Mechanical characterization of green Sitka spruce logs

Timber is by its nature a very heterogeneous material. Many different factors impact its mechanical properties, from tree species to environment (climate, competition, soil…). While a number of studies in the past dealt with... [ view full abstract ]

Timber is by its nature a very heterogeneous material. Many different factors impact its mechanical properties, from tree species to environment (climate, competition, soil…). While a number of studies in the past dealt with sawn timber and its mechanical properties, only a small number of them studied round logs. An even smaller number dealt with logs where moisture content is above fibre saturation point, e.g. green logs. The objective of this study is to investigate and describe the load-deflection response of green Sitka Spruce logs using a four point bending test. The trees for this study were felled at the age of 20 years and consequently the percentage of juvenile wood is relatively high.

Green logs behave differently when stressed in comparison to dry timber. The flexural stiffness is significantly lower and the load-deflection response is highly non-linear. The test results are useful for designing further green round log testing, especially when studying wind throw in standing trees, as they can give a clearer picture of the stresses that can be expected. There are significant differences in static and dynamic modulus of elasticity between bottom and top logs from the same tree. More logs will have to be tested to further characterize the nature of those differences.