Design and load-bearing behaviour of joints with fully-threaded-screws pre-drilled with laser-technique - continuation of projekt

由于其对气候变化的重要贡献，木材可持续建筑目前正经历着巨大的推动，特别是在多层建筑中。由于载荷增加，连接点通常需要全螺纹自攻螺钉（STS）进行加固。使用STS连接和加固木结构也体现在现行的欧洲规范5中，该规范将于2022年生效。与此同时，STS在新的应用领域的应用也在研究中，其中使用了大啮合长度（lE = 600 mm及以上）的螺钉。然而，STS的精确定位仍然具有挑战性，特别是对于高长度的STS。不均匀的木结构和高长度导致螺丝偏离其指定轴。利用激光辐射进行热裂解预钻是一种合适的、创新的精确插入STS的解决方法。在目前的DFG工程中，分析了热解预钻相对于机械预钻在胶合层中精确插入方面的优势。在该项目中，开发了在木材上钻深孔的合适工艺参数和激光钻孔工艺的设计知识。此外，研究了热输入对木材基体的影响，并在拉拔试验中确定了热输入对螺纹与胶合木基体之间粘合行为的影响。预钻孔径可达300毫米，钻头横截面相当稳定，平均直径为6毫米。在更深的深度，气体的演化会导致更多的碳化燃烧，从而对插入的STS的结合强度产生不利影响。这是由于在钻孔过程中，烧蚀产物与钻孔通道内的激光辐射相互作用时间较长，加热效果较强。因此，有必要改善对激光钻点的保护气体供应，以防止火焰和废气排放烧蚀到钻点。因此，对于拟建的延续项目，拟借助至少具有一个双截面的探头进行激光钻孔工艺，用于提供和去除保护气体和废气。新方法提高了惰性气体对烧蚀点的供应，并最大限度地减少了激光辐射加热烧蚀产物引起的热解烧蚀。在新开发的探头的帮助下，在这个延续项目中，目标钻孔深度将增加到至少600毫米。

Due to its important contribution to climate change, sustainable construction with wood is currently experiencing a significant boost, especially in multi-story buildings. Due to the increased loads, the joining points often require fully threaded self-tapping screws (STS) for reinforcement. The use of STS for joining and strengthening timber constructions is also represented in the current Eurocode 5, which will come into force in 2022. At the same time, the use of STS is being investigated in new fields of application, where screws with large engagement lengths (lE = 600 mm and more) are used. However, exact positioning of STS, especially with high lengths, remains challenging. The inhomogeneous wood structure and the high length cause the screws to stray from their designated axis. Pyrolytic pre-drilling utilizing laser radiation represents a suitable and innovative solution method for the exact insertion of STS. The advantages of pyrolytic pre-drilling compared to mechanical pre-drilling with regard to accurate insertion in glulam were worked out in the current DFG project. Within the project suitable process parameters for drilling deep holes in wood and the knowledge about the design of the laser drilling process were developed. Also, the effects on the wood matrix due to heat input were investigated and its influence to the bonding behaviour between the screw thread and the glulam matrix was determined in pull-out tests. Pre-drilled holes up to 300 mm were obtained with a reasonably constant drill cross-section with an average diameter of 6 mm. The gas evolution at greater depths causes more carbonizing burnup throughout the bore channel, adversely affecting the bonding strength of the inserted STS. This results from the stronger heating of the ablation products due to the longer interaction with the laser radiation in the drilling channel during the drilling process. Thus, it is necessary to improve the shielding gas supply to the laser drill point to prevent flames and waste gas disposal from ablating to the drill point. Therefore, for the proposed continuation project, it is planned to carry out the laser drilling process with the aid of a probe with at least one double cross-section for the supply and removal of shielding gas and exhaust gases. The new method improves the supply of inert gas to the ablation point and minimizes pyrolytic burn up due to the heating of the ablation products by the laser radiation. With the help of the newly developed probe, the targeted drilling depth is to be increased to at least 600 mm in this continuation project.