Interrelationship between the severity of heat treatments and sieve fractions after impact ball milling: a mechanical test for quality control of thermally modified wood

Thermal modification processes improve the durability and dimensional stability of wood, but strength properties, especially dynamic ones, are compromised. 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 by investigating heat-treated Picea abies Karst., Abies alba Mill. and untreated Robinia pseudoacacia L. The method is based on crushing small specimens by thousands of impacts from pounding steel balls in a heavy vibratory mill. The level of destruction was determined by sieving and analyzing the size distribution of the fragments. We calculated the resistance to impact milling (RIM) based on the mass of the size fractions. RIM shows a linear correlation with the intensity of the thermal treatment. The HEMI test method has the following advantages: small number of specimens, short time for specimen preparation, small variances, and high reproducibility of results.