Assessment of the Impact Strength Properties of Thermally Modified Wood by Non-Destructive Testing

Abstract This article examines the effectiveness of non-destructive testing (NDT) in assessing wood under impact loadings. Our research was to evaluate the feasibility of using the frequency resonance technique (FRT), to predict the behaviour under impact of thermally modified timber (TMT) compared with a control sample of untreated wood. Wooden planks from five different species were subjected to a thermal modification process (TMP) under two different regimes. Both the TMT and control samples were evaluated using NDT to measure their dynamic modulus of elasticity (MOED), logarithmic decrement of damping (LDD) and acoustic conversion efficiency (ACE). Subsequently, wood samples from the same species were tested using drop-weight impact tests to measure their inflicted maximum force and impact bending strength (IBS), while high-speed cameras recorded the impacts to measure the maximum deflection of the specimens. The results revealed that the only relatively efficient prediction of FRT was the relationship between MOED and IBS. The ACE and LDD results did not show any acceptable correlations with impact tests, indicating that NDT is not reliable for assessing maximum force and deflection in the wood species under impact. Our study also found that the efficiency of the results and predictions were influenced by the wood species and the TMP conditions, necessitating a large number of samples for each species and heat modification temperature to achieve accurate NDT results. Our study found that the efficiency of NDT predictions was significantly influenced by both wood species and the TMP conditions. Specifically, oak showed a relatively higher coefficient of determination, while ash had the lowest. The thermal treatment also had a varied effect on NDT's ability to determine IBS, increasing its efficiency for larch specimens while decreasing it for ash and beech, with no significant effect on oak and spruce. These findings imply that future NDT methodologies must be developed with a species-specific approach and calibrated for each unique modification condition. Consequently, achieving accurate NDT results will require comprehensive data sets with a large number of samples for each species and heat modification temperature.