Wavefield imaging of nonlinear ultrasonic Lamb waves for visualizing fatigue micro-cracks

Ultrasonic displacement measurements from local absorption of ultrasound in thermochromic liquid crystal sensors

Wavefield imaging can be used for measuring the wavefield produced by an ultrasound transducer for medical and industrial applications, or for the detection and monitoring of defects in non-destructive testing. Typical wavefield imaging methods include interferometry/vibrometry, and the use of microphones and hydrophones. These involve scanning, making them time consuming, and microphones have limited resolution. An alternative method presented here uses thermochromic liquid crystal sensors which react to heat generated due to absorption of ultrasonic waves. The result is a colour scale that varies with temperature, with the temperature change dependent on ultrasonic displacement. Measurements of the resonant modes of a flexural ultrasonic transducer were taken between 320 kHz and 6.77 MHz. Temperature maps were obtained from photographs of the TLC sensor using the true-colour image processing method. The obtained temperature change across the transducer face was compared with displacement measurements taken using interferometry, showing excellent agreement in the position of the mode features and good resolution at lower frequencies. Thermal measurements were also taken to directly observe the heating of the transducer cap, showing the effect of the thermal conductivity of the transducer along with confirming the increased heat generated by the ultrasound absorption when a backing layer is used. The sensors show promise for fast transducer characterisation, with further potential applications in structural health monitoring and defect detection.

Analytical and numerical modeling of nonlinear lamb wave interaction with a breathing crack with low-frequency modulation

To characterize fatigue crack, an analytical calculation and finite element (FE) simulation of Lamb wave propagating through the region of a breathing crack in a two-dimensional(2D) isotropic plate were studied. Contact surface boundary conditions between the two surfaces of the vertical crack were considered to study contact acoustic nonlinearity (CAN) from the breathing contact crack in conjunction with the modal decomposition method, Fourier transform, and variational principle-based algorithm. Reflection and transmission coefficients in the fundamental frequency and second harmonic frequency were calculated and analyzed quantitatively. Different ratios of incident wave amplitude to crack width were studied to calculate CAN results related to micro-crack width. In addition, a low-frequency (LF) vibration(10 Hz) excitation was introduced to perturb the free surface vertical crack to close, and an interrogating Lamb wave(1 MHz) was used to study crack-related CAN in different conditions for interpreting the modulation mechanism. The contact boundary conditions between two surfaces of vertical crack were set which were dynamically changed due to the low frequency modulation. The clapping effects when the crack closed due to the modulation of the contact boundary conditions between the crack surfaces were studied and analyzed to get the quantitative correlation between CAN and LF modulation. The results obtained from the analytical model were compared with those from the FE simulation, showing good consistency. Knowledge of these effects is essential to correctly gauge the severity of surface cracks in the plate, which can be spotlighted in its application to quantitative evaluation of micro fatigue cracks in structural health monitoring(SHM).

Acoustic field visualisation using local absorption of ultrasound and thermochromic liquid crystals

Acoustic field and vibration visualisation is important in a wide range of applications. Laser vibrometry is often used for such visualisation, however, the equipment has a high cost and requires significant user expertise, and the method can be slow, as it requires scanning point by point. Here we suggest a different approach to visualisation of acoustic fields in the kHz - MHz range, using paint-on or removable film sensors, which produce a direct visual map of ultrasound displacement. The sensors are based on a film containing thermochromic liquid crystals (TLC), along with a backing/underlay layer which improves absorption of ultrasound. The absorption generates heat, which can be seen by a change in colour of the TLC film. A removable sensor is used to visualise the resonant modes of an air-coupled flexural transducer operated from 410 kHz to 7.23 MHz, and to visualise 40 kHz standing waves in a Perspex plate. The thermal basis of the visualisation is confirmed using thermal imaging. The speed and cost of visualisation makes the new sensor attractive for use in condition monitoring, for fast assessment of transducer quality, or for analysis of acoustic field distribution in power ultrasonic systems.