海船断裂控制设计准则

传统的断裂控制设计准则，如脆性转变温度准则或最低工作温度准则等，在实际断裂评定中，有时会遇到困难，其原因在于建立这些准则的条件与实际结构的服役条件存在差异。本文在分析材料断裂韧度的温度效应和载荷速率效应的基础上，根据海船工作条件构造了断裂特征分析图（Fracture Characteristic Analysis Map),提出了一个计及材料性能（断裂韧度，屈服强度）、环境因素（服役温度，载荷速率）和结构特点（板厚及平面应变约束）在内的海船断裂控制设计准则。应用该准则可根据对海船工作特性的要求，选择材料和预测结构的断裂行为，从而为防断选材、结构完整性评定、可靠性分析以及失效分析、预测、预防提供了定量的依据。该准则克服了传统断裂控制准则的局限性，可以全方位地模拟海船结构特点及工作条件，具有更满意的适用性。

Design Criterion of Ship Fracture Control

Fracture control design for ships now is in accordance with the transition temperature criterions, such as nil ductility temperature (NDT) and fracture analysis diagram (FAD). Under certain circumstances, these traditional criterions, however, are difficult in practice for fracture control of engineering constructions, because there is distinction of service condition of ships from the testing condition from which the criterion is established. For example, NDT, measured by Drop Weight Test (DWT) in which loading velocity is only about 9m/s, leaves out of account of loading rate sensitivity in the range of higher loading velocity, and besides the change of material's properties in Heat Affected Zone (HAZ) near the bead welding is not considered either. So far as the FAD is concerned, the level of fracture toughness for different materials is not distinguished, so it is impossible to quantitatively evaluate the safety of practical constructions. The reason is that the tolerant crack size depends on the stress level in service and fracture toughness of materials In this paper, a fracture characteristic analysis map is constructed and a new design criterion of fracture control for ships is established on the basis of analysis of the temperature effect and the loading rate effect in fracture toughness and service condition of ships. As a consequence, the material properties (fracture toughness, yield strength), the environmental factors (service temperature, loading rate) and the structural characteristics (plate thickness, plane strain constraints) are all involved in this new criterion. According to the demands for service characteristics of ships, the suitable material could be selected among the available materials, and the fracture behavior of the constructions could be predicted with this criterion. From this time forward the quantitative analysis for the material choice in marine design and building, the evaluation of structural integrity and reliability, and the failure prediction and prevention will become possible in practice of engineering.