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Abstract
Detection of local wood inhomogeneities is important for accurate strength and stiffness prediction. In hardwood specimens, visual characteristics (e.g. knots or fibre deviation) are difficult to detect, either with a visual surface inspection or by the machine. Transversal ultrasound scan (TUS) is a non-destructive evaluation method with high potential for hardwoods. The method relies on differences in ultrasound wave propagation in perpendicular to the grain direction. The aim of this study is to estimate and analyse the capabilities of TUS for defect detection in hardwoods and prediction of mechanical property values. In the current paper, the TUS was applied to the hardwood species European ash (Fraxinus excelsior L.), Norway maple (Acer platanoides L.) and sycamore maple (Acer pseudoplatanus L.). In total, 16 boards of both specimens were completely scanned perpendicular to the grain using a laboratory scanner with dry-coupled transducers. The measurements were processed to 2D scan images of the boards, and image processing routines were applied to further feature extraction, defect detection and grading criteria calculation. In addition, as a reference for each board, all relevant visual characteristics and mechanical properties from the tensile test were measured. Using the TUS global fibre deviation, the size and the position of the knots can be detected. Knottiness correlates to the strength properties similarly or even better compared to the manual knottiness measurement. Between the global fibre angle measured using TUS and measured on the failure pattern, no correlation could be found. The ultrasound modulus of elasticity perpendicular to the grain does not show any meaningful correlation to the elastic properties parallel to the grain. In overall, TUS shows high potential for the strength grading of hardwoods.
Highlights
Temperate European hardwoods, such as ash, beech and oak of structural size are known for their excellent mechanical properties, which make them attractive for structural applications (Blass et al 2005; Ehrhart et al 2016a, b; Kovryga et al 2019)
Models based on dynamic modulus of elasticty (MOEdyn), which is the most common criterion for the strength prediction, show R 2 values between 0.18 and 0.36 (Nocetti et al 2016; Ravenshorst 2015)
The aim of the present study is to analyze the potential of Transversal ultrasound scan (TUS) for grading of European hardwoods, ash and maple
Summary
Temperate European hardwoods, such as ash, beech and oak of structural size are known for their excellent mechanical properties, which make them attractive for structural applications (Blass et al 2005; Ehrhart et al 2016a, b; Kovryga et al 2019). The same characteristics are more difficult to detect and measure with the available techniques compared to softwood (Olsson et al 2018; Schlotzhauer et al 2018). The relationship between the grading criteria and the mechanical properties, as well as application of grading methods, differ between softwoods and hardwoods. The most accurate strength prediction method, at least for softwoods, is achieved by means of machine strength grading. The grading machines can explain up to 62% of the strength variation (Bacher 2008). Machine grading allows only limited prediction accuracy. Higher prediction accuracy can be achieved only if M OEdyn is combined with visually measured knottiness (Frühwald and Schickhofer 2005; Kovryga et al 2019). Machine detection of the knottiness works less accurate for hardwoods compared to softwoods. Other methods are currently not available for the strength grading purposes
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