A testing method using circumferentially notched round bars for investigating mixed mode behaviour under loading in tension and torsion is applied to wood. The applicability of the method to ...anisotropic materials is investigated for two types of wood, beech and spruce, considering the longitudinal and radial orientation with respect to the stem axis of the tree. The strong anisotropy of wood requires different evaluation procedures for radial and longitudinal sample orientation. The K-concept of linear elastic fracture mechanics (LEFM) and concepts of non-linear elastic fracture mechanics (NLEFM) were used for the evaluation of radial and longitudinal samples, respectively. Differences between the investigated wood types under radial orientation, in their durability to withstand torsional loads, could be observed by examining ratios of the values of the fracture toughness in mode III against mode I. Micrographs of the fracture surfaces support the assumption that the higher amount of wood rays in beech is responsible for the higher toughness under torsion. In case of longitudinal specimen geometry it was found that at very high levels of torsional deformation beech and spruce reach similar values in their specific fracture energy in mode I.
Crack propagation in wood is strongly influenced by the microscopic structure of the material. The relationship between structure and function with regard to damage and fracture behaviour can only be ...understood with a sufficiently fine level of examination. An experimental approach to perform micro-wedge splitting tests on spruce and beech inside the chamber of an Environmental Scanning Electron Microscope and under atmospheric conditions is presented. The specimens are loaded in mode I in the TR crack propagation system. Based on the load-displacement diagram, the characteristic parameters of fracture energy, critical load and initial elasticity are determined. The load and displacement data for the in situ experiments are related to the obtained ESEM images and allow a discussion of the fracture process on the cellular level. Density was found to be an important factor for fracture mode and several crack arresting phenomena depending on the variation of elasticity could be identified.
Ultrasonic-assisted cutting of wood Sinn, G.; Zettl, B.; Mayer, H. ...
Journal of materials processing technology,
12/2005, Letnik:
170, Številka:
1
Journal Article
Recenzirano
Ultrasonic-assisted cutting experiments have been performed on two wood species, spruce and beech in dry and wet state. Cutting forces and normal forces are measured and analysed with respect to ...uncut chip thickness (i.e. infeed depth) and linear correlation is found. Compared to conventional cutting reduction of cutting forces in the order of 50% is achieved at relatively small vibration amplitudes of 8
μm. The results are interpreted in terms of reduced friction forces caused by ultrasonic vibration of the cutting knife.
Mixed mode fracture energy of sprucewood TSCHEGG, E. K; REITERER, A; PLESCHBERGER, T ...
Journal of materials science,
07/2001, Letnik:
36, Številka:
14
Journal Article
Recenzirano
The characterization of Mixed Mode (Mode I and Mode II) behaviour of wood was concentrated on concepts of linear fracture mechanics in the past. Using an adopted version of the splitting test it was ...possible to obtain complete load displacement curves under different Mixed Mode loading cases for crack propagation along the grain. Therefore fracture energy concepts (specific fracture energy) could be used to characterize the material behaviour. Additionally strength parameters were used in order to describe crack initation in two crack propagation systems. The values for specific fracture energies as well as the strength values were compared with pure Mode I fracture tests. Moreover, the size effect under Mixed Mode loading was investigated to guarantee size independent material characterizing values for the specific fracture energies.
The remarkable mechanical properties of biological materials reside in their complex hierarchical architecture and in specific molecular mechanistic phenomena. The fundamental importance of molecular ...interactions and bond recovery has been suggested by studies on deformation and fracture of bone and nacre. Like these mineral-based materials, wood also represents a complex nanocomposite with excellent mechanical performance, despite the fact that it is mainly based on polymers. In wood, however, the mechanistic contribution of processes in the cell wall is not fully understood. Here we have combined tensile tests on individual wood cells and on wood foils with simultaneous synchrotron X-ray diffraction analysis in order to separate deformation mechanisms inside the cell wall from those mediated by cell-cell interactions. We show that tensile deformation beyond the yield point does not deteriorate the stiffness of either individual cells or foils. This indicates that there is a dominant recovery mechanism that re-forms the amorphous matrix between the cellulose microfibrils within the cell wall, maintaining its mechanical properties. This stick-slip mechanism, rather like Velcro operating at the nanometre level, provides a 'plastic response' similar to that effected by moving dislocations in metals. We suggest that the molecular recovery mechanism in the cell matrix is a universal phenomenon dominating the tensile deformation of different wood tissue types.
The nanostructure of the wood cell wall and, in particular the tilt angle of the cellulose fibrils versus the longitudinal cell axis (microfibril angle, MFA), are known to play a key role in ...determining the mechanical properties of wood. A variation of microfibril angles during growth may therefore be regarded as a means to adapt to different loading situations. In the present study, a branch of Norway spruce (Picea abies) was used as a model system. The change of microfibril angles with increasing age and size of the branch and therefore increasing gravitational load was systematically investigated. Small angle X-ray scattering (SAXS) was applied to obtain a map of MFA all over the branch as a function of the distance from the trunk within each annual ring. It was found that in compression wood the MFA decreased continuously from the trunk towards the tip in all annual rings. In opposite wood, however, the course of microfibril angles was found to change considerably with the age of the branch: in the outer annual rings, very small microfibril angles occurred in the middle part of the branch. The results are discussed in view of the mechanical implications of different microfibril angles.
ABSTRACT Ultrasonic fatigue testing is appropriate to perform random loading tests in the regime of very high numbers of cycles. It has been shown that neither an endurance limit nor a threshold ...stress intensity exists under loading with randomly varying amplitudes even for materials that do show these limits under constant amplitude loading conditions. The technical features of the ultrasonic testing technique in order to perform random fatigue tests are shortly described. Endurance tests were performed on smooth specimens of AlSi7Mg (A356.0) aluminium alloy and on notched AISI 4142 and C45 steel specimens. The previous studies of crack propagation and threshold behaviour on AISI 420 ferritic chromium steel and GGG 100‐B cast iron are included. Experimental results on lifetime and fatigue crack growth measurements under randomly varying amplitudes, as well as lifetime predictions, based on constant amplitude measurements and damage accumulation calculations are reported.
Mode I and Mode III loading experiments were performed on side grooved CT specimens of two types of Laminated Veneer Lumber (LVL). Steady state crack propagation was maintained in order to detect ...complete load displacement diagrams. Fracture behaviour and influence of fiber orientation were studied and all important fracture mechanical values (stiffness/compliance, microstructural damage, crack initiation energy, specific fracture energy etc) were determined. Much higher crack initiation energies and specific fracture energies resulted in mode III loading than in mode I loading for both material types. Under external mode III loading, crack initiation occurs in mode III and crack propagation however takes place under mode I owing to crack surface interference. The influence of fiber orientation on fracture mechanical properties of LVL was discussed.
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