The effect of thermal modification (TM) on the chemistry, anatomy and mechanical properties of wood is often investigated using small clear samples. Little is known on the effect of growth-related and processing defects, such as knots and checks, on the bending strength and stiffness of thermally modified timber (TMT). Nine boards of Norway spruce with different combinations of knot types were used to study the combined effects of checks and knots on the bending behaviour of TMT. Digital image correlation (DIC) measurements on board surfaces at sites of knots subjected to bending allowed to study strain distribution and localise cracks prior to and after TM, and to monitor development of fracture (around knots) in TMT to failure. DIC confirmed that checking in knots was increased after TM compared to kiln-dried timber, specifically for intergrown knots and intergrown parts of encased knots. Effects appear local and do not affect board bending stiffness at these sites. Bending failure in TMT initiated mainly at knot interfaces or besides knots and fractures often propagated from checks. Scanning electron microscopy analyses of fracture surfaces confirmed this, and fractures were typically initiated around knots and at knot interfaces due to crack propagation along the grain in the longitudinal–radial plane (TL fracture) under mixed mode I and II loading, such that boards failed in simple tension like unmodified timber. Images of fracture surfaces at the ultrastructural level revealed details of the brittle behaviour of TM wood. This was especially apparent from the smooth appearance of transwall failure under mode I loading across the grain.

中文翻译：

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静态弯曲加载的热改性木材中的裂纹形成、应变分布和节周围的断裂面

通常使用小的透明样品来研究热改性 (TM) 对木材的化学、解剖结构和机械性能的影响。关于生长相关和加工缺陷（例如节和格子）对热改性木材 (TMT) 的弯曲强度和刚度的影响知之甚少。使用具有不同结类型组合的九块挪威云杉板来研究检查和结对 TMT 弯曲行为的组合影响。数字图像相关性 (DIC) 测量在板表面受弯曲的结点处可以研究应变分布并定位 TM 之前和之后的裂纹，并监测 TMT 中断裂（围绕结点）到失效的发展。DIC 证实，与窑干木材相比，TM 后检查节数增加了，专门用于相互生长的结和包裹结的相互生长的部分。效果是局部的，不会影响这些部位的板弯曲刚度。TMT 中的弯曲失效主要在结界面处开始，或者除了经常从检查传播的结和断裂之外。断裂表面的扫描电子显微镜分析证实了这一点，由于在混合模式 I 和 II 加载下裂纹沿纵向-径向平面中的晶粒传播（TL 断裂），通常在节点周围和节点界面处开始断裂，从而导致板失效在简单的张力中，就像未改性的木材一样。超微结构水平的断裂表面图像揭示了 TM 木材脆性行为的细节。这从在模式 I 下加载横穿晶粒的跨壁破坏的平滑外观中尤为明显。