【作者】 肖光春；

【导师】 荆洪阳；

【作者基本信息】 天津大学 ， 材料加工工程， 2011， 博士

【摘要】 钢材因其具有良好的力学性能、经济性能和使用性能等优点,在船舶、桥梁、压力容器、电力铁塔、高层建筑、油气管道等民用和工业设施领域得到广泛地应用,是国民经济建设和国防建设中的重要材料。但无论是在安装还是在实际服役过程中都会受到温度和塑性变形的影响。针对这两种影响因素,本文选取Q420高强结构钢和X80管线钢为研究对象,系统研究了温度和塑性预应变对钢材力学性能及其断裂韧度的影响,并基于局部法对其断裂行为进行了预测。首先根据冲击试验结果得出Q420高强结构钢和X80管线钢的韧-脆转变温度区间。然后对这两类钢材的原材料和预应变材料分别进行了不同温度下的拉伸试验。试验结果表明,钢材的屈服强度和抗拉强度随着温度的降低而增大,塑性随着温度的降低而减小;拉预应变因工作硬化提高了钢材的屈服强度与抗拉强度,而压预应变因包申格效应降低了钢材的屈服强度与抗拉强度,但两者都降低了钢材的塑性。此外,无论是拉预应变还是压预应变对屈服强度的影响程度更大。利用标准三点弯曲试样分别测试了Q420高强结构钢和X80管线钢原材料和预应变材料在不同温度下的断裂韧度。试验结果表明,温度对钢材的断裂韧度具有显著影响,其随着温度的降低显著减小,断裂形式也由延性断裂向脆性断裂转变;无论是拉预应变还是压预应变都降低了钢材的断裂韧度,进一步引起钢材断裂形式的转变。此外,本文并对不同断裂方式下的试样断口形貌进行了分析。通过有限元数值模拟分析发现,温度和预应变对试样断裂时裂纹尖端区域应力应变场的增大作用促进了材料的脆性断裂,即弹塑性材料的断裂主要取决于裂纹尖端的应力应变场及其微观断裂过程。本文将局部法应用于结构钢和管线钢预应变材料的断裂行为研究。研究发现,不同温度下的原材料试样和预应变材料试样发生断裂时,在相同断裂概率下的威布尔应力基本相同,并基于局部法理论由原材料试样的试验结果成功预测了不同温度下预应变试样的断裂韧度概率分布。此外,本研究提出了一个简单的参考温度ΔT_P评定方法:在服役温度T下的预应变材料试样的临界CTOD值可以由较低温度TΔT_ P下的原材料试样的临界CTOD值所代替,参考温度ΔT_ P是作为预应变引起的流变应力的变化值Δσ~P_f的函数提出的。由该评定方法得出的ΔσP~_ f-ΔT_ P评定曲线与试验直接得到的结果基本一致。更多还原

【Abstract】 Due to their outstanding mechanical properties, economic properties and service performance, steels have been widely applied in the civil and industrial facilities of ships, bridges, pressure vessels, power towers, high-rise buildings, oil and gas pipelines, etc. It has become an important material in the national economic construction and national defense construction. However, it may suffer from effects of the temperature and plastic deformation before and during operation. For these two factors, taking high-strength structural steel Q420 and pipeline steel X80 as examples, this paper systematically studied effects of temperature and prestrain on tensile and fracture toughness of steels, and evaluated the fracture behavior based on the local approach.Firstly, according to the impact test results, the ductile-brittle transition temperatures of high-strength structural steel Q420 and pipeline steel X80 were obtained. Then, tensile tests were conducted on the virgin and prestrained materials at different temperatures. The test results revealed that the yield stress and tensile strength of steels were increased with the decrease in temperature, but it reduced the plasticity. Furthermore, the yield stress and tensile strength of steels increased in steels with tensile prestrain due to work hardening and decreased in compressed steels due to the Bauschinger effect, but they all reduced the plasticity. In addition, the effect of prestrain could be more significant for the yield stress.The fracture toughness tests were performed under three-point bending loading conditions for the virgin and prestrained specimens of high-strength structural steel Q420 and pipeline steel X80 at different temperatures. The test results revealed that temperature significantly influenced the fracture toughness of steels and that fracture toughness was significantly reduced with the decrease in temperature, resulting in brittle fractures. Prestrain also reduced the fracture toughness of steels, and further increased the probability of brittle fractures. Meanwhile, specimen fracture morphologies of different fracture modes were also analyzed.Based on the finite element analysis, it was found that the near-tip stress/strain fields near the crack tip of different specimens were elevated by temperature and prestrain. Such activation of the near-tip stress fields facilitated the brittle fracture of steels. It can be concluded that the fracture toughness of elastic-plastic material mainly depends on the stress/strain field near the crack tip and its micro-fracture process.The local approach was used to study the fracture behaviors of the prestrained materials of structural steel and pipeline steel. The research found that the Weibull stress for all specimens were identical under the same fracture probability when the fracture initiation occurred for the virgin and prestrained specimens at different temperatures. Based on the local approach, the fracture toughness probability distributions of the prestrained specimens at different temperatures had been predicted from the test results of the virgin material specimens. Furthermore, this paper proposed a simplified reference temperature△T_ P evaluation method. The fracture toughness at the service temperature T with prestrain could be displaced by the fracture toughness of the virgin material at a lower temperature T△σ_T~P. The temperature shift△T_P was provided as a function of the flow stress elevation caused by the prestrain. The estimated△σ_ P~f-△T_ P curve was almost consistent with the experimental results.更多还原