Long Term Deformation Prediction Model For Laminated Bamboo Based On Accelerated Creep Method

Laminated bamboo is a low carbon,renewable oriented bamboo based fiber composite material,which has broad application prospects in engineering construction.However,as a new type of biomass material,it is prone to viscoelastic deformation,namely creep,under long-term loading.Once the creep deformation of materials exceeds the allowable deformation within the structural design reference period,the functional requirements for applicability will be lost,and even creep damage to the structure will occur.Therefore,it is of great significance to deeply study the creep properties of laminated bamboo and explore its creep laws during the design reference period.The main research contents and conclusions of this article are as follows:(1)Through conducting 500 h constant temperature and humidity creep tests on three groups of 15 laminated bamboo tension,compression,and bending specimens,the tension,compression,and bending creep deformation time curve of laminated bamboo was obtained,and the creep behavior under different stresses was analyzed.The results showed that during tensile creep,the material exhibits linear viscoelastic behavior under all stresses,and the creep curve has only two stages: transient and steady-state creep;In compression creep,when the stress level is below 40%,the material exhibits linear viscoelasticity,while when the stress level is above 40%,the material exhibits nonlinear viscoelasticity.The creep curve has three stages: transient,steady state,and accelerated creep;In bending creep,when the stress level is below 40%,the material exhibits linear viscoelasticity,while when the stress level is above 40%,the material exhibits nonlinear viscoelasticity,and the creep curve has only two stages: transient and steady-state creep.(2)Two creep mechanics models and two mixed creep models were used to fit the relative creep coefficient curve of laminated bamboo under compression/bending,and the corresponding constitutive model parameters were obtained.Further,the creep prediction of laminated bamboo during the design reference period of compression/bending was conducted,and the advantages and disadvantages of various creep models used for predicting the creep deformation of structures/components during their design reference period were analyzed.The results showed that there are obvious defects in the 3Kelvin model and the Burgers model.The former can lead to premature stabilization of the creep coefficient,while the latter can lead to nearly linear growth of the creep coefficient when the prediction cycle is long.The five-parameter model and the Kelvin-Log model exhibit a reasonable time-varying trend.(3)Accelerated creep tests were conducted on three groups of 30 laminated bamboo tension,compression,and bending specimens using the stepped isothermal method(SIM).The accelerated creep deformation time curve of laminated bamboo under tension,compression and bending conditions was obtained,and the creep behavior under different stresses was analyzed.The results showed that the tensile accelerated creep curves all exhibit a state of linear creep,with no creep in the third stage and ultimately brittle failure.In compression accelerated creep,when the stress level is above 40%,the creep curve changes from linear to non-linear,and finally enters the third stage of creep,with the specimen plastic failure.The bending accelerated creep curves all entered a nonlinear state from a linear state,and there was no significant third stage of creep,resulting in brittle failure.(4)The SIM method was used to fit the accelerated creep curve of laminated bamboo under tension/compression/bending,and the creep prediction curve within the 50 year design reference period was obtained.The prediction curves of the five-parameters and the Kelvin-Log model were further compared,and the advantages and disadvantages of the three prediction curves were analyzed.The results showed that the life of tension parallel to grain can be predicted using the SIM-Master curve;The creep change rate of compression parallel to grain can be predicted using a five-parameter model;The change rate of bending creep can be predicted using a five-parameter model and a SIM-Master curve,and the life of bending specimens can be predicted using a SIM-Master curve.The Arrhenius equation was used to fit the horizontal displacement coefficient,verifying the applicability of the SIM method for constructing the tensile/compressive/bending creep master curve of laminated bamboo.A prediction equation for the service life of compressed and bent laminated bamboo specimens was established based on the accelerated creep Master curve.(5)The regulations on long-term deformation in wood structure standards of the United States,Europe,China and Japan were summarized,and their differences were analyzed.Finally,based on the experimental results in this paper,a long-term deformation calculation formula for laminated bamboo under dead load and cyclic live load was established.