Mechanical behaviour of wood compressed in radial direction-part Ⅰ. new method of determining the yield stress of wood on the stress-strain curve

A test equipment was developed, which allows for real time observation of the deformation behaviour of wood cellular structure under a compression load applied in radial direction. Compression tests were performed on jack pine (Pinus banksians) and balsam poplar (Populus balsamifera) specimens to explore the relationship between the yield stress and the first failure occurring in wood cell layers during radial compression. The microstructural changes for P. banksians and P. balsamifera wood below and above the yield point were analyzed. The study results showed that for P. banksians the first failure of wood cells occurred at the first earlywood layer, while for P. balsamifera it occurred at the layer with the largest vessels. The first failure of wood cell layer for each species tested was found to correspond to the yield point on the stress-strain curve. A new method of determining the yield stress for wood specimens under radial compression was developed.