Experimental And Nondestructive Examination Studies On The Material Mechanical Properties Of Chinese Fir Structural Timber

The standard small specimens were widely used in the existing determination method such as China's national standards,Japan's JIS or the United States's ASTM to obtain the material properties of ancient architecture aged wood.However,almost all mechanical material indicators in the test result of these standard small specimens were larger than that of full-scale structural timber,which can not make an good evaluation on the mechanical analysis of real structures.Addressing this issue,it is necessary to carry out a mechanical material property studies based on full-scale structural timber.In this paper,the full-scale fir structural timber obtained from the renovation project of Yangzhou folk houses with the age of 50-100 years were employed as the tested specimen materials.By means of nondestructive examination(such as stress wave,ultrasonic and Pilodyn),standard small specimen and full-scale testing approach,the change rule of structural timber physical and mechanical properties were studied.This paper explored the relationship between the test results of various detection methods(such as nondestructive examination and small specimen test)and the mechanical properties of structural timber,which can provide a reference for the quantitative evaluation of nondestructive examination techniques,and a scientific basis for the state evaluation,safety limit and performance maintenance of ancient timber construction.The main research conclusions were summarized as followings:1.The material properties of structural timber was obviously different along radial direction.From sapwood to heartwood,the density,compressive strength and the elastic modulus of new fir decreased by more than 10%,while the properties of old fir kept unchanged,except the elastic modulus decreased by 6%-16.25%.Therefore,it is necessary to distinguish the sapwood and heartwood of the structural timber along the radial direction,when the standard small specimens were used to study the change law of the material properties.The test results indicate that the density,compressive strength,elastic modulus and the dynamic elastic modulus of nondestructive testing of aged fir heartwood increased by 9.31%-19.47%,and the correlation properties of aged fir sapwood decreased slightly or stay unchanged.The wood surface was also softened.2.There existed obvious difference on the change rule of old wood between the standard small specimens and the full-scale structural timber.The results of standard small specimens show that the compressive strength of old Chinese fir increased by 7.47%,and the elastic modulus of the grain was basically unchanged.The results of full-scale structural timber show that the compressive strength of old Chinese fir reduced by 23.4%,and the elastic modulus of the grain reduced by 8.6%.Therefore,the latter result was more suitable for mechanical analysis of real structures.The structural timber material strength can be calculated by the material strength of standard small specimens multiplied a certain reduction factor.3.The results of nondestructive examinationhas the strong link with the properties of structural timber.The propagation velocity of stress wave detection was significantly correlated with the moisture content oftimber.The dynamic elastic modulus of ultrasonic detection was expressively correlated with the density,the compressive strength and the elastic modulus of small specimen.The penetration depth of Pilodyn was meaningfully correlated with the moisture content,the strength of timber,the compressive strength and the elastic modulus of small specimen.On the basis of a large number of experiment data,the material properties of timber can be calculated by the nondestructive examination results,and the quantification can be realized.