Analysis of fracture mechanism of bolted joints and nail joints in LVL

研究了层压单板中螺栓连接的断裂，以及钉入层压单板中的裂纹扩展。钉入引起的材料内部裂纹比表面裂纹长。根据断裂力学概念，利用材料常数（密度和断裂韧性）估算出的裂纹长度与实验结果吻合较好。研究了垂直和平行两种剪切荷载作用下螺栓连接的断裂、螺栓位置对极限荷载的影响以及螺栓孔周围应力集中的测量。2“× 10”木材尺寸的LVL （Micro-lam; 2.0 E）由直径16mm的螺栓连接(d)。根据LVL屈服条件，给出LVL允许锚杆位置：在荷载垂直于颗粒的情况下，允许锚杆孔边距大于4d ~ 7.6d（=试件深度的一半），允许锚杆端距大于7d ~ 10d。当荷载与颗粒平行时，末端距离大于7d。这些距离比木材标准推荐的距离要长一点。螺栓孔周围的重要应力集中为垂直于晶粒的螺栓孔边界处的拉应力和平行于晶粒的螺栓孔端面处的拉应力。当端、侧距离不够时，接头的主要断裂方式为LVL沿晶断裂。根据断裂力学概念估计，LVL单板的车检等效边缘裂纹长度约为9 mm。声发射与接头断裂方式的关系。

The fracture of bolted joints in LVL, Laminated Veneer Lumber, and the crack propagation during driving nails into LVL were studied.1. The crack caused by driving nails was longer inside of materials than at surface. The crack length estimated from fracture mechanics concept using the material constants (density and fracture toughness) agreed well with that of experiments.2. The fracture of the bolted joints subjected to shear loads (perpendicular to grain and parallel to grain), the influence of the bolt position on the ultimate load, and the measurement of the stress concentration around a bolt hole were studied. The LVL (Micro-lam; 2.0 E) of 2" by 10" lumber size were jointed by bolts of 16mm in diameter(d). The allowable bolt position for LVL based on the yield condition of LVL was given as follows: In the case of load perpendicular to grain, the allowable side distance of bolt hole and the end distance were longer than 4d-7.6d(= half of the depth of a specimen) and longer than 7d-10d, respectively. In the case of load parallel to grain, the end distance was longer than 7d. These distances were a little longer than those of the recommendation of the standard for wood.3. The important stress concentrations around a blot hole were the tensile stress at the boundary of a bolt hole for the load perpendicular to grain and the tensile stress at the end surface for the load paralled to grain. The major fracture mode of joints was split of LVL along grain when the side and the end distances were not enough.4. The equivalent edge crack length of the lathe-check of veneers in LVL estimated from the fracture mechanics concept was about 9 mm.5. The acoustic emission related to the fracture modes of joints.