Geometric time-domain methods of vibration-based damage detection

GeometricTimeDomainMetho dsofVibrationBasedDamageDetectionStephenTrickeya,Michael ToddMarkSeaverandJon NicholsbaCo de5673, U.S.Naval ResearchLab oratory,Washington, DC20375bDepartmentofMechanicalEngineering andMaterial Science,DukeUniversity,Durham NC27708ABSTRACTWepresentanewmetho dologyforvibrationbaseddamagedetectionderivedfromthecharacterizationofchanges inthegeometricprop ertiesoftimedomainresp onse ofastructure.Manynewfeaturespresentthemselves when the geometry of attracting ob jects in phase space are considered.The most promising avenueof study are metrics that describ e changes to the attractor shap e or dimension.In particular, the utilityof afeature consisting of the ratio of average lo cal variance (or spatial disp ersion) of the input to the aerage lo calvariance of the resp onse is assessed.Presenting the results of the geometric time domain metho d in a statisticalframeworkhighlightsthemetho d'sincreasedsensitivitytosubtledamage-inictedchangesstructurewhencomparedtomoretraditionalmo dalbasedmetho ds.Inadditionthegeometricmetho ddemonstratesamorerobusthandlingofchangesduetoambientenvironmentaluctuation.Resultsarepresentedfrom niteelementmo delofa thinplate withweldlinedamage implementedthrougha relaxation of ab oundarycondition.Keywords:vibration based damage detection, time domain, chaotic attractor prop erty analysis1. INTRODUCTIONThe research area of structural health monitoring is rapidly maturing.New metho ds and techniques continue tob e prop osed.The essence of these structural health monitoring techniques lie in the damage detection problemwhich addresses ve basic questions.1) Is the system damaged?2) Where is the damage lo cated?3) What isthetyp e ofdamage that has b eendetected?4)Howsevere isthe damage?5)Whatisremaining usefullife of the structure?As one progresses through the hierarchy of these questions, it b ecomes increasingly morediculttoformulate ananswer.Itislikely thatnosingle metho dwillbeableaddress allofthedamagedetectionquestions.In fact a given technique mayonlyaddress one or two of the ve.Thus it is reasonableto assume that to correctly answer the set of damage detectionquestions, a suiteof metho ds will need to b eapplied.One class of metho ds under consideration for inclusioninsome futureset of damage detectionto ols havefollowed as a continuation of the system identi cation problem where a (typically linear) mo del is t in somesense to observed system resp onse data.Mo dal prop erties such as resonant frequencies, mo de shap es, damping,exibilityandstrainenergyfallintothiscategory.1, 2ARMA/ARMAX3, 4subspaceidenti cation5{7metho ds are additional techniques to construct mo dels from time history data.Another approachis to developa high delity large order nite element mo del of a system and then iterate using a mo del up dating approachuntil a desired level of agreementis achieved b etween the physical system and the mo del.Adapting system identi cation techniques for damage detection simply involves an additional system iden-ti cation for each p ossibly damaged structure.Newly identi ed mo dels that di er to a signi cant degree fromthe original mo del are considered damaged.The problem that many of these techniques have with regard tosuccessfully detecting damage lies in the fact that in order for them to succeed the mo del must b e able to captureuniquely the characteristics of the damage mechanism while at the same time account for or b e insensitivetoadditional variations that may o ccur due to uctuations in ambient conditions.