The influence of compression failure on the bending, impact bending and tensile strength of spruce wood and the evaluation of non-destructive methods for early detection

摘要

Es wurden die Biegefestigkeit und der Biege-E-Modul nach DIN 52186 und die Biege-E-Moduln errechnet aus Eigenfrequenz und Schallgeschwindigkeit an kleinen Fichtenproben mit und ohne Winddruckstauchungen untersucht. Ein Teil der Proben wurde vor der Prüfung bei Normalklima (20℃/65% relative Luftfeuchte) konditioniert, der andere Teil nach der Konditionierung bei Normalklima einen Monat in Wasser gelegt und danach geprüft. Bei den Proben mit Stauchungen war die Festigkeit gegenüber den Proben ohne Stauchungen sowohl bei den normalklimatisierten als auch bei den wassergelagerten Proben um mehr als 20% reduziert. Der E-Modul nach DIN war bei den normalklimatisierten Proben durch die Stauchungen nur wenig reduziert, bei den wassergelagerten Proben jedoch um mehr als 30%. Die aus der Eigenfrequenz und der Schallgeschwindigkeit errechneten E-Moduln zeigten dagegen nur eine geringe bzw. keine stauchungsbedingte Reduktion. Es war deshalb nicht möglich, mittels der zerstörungsfreien Prüfmethoden Eigenfrequenz und Ultraschall die durch die Stauchungen bedingte Festigkeitsreduktion der Proben zu erfassen.%Bending strength (MOR) and bending Young's modulus (MOE) according to DIN 52186 and MOE calculated on the basis of eigenfrequency and sound velocity were tested on small clear wood specimens of Norway spruce wood with and without compression failure. One group of specimens was climatised in a normal climate of 20℃ and 65% relative humidity, while the other group was stored for one month under water before testing. The MOR of specimens with compression failure decreased about 20% on average (normal climate and wet) compared with the specimens without compression failure. The MOE of the specimens with compression failure was reduced only minimally compared with the specimens without compression failure stored in a normal climate, but very distinct differences (more then 30%) were found under wet conditions. The MOE of the specimens with compression failure calculated on the basis of eigenfrequency and sound velocity were not reduced or only minimally compared with the specimens without compression failure. It is therefore not possible to detect compression failure and to determine reduction in MOR using eigenfrequency or sound velocity. In addition, impact bending (DIN 52189), tensile strength and tensile MOE (DIN 52188) were tested on small clear wood specimens of Norway spruce wood with and without compression failure. The specimens with compression failure revealed an average reduction in impact strength of about 40% and an average reduction in tensile strength of about 20% compared with the specimens without compression failure, whereas tensile MOE of the specimens with compression failure was not reduced compared with the specimens without compression failure. The detection of compression failure by computer tomography (CT) was tested on Norway spruce wood boards 10 cm in thickness, and detection by optical scanner was tested on planed Norway spruce wood boards. CT recognised large compression failures easily, whereas the scanner was not able to detect them.