Phase-delayed laser diode array allows ultrasonic guided wave mode selection and tuning

Abstract

(2013). Phase-delayed laser diode array allows ultrasonic guided wave mode selection and tuning. Journal of Applied Physics, 113 (14) Selecting and tuning modes are useful in ultrasonic guided wave non-destructive testing (NDT) since certain modes at various center frequencies are sensitive to specific types of defects. Ideally one should be able to select both the mode and the center frequency of the launched waves. We demonstrated that an affordable laser diode array can selectively launch either the S 0 or A 0 ultrasonic wave mode at a chosen center frequency into a polymer plate. A fiber-coupled diode array (4 elements) illuminated a 2 mm thick acrylic plate. A predetermined time delay matching the selected mode and frequency was employed between the output of the elements. The generated ultrasound was detected by a 215 kHz piezo receiver. Our results imply that this array permits non-contacting guided wave ultrasonic NDT. The solution is small, affordable, and robust in comparison to conventional pulsed lasers. In addition, it does not require experienced operators. V C 2013 AIP Publishing LLC.[http://dx

Phased laser diode array permits selective excitation of ultrasonic guided waves in coated bone-mimicking tubes

“…We showed that phased photo-acoustic excitation permits tunable and selective mode excitation in coated tube samples, in a manner similar to that we used previously with free plate samples (Karppinen et al, 2013). In coated tubes we observed, within the frequency range of interest, several modes separated into five zones, whereas in the plate, there were only two modes.…”

“…The illuminated spot on the coating was 2 mm in diameter. The laser diodes were successively excited using a tunable time delay, that is, phase delay (t pd ) (Karppinen et al, 2013). The fiber head (distance to the coating 2 mm) was translated along the long axis of the sample, in 0.5 mm steps, to provide sixty source-receiver distances within 20-50 mm range.…”

“…The material properties used for the theoretical predictions were consistent with the properties of the bone phantoms. The value of the phase-delay parameter of the phased excitation was predicted as in (Karppinen et al, 2013)…”

“…This creates a situation where certain modes hide other modes, modes that could contain important information. To solve this problem of choosing a particular mode, we previously proposed a phased laser diode array solution and demonstrated that we could select between the two fundamental Lamb modes (A 0 and S 0 ) propagating on plates (Karppinen et al, 2013). Related techniques of spatial and temporal modulation for mode selection have been investigated in several publications (Gr€ unsteidl et al, 2013;Sharples et al, 2006;Veres et al, 2013;and Song et al, 2016).…”

Non-contact damage detection on a rotating blade by Lamb wave analysis

“…To facilitate defect detection, one could use Lamb waves with a specific frequency and shape. This can be done by modulating the exciting signal spatially and/or temporally [33,34].…”