Air-coupled ultrasound inspection of glued laminated timber

Abstract

A novel air-coupled ultrasound (ACU) 120 kHz normal transmission system enabled successful imaging of bonding and saw cut defects in multilayered glulam beams up to 280 mm in height with a signal-to-noise ratio (SNR) of 40 dB. The main wave propagation paths were modeled; quasi-longitudinal and quasi-transverse modes were coupled in each lamella and the sound field was found to be shifted from the insonification axis as a function of the ring angle, leading to interference of wave paths in the receiver and to 15 dB amplitude variability in defect-free glulam. The assessment was improved with spatial processing algorithms that profited from the arbitrary scanning resolution and high reproducibility of ACU. Overlapped averaging reduced inband noise by 15 dB, amplitude tracking captured only the first incoming oscillation, thus minimizing diffraction around defect regions, and image normalization compensated 6 dB of systematic amplitude variability across the fiber direction. The application of ACU to in situ defect monitoring was demonstrated by using multiparameter difference imaging of measurements of the same sample with and without saw cut defects. The segmentation of the defect geometry was improved significantly and the amplitude variability was reduced by 10 dB. Further work is planned to model additional insonification setups and grain and density heterogeneities.

Delamination detection in a 90-year-old glulam block with scanning dry point-contact ultrasound

“…The polarization vector p of the transducer elements was aligned with the fiber direction L (Y) to couple efficiently the shear wave horizontal mode. This mode is most attractive for wood inspection due to its low level of anisotropic energy flux shift from the defied normal insonification direction (Sanabria et al 2011b;Sanabria 2012). For C-scan presentations, the positive maximum amplitude of the A-scan in the selected time gates was calculated and displayed as a 2D image.…”

Relationship between Chemical Retention and Velocity of Air-Coupled Ultrasonic Waves in Fire-Retardant-Treated Wood

“…The development of nondestructive inspection systems for chemical retention in fire-retardant treated wood has thus attracted considerable attention in the context of wood industries. Recently, air-coupled ultrasonic waves have been studied for use in the quality control of wood and wood-based composites (Vun et al 2008;Dahmen et al 2010;Hsu et al 2010;Sanabria et al 2011;Sanabria et al 2013;Mori et al 2016). This technique makes it possible to evaluate the current state of a wood sample without contact and without destroying the wood.…”

Analytical modeling, finite-difference simulation and experimental validation of air-coupled ultrasound beam refraction and damping through timber laminates, with application to non-destructive testing

“…normal or slanted incidence is coupled, and the re-radiated pressure fields are integrated by the receiver. While these setups provide high lateral resolution and sensitivity for centimeter-thick wood materials [7][8][9][10], their performance for thicker timbers is constrained by the position-dependent wave propagation paths, which induce variable shifts in the re-radiated beam coordinates, leading to partial or total signal fading at the receiver [6,[11][12][13]. Further development of these NDT technologies requires simultaneous acquisition of multiple receiver signals for each transmitter position, and the use of inversion methods to obtain spatially resolved information at relevant material planes [14,15].…”

“…The propagation of collimated ultrasound beams in anisotropic media, as generated by planar ACU transducers used in NDT, leads to beam skewing off the main propagation paths defined by the isotropic Snell law [19]. The modeling of these effects in wood has so far received very limited attention, beam-skewing effects have been analytically parameterized for unbounded media in function of grain [29] and ring angles [12]. Yet the quantitative modeling of ultrasound wave propagation paths through real composite structures including acoustic damping, heterogeneity and sample boundaries needs to be addressed.…”