Detection of Hidden Defects in Cross-Laminated Pine (Pinus sylvestris L.) Panels

Cross-laminated timber is one of the promising construction materials. The existing domestic regulatory and technical framework does not regulate non-destructive testing, as well as examination and monitoring of the technical condition of cross-laminated timber panels, while the results of the practical application of the basic methods of non-destructive testing for these purposes are limitedly presented in the scientific literature. The aim of the research has been to assess the possibility and accuracy of detecting hidden defects in cross-laminated timber panels using the acoustic method and measuring the drilling resistance. The research has been conducted on samples of 100×100 mm panels of different thickness, made of pine wood (Pinus sylvestris L.). Hidden defects include cavities inside the panels, rot in the billets for the layer, and areas with no adhesive layer. Acoustic non-destructive testing has been carried out by the velocimetric method using ultrasonic devices “Pulsar 2.2” and Pundit PL-200. To obtain the drilling resistance profile across the panel thickness, an IML-RESI PD 400 mobile device has been used with standard thin drill bits with a cutting diameter of 3 mm. The paper presents the results of zonal acoustic scanning of cross-laminated timber panels in the form of contour graphs, which reflect the ultrasonic signal speeds along the entire plane of the panel, as well as drilling resistance profiles in places of inherent defects. Based on a pairwise comparison of the medians using the Mann-Whitney U-test and the laws of velocity distributions of the ultrasonic signal for panels with a thickness of 120 and 200 mm, according to the Kolmogorov-Smirnov test (for panels measuring 120 mm), the absence of statistically significant differences between the samples has been established. The ultrasonic wave velocity of 1,400 m/s has been the boundary value characterizing the presence of an internal defect in the panel. It has been shown that the method of measuring drilling resistance does not allow detecting the presence or absence of an adhesive line in the studied panels. Zonal scanning using the velocimetric method and determination of drilling resistance make it possible to visually represent the presence of internal defects in the form of drilling resistance profiles and contour velocity graphs.