Bonded Interface Surface Testing Via Differential Interferometric Stoneley Wave Measurements

AbstractDifferential optical interferometry, initially developed to characterize opticalrecording media surfaces and subsequently used to detect continuous and pulsed ultrasonicelastic waves, has been applied to the measurement of the interface properties of bondedtransparent solids. Acousto -optic interaction between dual laser probe beams and the peri-odic particle displacements induced by acoustic waves on the boundary permits the qualita-tive identification of surface roughness defects at the interface. In this paper, thetheory and operation of the real -time interferometric measurement system, an analysis ofthe acousto-ontic interaction, and surface defect testing results for bonded glass speci-mens are presented.IntroductionThe development of a technique for the nondestructive evaluation (NDE) of bond charac-teristics in adhesively bonded materials is necessary for the acceptance of adhesive bondingas a substitute for conventional mechanical fastening techniques in high -strength aerospacestructure applications. Several existing mechanical NDE methods are capable of sensinglarge voids and particle inclusions at bond interfaces but cannot determine effects due tosmall defects, bond thickness variations, or localized bond weaknesses.' We have experi-mentally investigated the potential of using ultrasonic Stoneley waves, high frequencyperiodic elastic waves which may propagate along the surface between adhesively bondedmaterials, to measure these bond parameters. Stoneley waves are ideal for these measure-ments because Stoneley wave energy is confined to a thin region on both sides of theboundary and significant interaction between the waves and boundary structure thus occurs.The bonded structural materials of engineering interest are usually opaque solids.Measurements of pulsed Stoneley waves on the boundaries between such materials have beenobtained using mechanical transducers mounted on external surfaces in a pitch catch arrange-ment and employing several methods of acoustic wave mode conversion.2 These pulsed measure-ments are quite useful but intrinsically indirect; the receiving transducer does not senseparticle displacements directly at the boundary and effectively integrates the output soundpressure distribution.Optical measurements of CW Stoneley waves directly at the boundary are possible if oneor both of the selected bonded media are transparent, allowing the optical probing beams tobe directed through that medium. Several acousto -optic techniques may be applied to thedetection of Stoneley waves on such boundaries although most require large wave amplitudeand the interaction between optical and acoustic beams in a region several acoustic wave-lengths wide.3 Differential interferometry, however, allows the sensitive localizedprobing of the Stoneley wave surface displacements as well as the flexibility necessary tomeasure associated surface and bulk field effects. In this paper we describe directdifferential interferometric measurements of Stoneley wave attenuation due to surfaceroughness.TheoryIn this section, we consider the propagation of ultrasonic Stoneley waves, differentialinterferometry, and the acousto-ontical interaction necessary for wave detection.