Development of strength grading of timber using combined measurement techniques: Report of the Combigrade-project - phase 2

More than 1000 pieces of spruce (Picea abies) logs and 1000 pieces of pine (Pinus sylvestris) logs were sampled mostly from Finland but also from North-Western Russia. Non-destructive measurements were first made on the logs and sawn timber of five different cross-section sizes were produced by sawing and kiln drying. Next, the sawn timber boards and planks went through several non-destructive measurements. NDT-measurements were made by 7 organisations producing some 50 different measured quantities of both logs and of each test piece. Finally, after all the non-destructive measurements, the test material was loaded to failure in bending, and grade determining properties (modulus of elasticity, bending strength and density) were measured. Degrees of determination were calculated between the non-destructively measured indicators and grade determining properties. It could be concluded that the coefficients of determination - the r2 values - between strength and most non-destructive indicators were remarkably higher for pine than for spruce. Especially, knot size and density are better grading parameters for pine than for spruce. This does not, however, indicate that yield to high grades would be in general better with pine than spruce, but it indicates that pine has larger variability of knot sizes and density, and consequently higher variability of strength. Log scanning can also produce strength indicators which are on the same level as some existing grading methods. In parallel to this project another project that dealt primarily with grading of spruce based on tension strength was carried out and some results of that project are also reported.