第1讲 断裂力学导论PPT.ppt

断裂力学 第一讲;断裂力学;形形色色的断裂现象;疲劳断裂;Facts on Liberty Ships;The Aloha Boeing 737 Accident;The Aloha Boeing 737 Accident;一个不断发展的学科： 强度问题、裂纹问题、断裂问题、疲劳、损伤、分析和规范、结构完整性问题 ，等等;损伤分布;与裂纹共生的结构;结构完整性和寿命评估系统;从强度理论到裂纹力学;应力状态和主应力参数;强度函数;常用的强度准则;孔洞导致应力集中 - Inglis理论;Fracture stress for realistic material Inglis (1913) analyzed for the flat plate with an elliptical hole with major axis 2a and minor axis 2b, subjected to far end stress The stress at the tip of the major axis (point A) is given by The ratio is defined as the stress concentration factor, When a = b, it is a circular hole, then When b is very very small, Inglis define radius of curvature as And the tip stress as ;Inglis理论推广到裂纹;玻璃纤维的断裂强度（示意图）;Griffith关于裂纹扩展的概念;Griffith’s Energy balance approach;A A Griffith laid the foundations of modern fracture mechanics by designing a criterion for fast fracture. He assumed that pre-existing flaws propagate under the influence of an applied stress only if the total energy of the system is thereby reduced. Thus, Griffiths theory is not concerned with crack tip processes or the micromechanisms by which a crack advances. ;Griffith’s Energy balance approach (Contd.);Griffith’s Energy balance approach (Contd.);Griffith’s Energy balance approach (Contd.);Griffith’s Energy balance approach (Contd.);Modification for Ductile Materials For more ductile materials (e.g. metals and plastics) it is found that the functional form of the Griffith relationship is still obeyed, i.e. . However, the proportionality constant can be used to evaluate gs (provided E is known) and if this is done, one finds the value is many orders of magnitude higher than what is known to be the true value of the surface energy (which can be determined by other means). For these materials plastic deformation accompanies crack propagation even though fracture is macroscopically brittle; The releas