Research On Rolling Shear Failure Mechanism Of Cross-laminated Timber Based On Acoustic Emission

Cross-laminated timber(CLT)is widely used in mid-rise and high-rise timber constructions,owing to a series of its excellent properties such as high strength,fire resistance and earthquake resistance.When CLT is used as a load-bearing structure such as floor sheathing or roof sheathing,it bears the out-of-plane load perpendicular to the sheathing surface,making the transverse layer prone to rolling shear failure.Thus,it has strong significance in practice to study the damage evolution law of CLT rolling shear failure for better understanding the structural properties of CLT and ensuring the safety and stability of large-scale timber constructions.Based on Acoustic Emission(AE)and Digital Image Correlation(DIC)method,real-time monitoring throughout the rolling shear failure of CLT was carried out in this paper.And AE parameter analysis was used to study the AE characteristics(micro-crack initiation and propagation)in the transverse layers of CLT with different longitudinal layer materials(SPF and recombined bamboo)and different transverse layer species(SPF,Douglas fir and Scots pine).Meanwhile,according to the strain information collected by DIC,the surface strain field characteristics of CLT with different transverse layer species were analyzed.It was further studied whether the AE parameter analysis has a predictive effect on the damage evolution of CLT rolling shear failure.And the law of the initiation and propagation of micro-cracks in the transverse layer of CLT during rolling shear failure.In addition,cluster analysis was performed on the AE signals obtained during the CLT rolling shear failure process to establish an intuitive connection between the rolling shear failure mechanism and the AE signals.First of all,the thesis studied the rolling shear strength and failure modes of CLT specimens with different longitudinal layer materials and different transverse layer species.The results showed that the average rolling shear strength of SPF and recombinant bamboo were 2.13 MPa and 2.62 MPa respectively in CLT specimens of different longitudinal layer materials.And the rolling shear strength of the recombinant bamboo was 18% higher than that of SPF.It indicated that the longitudinal layer material had a significant effect on the CLT rolling shear strength.Among CLT specimens of different transverse layer species,the average rolling shear strength of Douglas fir and Scots pine were 2.20 MPa and 1.69 MPa,respectively.The average rolling shear strength of Douglas fir was close to that of SPF,and was only 3% higher than that of SPF.Scots pine had the lowest average rolling shear strength,30% and 26% respectively lower than that of Douglas fir and SPF.The CLT rolling shear strength was significantly influenced by the transverse layer species.The cracks originated from the weak point of the transverse layer,and then extended along the direction of the wood ray or the limit of the early wood and late wood,which eventually caused the rolling shear failure of CLT specimens.The rolling shear failure process of with recombinant bamboo as the longitudinal layer material was short and severe.Then,the rolling shear failure test of CLT specimens was monitored in real time using AE technology and DIC method to characterize and evaluate the rolling shear failure process.It was found that the cumulative energy curve and cumulative ringing count curve had similar trends over time during the rolling shear failure process of CLT specimens.There was a good correlation between the amplitude,energy,and ringing count.Among CLT specimens of different transverse layer species,Douglas fir specimen was more prone to AE signals than SPF specimen and Scots pine specimen.Its interior was the first to be damaged in the early stages.The damage evolution process of CLT rolling shear failure could be divided into two stages: damage accumulation and failure.The amplitude of the AE signals in the first stage was low,and the cumulative energy curve and cumulative ringing count curve grew slowly,while the cumulative energy curve and ringing count curve rose sharply in the second stage.AE technology could accurately characterize the internal damage evolution(crack initiation,propagation,etc.)of the transverse layer.The DIC method could effectively and intuitively measure the surface strain of CLT,and predict the location of the crack initiation of the transverse layer in advance according to the evolution of the strain concentration area.The combination of AE technology and DIC method could monitor the damage evolution process of CLT rolling shear failure from the inside-out.Finally,the cluster analysis of the AE signals was used to establish the connection between the rolling shear failure mechanism of CLT specimens and the AE signals.The results indicated that the AE signals generated by the rolling shear failure process of four different CLT specimens were divided into two or three clusters,of which the AE signals that dominated the rolling shear failure mainly fell into two clusters,namely the major AE signal and the secondary AE signal.The major AE signal accounted for 70-88% of all AE signals,the amplitude range was 52-70 dB,and the duration was long.It originated from the initiation of micro-cracks in the transverse layer of CLT(internal damage accumulation).While the secondary AE signal accounted for 12-29%,and it included all high-amplitude AE signals.Its duration was shorter than the major AE signal.And it was caused by the extension of the cracks.