Dynamic crushing characteristics of spruce wood under large deformations
An extensive series of large deformation crushing tests with spruce wood specimens was conducted. Material orientation, lateral constraint and loading rate were varied. Regarding material orientation, a reduction in the softening effect and the general force level was observed with a higher fiber-load angle. A comparison with characteristics gained by application of Hankinson’s formula showed discrepancies in compression strength and the beginning of the hardening effect. Lateral constraint of the specimens caused a multiaxial stress state in the specimens, which was quantified with the applied measuring method. Further, a higher force level compared to specimens without lateral constraint and significant hardening effect at large deformations resulted. Thus, the influence of a multiaxial stress state on the force level could be determined. An increase in the loading rate led to higher force levels at any displacement value and material orientation.
Compressive behaviour of axially loaded spruce wood under large deformations at different strain rates
Impact limiting components of packages for the transport of radioactive materials are often designed as wood filled steel constructions. Wood absorbs major part of the impact energy in order to minimise the impact load acting upon the containment. Dynamic impact experiments with wood filled impact limiters showed different crushing mechanisms for axially loaded wood depending on their lateral constraint. Tests on spruce wood samples (Picea abies) were performed in order to clarify the influence of strain rate from static to 30 s−1 on a) compression strength, b) stress at a global strain level of 50%, and c) energy absorption capacity at 50% deformation, including statistical evaluation of the results. Results were as follows: strain rate increase led to significantly higher compression strength, stress and strain energy at a strain level of 50%. Lateral strain restriction had no effect on compression strength; it had a significant effect on stress and strain energy at strain level of 50%. Therefore, the definition of a general yield curve for wood under large deformations is not possible, the yield curve has to be chosen taking into account lateral constraints.ZusammenfassungStoßdämpfende Bauteile von Transportbehältern für radioaktive Stoffe sind oft als holzgefüllte Stahlkonstruktionen konstruiert. Um die Stoßbeanspruchung der dichten Umschließung zu minimieren, absorbiert das Holz einen Großteil der Aufprallenergie. Fallversuche mit stoßdämpfenden Bauteilen haben gezeigt, dass abhängig von seitlicher Dehnungsbehinderung unterschiedliche Kompressionsmechanismen für axial beanspruchtes Holz ersichtlich waren. Versuche mit Fichtenholzproben (Picea abies) wurden durchgeführt, um mit Hilfe statistischer Methoden den Einfluss der Dehnrate von statisch bis 30 s−1 auf (a) Druckfestigkeit, (b) Spannung bei globaler Stauchung von 50 % und (c) Energieabsorption bei einer Stauchung von 50 % zu ermitteln. Folgende Resultate wurden erzielt: Eine höhere Dehnrate führte zu signifikant höheren Druckfestigkeiten, Druckspannungen und Energieabsorptionen bei 50 % Stauchung. Die seitliche Dehnungsbehinderung hatte keinen Einfluss auf die Druckfestigkeit, mit seitlicher Dehnungsbehinderung zeigten sich aber signifikant erhöhte Druckspannungen und Energieabsorptionen bei 50 % Stauchung. Daher ist die Definition einer allgemeingültigen Fließkurve für Holz bei großen Deformationen nicht möglich, die Fließkurve muss unter Einbeziehung der seitlichen Dehnungsbehinderung gewählt werden.
Damping and Poisson factor behaviour in timber considered as an orthotropic material, part 2: the Poisson factor
Abstract In this paper measurements are described which were used to determine damping (loss factors) in two timber species, King William pine and spruce. Tests were conducted by loading uniaxially in each of the three principal material property directions. Testing was conducted over the frequency range 0·5–70 Hz by applying axial forces well below system resonance and measuring the phase between force and response. The investigation allowed for assessment of the effect of frequency, stress level and stress history on loss factor. Stress levels investigated were between 1·3 MPa and 11·3 MPa for axially loaded specimens (i.e., strain levels between 145 μe and 2020 μe). In radially loaded specimens the stress range was 0·5–3·9 MPa (230–2040 μe) while tangentially loaded specimens were loaded in the range 0·5–1·5 MPa (155–3330 μe). Measured values have shown that loss factors can be dependent on frequency, stress level and conditioning particularly in the radial and tangential directions.