Abstract
The acoustic non-linearity parameter of Rayleigh waves can be used to detect various defects (such as dislocation and micro-cracks) on material surfaces of thick-plate structures; however, it is generally low and likely to be masked by noise. Moreover, conventional methods used with non-linear Rayleigh waves exhibit a low detection efficiency. To tackle these problems, a method of exciting reversed-phase Rayleigh waves in opposite directions is proposed to measure the acoustic non-linearity parameter of materials. For that, two angle beam wedge transducers were placed at the two ends of the upper surface of a specimen to excite two Rayleigh waves of opposite phases, while a normal transducer was installed in the middle of the upper surface to receive them. By taking specimens of 0Cr17Ni4Cu4Nb martensitic stainless steel subjected to fatigue damage as an example, a finite element simulation model was established to test the proposed method of measuring the acoustic non-linearity parameter. The simulation results show that the amplitude of fundamentals is significantly reduced due to offset, while that of second harmonics greatly increases due to superposition because of the opposite phases of the excited signals, and the acoustic non-linearity parameter thus increases. The experimental research on fatigue damage specimens was carried out using this method. The test result was consistent with the simulation result. Thus, the method of exciting reversed-phase Rayleigh waves in opposite directions can remarkably increase the acoustic non-linearity parameter. Additionally, synchronous excitation with double-angle beam wedge transducers can double the detection efficiency.
Highlights
Recent theories and related experimental studies have shown that the fatigue damage of metal materials in the early stage is related to the non-linear effect of ultrasonic waves [1,2,3,4,5]
In the early stage of fatigue damage to metals, the waveform is distorted when ultrasonic waves at a single frequency propagate therein due to the presence of various micro-defects such as dislocations, persistent slip bands (PSB), and micro-cracks, generating second harmonics
The results show that, compared with conventional methods using non-linear Rayleigh waves, the fundamental amplitude is significantly reduced due to offset effects, while the second harmonic amplitude increases due to superposition because of the reversed phases of the excited signals, and β increases
Summary
Recent theories and related experimental studies have shown that the fatigue damage of metal materials in the early stage is related to the non-linear effect of ultrasonic waves [1,2,3,4,5]. When detecting the fatigue damage to large-area thick-plate parts by applying non-linear Rayleigh waves, β is low, causing a low detection efficiency. To solve this problem, a method of exciting reversed-phase Rayleigh waves in opposite directions is proposed to measure β. The results show that, compared with conventional methods using non-linear Rayleigh waves, the fundamental amplitude is significantly reduced due to offset effects, while the second harmonic amplitude increases due to superposition because of the reversed phases of the excited signals, and β increases. Principle of the Method for Exciting Reversed-Phase Rayleigh Waves in Opposite Directions
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