Non-destructive testing (NDT) can be defined as the science of testing the mechanical and physical properties or the defects of a piece of material or structure, without having to damage or alter the integrity. This allows the end-user greater capabilities.
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Some Common NDT Methods Applied to Wood
Stress Wave Method
This method is based on the speed of sound and the rate of its attenuation. The strength and stiffness of wood are determined on the same fundamental quantities, which depend on the combination of these factors.
This is a simple in-situ technique that is very simple to apply and does not require any preparation before it can be used. Therefore, it is an effective tool that is used by structural inspectors. It has significantly reduced the error of judgment during the wooden member’s structural analysis.
A grid system is the base of detection of the levels of decay. The grid is drawn on the opposite sides of the surface that is being inspected. The stress wave transmitter and the receiver transducer are placed at opposite sides of the grid system of the member. As a result, the measurement is in a transverse direction. The speed of the sound wave is significantly reduced in the decayed portions of the wooden member when compared with the intact areas.
Ultrasound can be defined as a high frequency of sound that is not audible. For wood, the ultrasound spectrum lies between 20 kHz to 500 kHz. This relatively low frequency is used because of wood's high attenuation of sound waves. The through transmission and the pulse-echo methods are the two most commonly used methods in this category.
For the electric pulse generator, a signal is sent through a transmitter transducer and if there exists a flaw or defect in the path of the ultrasound, it will be partially reflected. The reflected signal is received by a transmitter while the reduced one will be received by a receiver. The proportion of the two signals is used as a basis for further investigation of the defects that exist internally.
A pulse-echo method will only have a single transducer that serves as both a receiver and transmitter of signals. Therefore, only the reflected pulse will be measured using a pulse-echo method. The time between the initiation of the signal and its reflection will be shorter if the wave is reflected by the defects in the wood.
The velocity of ultrasound also increases with the reduction of moisture content in the wood. Decay and discontinuities in the wood will slow the propagation of the ultrasound. However, this method is not suitable for detecting defects such as wormholes, splits, and cracks.
This method is based on the resistance that a small drill bit faces as it penetrates the wooden material at a constant speed which correlates with density. A seismograph is a type of equipment that has been based on this principle. The needle of this piece of equipment records the resistance of the wooden material and translates this resistance into the density of the wooden timber.
An x-ray is also used to measure the density of the wood. The wood defects which can be a result of splits, knots, or insect damage will significantly alter the density of the wood and will sufficiently change the absorption of x-rays. This helps in detecting the defect successfully. This method is widely used in grading lumber, preservative distribution check, dendrochronology, and inspection of utility poles.
Applications Of NDT In Forest Products
Detection Of Defection In Solid Wood
Wood being a natural material will contain intrinsic flaws such as knots, resin pockets, knots, etc. Insects’ attraction to wood or fungi can easily deteriorate the integrity of the wood structure. Defect detection is an accurate method that can result in an optimized sawing decision to manufacture forest products, and consequently increases the yield of the process of production, the finished value product, and its uniformity.
Grading Or Sorting Of Structural Products
Wood products show a variety when compared to synthetic products such as plastics, metal, or concrete. Therefore, the properties of the wood such as its stiffness and strength will have to be assessed individually to sort or grade each wood product. Visual inspection has been the traditional way to detect any defects on wooden material. NDT’s are more reliable and repeatable to grade or classify wooden products as compared to using visual inspection.
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A variety of conditions such as the change in moisture content as a result of insect and fungal attacks can be tested on timber with the help of NDT’s. These conditions will affect the properties of the wooden product such as its stiffness, density, the slope of grain which can all be tested at the same time with a combination of different NDT’s.
In-Situ Evaluation Of Wooden Members
For structural safety, a wooden structure will need to pass a minimum strength level. Because of the traditional visual inspection of wood, significant amounts of old wood has in the past had to be replaced despite it still being structurally intact. NDT techniques have been introduced to minimize such misjudgments.
Monitoring The Health Of Living Trees
Better cutting decisions can be made by foresters who monitor the health condition of the forests to help increase the yield of the forest products. Most NDT techniques used will indirectly estimate its strength properties.
Non-destructive testing is a necessary technology that is applied not only to timber but also to other construction materials to test the mechanical and physical properties of concrete, bamboo, steel, etc. For example, common methods for testing concrete testing include the Windsor probe and the concrete test hammer whereas for steel, magnetic particle testing, ultrasonic weld testing, thickness testing, and the hydraulic anchor testing are some common methods used.
References and Further Reading
Twi-global.com. n.d. What is Non-Destructive Testing (NDT)? Methods and Definition. [online] Available at: <https://www.twi-global.com/technical-knowledge/faqs/what-is-non-destructive-testing>
Zombori, B., n.d. "In situ" Nondestructive Testing of Built in Wooden Members. [online] Ndt.net. Available at: <https://www.ndt.net/article/v06n03/skatter/skatter.htm>
Limble. n.d. Non Destructive Testing: Practical Applications Of NDT Methods. [online] Available at: <https://limblecmms.com/blog/non-destructive-testing-ndt/>