Research On CLT Rolling Shear Performance Based On Digital Image Correlation Method

The rolling shear properties of the cross-laminated timber(CLT)transverse layer are the key factors affecting the bending performance.How to improve and improve the rolling shear performance has become the focus of current CLT research.The rolling shear performance of CLT is mainly affected by factors such as sawn timber species and materials,structural features,processing techniques and test methods.Based on the existing research,the paper further analyzes the effects of different tree species,width-thickness ratio and test methods on CLT rolling shear performance.Combined with Digital Image Correlation(DIC),the CLT rolling shear failure process Measurement and analysis of full field strain and displacement correlates the strain distribution with the structural characteristics of the sawn timber.Firstly,the paper studies the influence of tree species with different structural characteristics on CLT rolling shear performance.The test results show that the average values of rolling shear modulus and strength of the ring-hole eucalyptus are 183 MPa and 3.19 MPa,respectively,and the strength characteristic value is 2.71 MPa.The rolling shear modulus and strength of the birch material were 198 MPa and 3.62 MPa,respectively,and the strength characteristic value was 2.95 MPa.The rolling shear modulus and strength of eucalyptus were 7.6% and 11.9% lower than that of birch,respectively.The rolling shear strength of tree species with different pipe hole types is quite different,and the rolling shear performance of the loose hole material is better than that of the ring hole material;the rolling shear modulus and the average value of the intensity of the ash(fine wood ray)are respectively It is 243 MPa and 3.58 MPa,and the strength characteristic value is 2.76 MPa.The average rolling shear modulus and strength of eucalyptus(wide wood ray)were 489 MPa and 3.38 MPa,respectively,and the strength characteristic value was 2.98 MPa.The rolling shear modulus of ash is 50.3% lower than that of eucalyptus,and the rolling shear strength is 5.9% higher than that of eucalyptus.The rolling shear modulus of different tree ray broad and narrow trees has a large difference;there are significant differences in CLT rolling shear performance of different early and late wood species.The average rolling shear modulus and strength of Douglas fir(early and late material)are 173 MPa and 2.34 MPa,respectively,and the strength characteristic value is 1.74 MPa.The average rolling shear modulus and strength of Pinus sylvestris var.mongolica(later and late material)are 109 MPa and 1.73 MPa,respectively,and the strength characteristic value is 1.32 MPa.The rolling shear modulus and strength of Douglas fir were 58.7% and 35.3% higher than that of Pinus sylvestris var.Secondly,the strain change trend of CLT specimens with different structural features was tested by DIC,and the influence of different structural features on the shear failure mechanism of CLT was analyzed.The test results show that when subjected to shear stress,most of the CLT specimens firstly generate stress concentration at the unbonded side of the transverse layer,and finally the rolling shear failure occurs in the weak structural area of the wood near the unbonded side.For the annular hole material in the hardwood material,the dense hole in the early material area belongs to the weak area of the sawing structure;the loose hole material in the hardwood material usually breaks along the annual ring or wood ray direction.For coniferous wood,the early wood area at the junction of the early and late materials was the first to be destroyed,which was a relatively weak area on the structural characteristics of the wood.At the same time,the different transition forms of the early and late materials did not significantly affect the failure mode of the CLT specimen.The paper also used domestic fast-growing eucalyptus as the research tree species,and tested the effects of different width-thickness ratios on its rolling shear performance.The test results show that the rolling shear modulus and strength of the specimens with width to thickness ratio of 2,4 and 6 are 370 MPa,427 MPa,503 MPa and 3.38 MPa,3.63 MPa and 3.74 MPa,respectively.The rolling shear modulus and strength of CLT are both As the aspect ratio increases,it increases.When the aspect ratio is 6,the rolling shear modulus and strength are the largest,35.9% and 10.7% higher than the width to thickness ratio of 2,respectively,and 17.8% and 3% higher than the width to thickness ratio of 4.Different width-to-thickness ratios have a significant effect on the rolling shear modulus,and have no significant effect on the rolling shear strength.When the width-to-thickness ratio of the test piece is increased,the gap of the unbonded area on the side of the transverse layer is also reduced,thereby reducing the high shear strain area of the test piece and improving the ability of the test piece to resist shear stress,thus being thick and thick.The rolling shear performance of the eucalyptus specimens with a ratio of 6 is significantly better than that of the eucalyptus specimens with a width to thickness ratio of 2.At the same time,compared with SPF,domestic fast-growing eucalyptus has higher rolling shear modulus and strength,which are 261.9% and 89.1% higher than SPF,respectively.In the preparation of CLT,the use of fast-growing eucalyptus as a transverse layer can significantly improve its rolling shear performance,and eucalyptus can be used as a candidate tree species for CLT products.Finally,SPF and Douglas fir were used as the transverse layers of CLT specimens in this paper,and the rolling shear modulus was measured based on the testing principle of DIC.Meanwhile,the measurement results of the traditional displacement meter test method were compared.Test results show that the DIC SPF and fir rolling shear modulus were 115 MPa and 127 MPa,the traditional displacement meter testing SPF and fir of rolling shear modulus were 118 MPa and 131 MPa,and test method of traditional displacement test values higher than DIC test method test values respectively 2.6% and 3.1%,two methods of test test values of relative error is within 5%.