Species independent machine stress grading of hardwoods

摘要

Machine strength grading has traditionally been focussed on softwoods. With the introduction of many 'new' (hard)wood species from all around the world on the market, especially from sustainable managed forests, there is a need for strength grading of 'unknown' species and assign them into strength classes. In this project, over 2000 beams from 35 different species covering the whole range of strength, stiffness and density profiles and originating from South-America, Africa, Asia and Europe have been tested using both non-destructive and destructive testing techniques. The dataset was used to develop a species independent strength grading model in order to classify timber without the need of extensive and expensive laboratory testing. Since optimisation of the yield of a hardwood species with visual grading is hardly possible, non-destructive measurements on hardwood allows it to be assigned to higher strength classes, greatly improving the yield. The main research goal was to determine a common ground on which hardwood beams can be graded using stress waves. On the basis of the different species tested, a species independent strength grading model was developed. With the derived model, timber can be assigned to strength classes without destructive testing. This method can be used to economically determine the strength class for use with visual grading of 'new' hardwood species that are commonly introduced on the market in rather small amounts. Now, even in remote areas, hardwood timber can be machine graded into strength classes as used in the Pacific, Africa, America and Europe improving their possibilities to sell timber to these high end markets. The aspects on grading and design of hardwood are relevant because presently many unknown timber species are introduced on the market as a result of the demand for timber from forests that are managed in a sustainable, environmentally friendly manner.