Proof of the Elongation Reserve of Longitudinally Compressed Wood by Tensile Tests

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This paper deals with the proof of the theory of elongation reserve remaining in wood after compression parallel to the grain. After compression, the wood becomes much more pliable and the force required for bending is reduced. At the end of the 1-minute fixation following 20% compression, the compressive stress in the beech samples was reduced to an average of 72.3%, while the oak samples showed a 65.6% change. The remaining shortening at this time was 3-5%. At the end of the 3-hours fixation, the compressive stress had decreased to 37.1% for beech and 27.9% for oak, resulting in a residual shortening of 12-18%. An average maximum tensile force of 1.76 kN was required for untreated beech samples, which resulted in a 1.55 mm increase in size parallel to the grain. For specimens fixated for a short-time, a tensile force of 1.06 kN caused a 3.66 mm increase in size, while for specimens fixated for a long-time, a force of 0.85 kN caused an 8.79 mm increase in size. The trends were similar for oaks. The higher moisture content provided a significantly larger increase in size during the tensile tests. The existence of the elongation reserve was clearly confirmed.