Nondestructive ultrasonic determination of disbonds in adhesively bonded submarine shaft vibration reducers

Nondestructive Ultrasonic Determination Of Disbonds In Adhesively Bonded Submarine Shaft Vibration ReducersN. K. Batra, K.E. Sinimonds and H.H. ChaskelisNaval Research LaboratoryWashington, DC 20375-5343ABSTRACTGood adhesive bond between the laminates is extremely important for the shear strength ofadhesively bonded poly-rubber-steel concentric cylinders used as marine shaft vibration reducers (SVR). The SVR are inspected nondestructively by using ahigh speed PC-based transmission ultrasonic C-Scan system capable of scanning and producing images. Transmission of ultra-sound at frequencies between 5-20MHz and time-of-flight variations of these signals through these multilayered structures isused to determine the disbonds at the interfaces. Two and three- dimensional images are generated and analyzed statistically toquantify the areas of disbond. Mathematical models for the propagation ofultrasound through such a layered structure are ap-plied for the interpretation of these images.Keywords: NDE, ultrasonic, multilayered structure, delamination, disbonds, strain energy density1. INTRODUCTIONAdhesively bonded structures are increasingly being used for marine applications. One such application involves the useof adhesively bonded cylinders of polyethylene- rubber-steel as marine shaft vibration reducers (SVR). Steel forms the inner-most cylindrical lamina and polyethylene forms the outermostlayer ofthis component. The adhesively sandwiched lamina, rub-her, is subjected to axial shear loads tangential to the curved surfaces. For this component to perform well under load it is nec-essary that the adhesive bonds at the polyethylene-rubber and steel-rubber interfaces be strong and free of any deleteriousdelaminations. Any surface areas which are devoid ofadhesive, have trapped gas, are not chemically bonded by the adhesive orare simply in mechanical contact, form areas of delaminations detrimental to the performance of these components.In this paper we discuss the effect ofdelaminations on the propagation ofultrasonic waves through such a component. Weshow how the variation in the intensity of transmitted ultrasound can be digitized, transformed into a two dimensional imageand interpreted. The digitized data is wrapped to depict the three dimensional image of the transmitted amplitude. Image anal-yses, such as statistical histograms, are used to interpret the variation in the amplitude and the delaminations.Since these adhesively bonded laminates are designed to bear shear stress, the effect of the disbonds under load must bepredicted "a priori." We predict the strain energy distribution in the rubber at the adhesively bonded interfaces. Using finite el-ement analysis and a mathematical model for such a multilayered structure, strain-energy density is computed and plotted forvarious simulated delaminations ofknown shape at the interfaces.2. THEORYWe review briefly the effect of a delamination on the propagation of ultrasonic waves through cylindrical layers as shownin Fig. 1. Let us assume the incident wave is normal to surface of the multilayered structure, i.e. only compressional waves arepropagated. It can be shown1 that the reflection pressure amplitude from this entire set of layers is given byR