Vol.3, No 13, 2003 pp. 745-754
UDC 537.856  539.421
EVALUATION OF EDDY CURRENT SENSITIVITY
FOR THE INSPECTION OF CRACK PROPAGATION
UNDER A COMPOSITE PATCH
Zaira P. Marioli-Riga1, George J. Tsamasphyros2,
George N. Kanderakis2
1Hellenic Aerospace Industry Ltd., R&D Department, Shimatari, Greece, 32009
E-mail: zmarioli@haicorp.com
2National Technical Univ. of Athens, Dept. of Eng. Science, Section of Mech. Athens, Greece, 15773
The increasing demand for life extension of both military and civilian aircrafts leaded to significant advances in repair technology of cracked metallic structures. Thus, composite patch repair of metallic structures became a rapidly grown technology in the field of aerospace. The eddy current method is used to trace crack propagation under a composite patch repair of a cracked metallic structure, after mechanical testing in fatigue. The capability and the reliability of the eddy-current method to detect cracks under a composite obstacle of significant thickness are checked for several patch thicknesses. Notched specimens 6mm thick were fabricated using 2024-T3 Aluminum. Boron Epoxy patches bonded with film adhesive were applied to the one side of the metallic specimens. Initial notches were 10mm long, while the thickness of the reinforcement was varying from 2 layers (0.25mm) to 7 layers (0.875mm) in order to represent actual structural composite patch repairs. Crack propagation from the tip of the notches was achieved by fatigue loads. The estimation of required loads to cause fatigue crack propagation was done by means of three-dimensional finite elements analysis. The eddy current method was then applied to trace the crack tip under the patch after their mechanical testing. Accuracy of the eddy-current method was verified by measuring the crack lengths on both sides of the specimen and comparing the results. The eddy-current method was found to be fully capable of tracing the crack propagation under the composite patch, requiring only proper calibration for the generator. Small differences in the crack lengths between the patched and the unpatched side of the specimen were explained by their non-symmetric configuration, which induced different stress intensity factors at the patched and the unpatched sides, as finite elements analysis has clearly shown.
Key words:  Composite Patch Repair, Eddy-Current, Fatigue, Non Destructive Inspection, Composite Materials.
OCENA OSETLJIVOSTI NA VRTLOŽNU STRUJU
KOD NADGLEDANJA PROSTIRANJA PRSLINE
POD MEŠOVITOM ZAKRPOM
Rastuća potreba za produženjem trajanja kako vojnih, tako i civilnih vazduhoplova dovela je do značajnih pomaka u tehnologiji popravke naprslih metalnih struktura. Time je popravka metalnih struktura mešovitim krpljenjem postala tehnologija koja se naglo razvija u oblasti avioprostora. Metoda vrtložne struje koristi se za praćenje prostiranja prsline kod opravke mešovitim krpljenjem naprsle metalne strukture, nakon mehaničkog testiranja na zamor. Sposobnost i pouzdanost metode vrtložne struje u otkrivanju prsline pri mešovitoj smetnji značajne debljine provereni su za nekoliko različitih debljina zakrpe. Zarezani uzorci debljine 6mm proizvedeni su korišćenjem 2024-T3 aluminijuma. Borne Epoksi zakrpe spojene slojnim vezivnim sredstvom primenjene su na jednu stranu metalnih uzoraka. Početni zarezi bili su dugi 10mm, dok je debljina ojačanja varirala od 2 sloja (0,25mm) do 7 slojeva (0,875mm) da bi se prikazale stvarne  popravke modela mešovitim krpljenjem. Prostiranje prsline od vrha zareza ostvareno je opterećenjima do zamora. Procena opterećenja potrebnih da se izazove prostiranje prsline usled zamora uradjena je putem trodimenzionalne analize konačnih elemenata. Potom je metoda vrtložne struje primenjena da se prati vrh prsline ispod zakrpe nakon njihovog mehaničkog testiranja. Preciznost metode vrtložne struje potvrdjena je merenjem dužine prslina na obe strane uzorka i poredjenjem rezultata. Nadjeno je da je metoda vrtložne struje u potpunosti u stanju da prati prostiranje prsline pod mešovitom zakrpom, pri čemu je samo neophodno odgovarajuće kalibrova-nje generatora. Manje razlike u dužini prslina izmedju krpljene i nekrpljene strane uzorka objašnjene su njihovom nesimetričnom konfiguracijom, koja je izazvala različite faktore intenziteta napona na krpljenoj i nekrpljenoj strani, kao što je to jasno pokazala analiza konačnih elemenata.