船舶結構破壞抗力評估簡化方法

Simplified Fracture Resistance Assessment in Ship Structure

本研究探討具有疲勞裂縫之船舶縱向構件的破壞失效問題。船的一生中遭遇極端海況時，部分疲勞裂縫可能成長達到一臨界長度進而造成迅速破壞。臨界裂縫長度之大小可表現結構對於破壞之抵抗能力。本研究提出一簡化法以求取此臨界裂縫長度，所採用之方法為破壞評估圖，其可用來評估裂縫尖端之破壞狀況。本研究為兩種不同型式之裂縫，以有限元素分析法回歸出裂縫尖端狀況之解析化公式，並將材料韌性以裂縫尖端開口位移來表示。此簡化法可快速計算具有裂縫之縱向構件的破壞應力，將破壞應力表示為裂縫長度之函數。再將長期極端負荷下所造成之最大縱向應力代入，即可得到臨界裂縫長度。本研究以一艘海岬型散裝船為例，發現其裂縫臨界長度很大，破壞抗力良好。 This study investigates the fracture failure of ship longitudinal members, in which a crack has propagated by fatigue. During the ship life, some fatigue cracks may propagate till lengths that can be critical in view of the fast fracture, when the ship meets extreme sea conditions. This critical crack length characterizes the fracture resistance of a structure. This study proposes a simplified approach to evaluate the critical crack length. Specifically, this study employs the failure assessment diagram methodology to assess the conditions of failure at the crack tip. Based on two crack configurations, this study establishes the analytical formulations of the crack-tip condition that are regressed using finite element analyses. The material toughness is expressed in terms of crack-tip opening displacement. This simplified approach can thus rapidly compute the failure stress of cracked longitudinal members as a function of the crack length. The critical crack lengths corresponding to the maximum longitudinal stresses derived from long-term extreme loads can thus be determined. For the ＂Capesize＂ bulk carrier considered in this study, the critical crack lengths are found to be very large. So, the fracture resistance is found satisfactory.