In nature, we often divide living things into animals and plants. Wood belongs to the category of plants. Different tree species give people different feelings of touch. Wood has unique natural properties and is a natural biological material. Wood has been accompanying humans from low-level primitives to advanced stages of chemical and mechanical processing. It is a material that humans have used for thousands of years. It is not only our means of living but also our means of production. Therefore, wood protection is particularly important today with highly developed science and technology. One of the main areas of application of non-destructive testing technology is ancient and valuable trees and ancient wooden structures. Their protection is an important part of wood protection.
Non-destructive testing, also known as non-destructive testing, uses the different chemical properties or physical mechanics of the material to test the relevant properties of the target object (such as mechanical properties, optical properties, shape, stress , displacement, fluid properties, etc.) for inspection and testing, the measurement of various defects is particularly important. The biggest feature of non-destructive testing is that the results can be obtained in a short time without damaging the original characteristics of the material. The operator can make quick judgments and make correct decisions.
1. Application of non-destructive testing technology
1.1 Observe with the naked eye
This non-destructive testing method is the oldest and simplest method still used today and can help verify and judge non-destructive testing results. When judging changes in components and products, visual identification and observation are required. Through careful analysis and observation of late decay, mechanical damage, severe insect infestation and broken fragments, etc, the results are analyzed and judged based on visual observation to determine the detection Whether products and parts are good or bad or obsolete or qualified.
1.2 Approximate non-destructive testing method
This method is often used together with visual inspection. Mainly include: (1) Use wood resistance tester to measure internal decay of wood. (2) Detect the internal decay of the wood by drilling the wood core; (3) Use a hammer to determine whether the log or pillar is decayed or has holes. (4) Check the surface hardness of the wood, scrape the bark with a knife, and initially judge the degree of decay based on the hardness.
To help the naked eye obtain more information is to use approximate non-destructive testing methods. For example, growth cone wood cores can be used to determine the size and depth of decay. Currently, hammer tapping and naked eye observation are mainly used to determine whether wooden structural components are rotten or hollow, so as to protect ancient trees and repair ancient buildings in my country, and determine whether wooden components need to be replaced based on these.
1.3 Stress wave
Acoustic stress waves are made to vibrate by using a given stress or impact. The main method currently used is impact to generate vibration. It is the most commonly used non-destructive testing method for safety evaluation of wooden structures of ancient buildings. The acoustic stress wave method often uses the method of measuring vibration spectrum and measuring the speed of sound propagation for analysis. Because the measurement of sound velocity is simple and easy, it is commonly used to calculate the residual dynamic elastic modulus of wooden components by measuring the sound velocity. When the propagation speed perpendicular to the direction of the wood grain increases rapidly, the wood is infested or decayed. Under normal circumstances, when wood is seriously damaged, the propagation speed of stress waves increases by 50%; when the strength loss of wood has reached 50%, the propagation speed of stress waves increases by 30%. The best way to detect decay is laterally (radial or chordal). An important reference and basis for non-destructive testing to determine and analyze the longitudinal propagation speed of wood and whether it is decayed is the propagation speed of stress waves in healthy wood.
There is also a method of measuring the stress wave vibration spectrum, which can overcome the shortcomings caused by detecting the propagation speed, because the acoustic stress wave spectrum can select multiple variables from its spectrum, such as main frequency, frequency displacement, maximum energy, etc. It can provide all the characteristics and conditions of the material being tested, and can detect variables that have little to do with the environment.
1.4 Ultrasonic (stress) waves
The ultrasonic stress wave method is basically the same as the acoustic stress wave method. The main difference between them is that the frequency of ultrasonic wave application exceeds 20kHz.
The principles of ultrasonic measurement are divided into two types: pulse-response system and penetrating stress wave system. The pulse-response system can measure the depth of wood decay and measure and record the characteristics of the echo waves that propagate to the internal surface of the material; penetrating stress wave system It means that the ultrasonic wave propagates along the thickness direction of the wood being detected, and the acoustic wave characteristics of the wood being detected are recorded on the other side.
1.5 Other non-destructive testing technologies
(1) X-ray: This is a method commonly used in production lines and laboratories. It is mainly used to detect wood microdensity, wood knots, internal decay of wood, etc., such as common X-ray wood defect detection. System, soft X-ray wood microdensity tester, etc.
(2) Electrical method: Determine the moisture content of the wood, and use the correlation between the moisture content of the wood and the resistance of the wood to conduct non-destructive testing. Detecting wood decay in the field can exploit wood resistance characteristics.
(3) Gamma rays: Gamma rays can be used to quantitatively determine the distribution of preservative trace elements in wood, and can also quantitatively detect the degree of internal decay of wood. The disadvantage of this detection method is the use of radioactive elements.
2 Main uses of non-destructive testing in wood protection
2.1 Non-destructive testing of wood moisture content
An important indicator that determines and affects the use of wood is the moisture content of wood. For wooden components of ancient buildings, moisture content is of even greater significance. Generally speaking, wooden components with too high moisture content mean that the possibility of diseases and insect pests in ancient building wooden components increases, which must be paid attention to. Commonly used non-destructive testing instruments for wood moisture content are developed and manufactured based on principles such as microwave, high-frequency current, infrared, direct current, and dielectric constant.
2.2 Application of non-destructive testing on ancient and valuable trees
Ancient and valuable trees are important cultural heritage, important natural landscapes and resources, and are valued and protected in all countries around the world. Strengthen the protection of ancient and famous trees, and detect internal defects of ancient trees without causing new disasters or damaging their growth. At present, some developed countries abroad also use non-destructive testing technology to protect urban trees.
2.3 On-site inspection of wooden structural components of ancient buildings
Use non-destructive testing technology to protect and repair the wooden structures of ancient buildings, and evaluate the safety of the wooden structures. Without destroying the original wooden components, we can provide strong evidence through non-destructive testing, so that we can design and replace wooden components or determine repairs before repairs. Waiting for work. Non-destructive testing mainly detects internal defects of wooden components and residual strength of wooden components, and evaluates the service life, safety and reliability of wooden structures.
Conclusion: Human beings must pay attention to protection when using natural resources. Only when nature has a virtuous cycle can human beings develop sustainably. Timber has a large demand and a wide range of applications. When human beings ensure the needs of life and production, they must also protect the ecology and wood. This also shows that non-destructive testing technology has a broader space and a better prospect.
