石墨是高温气冷堆的反射层和堆芯结构材料，准确了解其力学性能对石墨构件的安全分析具有重要意义。当前对不同制造工艺石墨材料力学特性的对比研究有限，动态力学特性的研究基本空白。本文通过实验加载和实时观测方法，研究三种不同制造工艺石墨的裂纹扩展规律及破坏过程，获得静态、动态拉伸强度及断裂韧性，与微观分析相结合，研究石墨的试样尺寸、颗粒尺寸、加载速率等对其力学性能及断裂行为的影响规律，揭示了石墨损伤破坏的机理，为反应堆石墨构件材料选择和动态载荷下的完整性分析与评价提供了依据。主要研究成果如下： 1.通过圆盘劈裂实验测得系列石墨材料的劈裂拉伸强度，发现劈裂拉伸强度随着试样尺寸的增大呈下降趋势、随着石墨颗粒尺寸的减小显著增大，试样尺寸较小时要综合考虑试样尺寸和石墨颗粒尺寸的影响。石墨的劈裂拉伸强度符合两参数威布尔分布，且石墨颗粒尺寸越大其劈裂拉伸强度的分散性越强。 2.验证了分离式霍普金森压杆加载圆盘劈裂实验测量石墨强度的有效性。实验结果表明石墨劈裂拉伸强度随应变率增大显著增加，粗颗粒石墨动态增长因子为1.2~1.6，显著大于细颗粒石墨（1.05~1.2），断裂应变随应变率增大则基本不变。 3.通过三点弯实验测得三种石墨材料的平均断裂韧性为1.08~1.29 MPa·m1/2，石墨裂纹扩展速度呈现先迅速增大后迅速减小的变化趋势，随着石墨颗粒尺寸的增大，断裂韧性明显增大而最大裂纹扩展速度显著降低。 4.采用冲击试验机动态加载的三点弯实验测量粗颗粒、细颗粒石墨的动态断裂韧性，发现其均随冲击速率的增大而显著增大，且细颗粒石墨的增长幅度更大。石墨裂纹尖端张开位移和裂纹扩展速度均呈现复杂变化模式且具有波动特性。 5.孔隙等原生缺陷及其附近的应力集中导致微裂纹萌生并影响其扩展路径，超细颗粒石墨裂纹沿着粘结剂撕裂扩展、细颗粒石墨裂纹沿着粘结剂及部分大颗粒撕裂扩展，粗颗粒石墨裂纹沿着石墨颗粒内部撕裂扩展。加载速率越大断口表面越平直，裂纹扩展路径中存在裂纹偏转、弯曲以及桥接现象，断裂模式也发生改变，静态加载时遵循最小能耗原则，动态加载时遵循最短路径原则，由此石墨的动态拉伸强度和断裂韧性与静态有显著的区别，在评价石墨构件在地震等动载荷下的结构完整性时应考虑石墨材料性能的变化。

Investigation of the mechanical properties of graphite has significant importance for the structural integration of the graphite components, as they serve as both reflectors and structural material in the High Temperature Gas-cooled Reactors (HTGR). At present, the researches on the comparison between the material properties of graphite with different manufactural procedures are limited and the studies on the dynamic tensile strength and fracture toughness of graphite are absent. In the present paper, an experimental work was carried out to investigate the mechanical properties and fracture behaviors of 3 different graphite, which has different grain sizes and manufacture procedures. The static and dynamic splitting tensile strengths and fracture toughness were measured and the influence of the size of specimens, grain size and loading rate were investigated. Furthermore, the microstructures of the fractured specimens were observed by means of scanning electron microscope, polarization microscope and super-depth microscope to study the damage mechanisms of the graphite with different morphology under different loading conditions. The results will provide important guidance for the selection of graphite for the application in HTGR, as well as the design and evaluation of the graphite components under dynamic loadings, such as earthquake. The major work of the thesis is as follows: 1.The splitting tensile strengths of different graphite materials were obtained by the disc compression tests. The trend that the strength decreases with the size of the specimens was observed and the grain size affects the strength significantly, indicating that both factors need to be considered when small specimens are used. The distribution of the strengths of the graphite can be well described by the two-parameter Weibull distribution. The divergence increases with the increase of the grain size. 2.Split Hopkinson Pressure Bar (SHPB) test system was used to perform the disc compression tests in order to study the dynamic splitting tensile strengths of graphite materials with different grain size. The results show that the SHPB test method is capable of performing the disc compression tests of graphite and the dynamic tensile strength of graphite increases with the strain rate. Fine-grained graphite has a dynamic increase factor (DIF) of 1.05 to 1.2, whereas coarse-grained graphite is about 1.2 to 1.6. It was discovered that fracture strain maintains the same as strain rate increases. 3.The fracture toughness of the 3 graphite materials is in the range of 1.08~1.29 MPa·m1/2. The crack propagation rate during fracture increases dramatically at the beginning and decreases at the later stage. With the increase of the grain size, the fracture toughness increased and the crack propagation rate decreased. 4.The dynamic fracture toughness of two graphite materials (coarse grain and fine grain), which were measured using instrumented impact tester, increase significantly with the loading rate. The increase of the amplitude was larger for the fine-grain graphite. The crack tip opening displacement and crack propagation rate were unstable. 5.Examination of the microstructures of the graphite specimens after the fracture shows that the stress concentrations around the defects may cause the initiation of the cracks and affect the crack path. The cracks in the super-fine-grain propagate mainly along the binder, while those in the fine-grain graphite propagate along both the binder and filler. Cracks within the filler particles were observed for the coarse-grain graphite. The fracture surfaces are smoother for both splitting tensile tests and fracture toughness tests when the load rates were increased. The fracture modes seem to follow the minimum energy consumption principle for the static loading and the shortest crack path principle for the dynamic loading. Differences between the mechanical properties as well as the fracture mechanism indicated that dynamic properties shall be considered when evaluating the structural integration of graphite components under seismic loading.