Compressive strength of beech and birch at different moisture contents and temperatures

Abstract

Owing to increased environmental demands to replace petroleum-derived materials with more environmentally friendly materials, research has been directed towards the potential of using wood in the automotive industry. This study extensively investigates the compressive strength (fc) of two hardwood species (beech and birch) with different anatomical directions, at various moisture and temperature levels, in an attempt to understand the wood’s behaviour during the forming process. The experimental tests were performed at 20, 100 and 140 °C on specimens with five moisture levels, ranging from completely dry to wet conditions. Overall, irrespective of the investigated direction, the measured compressive strength exhibited a clear exponential trend with increasing moisture content over the whole temperature range. This was capitalised on to present a simple predictive equation to roughly estimate the fc of beech and birch in different moisture and temperature conditions by relating to their dry fc at a reference temperature of 20 °C. The proposed approach was compared with other procedures and trends reported in the literature for the effect of moisture and temperature on the fc of wood. Furthermore, the reduction factors for the effect of temperature on the fc of softwoods, as set out in Eurocode 5 (EN 1995-1-2), were discussed in comparison with the present study findings. It was shown that, although the Eurocode approach is conservative, it may still be applicable for estimating the fc of hardwood species.

The Effect of Rays on the Mechanical Behaviour of Beech and Birch at Different Moisture and Temperature Conditions Perpendicular to the Grain

“…The cubic samples were then conditioned according to their individual temperature level (except testing sets of 20 • C) with a laboratory oven for 7-15 min, determined from preliminary studies performed on equivalent samples to the testing series. Further details concerning sample preparation, moisture, and temperature conditioning are given elsewhere [15].…”

“…Microscopic in-situ investigations of wood require very small samples and labour-intensive preparation methods [18]. However, compression tests have shown that very small samples dry out or cool down very quickly [15]. Compression tests with small specimens at high MCs and elevated temperatures would, therefore, have to be carried out in a climate chamber.…”

“…Wood samples with higher strength levels are depicted in Figure 1a, while those with lower strengths are shown in Figure 1b. A detailed discussion about the effect of moisture content and testing temperature on the perpendicular-to-grain compressive strength (f c ) of beech and birch is given elsewhere [15] and will only be briefly discussed here.…”

“…The average stress-strain curves of beech and birch are shown in Figure 13 for different moisture contents and temperature levels. As with radial fc, a detailed discussion about the effect of moisture content and testing temperature on the tangential compressive strength (fc) of beech and birch is given elsewhere [15]. Overall, the material is weaker and At any rate, when wet beech and birch were compressed along the grain [26], the stability of the material appeared to improve with temperature for beech, but not for birch.…”