Editor's note: Recently, Zhejiang Agriculture and Forestry University has developed a wood stress wave tomography imager in the final of the thirteenth "Challenge Cup" National Bank of China University Extracurricular Academic and Scientific Works Competition. This technology is in line with international standards and adopts two-dimensional imaging, which allows people to see the image of the wood interior at a glance.
Stress wave propagation allows non-destructive testing of trees
Using the stress wave propagation time as the principle for non-destructive testing of the internal structure of the tree, you can obtain a two-dimensional or three-dimensional image of the internal cross-section of the tree, from which you can find the internal health and decay of the wood and the degree of decay to obtain ancient trees or other Measure the health status of the subject.
The propagation of stress waves in wood is a dynamic process directly related to the physical and mechanical properties of wood. The principle of the stress wave method to detect wooden materials is to use mechanical materials to propagate mechanical waves within the wooden materials after being impacted, and according to the relationship between the elastic modulus E of the wooden materials, the stress wave velocity C and the wooden material density P: E = C2p, determine the elastic modulus of the wood material by measuring the propagation speed of the stress wave, and finally estimate the mechanical strength of the wood material.
The stress wave detection endoscope method belongs to the mechanical wave behavior, which is not harmful to the human body. The industrial endoscope is not restricted by the shape and size of the measured wood. No coupling agent is required between the sensor and the measured wood, and it is convenient to carry. It is widely used in the evaluation of standing wood quality, on-site endoscope inspection of wooden structural parts and evaluation of the mechanical properties of various wooden products.
Know the trees are good or bad without cutting them
A large tree that grows, is there any vacancy in the trunk? Experienced masters tapping with their hands to listen to the sound may be able to identify it. But there will still be missteps. Just like cutting down trees in a forest farm to make materials, I often encounter hollowed trees that are useless and cut in vain.
Professor Li Guanghui, who is responsible for the development of wood stress wave tomography, said that the instrument mainly uses the pulse hammer to hit the tree to cause the propagation of stress waves inside it. During this period, the wood stress wave tomography imager coupled with the pulse hammer , The sensor output signal will be automatically used, processed by the analog circuit and then output to the digital processing circuit, and then the digital processing circuit processes the signal output by the previous stage to obtain a waveform diagram of the stress wave propagation time, and through the wood stress wave tomography imager The display shows. Users can determine the internal defects of wood by measuring the changes of stress wave propagation time and propagation speed, and calculating parameters such as the elastic modulus of wood, and related data of similar tree species than normal.
"Using our instruments, you will know before you cut the tree." Wang Lixia, a student of the School of Information Engineering at Zhejiang Agriculture and Forestry University, said that you only need to nail some nails to the tree to support the sensor. The situation in the trees. "According to the speed of the sensor, the image can be displayed on the computer. The slower the speed of the return, the greater the possibility of showing red and yellow on the computer, because these two colors represent problems with trees, where red indicates problems The most serious, if it shows green, it means the inside of the tree is good. "
A generation of instruments have been used in the Forbidden City
"Our instruments can be used not only on growing trees, but also in wood inspection of ancient buildings. A generation of instruments two or three years ago also tested the pillars in the Forbidden City in Beijing to help management units find problems in advance. Maintenance. "Wang Lixia was proud of the instruments he was involved in research and development.
This pride is not without reason. "Before this, although there were infrared detection method, ultrasonic detection method, nuclear magnetic resonance detection method, etc. in the field of wood research, it was difficult to promote because of the expensive and complicated equipment. The biggest feature of the product we developed is that it does not destroy the original characteristics of the material. In addition, the test results can be continuously obtained in a short time. In addition, our instrument also does not require any coupling agent between the sensor and the measured wood; it is convenient to use and is not affected by the size and shape of the wood; it does not cause harm to the human body, etc. The advantages are much lower than those of similar products at home and abroad. "
The new generation of wood stress wave tomography imager has been technically in line with the international standards. They all use two-dimensional imaging, which allows people to see the image of the wood interior at a glance. However, the price is a fraction of foreign products. "Germany-made instruments are priced at more than 200,000, Hungary's instruments are priced at 300,000, our price is only about 30,000, and the effect is similar." Now, the provincial forestry department has used wood stress wave tomography developed by Zhejiang Agriculture and Forestry University instrument.
Progress at home and abroad
As early as 1950, international research began to use stress wave technology to evaluate the properties of wood. In the 1960s, foreign researchers discovered that due to the presence of wood defects affecting the strength of the wood, the propagation time of the shock wave will be affected, and the nature of the material can be judged by detecting the difference in time. However, due to the limitation of material size and measurement technical conditions at the time, this time difference is difficult to detect. In addition, the specific relationship between the elastic modulus value and the speed is also unknown. It was not until 1988 that in the research of Ross and Pellerin, the relationship between the elastic modulus of wood, the velocity of stress waves and the density of wood materials was clearly defined as E = C2p / g. From then on, the elastic modulus of wood is determined by measuring the stress wave velocity. Furthermore, the evaluation of wood properties can be realized.
In the following years, foreign scholars used statistical analysis methods to conduct statistics and analysis of various measurement parameters by conducting non-destructive testing tests on a large number of wood to evaluate physical and mechanical properties such as elastic modulus and tensile strength of wood. Beijing Delang researchers further verified the feasibility of the stress wave method in wood defect detection and explored its application trend in the field of wood nondestructive testing.
In 1995, Comrade Wang Zhi used stress wave non-destructive testing technology to detect the elastic modulus of medium-density fiberboard. This is the first domestic report on the use of stress wave method to detect wood materials. The results show that the bending elastic modulus of MDF measured by the stress wave nondestructive testing method is sufficiently accurate, and the test data has high accuracy and repeatability, thus laying a foundation for the online automated control of product quality in China's MDF production line basis.
In recent years, many scholars have conducted a lot of wood nondestructive testing research based on the stress wave method, and have achieved certain results in the establishment of stress wave propagation models and the development of stress wave signal acquisition systems, and are more prominent in practical applications. Based on the stress wave detection method, Shang Dajun et al. Used Pilodyn endoscopic detection equipment to conduct on-site non-destructive testing on the decay and moth-eaten wood components of some ancient buildings in Tibet. Through quantitative and qualitative comparisons of the decay and moth-eaten degree of wood components The feasibility of the stress wave endoscope detection method for the detection of the decay and moth-eaten degree of ancient wooden components. In the same year, Duan Xinfang and other circles conducted a preliminary study on the stress wave technique to detect the residual elastic modulus of ancient wooden components. The results show that the nondestructive testing of stress waves can effectively determine the mechanical strength of residual wooden components, and the retention rate of elastic modulus of different types of wooden components It is related to its bearing effect. Zhu Jingle and others summarized the research progress of the stress wave equipment Pilodyn endoscope in the application of genetic improvement of forest trees, and confirmed the important role of stress wave nondestructive testing in the selection of forest wood properties. Li Hua and others innovatively applied the three-dimensional stress wave tomography scanner and resistance meter to the investigation of the wooden structure of ancient buildings, and obtained more accurate evaluation results.
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