High-energy multiple impact (HEMI)-test – Part 1: A new tool for quality control of thermally modified timber
C Brischke, A O Rapp, C R Welzbacher
Thermal modification processes improve durability and dimensional stability of wood, but the strength properties, especially the dynamic ones, are compromised and need to be considered with respect to industrial quality control. Results from standard dynamic strength testing, such as impact bending tests, suffer from high variability, and therefore require a high number of replicates. To overcome this, a new test method named high-energy multiple impact (HEMI) was developed. In the present paper heat treated beech (Fagus sylvatica L.), spruce (Picea abies Karst.), silver fir (Abies alba Mill.), and larch (Larix decidua Mill.) were investigated. The HEMI method is based on crushing small specimens by thousands of impacts of pounding steel balls in a heavy vibratory mill. The level of destruction was determined by slit sieving and analysing the size distribution of the fragments. We calculated the resistance to impact milling (RIM) based on the mass of the size fractions. RIM showed a linear correlation with the decrease in mass of the wood by the thermal treatment. The HEMI-test method has the following advantages: small number of specimens, short time for specimen preparation, small variances, high reproducibility of the results, and applicability to timber out of service for a subsequent quality control.