Excitation Method for Thermosonic Non-destructive Testing

Abstract

Thermosonics is a non-destructive testing method in which cracks in an object are made visible through the local generation of heat caused by friction and/or stress concentration. The heat is generated through the dissipation of mechanical energy at the crack interfaces by vibration. The temperature rise around the area close to the crack is measured by a high-sensitivity infrared imaging camera whose field of view covers a large area. The method therefore covers a large area from a single excitation position so it can provide a rapid and convenient inspection technique for structures with complex geometry and small and closed cracks. An ultrasonic horn, originally designed for welding, has generally been used for thermosonic testing. However, it is diffcult to obtain reproducible and controllable excitation with the existing horn system because of non-linearity in the coupling; surface damage can also be produced by chattering caused by loss of contact between the tip of the horn and the structure. Therefore, the general aim of the study was to develop a reliable and convenient excitation method that should excite sufficient vibration for the detection of the defects of interest at all relevant positions in the structure and must also avoid surface damage. In this thesis, a numerical and experimental study for the development of the ex- citation method for reliable thermosonic testing is presented. Successful excitation methods for the detection of delaminations in composites and cracks in metal struc- tures are described. A simple, small wax-coupled PZT exciter is introduced as a con- venient, reliable thermosonic test system in applications where relatively low strain levels are required for damage detection such as composite plates. A reproducible vibration exciter may be su cient for thermosonic testing in some metal structures such as a thin plates. However, higher strain levels are often required in metal structures, though the required strain level is dependent on the crack size. This level of strain is not easily achieved within the reproducible vibration range because of non-linearity in the contact between the exciter and the structure. Therefore, studies are conducted with an acoustic horn with high power capability to investi- gate the characteristics of the vibration produced in a real structure with complex geometry and to develop a excitation method for achieving reliable excitation in the non-linear vibration range for thermosonic testing. An excitation method for a complicated metallic structure such as a turbine blade is also investigated and the in uence of the clamping method and the excitation signal that is input to the horn on the vibration characteristics generated in the testpiece is presented. As a result, a fast narrow band sweep test with a general purpose amplifier and stud coupling is proposed as an excitation method for thermosonic testing. This method can be ap- plied to different types of turbine blades and also to other components. One typical characteristic of a thermosonic test using non-linear vibration is the lack of repeata- bility in the amplitude and the frequency characteristic of the vibration. Therefore, vibration monitoring is necessary for reliable thermosonic testing and a Heating In- dex(HI) has been proposed as a criterion indicating whether su cient vibration is achieved in a tested structure or not. The HI is calculated from different vibration records measured by different sensors and these results are compared in this thesis. A microphone can provide a cheaper and more convenient non-contacting vibration monitoring device than a laser or strain gauge and the heating index calculated by a microphone signal shows similar characteristics to that calculated from the other sensors.