Duration Of Low Cycle Fatigue Load Effect On CLT Fabricated With Fast-growing Chinese Fir

At present,the research on the mechanical properties of cross laminated timber（CLT）mainly focuses on the short-term mechanical behavior.As a load-bearing member,the mechanical properties of CLT under long-term load or fatigue load are also the key properties of CLT members.Developing domestic fast-growing Chinese fir as CLT laminate material is conducive to the development and application of domestic CLT materials.The research of this paper is helpful to improve the mechanical properties of CLT and promote the application of fast-growing Chinese fir in CLT materials.Firstly,the dynamic elastic modulus of 600 fast-growing Chinese fir sawn timber was measured by stress wave nondestructive testing method,and the fast-growing Chinese fir sawn timber was graded;Then,different grades of fast-growing Chinese fir sawn timber were used to form CLT members with two different laminate grades.The bending and shear properties of CLT specimens under static and low cycle fatigue（trapezoidal wave and triangular wave）loading were evaluated;Finally,the damage accumulation model is calibrated according to the low cycle fatigue test data,and the adjustment coefficient of duration of load effect（DOL）of fast-growing Chinese fir CLT is evaluated based on the stress ratio.The main conclusions are as follows:（1）Nondestructive testing and grading of fast-growing Chinese fir sawn timber.The maximum,minimum and average density of fast-growing Chinese fir sawn timber are 0.57g·cm^-3,0.27g·cm^-3 and 0.40g·cm^-3 respectively,and the dynamic elastic modulus is 13.58GPa,6.00GPa and 9.36GPa respectively.The relationship between the density of full-scale fast-growing Chinese fir sawn timber and the elastic modulus is not obvious.After grading,the average value of dynamic elastic modulus and density in each grading interval has a good correlation,and the correlation coefficient R² is 0.9116.The density conforms to lognormal distribution but does not conform to normal distribution and two parameter Weibull distribution,and the dynamic elastic modulus conforms to normal distribution but does not conform to lognormal distribution and two parameter Weibull distribution.The fitting goodness of fitting density and dynamic elastic modulus with lognormal distribution and normal distribution is the best,and the K-S test value is the smallest.The coefficient of variation of dynamic elastic modulus in each classification interval is significantly lower than that of the whole.The larger the classification interval is,the smaller the coefficient of variation is.（2）Static bending and shear properties of fast-growing Chinese fir CLT.The bending elastic modulus,bending strength and shear strength of fast-growing Chinese fir CLT（HLH）made of high-grade longitudinal laminates and low-grade transverse laminates are higher than those of fast-growing Chinese fir CLT（LLL）made of three-layer low-grade laminates.The dynamic elastic modulus（8155MPa and 10798MPa）of low-grade and high-grade laminates respectively meet the elastic modulus requirements（6500MPa and 10300MPa）of E3 CLT secondary strength direction and E2 CLT primary strength direction laminates in ANSI/APA PRG 320-2018.The average values of bending elastic modulus,bending strength and shear strength of specimens with LLL and HLH lamination layer grade are significantly different at the level of 0.01.The relative errors between the apparent bending stiffness,effective bending strength and effective shear strength of LLL and HLH lamination layer grade specimens and the test values are within10%.The low-grade longitudinal laminate specimens mainly have transverse layer rolling shear failure and bottom tensile failure,while the high-grade longitudinal laminate specimens mainly have transverse layer rolling shear failure.（3）Low cycle fatigue properties of fast-growing Chinese fir CLT.The average failure cycle number of low cycle bending and shear fatigue specimens from small to large is low-grade laminate specimen under trapezoidal wave loading<high-grade laminate specimen under trapezoidal wave loading<low-grade laminate specimen under triangular wave loading.The number of failure cycles of specimens with different loading waveforms is significantly higher than that of specimens with different lamination layer grades.Under the same cumulative probability level and stress ratio,the number of failure cycles of triangular wave specimen and high-grade longitudinal laminate specimen are higher than trapezoidal wave specimen and low-grade longitudinal laminate specimen respectively.The failure mode of low cycle fatigue specimen is basically the same as that of static specimen.It is impossible to distinguish the difference between bending and shear fatigue specimens under different cyclic loading waveforms only from the failure mode;The high-grade longitudinal laminate CLT mainly occurs transverse layer rolling shear failure,while the low-grade longitudinal laminate CLT mainly occurs rolling shear failure and bottom tensile failure.The proportion of bottom tensile failure of low-cycle fatigue specimens of low-grade and high-grade longitudinal laminates is like that of static specimens.（4）Duration of load effect on fast-growing Chinese fir CLT.The predicted results of the cumulative damage model are consistent with the test results.The fitting effect of PG-H-T group is the best,and there are acceptable errors between the test values and predicted values of other groups.The number of failure cycles of bending specimens is generally higher than that of shear specimens.The DOL adjustment coefficient of triangular wave specimen,high-grade laminate specimen and bending specimen under the continuous action of standard period and long-term constant load is higher than that of trapezoidal wave specimen,low-grade laminate specimen and shear specimen respectively,showing better long-term strength.The DOL adjustment coefficient of fast-growing Chinese fir CLT under the continuous action of long-term dead load for 50 years without considering the service conditions is close to or higher than the DOL adjustment coefficient（0.65）recommended by CSA O86-14 in CLT handbook except PG-L-T group（0.41）.The DOL adjustment coefficient of fast-growing Chinese fir CLT in PW-L-T group（0.64）,PW-H-T group（0.68）and PG-H-T group（0.66）under 50 year long-term constant load is equivalent to that of common solid wood.