The moisture-dependency of the fracture energy for unmodified and acetylated Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth) has been investigated. Specimens were conditioned at relative humidity levels of 20, 75, and 97%, as well as dry and water-saturated. At moisture contents below 15%, the fracture energy increased with increasing moisture content for both unmodified and acetylated wood, while it decreased for untreated wood at higher moisture contents. A significant difference in moisture-dependency was found, indicating higher fracture energy for unmodified wood compared to acetylated wood at similar moisture contents. Additionally, to assess the impact of the increased brittleness for structural applications, the fracture energy was compared at equal relative humidity levels. The largest difference was seen at 75% relative humidity with approximately 50% lower fracture energy for acetylated wood. No significant differences were found for water-saturated samples. The moisture-dependency of the fracture energy, combined with the reduced hygroscopicity of acetylated wood, is suggested to be one, but not the only, contributing factor to the lower fracture energy of acetylated wood compared to unmodified wood at equal humidity levels. These observations have importance for structural design since design codes often assess material parameters based on ambient humidity.

中文翻译：

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木材断裂能的水分依赖性：未改性和乙酰化的苏格兰松树和桦树的比较

未改性和乙酰化的苏格兰松树（Pinus sylvestris L.）和桦树（Betula pendula）断裂能的水分依赖性罗斯）已被调查。将样品调至相对湿度为20％，75％和97％的条件下，并使其处于干燥和水饱和状态。当含水量低于15％时，未改性和乙酰化木材的断裂能都随含水量的增加而增加，而对于含水量较高的未经处理的木材，其断裂能会降低。发现在湿度依赖性方面存在显着差异，这表明与在相似水分含量下的乙酰化木材相比，未改性木材的断裂能更高。另外，为了评估脆性增加对结构应用的影响，在相等的相对湿度水平下比较了断裂能。相对湿度为75％时最大的差异是乙酰化木材，其断裂能降低了约50％。对于水饱和的样品，没有发现显着差异。断裂能的水分依赖性，加上乙酰化木材的吸湿性降低，被认为是（但不是唯一的）促成乙酰化木材与相同湿度水平下未改性木材相比断裂能更低的一个因素。这些观察结果对于结构设计非常重要，因为设计规范经常根据环境湿度评估材料参数。