Study On Interlaminar Shear Performance And Its Influencing Mechanism Of Cross-laminated Timber(CLT)

Cross-laminated timber(CLT)is a kind of engineering material made of odd layers of sawed timber at the Angle of vertical intersection.The orthogonal structure of CLT gives it excellent in-plane bi-directional dimensional stability,high strength,high stiffness et al.In wooden structure,CLT is often used as floor panel sand roof panels where load-bearing performance is needed.The CLT in these positions is mainly affected by bending stress,and the main damage is interlaminar shear failure,which is the key to the failure of the whole timber structure components and has a great influence on the safety performance of wooden structures.In view of this,this paper systematically studied the factors influencing the interlaminar shear performance of CLT,the interlaminar shear failure mode and the interlaminar shear failure process,providing scientific basis and technical support for the preparation process.The conclusions are as follows:(1)The performance of bonding line is an important factor of interlaminar shear performance,which is mainly affected by the type of adhesive and the bonding pressure.For Larch wood,1.2 MPa bonding pressure and PUR adhesive are the optimal bonding process,and sufficient amount of PUR can penetrate into wood cells under 1.2 MPa pressure to form sufficient mechanical binding force.(2)When CLT was prepared by mixing poplar and OSB with larch,the bonding pressure of 1.2MPa could ensure the permeability of PUR adhesive in different laminate.The maximum shear strength of CLT laminated with poplar as perpendicular layer is 2.41 MPa,and the lowest shear strength of CLT laminated with OSB is 1.98 MPa.The durability of CLT prepared by poplar,OSB and Larch was better than that prepared by larch single species.PUR adhesive should be selected for laminates with different p H and buffer capacity,because the curing mechanism of PUR is wet curing.(3)The interlaminar shear strength of CLT is significantly affected by the type of vertical laminates.The average interlaminar shear strength of CLT prepared by three kinds of laminates as vertical layers is3.52 MPa(Larch-Larch-Larch),4.18 MPa(Larch-poplar-Larch)and 1.93 MPa(Larch-OSBLarch),respectively.The CLT of poplar with vertical layer showed excellent interlaminar shear performance in strong strength direction,because of the homogeneous structure of poplar in transverse plane and the shear failure along the load path,which was different from the rolling shear failure of larch and the shear failure at the core layer of OSB board.(4)The difference of cell wall structure between early wood and late wood is the main factor causing the location and mode of failure of rolling shear.In the shear plane,the earlywood belongs to the weak phase structure in the plane,so the failure occurs in the early wood position.Earlywood cell wall is thin and microfibrillar angle is large,resulting in rolling shear failure in early wood and along the direction of growth rings.The failure of earlywood cell is trans-wall fracture,and the cracks on cell wall mostly expand along the direction of microfibrillar angle When the crack extended to the latewood,the crack extended in radial direction until the crack extended to the earlywood,the cracks continued to expand along the earlywood,and the shear failure at the latewood was mainly caused by intercellular shear slip separation,or the separation of S2 and S1 layers of cell wall.By changing the grain direction of sawn timber,the stress of weak position such as earlywood can be reduced.The minimum rolling shear strength of the half-heart board is 1.59 MPa,which should be avoided as far as possible when sawing the laminated board.The rolling shear strength and rolling shear modulus of the plain-sawn timber are the highest.The reason is that the plain-sawn timber changes the failure path of the shear and avoids the rolling shear failure.(5)The crack defect has a significant effect on the interlaminar shear performance of CLT.When the ratio of prefabricated crack length to span(a:L)≥ 0.29,the interlaminar shear strength and primary failure load are significantly reduced.When a: L reaches 0.55,the interlayer shear failure mode changes.