Research On Ultrasonic Testing For Micro-crack Based On VAM Method

In recent years, microscopic-crack detecting has become the frontier of ultrasonic non-destructive testing. Bounded by its principles, conventional ultrasonic detection is insensitive to micro-cracks caused by early fatigue damage. After years of study, the early researchers have found out that non-linear ultrasonic detection method can be applied to detect micro-cracks. The mechanism of vibro-acoustic modulation of nonlinear ultrasonic detection was studied, and micro-cracks in aluminums were studied experimentally.In chapter one, the background and significance of the research were stated; the current progress of nonlinear ultrasonic detection and vibro-acoustic modulation were reviewed; and the primary coverage of this paper is given.In chapter two, two kinds of non-linear detection method, finite amplitude & vibro-acoustic modulation, were introduced; then the mechanism of the latter was elaborated, and non-linear spring model was established to discuss the detection principle of vibro-acoustic modulation detecting micro-cracks from the microscopic point of view.In chapter three, the effectiveness of vibro-acoustic modulation in respect of micro-cracks was verified through contrast simulation analysis based on finite element model; main factors affecting the result were analyzed, the analysis can be used as guidance setting experimental parameters.In chapter four, a vibro-acoustic modulation signal analysis method based on empirical mode decomposition(EMD) and singular value decomposition(SVD) was proposed, which can be used to extract the feature signal submerged in noise.In chapter five, a vibro-acoustic modulation detection system was established to experimentally study the micro-cracks in aluminum plate; through quantitative characterization study of crack length, it was found that the main-band amplitude and the sideband amplitude were not suitable for quantitative evaluation of crack length; combining vibro-acoustic modulation with information theory, entropy was proposed as a damage evaluating indicator, the quantitative characterization of damage was realized.Chapter six summarized former chapters and pointed out the deficiencies.