Högtemperaturtorkat virke - varför minskar hållfastheten?

The strengths of conventionally and high temperature dried spruce timber have been compared. Material from both southern and northern Sweden was included in the study. A total number of 1050 specimens (50mm x 150mm x 5m) were dried by using five different drying methods and then strength tested. The drying temperatures varied between 70°C and 125°C. The material was also strength graded by using a Cook Bolinder grading machine and the yield in class C30 was evaluated. Fracture energy (perpendicular to the grain direction) was measured for 25% of the tested material. The results show a decreased bending strength by, on the average, 5% for timber dried at 125°C compared to conventionally dried timber (at 70°C). For the 5th percentile value (characteristic value) corresponding decrease was 13%. The tensile strength was reduced more than the bending strength. The stiffness was not affected by high temperature drying, neither in bending nor in tension. The results also indicate that material from northern Sweden is more affected by high temperature drying than material from southern Sweden. No difference in fracture energy was found for the material dried by using the different drying methods. However, the maximum force measured during the fracture energy testing was significantly lower for the high temperature dried material. This indicated that the tensile strength perpendicular to the grain decreased during high temperature drying. The bending strength was lower after machine strength grading (class C30) for the high temperature dried timber compared to the conventionally dried timber. This was expected as the grading machine measures stiffness and the stiffness is, as shown, not affected by high temperature drying