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

Abstract

Glued laminated timber (glulam) is known in timber constructions since more than 100 years. Glulam members can delaminate due to aging and excessive changes of temperature and humidity. This results in significantly reduced load bearing capability of the affected structural members. This contribution focuses on the ultrasonic point-contact inspection of gluing plane delamination as a nondestructive method. Ultrasonic measurements on a section of a 90-year-old roofing glulam member are presented. The results are compared with manual detection and evaluation of delamination with a feeler gauge, with X-ray computed tomography analyses, and with numerical simulations. Appropriate data evaluation of the mechanized ultrasonic results allows the determination of material separation that are deeper than 20 mm in the signature of the surface wave and large-scale delamination ( > 80% of the complete bonding width) in the back-wall echo. Numerical simulations based on the finite-difference time-domain method shed light into the details of the wave propagation and support the experimental findings.

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

“…Although the SH mode is not coupled in ACU measurements, its relatively isotropic behavior (v < 6°) and low damping favor its use in contact inspection with shear waves. This agrees with experimental observations [6,27], for which optimum shear wave transmission has been observed with p aligned with the fiber direction. [25].…”

“…Its application is therefore not limited to wood research, but can be extended to other complex materials of relevance in NDT, for instance, biological tissue, fiber-reinforced composites, concrete or austenitic stainless steel. Moreover, the direct solver for pressure fields in fluids allows efficient incorporation of air-coupled transducers in arbitrary imaging configurations, contact ultrasound testing has been as well successfully modeled in a separate work [27].…”

“…(3). For the out-of plane shear mode (SH), Neuenschwander et al [27] observed lower tan d ¼ 0:05 at 50 kHz, which may be explained by the different frequency regimes.…”

“…1). Exemplary point source wavefronts have been simulated with different degrees of material simplification [4,14,27,28]. However, an in-depth discussion and quantitative validation of the wave propagation paths are still required.…”

Bonding Defect Imaging in Glulam with Novel Air-Coupled Ultrasound Testing

“…With appropriate data evaluation, edge delaminations deeper than 20 mm can be successfully detected in the signature of the surface wave and large scale delaminations (>80% of beam width) in the backwall echo. The data interpretation was supported by finite-difference time-domain simulations (Neuenschwander et al, 2013;Sanabria et al, 2011b).…”