Air Coupled Ultrasonic Inspection of Steel Rubber Interface

This paper describes the investigation for the feasibility of inspection of steel-rocasin rubber specimen used in rocket motor for detection of debond by using air-coupled ultrasonic. A pair of high performance air-coupled transducers was used to generate and receive the high intensity ultrasonic pulses. Probes were placed at the opposite side of the specimen during inspection. It was shown that debonds between the steel and rocasin rubber interface can be detected by this technique. Debonds were confirmed to be present by testing with conventional contact ultrasonic through transmission method. Introduction A rocket motor [1] case is made up of steel or composite. Layers of insulator are bonded to the casing to prevent the excessive heat flow to the casing during burning of propellant. Other side of the rubber is bonded to the propellant during the casting and curing process. The proper bonding between of casing and rubber layer is essential in view of the requirement of structural integrity. The bonding at the interface is checked by ultrasonic Non-destructive evaluation (NDE) method [2]. For the purpose of ultrasonic testing (UT), the total outer and/or inner region of the motor is divided into grids and the testing is carried out on the specified grids. Pulse echo, through transmission or multiple echo technique of UT is basically used with straight beam contact probes for detection of interfacial flaws. Continuous scanning of the region is tedious and impractical because of the contact method of testing. In contrast to that, air-coupled ultrasonic [3] has the major advantage of possibility of continuous scanning and can overcome the problem due to couplant and pressurization of probes during testing. In recent past, air-coupled ultrasonic technique is being utilized for testing of material with low acoustic impedance like composites [4][5]. Metallic specimens [6][7] were also inspected by air-coupled ultrasonic with generation using laser and detection by air coupled transducer . As there is a significant difference in the acoustic impedance of metals with air, most of the energy gets reflected back to the air for inspection with air-coupled ultrasonic. Hence, air-coupled ultrasonic using pair of transducer was not treated as good method inspection of metallic specimen. However this created interest to study the metal-rubber specimen with lower thicknesses with air-coupled ultrasonics. This paper describes the investigation for feasibility of inspection of steel-rubber specimen using air coupled transducers. The paper presents the experimental results of air-coupled ultrasonic inspection of metal-rubber interface for presence of debond. Air-coupled ultrasonic Air coupled ultrasonic [3] method utilizes air as the coupling media. In usual sense a couplant is utilized to match the acoustic impedance between the probe and the inspected material so that there will be less loss of energy. In case of air as coupling media, most of the M or e In fo a t O pe n A cc es s D at ab as e w w w .n dt .n et /? id = 15 20 4 energy gets reflected back to the incoming media from the interface. Hence very small energy is transmitted through the object which is usually less than the detectable limit of the detector. Material properties of air, steel and rocasin rubber is tabulated in table 1. The reflection and transmission co-efficient of ultrasonic intensity at the interface is tabulated in table 2. Table 1: Ultrasonic properties of relevant materials Material Density (ρ) (kg/m) Ultrasonic Velocity (ν) (m/s) Acoustic Impedance(kg/ms × 10) Steel 7850 5940 46.62 Rocasin Rubber 115