Nondestructive testing of finger-jointed structural timber: Overview of possible methods, results of preliminary evaluations, and possibilities for industrial implementation

Structural timber including finger joints is an economically interesting product, because it is made from timber with smaller dimensions. Requirements for material properties and guidelines for the manufacturing process and the factory's internal quality control are composed in DIN 68140-1 and DIN EN 385. However, to support quality assurance and to avoid quality problems, nondestructive testing (NDT) methods are requested. Possible methods for NDT of finger joints can be taken from the numerous publications about NDT of wood. These methods are reviewed and evaluated regarding the special features of finger jointed timber: For ultrasound transmission (perpendicular to the wood fiber or diagonally through the joint) transmitters and receivers have to be coupled well to the timber. Stress-wave techniques along the beam are easier to apply but the measured parameters (stress wave velocity, frequency spectrum) and evaluation methods have to be selected very carefully to localize weak zones. Excitation and measurement of bending vibrations is easy to handle as well but this method works properly only if weak joints in the beam affect the vibration modes. A novel method is local ultrasound excitation together with a thermographic detection of heat which is generated by friction in splits or poorly glued joints. Among the mechanical techniques stress-grading (bending test) is not suitable for large cross-sections and proof-loading (tension test) is a destructive method. Tomography using X-rays is also available but at high expenditures. Several of these possible methods have been tested but for the time being there is no favorite technique which has already proved to solve the problem. The wood industry is still looking for a safe technique with only small expenses for sensors and data acquisition but the first steps to achieve this goal have been made.