Simon Aicher

Nichtlineares Bruchmechanik-Maßstabsgesetz für Fichte bei Zugbeanspruchung senkrecht zur Faserrichtung

ZusammenfassungEs wird über Untersuchungen zur bruchmechanischen Größengesetzmäßigkeit von fehlerfreiem Fichtenholz bei Modus I Rißöffnungsbeanspruchung im RL-Rißsystem berichtet. Die Versuche wurden mit einseitig gekerbten, geometrisch weitgehend 2D-ähnlichen SENB-Biegeproben durchgeführt. Die Prüfkörper-Querschnittshöhend variierten von 10 bis 320 mm, die Kerblängen betrugen konstant das 0,6fache der Querschnittshöhe. Die Gesamtheit der Versuchsergebnisse, ließ sich gut durch ein stetiges, nichtlineares Bruchmechanik-Maßstabsgesetz nach Bazant approximieren, während die Weibull-“Weak-Link”-Hypothese, bedingt durch den bruchmechanischen Versagensmechanismus, eher unzutreffend ist. Der festgestelle Maßstabseffekt entsprach bei den größeren Prüfkörpern (d≥160 mm) nahezu exakt dem Maßstabsgesetz der linearen Bruchmechanik (LEFM). Bei zunehmend kleineren Größen war übereinstimmend mit den Annahmen der nichtlinearen Bruchmechanik (NLFM), das heißt infolge des zunehmenden Einflusses der dehnungsentfestigenden Bruchprozeßzone, eine abgeschwächter Maßstabseffekt zur verzeichnen. Der erhaltene NLFM-LEFM-Übergangsbereich kann durch eine von der Versuchskonfiguration unabhängige bruchmechanische Kenngröße. die Sprödigkeitsziffer β∼10, beschrieben werden.AbstractA report is given on investigations, concerning fracture mechanics based size effect of clear spruce wood loaded in mode I in RL-crack system. The tests were performed with single notched, geometrically almost 2D-similar SENB bending specimens. The beam depths,d, varied from 10 to 320 mm, the notch lengths were constant 0,6 timesd. The entity of test results could be approximized well by means of a steady nonlinear fracture mechanics size effect law by Bazant, whereas the Weibull weak link hypothesis, according to fracture mechanics based failure, is rather inapt. The received size effect conformed nearly exactly to the size effect law of linear fracture mechanics (LEFM) with the larger pecimens (d≥160mm). Along with, increasingly smaller sizes a reduced size effect was abtained agreeing well with assumptions of nonlinear fracture mechanics (NLFM), i.e. growing influence of softening fracture prozess zone. The experimental transition region between NLFM and LEFM can be specified via brittleness number β∼10, a fracture mechanics parameter independent of individual test configuration.

Rolling shear modulus and strength of beech wood laminations

Abstract Previous research indicated that the rolling shear properties of European beech wood (Fagus sylvatica) are considerably higher than those of softwood. The aim of the presented investigation was to substantiate previous data on rolling shear modulus and strength of European beech wood and to further evaluate its substitution of softwoods in applications where shear properties are influential, namely as cross layers in cross-laminated timber (CLT). Further, the effect of the annual ring orientation within the boards on shear modulus and strength was of major interest. The beech specimens comprised four different sawing patterns, classified unambiguously with reference to the pith location. The shear properties were determined by 50, two-plate shear tests with specimen cross-section dimensions of 33 mm × 135 mm. A mean rolling shear modulus of 370 N mm-2 was obtained, whereby no significant detrimental effect for pith boards with cracks was observed. In agreement with continuum mechanics, the semi-quarter-sawn boards revealed the highest shear moduli whereas the quarter-sawn boards showed roughly 30% lower values. The mean rolling shear strength was 5.6 N mm-2 for all specimens, whereby pith specimens resulted in generally lower values. The 5% quantile, disregarding pith specimens, was 4.5 N mm-2. In conclusion, the rolling shear strength and modulus exceed the respective characteristic values for softwoods by roughly factors of 5 and 7, indicating great potential for beech wood cross-layers in CLT.

Materials and joints in timber structures

Description based on online resource; title from PDF title page (ebrary, viewed October 11, 2013).

Creep Testing of One-Component Polyurethane and Emulsion Polymer Isocyanate Adhesives for Structural Timber Bonding*

Abstract Moisture hardening one-component polyurethane (PU) adhesives and emulsion polymer isocyanate (EPI) adhesives have recently gained a considerable market share in structural timber gluing, e...

Novel Lightweight Timber Composite Element: Web Design in Shear and Compression

The paper examines the build-up and some aspects of the mechanical behavior of a novel lightweight timber composite called the Keel-web element. The element is a double-skinned composite similar to a multiple box beam element. The flanges consist of finger jointed lumber chords arranged and glued in parallel. The name-giving specific characteristic of the element consists of the multiple S-shaped webs made of plywood or OSB, resembling ship keels, glued in between the flanges. The element, which can be produced in a fully automatized process, as straight or cambered, with lengths of up to 35 m, has recently obtained a German technical building approval. The build-up will first be examined, and afterwards, the engineering design approach for the shear force and compression capacity, partly following Eurocode 5, is shown.

CLT Plates under Concentrated Loading – Experimental Identification of Crack Modes and Corresponding Failure Mechanisms

Since wood products for structural elements, such as cross-laminated timber (CLT), have gained importance in the building sector, the need for appropriate and reliable design codes has become essential. For this reason, this work focuses on global failure mechanisms and the corresponding evolution of different crack modes in CLT plates, depending on geometric and/or material related influence quantities. Therefore, plate-bending experiments on 3- and 5-layered CLT-plates were carried out. In addition to standard evaluation methods, each specimen were cut into cubes (10/10/10 cm) to get information about the failure modes inside the plates. Areas and location of dominant shear failure, tensile failure, delamination, and mixed failuremodes could be clearly defined and connected to geometry and loading situation. Based on this evaluation well known but not yet in detail described effects, such as the ductile structural behavior of CLT plates, can be explained.

Glulam Composed of Glued Laminated Veneer Lumber Made of Beech Wood: Superior Performance in Compression Loading

Compared to most softwoods, the hardwood species beech (Fagus sylvatica) exhibits higher strength and stiffness properties, but – for structural use – also a number of drawbacks. The drawbacks, however, can be overcome partly by processing beech wood to laminated veneer lumber (LVL). In order to utilise beech LVL not only for plate-like structures, but also for beams or columns with deliberate cross-sectional dimensions, the beech LVL can be further processed to glued laminated beams (glulam) made of LVL laminations. The paper reports on experimental investigations of innovative high end structural beech glulam and demonstrates the system effect on the load capacity in compression loading parallel to the grain.

Bondline shear strength and wood failure of European and tropical hardwood glulams

The study reports on block shear investigations with bondlines of face-glued laminations and matched solid wood specimens from hardwood glulam (GLT) beams produced industrially from eight technically and stand volume-wise important species. The European hardwoods comprised oak, beech, sweet chestnut and ash and the tropical species were teak, keruing, melangangai and light red meranti. The adhesives were phenol-resorcinol and melamine-urea. When combining all species in one sample, a rather strong linear relationship of bond and wood shear strength was observed. The ratio of bond vs. wood shear strength was for all species on the mean value level ≥ 0.9, and likewise (with one exception) for the respective strengths’ 5%-quantiles. Consistent with literature, the test results showed no significant correlations between bond shear strength and density, wood shear strength and wood failure percentage of individual species, respectively. The investigations render the methodological basics of some international standards on bond quality verification as being inappropriate. New, empirically validated hardwood GLT bond requirements are proposed for discussion and implementation at the CEN and ISO levels. The strength ratio specifications reflect respective ANSI provisions, yet the reference quantity wood shear strength is now determined in an unbiased manner from matched GLT specimens. The wood failure verification proposal is based on the 10%-quantile and mean level for initial type testing and factory production control. The requirements further account for the pronounced difference observed in scatter of wood failure between European and tropical species.

Glulam from European White Oak: Finger Joint Influence on Bending Size Effect

Glued laminated timber consists today predominantly of softwoods as they are currently the main source of structural timber in the northern hemisphere. Due to several reasons, hardwoods will increasingly gain a more important role as a sustainable material resource which will of course therefore also impact glulam production. In Europe, the wood species white oak (Quercus Robur, Quercus Petraea) is the most important species other than beech, and is being increasingly used in structural applications. Recently, the first national and European technical approvals were issued for oak glulam. The design strength values were based on extensive testing campaigns, with emphasis on the differences inherent between growth regions of the raw material itself. The paper reports first on basic strength properties and requirements for oak laminations and beams as specified in the technical approvals. Further, essential test results which form the basis for the characteristic strength and stiffness parameters are given, showing a clear effect of size on the bending strength. In order to assess the beam characteristics based on basic lamination and finger joint properties, the applicability of the new European strength equations for softwood glulam is discussed. In the case where the important ratio of finger joint vs. lamination strength is at least one, as is typical for softwoods, the model applies for the considered hardwood databases as well. However, when the lamination strength reaches very high values with means in the range of 80 MPa to 120 MPa, finger joint strength vs. lamination strength drops significantly below one. It is then shown that the bending capacity and size effect can be well predicted for specific high strength hardwood glulams by a serial model, effectively including the bending stress gradient between adjacent laminations.