Investigation On Screening Internal Defects Of Historic Wood Structural Members Based On The Tap Force Characteristics

There are a large number of ancient wooden structures in China,and the main structural members of the wooden structures are columns,beams,rafts,and so on.Since the wood itself is an organic material,the wood will have different defects after a long period of use,such as holes,decay,cracks,and so on.Internal holes and internal decay occur inside the wood members and are not easily noticeable.The presence of these defects seriously affects the safety and use of the entire building.Non-destructive testing is the main method used to detect internal defects in wood members of ancient buildings.The tapping method belongs to the non-destructive testing method and is also the main method in the preliminary screening stage.However,the accuracy of the screening results of the internal defects of the wood components is very low.This paper attempts to screen the internal defects of wood components from the perspective of the impact force signal,and tries to lay the foundation for improving the scientificity and screening accuracy of the percussion screening method,as well as instrument the screening method and getting rid of the constraints of human factors.Based on the principle of local tapping of the tapping force signal,this paper took the circular section wood column and rectangular section plate which simulates the hole defect as the research object,and used the self-written Lab VIEW program to collect and analyze the local tapping signal.The tapping impact time ? and the ratio R were selected as the time-frequency domain identification parameters of this paper.The relationship between parameters and impact force,tapping position,surface hardness,defect size,hammer material and other factors were studied by tapping test.The recognition effect of the two parameters was compared.The results showed that the tap impact time ? and the ratio R were basically unchanged with the change of the tapping force.The reasonable range of the tapping force was 150?450N in general.Under the three kinds of contact materials,the aluminum alloy hammer The material identification effect was the best;the r and ratio R had significant correlation with the length of the defect,and the determination coefficients were 0.865 and 0.904 respectively;r had a good linear relationship with the surface hardness,and the regression formula was y=-0.04x+3.09.The coefficient of determination was up to 0.93.The criteria for identifiable internal defects were determined:when the rate of change of the two tapping parameters was greater than the maximum volatility of the two parameters of a good region of the same hardness,it was considered that there is an internal defect at the tapping position.Considering other random factors and engineering detection convenience,determining the two parameters could identify whether the internal defect has a change rate threshold of 15%and 5%.Both the defect size and the depth affected the defect stiffhess.The stiffness of the smallest defect that can be tapped was related to the surface stiffness.When the defect stiffness kd was 3.1 times the surface stiffness kc,the internal defect can be identified by tapping test Verification testing of the actual defective column showed that the tapping method can effectively identify internal defects with a defect stiffness less than a certain value.