibration-base structural amage etection methos base on structural ynamics characteristic parameters. They ivie vibration-base structural amage etection methos into traitional-an moern type. The traitional type refers to etection methos for structural amage only utilizing itself mechanical characteristic of structures, such as natural frequencies, moal amping, moal strain energy or moal shapes, etc. This kin of metho generally requires experimental moal analysis or transfer function measure, an the authors fin it not convenient for online etection of structures in service, because these experimental measures often nee multifarious instrument or manual operation. The moern type refers to etection methos for structural amage base on online measure response signal of structures in service. Among the moern type methos for structural amage etection, the representative ones inclue Wavelet analysis, Genetic algorithm an Artificial eural etwork, etc. The cite authors pointe out the main topics for the future research in amage etection base on measure vibration response of a structure: Minimizing the epenence on prior analytical moels an/or prior test ata for the etection an location of amage. Developing of methos that have ability to account for the effects of nonlinear structural response. copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA 297

3 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) Optimization of the position an number of measurement sensors. Construction an extraction of feature inex for small structural amage from structural vibration response. Ability of iscrimination of changes in the moal properties resulting from amage from those resulting from variations in the measurements (resulting from changing environmental an/or test conitions an from the repeatability of the test) Reucing the epenence upon measurable excitation forces, using vibrations inuce by ambient environmental or operating loas for the assessment of structural integrity. Comparison of ifferent amage etection methos by application to a common set of ata. Integration of local nonestructive inspection an global vibration testing. Damage prognosis which estimates the remaining service life of a structure. 3. CLASIFICATIO OF DAMAGE IDETIFICATIO METHODS In orer to etect structural amage from structural ynamic response signal, the first problem is to select amage feature inex to be constructe. The physical variable use to ientify amage may be a global one, but the physical variable use to etermine amage location is better to be local one an must be sensitive to structural local-amage. Determination of structural amage location is equivalent to etermining a region where structural stiffness an loaing capacity ecreases using a measurable quantity. The key factor of vibration base amage etection is to establish the calculation moel an to estimate the vibration parameter to be measure. Doebling [] an Farrar [2] categorize amage ientification methos accoring to: the type of measure ata (feature) use the techniques use to ientify the amage from the measure ata. Common features use in vibration base amage etection stuies are: ) moal frequencies, 2) frequency response functions, 3) moe shapes 4) moe shape curvatures 5) moal strain energy 6) ynamic flexibility, etc. The techniques use to ientify the amage from the measure ata can be classifie as: ) methos base on frequency changes (forwar or inverse problem methos) 2) methos base on moe shape changes 4) methos base on moe shape curvature changes 3) methos base on ynamically measure flexibility: comparison of flexibility changes, stiffness error matrix metho, effects of resiual flexibility, changes in measure stiffness matrix, 4) matrix upate methos, 5) neural-network base methos, 6) time-history an spectral pattern methos, 7) nonlinear methos, 8) statistical pattern recognition methos, etc. He [4] classifies amage etection methos as: Damage etection using experimental ata only. Damage etection without a reaily available spatial moel of the unamage structure is an approach eparte completely from moel upating path. Usually, the ata available are the experimental ata before an after amage occurre. As a result, we are ealing with two sets of moal or FRF ata. The comparison of these two sets shoul yiel the information about the existence an location of amage. The main question is how to relate the ifferences between moal an FRF ata before an after amage to the spatial stiffness changes that resulte in the ifferences. Damage etection using moal ata an analytical ata. Damage etection using moal ata is an approach that was largely aopte from moel upating. Its algorithm aims to etermine amage by using the moal ata from a amage structure an an analytical moel for its counterpart. Although many moel upating methos utilizing moal ata can be aopte irectly for amage etection, metho base on moal force vector is more convenient an efficient. Damage etection using FRF ata. Using measure FRF ata for amage etection has many avantages over the traitional methos using moal analysis ata: ) any numerical errors inherent in moal analysis results cause by inaccurate curve fitting an unavailable resiual terms are avoie; 2) no more efforts is neee to process FRF ata in orer to erive moal ata; 3) the most significant avantages of using measure FRF ata over erive moal analysis ata lies in the fact that FRF ata provie abunant information on the ynamic behavior of a structure. Moal analysis ata lose much of the information that FRF ata have, ue to the necessary numerical process to extract them. 4. OE APPROACH TO DAMAGE DETECTIO USIG FRF DATA Sampaio an Maia [8] present some new evelopment of the Detection an Relative amage Quantification inicator, concerning the etection, localization an the relative severity of amage. 298 copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA

4 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) This metho belongs to the class of methos that use the change in the frequency response functions to etect, locate an relatively quantify the amage. The main avantages of the metho are: ) it is not necessary to perform 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 frequency response functions, without further treatment. Theoretical escription: The equation of motion of a multiple-egree of freeom system with hysteretical amping, which is often use in escribing of complex structure s ynamics [5], is: [ M ]{ x& () t } + i[ D ]{ x ( t )} + [ K ]{ x ( t )} { f ( t )} () If the excitation is harmonic, the realtion between the response an the excitation at each frequency of the analysis is given by: { X} [ α ]{ F} (2) where 2 [ α ( ω )] ([ K + id ] ω [ M ]) (3) is the system receptance matrix, containing all the inforamtion about the ynamic characteristic of the system. Each element α k (ω) of the matrix correspons to an iniviual FRF escribing the relation between the response at a particular coorinate an a single force excitation applie at coorinate k: X α k (ω) F i 0, i... ; i k (4) Fk The column vector, k, of the receptance matrix, { α k }, escribes the shape (in space) exhibite by the structure at each excitation frequency ω, given by the responses normalize by the applie forces. When a structure is amage its stiffness an amping change an, in consequence, so oes the receptance matrix: 2 [ α ( ω )] ([ K + i D ] ω [ M ]) (5) where the superscript stans for amage. It is reasonable to assume that the smaller the egree of correlation between the column vectors,{ α k (ω)} an { α k (ω)}, the larger the amage. To measure the egree of correlation between two vectors, R. Pascual et al. [5,6] propose the Frequency Domain Assurance Criterium (FDAC): where FDAC 2 α k ( ω2 ) α k ( ω ) ( ) k _ ω, ω2 (6) [ α k ( ω2 ) α k ( ω2 )] α k ( ω ) α k ( ω ) [ ] represents the conugate operator an is the total number of co-orinates or measuring points. W. Heylen et al. [7] efine a simplifie form of FDAC, known as RVAC or Response Vector Assurance Criterion (RVAC) (see Figure ), with only one applie force (so that the receptance matrix turns to be ust a vector): ( ω ) α ( ω ) α RVAC _ ( ω ) (7) [ α ( ω ) α ( ω )] α ( ω ) α ( ω ) [ ] The Detection an Relative amage Quantification inicator is formulate as [8]: RVAC ω DRQ (8) ω ω where is the number of frequencies an, so, DRQ will vary between 0 an. 2 copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA 299

5 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) The proceure escribe previously is the strategy for amage etection that is level accoring to Rytter s classification of the structural amage etection. The similar proceure can be further applie to the localization stage, that is level 2 accoring to Rytter. A comparison between shape vectors can be taken as before, but now taking consecutive pairs of coorinates (p an p+) along the structure, instea of oing it for all the coorinates simultaneously (whole structure). The calculation is entirely similar an the local RVAC is efine as Figure. Graphical presentation of the RVAC [8] RVAC_ ( p, ) p + α p p + p ( ω ) α ( ω ) p + [ α ( ω ) α ( ω )] α ( ω ) α ( ω ) ω (9) p 2 [ ] For the sake of comparison with the propose metho, one shall also compute the Generalize Damage Inex, in terms of shape vectors, efine as: β i α i + α i α i ( i i α + i α i α i i i The evelope inicators can also give information about the relative extent of the amage, i.e., it can be use as relative amage quantifiers, which satisfy level 3 of structural amage etection accoring to Rytter. ω) 5. EXPERIMETAL DAMAGE DETECTIO O THE BEAM LIKE STRUCTURE Sampaio an Maia state in [8] that there is no nee for any analytical or numerical moel for structural amage etection base on the Detection an Relative Damage Quantification inicator. They showe numerical performance of the metho an its experimental application on the example of the free-free supporte beam, exciting it by shaker an collecting responses with 2 uniformly istribute accelerometers along the beam. (0) Figure 2. Two bounary conitions: the freely supporte beam an the cantilever beam However, it was intereste to investigate how propose metho operates in the case of impulse excitation an the limite number of accelerometers on isposal. Thus, we ecie to experimentally investigate performance of the propose metho applying it to the freely supporte beam an cantilever beam, Figure 2. Moal testing was performe by means of hammer excitation, using two 300 copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA

6 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) ifferent testing methos: roving hammer metho an roving accelerometers metho. It was necessary to collect FRFs for chosen number of DOFs from the unamage beam an from the amage beam for a few levels of amage. The amage was simulate introucing a cut at a certain location of the beam, an the amage was propagate by eepening the cut for mm at every level of amage. 5.. An equipment use for moal testing Measurement ata were collecte using the Multi-channel Data Acquisition Unit Portable Pulse, Bruel&Kaer type 3560 C, an analyze in the Pulse LabShop 9.0 software, in the frequency range of Hz, Figure 3. An impact hammer Enevco type generates excitation, while the responses were capture by moal accelerometer, B&K type 4507, attache to the structure. Both signals were weighte by some winow functions: the excitation signal by transient winow function an response signal by exponential winow function. Frequency resolution was chosen to be Hz, an the number of averaging was 5 per DOFs. Figure 3. Equipment use for moal testing 5.2. Damage etection of the freely supporte beam The steel beam of imensions mm was use for experimental investigation. The crack of mm with was introuce by wire-cut. The beam was suspene with common strings to simulate free-free conitions. Two test were one with two ifferent moal testing proceure: test : so calle roving hammer test with fixe response measurement position test 2: so calle roving accelerometer test with fixe hammer excitation position. In the roving hammer test the accelerometer was attache to noe 5 to capture the vibration response signals, while the impact hammer generates excitation on the each of 7 noes uniformly arrange along the beam, Figure 4. In the roving accelerometer test the excitation position was fixe at no 8, but the accelerometer was move along the 7 DOF of beam to capture the vibration response signals, Figure 5. To avoi the loaing mass effect ue to local ae transucer mass, 6 ummy transucers were use uring test, Figure 6a an 6b. The ummies were systematically replace by the real transucer for the measurement, until all of the measurement locations have been covere. Thus the systematic error incurre as a result of moving the transucer aroun on the structure uring test was turne into single error by the use of ummy transucer. The beam was cut with 7 epths in the mile of location 4 an 5, Table. As one can see from table the beam was measure in 0 conitions: the unamage (or reference), two unamage but ifferent from reference state, an 7 levels of amage at certain location. Table. Levels of amage of the free-free beam Level of amage Depth of cut [mm] Figure 4. Location of amage an response measurement location for the roving hammer test Figure 5. Location of amage an excitation position for the roving accelerometer test copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA 30

7 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) Figure 6. Dummy transucers use in the roving accelerometer test an the free-free beam equippe by ummy transucers [(m/s²)/] Frequency Response H(ogovor,pobua) - File (Magnitue) After 0 moal tests of the free-free beam were one, all FRFs measure at location 5 were OVERLOAD D:\Valentina\DOKTORAT\G\G bo\frequency Response H(ogovor,pobua) - Mark overlappe, 0k Figure 7. It is obvious from Figure 7, that there is some frequency shift ue to increasing of the amage, that is frequencies move to the left (ecreasing) ue to of ecreasing of the stiffness of the k beam (when level of amage increasing). Moal frequencies for 0 levels of amage an two moal tests are 00 shown in Table 2. a k.6k 2k 2.4k 2.8k 3.2k [Hz] [B/.00 (m/s²)/] Frequency Response H(ogovor 2,pobua) - File7 (Magnitue) Valentina\DOKTORAT\G OVERLOAD va acc\g_5a va acc\frequency Response H(ogovor 2,pobu b k.6k 2k 2.4k 2.8k 3.2k [Hz] Figure 7. Overlappe FRFs measure for 0 levels of amage: a. roving hammer test, b. roving accelerometer test Test Test 2 Table 2. Values of moal frequencies for unamage an amage beam Moal frequencies (Hz) Level of amage "" f f f f f f G - inikator DRQ Test :DRQ G inikator DRQ Test 2:DRQ DRQ DRQ Level ivo of amage oštećena Level ivo of oštećena amage Figure 8. Detection an quantification of the amage using the DRQ inicator for two moal tests After calculation the Response Vector Assurance Criterion (eq.7.) an the Detection an Relative amage Quantification inicator (eq.8.), results are graphically interprete as follow on Figure 8. It is obvious that DRQ inicator shows ecreasing tren as increasing the level of amage, for both moal tests. However, roving accelerometer test shows better tenency of ecreasing the DRQ inicator 302 copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA

8 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) value as the level of amage increasing. Generally, it can be state that the DRQ inicator is able to etect an relatively quantify amage, which satisfy the level an the level 3 of amage etection accoring to Rytter. For the purpose to locate amage, much better results showe the General Damage Inex - GDI (eq. 0) than the local RVAC inicator (eq.9). To calculate GDI, some correction has been one to the inex efine with the equation as follows: α ( ) ( ) ( ) ( ) f + α f α ( ) ( f ) ( ) β p, f () α ( ) ( ) + ( ) ( ) ( ) f ( f ) α However, GDI efine from equation was still not enough sensitive to a low level of amage, so that the new inex, name cumulative GDI was propose [3]. The Cumulative GDI was calculate by successive aing the values of GDI for the each level of amage. Some measurement inaccuracies occurre on the certain locations uring testing coul be average, but it is suppose that GDI shoul increase continuously on the location on amage. Figure 9 shows cumulative GDI inicating the location of amage between measurement location 4 an 5, for both moal tests. a.05 G Inikator GDI - Cumulative suma sukcesivnih GDI nivoa oštećena 5 4 b Cumulative G inikator GDI_4 4 5 GDI GDI Location Lokacia (noe (bro čvora) number) Location Lokacia (noe (bro čvora) number) Figure 9. Location of amage element using the GDI inicator: a) roving hammer test, b) roving accelerometer test 5.3. Damage etection of the cantilever beam The steel beam of imensions mm was use for experimental investigation. The beam was clampe at one en, forming bracket of 300 mm length, Figure 0. An impact hammer generates excitation on the each of 4 noes uniformly arrange along the beam. An accelerometer was attache to noe to capture the vibration response signals. The amage was simulate by cut of 0.5 mm with introuce the mile of location 4 an 5, with 5 level of epths, Table 2. As one can see from Table 3 the cantilever beam was measure in 7 conitions: the unamage (or reference), one unamage but ifferent from reference state, an 5 levels of amage at certain location Figure 0. Location of amage an response measurement location of the cantilever beam Table 3. Levels of amage of cantilever beam Level of amage Depth of cut [mm] After 7 moal tests of the cantilever beam were one, all FRFs measure at location were overlappe, Figure. It is obvious from figure, that there is some frequency shift ue to increasing of the amage, that is frequencies move to the left (ecreasing) ue to of ecreasing of the stiffness of the beam (when level of amage increasing). Moal frequencies for 7 levels of amage are shown in Table 4. copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA 303

9 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) Table 4. Values of moal frequencie for unamage an amage beam Moal frequencies (Hz) Level of amage "" Moe: f Moe 2: f Moe 3: f Moe 4: f [(m/s²)/] Frequency Response H(ogovor,pobua) -50% (Magnitue) OVERLOAD D:\Valentina\DOKTORAT\U k3\k3 50%\Frequency Response H(ogovor,pobua) - Mar k m k.6k 2k 2.4k 2.8k 3.2k [Hz] Figure. Overlappe FRFs measure for 7 levels of amage DRQ GDI K3 inikator DRQ DRQ Damage ivo level oštećena Figure 2. Detection an quantification of amage using the DRQ inicator K3 suma sukcesivnih GDI nivoa oštećena Location (noe Lokacia number) Figure 3. Location of amage element using GDI inicator After calculation the Detection an Relative amage Quantification inicator, results are graphically interprete as follow on Figure 2. It is obvious that DRQ inicator shows ecreasing tren with increasing the level of amage. Therefore, the DRQ inicator is able to etect an relatively quantify amage. As previously mentione for the freely supporte beam, to locate the amage much better results showe the General Damage Inex (GDI) than the local RVAC inicator. The Cumulative GDI was calculate by successive aing the values of GDI for the each level of amage. Figure 3 shows cumulative GDI inicating the location of amage between measurement location 4 an 5 of cantilever beam. 6. COCLUSIOS This paper presents one approach in amage etection using frequency response functions ata. It is point out that using measure FRF ata for amage etection has many avantages over the traitional methos using moal analysis ata, especially that FRF ata provie abunant information on the ynamic behavior of a structure. By reviewing accomplishment in amage etection methos base on vibration responses, one of the observe tasks for future evelopment an avancing of amage etection methos is minimizing the epenence on prior analytical moels an/or prior test ata for the etection an location of amage. So, the metho base on Detection an Relative amage Quantification inicator is investigate an approve. Experimental investigation was conucte on the free-free beam an the cantilever beam, using hammer excitation an roving hammer metho of moal testing. The results of experiments show that DRQ inicator is able to etect an relatively quantify the amage that is to recognize the pattern of amage variation. It was foun that Generalize Damage Inex gave better location of amage than local RVAC inicator. As certain measurement inaccuracies happene on the certain locations uring testing it coul be average by successive aing the values of GDI for the each level of amage. However, it is suppose that GDI shoul increase continuously on the location on amage. So, some improvement of the GDI inicator is propose that is the Cumulative GDI. Propose amage etection metho showe 304 copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA

10 AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG. Tome VIII (Year 200). Fascicule 3 (ISS ) goo performance even for the hammer excitation an one response transucer available, which is important consiering the practical implementation of the metho in the frugally equippe laboratories. REFERECES [.] Doebling S.W., Farrar C.R., Prime M.B., Shevitz DW., Damage Ientification an Health Monitoring of Structural an Mechanical Systems from Changes in their Vibration Characteristics: A Literature Review, Los Alamos ational Laboratory report LA-3070-MS.,996. [2.] Farrar C.R., Doebling S.W., Vibration-Base Damage Detection, Los Alamos ational Laboratory Report, 999. [3.] Golubović-Bugarski V., Moels of correlation between structural amages an ynamics response of mechanical structure, Ph.D.Dissertation, University of Bana Luka, B&H, 200. [4.] He J., Damage Detection an Evaluation I, Moal Analysis an Testing, ATO Science Series, 999. [5.] Maia,.M.M, Silva, J.M.M. et al., Theoretical an Experimental moal Analysis, Research stuies Press Lt., 997. [6.] Richarson M.H., Detection of Damage in Structures from Changes in their Dynamic (Moal) Properties- A survey, UREG/CR-43, U.S. uclear Regulatory Commission, Washington, D.C., 980. [7.] Rytter A., Vibration base inspection of civil engineering structures, Ph.D.Dissertation, Department of Builing Technology an Structural Engineering, Aalborg University, Denmark, 993. [8.] Sampaio R.P.C., Maia.M.M., Strategies for an efficient inicator of structural amage, Mechanical system an Signal processing, Vol.22, [9.] Sohn H., Farrar C.R., A Review of Structural Health Monitoring Literature: , Los Alamos ational Laboratory report LA-3976-MS, [0.] Farrar C.R., Doebling S.W., Damage Detection an Evaluation II, Moal Analysis an Testing, ATO Science Series, 999. [.] Yan Y.J., Cheng L., Wu Y.Y., Yam L.H., Development in vibration-base structural amage etection technique, Mechanical system an signal processing 2, , AALS OF FACULTY EGIEERIG HUEDOARA ITERATIOAL JOURAL OF EGIEERIG copyright University Politehnica Timisoara, Faculty of Engineering Huneoara, 5, Revolutiei, 3328, Huneoara, ROMAIA copyright FACULTY of EGIEERIG - HUEDOARA, ROMAIA 305