| The rolling shear stiffness and strength of CLT are key to determine its mechanical properties as floor and roof panels.The cross section of the transverse layer in CLT is prone to rolling shear failure under shear force.Rolling shear is not material’s inherent property.It mainly depends on species,density,ring width,growth ring orientation.This paper mainly study on the effect of macroscopic characteristics of poplar sawn timber and tree species on the rolling shear properties of CLT.In order to find out the superior macroscopic characteristics and species.Apply them to CLT buildings so as to improve construction safety.Expand the range of applications of poplar and make full use of China’s rich poplar resources.Firstly got three kinds of poplar sawn timber with different growth ring orientations(quarter sawn,semi-quarter sawn and flat sawn),flat sawn timber with different radial distances(42.5mm,72.5mm and 102.5mm)and quarter sawn timber with pith through log sawing,drying and sanding.Then used two-plate shear test to measure the rolling shear properties of poplar.The results indicated that: the rolling shear modulus of flat sawn,semi-quarter sawn and quarter sawn timber were 165 MPa,196 MPa and 180 MPa,and the rolling shear strength were 3.28 MPa,3.09 MPa and 2.82 MPa.The rolling shear modulus firstly increased and then decreased with the increasing growth ring orientation from flat-sawn orientation.Semi-quarter sawn timber had maximum modulus,which was 8.9% higher than that of quarter sawn timber and 18.8% higher than that of flat sawn timber.The rolling shear strength decreased gradually with the increasing growth ring orientation.Flat sawn timber had maximum strength,which was 6.1% higher than that of semi-quarter sawn timber and 16.3% higher than that of quarter sawn timber.Growth ring orientations did not have a significant effect on rolling shear modulus and strength.The rolling shear modulus of specimen with the radial distances of 42.5mm,72.5mm and 102.5mm were 176 MPa,200 MPa and 204 MPa,and the rolling shear strength were 3.26 MPa,3.3 MPa and 3.42 MPa.The rolling shear modulus and strength increased with the increase of radial distance.Radial distances did not have a significant effect on rolling shear modulus and strength.The rolling shear modulus and strength of specimen with pith reduced significantly,and the modulus and strength were 119 MPa and 2.09 MPa.The rolling shear modulus was 32.4% lower than the specimen with the radial distances of 42.5mm.The rolling shear strength was 35.9% lower than the specimen with the radial distances of 42.5mm.Pith had a significant effect on rolling shear modulus and strength.Four different coniferous tree species were used as the cross layer of CLT.The rolling shear tests were conducted by a modified sandwich wood specimen.The results indicated that: The rolling shear modulus of SPF,larch,scotch pine and hemlock were 96 MPa,168 MPa,56 MPa and 104 MPa,and the rolling shear strength were 1.27 MPa,1.66 MPa,1.29 MPa and 1.35 MPa.The rolling shear modulus and strength of larch were the highest.Scotch pine had the lowest rolling shear modulus,and SPF had the lowest the rolling shear strength.For poplar and different coniferous tree species,there was a positive correlation between rolling shear modulus and density,rolling shear strength and density and rolling shear strength and modulus.The correlation among the same tree species was lower than it among different species.At last,the fracture surface of three typical failure modes of poplar was observed by scanning electron microscopy.Summarized the failure modes of poplar and four coniferous trees.It was found that failure spots mainly lie along the wood rays and the boundary between springwood and summerwood.For poplar with different growth ring orientations,the quarter and flat sawn timber were mainly failed along the annual ring,and the semi-quarter sawn timber was mainly failed along the wood rays.The poplar specimen with pith all failed at the pith area. |
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