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

研究了单板层积材、单板层积材螺栓连接接头的断裂以及钉入单板层积材过程中的裂纹扩展.钉击产生的裂纹在材料内部比表面长。利用材料常数（密度和断裂韧性），由断裂力学概念估算的裂纹长度与实验结果吻合较好.研究了螺栓连接在顺纹和横纹剪切载荷作用下的断裂、螺栓位置对极限载荷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）通过直径（d）为16 mm的螺栓连接。根据单板层积材的屈服条件，给出了单板层积材的螺栓允许位置：在垂直于纹理的荷载作用下，螺栓孔的允许侧距大于4d ~ 7.6d（=试件深度的一半），螺栓孔的允许端距大于7 d ~ 10 d。顺纹加载时，端距大于7 d。这些距离比木材标准推荐的距离稍长。螺栓孔附近的重要应力集中是：垂直于晶粒方向载荷作用下螺栓孔边界处的拉应力和平行于晶粒方向载荷作用下螺栓孔端面处的拉应力。当侧端距离不够时，单板层积材沿沿着开裂是接头的主要断裂方式.根据断裂力学的概念，单板层积材车裂的等效边裂长度约为9 mm.声发射与接头的断裂模式有关。

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.