Articles containing the keyword 'mechanical strength'

According to the literature, the mechanical strength of the green reaction wood of softwood species (compression wood) is greater than that of normal wood. Drying increases the mechanical strength but less in reaction wood than in normal wood. In particular, the tensile strength along the grain and the impact strength are lower than in normal wood. The compression strength and possibly bending strength are greater, however.

The properties of the reaction wood of hardwood species (tension wood) differ from those of softwoods. When green, all mechanical properties are weaker than those of normal wood. When dried, the tensile strength and impact strength are better and compression strength lower. There is no great difference in the bending strength.

When the higher density of reaction wood is not taken into account and there are no impact forces, the mechanical strength of reaction wood in sawn goods etc. does not differ so much from that of normal wood. The harmful effect of knots, for example, can in practice be much greater.

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• Kärkkäinen, E-mail: mk@mm.unknown
• Raivonen, E-mail: mr@mm.unknown

The objective of the investigation was to determine the differences between faultless timber grown on a peatland before and after draining, in respect of compressive strength to the grain, volume weight, and shrinkage. In addition, the influence of the boundary zone between the close-ringed wood formed before draining and the wide-ringed wood produced after draining on strength of the timber was studied. The material consisted of 15 sample trees of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.), white birch (Betula pubescens Ehrh.) and silver birch (B. Pendula Roth).

The volume weight of wood of the tree species in ascending order is; spruce, pine, white birch, silver birch. The volume weight of Scots pine seems to decrease from the butt end upwards, while no trend was revealed for spruce. In the coniferous trees, the wide-ringed wood formed subsequent to draining was slightly lighter than the close-ringed wood produced prior draining. No distinct trend was seen in the birch species. The volume weight of pine and spruce increased with decreasing width of the growth rings up to a certain limit, after which the conditions inverted.

The compressive strength of the different kinds of wood seems to increase from the butt end upwards, but after height of two meters it begins to decrease considerably. In birch, this point of inversion is in somewhat greater height. In spruce timber, the compressive strength parallel to the grain is lowest for wood which contains exclusively wide-ringed wood formed after draining. The boundary zone between the woods formed before and after draining is very distinguishable, but has no remarkable influence on the compressive strength parallel to the grain. Shrinkage of close-ringed wood is higher in all three principal directions than that of wide-ringed wood. This can be explained by the variations in volume weight and fibrillar orientation of the tracheid walls.

The PDF includes a summary in English.

• Ollinmaa, E-mail: po@mm.unknown

Compression wood of the tree species studied in this investigation, Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and common juniper (Juniperus communis L.), was found to be characterized in its cross section by the thick walls and rounded shape of its tracheids and the profuse occurence of spaces. Tension wood of aspen (Populus tremula L.) and alder (Alnus incana (L.) Moench) was found in microscopic examination to be characterized by the gelatinous appearance of the wood fibres, by its small cell cavities and by the thickness and buckling of the inner layer of the cecondary wall. Tracheids of the compression wood were found to have shorter length than normal on an average, while the tension wood fibres were found to be longer.

The microchemical studies suggest a higher than normal lignin content in compression wood and lower than normal lignin content in tension wood, as compared to normal wood. The reverse would be true for the cellulose contents. Volume weight of absolute dry reaction wood was distinctly higher than that of normal wood. The longitudinal shrinkage of reaction wood, particularly of compression wood, is several times that of normal wood. Transversal shrinkage of compression wood is much less than normal wood. Swelling tests revealed pushing effect of compression wood on elongation and pulling effect on tension wood on constraction. Volume shrinkage of compression wood is less than that of normal wood, in contrast to tension wood. The strength of compression wood in absolutely dry condition was nearly same as that of normal wood.

The PDF includes a summary in English.

• Ollinmaa, E-mail: po@mm.unknown

The article includes a detailed review on the technical properties of wood. The weight, water content, strength and conductivity of the wood, and the factors affecting them are discussed. The mechanical and technical properties of wood are influenced, for instance, by tree species, age and part of the tree, geographical situation, site and growth conditions of the stand, anatomical structure of the wood, and temperature. The author summarizes the topics where further research should be addressed. For instance, the forest site types developed in Finland could be utilized in studies of the mechanic technical properties of wood.

The PDF includes a summary in English.

• Lassila, E-mail: il@mm.unknown

• Raiskila, Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, FI-01301 Vantaa, Finland E-mail: sr@nn.fi
• Saranpää, Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, FI-01301 Vantaa, Finland E-mail: pekka.saranpaa@metla.fi
• Fagerstedt, Department of Biological and Environmental Sciences, Plant Biology, P.O. Box 65, FI-00014 University of Helsinki, Finland E-mail: kf@nn.fi
• Laakso, Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, FI-01301 Vantaa, Finland E-mail: tl@nn.fi
• Löija, VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland E-mail: lm@nn.fi
• Mahlberg, VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland E-mail: rm@nn.fi
• Paajanen, VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland E-mail: lp@nn.fi
• Ritschkoff, VTT Building and Transport, P.O. Box 1806, FI-02044 VTT, Finland E-mail: acr@nn.fi