Study On Mechanical Properties Of Compressed Wood By Water Heat Treatment And Application

In the northern areas in China,there are abundant poplar resources.Poplar has the characteristics of rapid growth cycle and light weight.Because of its poor mechanical strength,its application in wood structure is limited.Poplar can be compressed in the direction of its horizontal grain under certain temperature and pressure,and compression of wood is an effective way to improve the performance of wood mechanics.This method not only improves the mechanical properties of wood,but also preserves the excellent properties of wood itself.The existing researches mainly focus on dimension stability of compressed wood after hot pressing.In this paper,compressed wood is obtained by using water heat treatment,the effect of compression rate(33.3% 60%)?compression time(0.5h 1h and 2h)and dry and wet circulation environment on mechanical properties and dimensional stability of compression wood were analyzed.Using compressed wood into pin as wood-wood connection fittings,the single pin double shear connectors used wooden pin diameter is 12 mm,analyzed the load and the wooden dowel fiber direction to 0,45,90,the bearing capacity of the connection stiffness and ductility.Compressed wood take process is: first,the poplar material is put into a 100? constant temperature electric water bath for 1 hour;after fully softened,with 200 t test hot-pressing machine along the direction of the cypress horizontal grain for hot pressing,hot pressing temperature is 150?,the loading mode is hierarchical loading;finally,the deformation of compressed wood is fixed through pressure preservation treatment.The compression ratio has an effect on the performance of the compressed wood: as the compression ratio increases,the bending strength and hardness of the compressed wood also increase.The bending strength of compressed wood is nearly twice as high as that of poplar in 60% compression ratio and 0.5h holding time.The hardness of the compressed wood was increased by 120.50% under the 60% compression ratio and the holding time of 1h.The hot holding time has an effect on the performance of the compressed wood: with the increase of the holding time,the flexural strength and elastic modulus of the compressed wood showed a decreasing trend.The hardness of the compressed wood increases with the increase of the pressure keeping time.At the compression rate of 60%,the hardness of compression wood at 1h and 2h was 17.94% and 14.95% higher than that at 0.5h.The best ductility of compressed wood produced by water heat treatment method is: the compression rate is 33.3% and hot holding time is 1h.The mechanical properties of the compressed wood can be affected by the dry and wet circulation environment.The mechanical properties of the compression wood are obviously decreased with the increase of the number of dry and wet cycles.Under the working condition of 2 hours,the deformation recovery rate of the compression wood was the lowest.After five dry and wet cycles,the deformation recovery rate of the compression wood was controlled at 2%.The results of double shear test of compression pin connection show that the joint has good bearing capacity.The joint also has good ductility because the plastic hinge is formed in the wooden pin.