Integrated strength grading
Sammanfattning: This work comprises strength grading for structural timber according to European standards and prediction of grade-determining properties by various technologies and in various stages of the sawmilling process. The detection technologies applied on logs were outer shape scanning, laser scattering, x-ray scanning and resonance analysis. For boards, microwaves, laser scattering, x-ray scanning, resonance analysis and visual scanning applied. Tracing of log rotation angle has been done to enable reconstruction of boards based on the log-scanning results. Modelling of strength, modulus of elasticity and density has been done using partial least squares. A method for deriving settings for structural grades based on several modelled properties has been proposed: the smallest increment algorithm. Classification to structural grades has been made by this algorithm and by classification trees, all validated according to the method stipulated in the standard. Visual override was studied by grading strictly on deformation criteria. Profitability of a grading process has been studied, based on relative net mill prices and production costs for structural timber and alternate grades from 2007. The quality of the grades has been briefly studied by analysis of the lower tail of some strength distributions. The result shows that all grade-determining properties must be estimated to achieve high raw material utilization and quality. The means for such estimation is not important, but the quality of the variables derived, based on the technology, is critical to the result. It was shown that tracheid-effect scanning by laser scattering gave variables valuable for strength prediction and that microwave signals in a very basic form provided more information than density alone. Log scanning by x-ray and resonance analysis enabled machine strength grading to the strength grade C40 already at the timber yard. It was indicated that the log-graded material below the 5th percentile was weaker than the board-graded material of the same grade. The result from pregrading by log grading showed itself in improved profitability for certain grade combinations and market conditions. As expected, the financial result is strongly dependent on the structural timber and alternate grade properties and price, grading accuracy and properties of the species. Different equipment can classify a specimen differently, although the final properties of the final grade fulfil the requirements for both machines. For this reason, it is important to select the stronger material in pregrading. Doing the opposite will alter the distribution to the unsafe side. The conclusion is that an integrated strength-grading process gives flexibility and enables the producer to meet well-defined customer specifications for structural wood products.
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