断裂力学-1绪论课件.ppt

1、Shanghai University断裂力学Fracture Mechanics郭战胜郭战胜E-mail：1.)Develop basic fundamental understanding of the effects of crack-like defects on the performance of aerospace,civil,and mechanical engineering structures.2.)Learn to select appropriate materials for engineering structures to insure damage toler

2、ance.3.)Learn to employ modern numerical methods to determine critical crack sizes and fatigue crack propagation rates in engineering structures.4.)Gain an appreciation of the status of academic research in field of fracture mechanics.Course Objectives Liberty shipsArthur M.Huddellone of the last Wo

3、rld War II Liberty shipslUnder the guidance of Henry Kaiser,a famous construction engineer whose previous projects included the Hoover Dam,the United States developed a revolutionary procedure for fabricating ships quickly.These new vessels,which became known as the Liberty ships,had an all-welded h

4、ull,as opposed to the riveted construction of traditional ship designs.lThe Liberty ship program was a resounding success,until one day in 1943,when one of the vessels broke completely in two while sailing between Siberia and Alaska.Subsequent fractures occurred in other Liberty ships.Of the roughly

5、 2700 liberty ships build during World War II,approximately 400 sustained fractures,of which 90 were considered serious.In 20 ships the failure was essentially total,and about half of these broke completely in two.S.S.SchenectadyOn 16 January 1943,Without warning,and with a noise audible for at leas

6、t a mile,the hull cracked almost in half.Only the bottom plates of the ship held.This was not the first of the war-built merchant fleet to fracture in this way there had been ten other major incidents,and several more would followbut it was perhaps the most prominent;it occurred in full view of the

7、city of Portland,and was widely reported in the newspapers even under wartime conditions.格氏1893年出生于伦敦，1911年毕业于曼岛的一所中学，获得奖学金进入利物浦大学读机械工程，1914年以一等成绩获得学士学位，并获得最高奖章。1915年，格氏到皇家航空研究中心工作，并与G.I.Taylor一起发表了用肥皂膜研究应力分布的开创性论文，该文获得机械工程协会的金奖。同年，格氏获得利物浦大学工程硕士学位。1921年，格氏以他的断裂力学成名作获得利物浦大学工程博士学位。其后，格氏历任空军实验室首席科学家，航空

8、研究中心工程部主管等职，在航空发动机设计方面做出了同样卓越的贡献，与他在断裂方面的名望相比，这些成就就少为人知了。格氏于1939年加盟劳斯莱思公司，1941年当选皇家学会院士，1960年退休，1963年辞世，享年70岁 The theory was considered to apply only to a limited class of extremely,such as glasses or ceramics bachelors degree in physics at 1931)lAn elder brother,William,had studied engineering at t

9、he University of Illinois(later to be Vice-President of the Springfield Boiler Co.),so that GRI was known there,and was invited after graduation at Knox to fill a graduate assistant teaching post at the University of Illinois,which he accepted.lIt permitted him to acquire his masters degree in 1933,

10、and subsequently a PhD in physics in 1937.Meanwhile,a unique event in Georges life occurred in 1933,with marriage to Georgia Boynton Shearer,creating a strong partnership that was to last for 65 years.lA Member of the American National Academy of Engineering,Royal Society of London,an Honorary Membe

11、r and winner of prizes and awards of many American and European scientific societies.l1986 Timoshenko MedallDiscussion,J.Appl.Mech.,25,No.2,299-303(1958).lHis ideas very important for practical use and subsequent works devoted to their improvement and development caused a decisive influence on the p

12、rogress in fracture mechanics as a new science and powerful engineering tool applicable to numerous problems connected with manufacturing and processing of materials and their failureproof operation in critical structures of various materials.1949 Rep.Prog.Phys.12 185-232was born in Obuda,a part of

13、Budapest Courtesy of MIT Museum The X-ray studies conducted earlier by Orowan(1955)showed that even in materials fracturing in a purely brittle manner,there was evidence of extensive plastic deformation on the crack surfaces.Fellow of the Royal Society,(1947)Member of the American Academy of Arts an

14、d Sciences,(1951)Member of the National Academy of Sciences,(1969);Bingham Medal of the American Society of Rheology,(1959);Gauss Medal of the Braunschweiger Wissenschaftliche Gesellschaft,(1968);Vincent Bendix Gold Medal of the American Society for Engineering Education,(1971);Paul Bergse Medal of

15、the Danish Metallurgical Society,(1973);The Acta Metallurgica Gold Medal,(1985).我国断裂力学工作起步至少比国外晚了20年，直到上世纪70年代，断裂力学才广泛引入我国，一些单位和科技工作者逐步开展了断裂力学的研究和应用工作。凌晨1点45分左右5名工人不同程度受伤。原定恒压0.92兆帕。当压力值还在0.7至0.8兆帕时，容器突然爆炸，釜门连接处断裂。2吨多重的釜门受巨大冲击力作用，瞬间飞过围墙。图说明：过山车最后停车点;半轴断裂段及撞伤轨道点共7条横档及右侧轨道变形处;撞断脱件飞出砸伤雷车停车港防雨棚;半轴断裂后被撞伤

16、轨道的第二段共14条横档;过山车始发车站台。半轴的断裂属于疲劳断裂 Ariel view of damage from East Ohio Gas Explosion Oct.27,2009-Dec.22,2009 At 2:20 a.m.,April 15,1912,the Titanic sank with the loss of more than 1,500 lives.A metallurgical analysis of steel taken from the hull of the Titanics wreckage reveals that it had a high duc

17、tile-brittle transition temperature,making it unsuitable for service at low temperatures;at the time of the collision,the temperature of the sea water was-2C.The analysis also shows,however,that the steel used was probably the best plain carbon ship plate available at the time of the ships construct

18、ion.lCylinder stress failureDamage to the Boston Elevated Railway due to the flooda molasses tank 15 m tall,27 m in diameter and containing as much as 2,300,000 US gal(8,700 m3)collapsed.Witnesses stated that as it collapsed,there was a loud rumbling sound,like a machine gun as the rivets shot out o

19、f the tank,and that the ground shook as if a train were passing by.The collapse unleashed a wave of molasses 25 feet(7.6 m)high at its peak,moving at 35 miles per hour(56 km/h).The molasses wave was of sufficient force to damage the girders of the adjacent Boston Elevated Railways Atlantic Avenue st

20、ructure and tip a railroad car momentarily off the tracks.Author Stephen Puleo describes how nearby buildings were swept off their foundations and crushed.Several blocks were flooded to a depth of 2 to 3 feet(60 to 90 cm).Puleo quotes a Boston Post report:The Silver Bridge was an eyebar-chain suspen

21、sion bridge built in 1928 and named for the color of its aluminum paint.The bridge connected Point Pleasant,West Virginia and Gallipolis,Ohio,over the Ohio River.On December 15,1967,the Silver Bridge collapsed while it was full of rush-hour traffic,resulting in the deaths of 46 people.Two of the vic

22、tims were never found.The collapsed Silver Bridge,as seen from the Ohio sideThe bridge failure was due to a defect in a single link,eyebar 330,on the north of the Ohio subsidiary chain,the first link below the top of the Ohio tower.A small crack was formed through fretting wear at the bearing,and gr

23、ew through internal corrosion,a problem known as stress corrosion cracking.The crack was only about 0.1 inch(2.5 mm)deep when it went critical,and it broke in a brittle fashion.Growth of the crack was probably exacerbated by residual stress in the eyebar created during manufacture.When the lower sid

24、e of the eyebar failed,all the load was transferred to the other side of the eyebar,which then failed by ductile overload.The joint was then held together only by three eyebars,and another slipped off the pin at the center of the bearing,so the chain was completely severed.Collapse of the entire str

25、ucture was inevitable since all parts of a suspension bridge are in equilibrium with one another.Witnesses afterward estimated that it took only about a minute for the whole bridge to fall.King Street Bridge(Melbourne)Overlooking the King Street Bridge and Crown Casino and Entertainment ComplexThe b

26、ridge was designed in 1959 by Utah Australia for the Country Roads Board,and constructed over the next two years,being opened on 12 April 1961.It was of a welded,deck-girder,suspended-span construction with spans up to 160 feet(49 m)long.King Street Bridge(Melbourne)Soon after completion,on 10 July

27、1962,one span collapsed under the weight of a 47 ton semi-trailer,though the weight was within the bridge limits.The subsequent Royal Commission found that bridge failed with a brittle fracture on a very cold Melbourne winter day.The Commission identified the cause of girder failure was due to crack

28、ed welds.King Street Bridge(Melbourne)The fabricator was not familiar with welding low-alloy steel and the steel supplied was high in carbon making it difficult even for an experienced fabricator.The cracks in the welds were not discovered by inspectors of both the fabricator and the Country Roads B

29、oard.The south end of the bridge has been undergoing differential settlement between the approach ramps resting on fill,and the suspended section supported by bored piles.The embarrassing incident was widely satirised at the time;most notably by the Melbourne TV clowns Zig and Zag and was the inspir

30、ation for a Spike Milligan radio comedy.Structural Failure of Alohas 73728 April 1988caused by fatigue failureFractured leg in plaster cast Cracked concretebullet hole x-ray showing femur fracture from auto accident Winter ice layer 1943-47年,美国近500艘全焊船1000多起脆性破坏，238艘报废。总是在焊接缺陷等应力集中区域，-34C水1947年，苏联45

31、00m3石油储罐，-43C，底部和下部壳连接处，大量裂纹。（低温、脆性、焊点应力集中、内外温差）五十年代，美国北极星导弹固体燃料发动机壳体试验，发生爆炸。高强度合金，传统强度和韧性指标合格，爆炸时工作压力远低于许用应力。（裂纹）1963年，美国F-111飞机训练中，左翼脱落，飞机坠毁，而当时飞行速度、负荷远低于设计指标。（热处理不当、机翼枢轴出现缺陷，疲劳载荷，裂纹）maxssnsmaxbbnb1max1n（屈服），（屈服），为屈服应力，塑性材料为屈服应力，塑性材料（破坏），（破坏），为强度极限，脆性材料为强度极限，脆性材料 循环加载循环加载 1400sMpa5%这些结构大多由高与超高强度材料

32、制造，高强度刚屈服应力这些结构大多由高与超高强度材料制造，高强度刚屈服应力而普通钢仅而普通钢仅200MPa，且材料韧性也很好，且材料韧性也很好(材料可按延伸律分为脆性和韧性两大类，材料可按延伸律分为脆性和韧性两大类，5为界限为界限)由上述典型事故看出，脆性断裂总是由宏观裂纹引起由上述典型事故看出，脆性断裂总是由宏观裂纹引起的。而这些裂纹要么由冶金夹杂物及加工和装配引起，的。而这些裂纹要么由冶金夹杂物及加工和装配引起，要么由疲劳载荷及工作环境引起。对于大多数结构和零要么由疲劳载荷及工作环境引起。对于大多数结构和零件来说，宏观裂纹的存在是不可避免的。含裂材料的强件来说，宏观裂纹的存在是不可避免的。

33、含裂材料的强度，取决于材料对裂纹扩展的抗力，这种抗力由材料的度，取决于材料对裂纹扩展的抗力，这种抗力由材料的内部属性决定。内部属性决定。断裂力学的目的，就是应用弹塑性理论和新的实验技术，断裂力学的目的，就是应用弹塑性理论和新的实验技术，研究裂纹尖端附近的应力、应变场和裂纹的扩展规律。研究裂纹尖端附近的应力、应变场和裂纹的扩展规律。断裂力学的研究对象断裂力学的研究对象宏观裂纹产生和发展的几个研究阶段：宏观裂纹产生和发展的几个研究阶段：断裂力学和材料力学的差别断裂力学和材料力学的差别 断裂力学的理论基础：弹性断裂力学的理论基础：弹性力学、塑性力学及粘弹性力力学、塑性力学及粘弹性力学等等。断裂力学是

34、材料力学等等。断裂力学是材料力学的发展和补充，但两者的学的发展和补充，但两者的设计思想还是不同的设计思想还是不同的 注意：由于断裂力学考虑了裂纹，根据裂纹失稳准则得注意：由于断裂力学考虑了裂纹，根据裂纹失稳准则得到的断裂应力与传统强度条件得出的结果不一定相同。到的断裂应力与传统强度条件得出的结果不一定相同。静载荷情况静载荷情况循环载荷情况循环载荷情况11max1n1n传统的疲劳设计，是用光滑试件作传统的疲劳设计，是用光滑试件作SN曲线，求出曲线，求出下界限应力疲劳极限。下界限应力疲劳极限。如果最大工作应力满足下式如果最大工作应力满足下式为循环载荷下的安全系数，并认为凡是有缺陷的构件都为循环载荷

35、下的安全系数，并认为凡是有缺陷的构件都不能应用不能应用断裂力学认为：含裂纹构件，只有裂纹未达到临界长度仍断裂力学认为：含裂纹构件，只有裂纹未达到临界长度仍可使用；在循环载荷作用下，裂纹先缓慢扩展，直至达到可使用；在循环载荷作用下，裂纹先缓慢扩展，直至达到临界长度，构件才失稳破坏。并选用指标临界长度，构件才失稳破坏。并选用指标dadN作用载荷每循环一周裂纹的扩展量，代表材作用载荷每循环一周裂纹的扩展量，代表材料抵抗裂纹扩展的能力。料抵抗裂纹扩展的能力。腐蚀介质下的情况腐蚀介质下的情况断裂力学抛弃了物体的连续性假设断裂力学抛弃了物体的连续性假设,而从物体中含有裂纹而从物体中含有裂纹这一前提出发这一

36、前提出发,以弹性力学和塑性力学为理论工具以弹性力学和塑性力学为理论工具,确定确定含裂纹体的应力场、位移场分布含裂纹体的应力场、位移场分布,据此找出决定位移扩展据此找出决定位移扩展的物理量的物理量,同时同时,通过试验测定出材料抵抗裂纹扩展的纯通过试验测定出材料抵抗裂纹扩展的纯力力,并建立二者之间的关系并建立二者之间的关系,即建立断裂准则即建立断裂准则断裂的分类断裂的分类针对晶体材料分析针对晶体材料分析:结晶体结晶体:由原子由原子(或离子、分子或离子、分子)规则排列形成规则排列形成.解理断裂解理断裂:金属在一定条件下金属在一定条件下,当应力达到一定数值时当应力达到一定数值时,便以极快的速度沿一定的

37、结晶学平面发生断裂便以极快的速度沿一定的结晶学平面发生断裂.断裂面断裂面平滑而光亮平滑而光亮.解理断裂是脆性断裂解理断裂是脆性断裂.解理断裂通常是由于解理面的正应解理断裂通常是由于解理面的正应力的作用破坏了晶体原子间的合力而引起的力的作用破坏了晶体原子间的合力而引起的.断裂的晶体学分类断裂的晶体学分类 晶体学解理断裂与滑移断裂晶体学解理断裂与滑移断裂 滑移断裂滑移断裂:受剪应力作用破坏了晶体原子间的结受剪应力作用破坏了晶体原子间的结合力而引起的断裂合力而引起的断裂.特点特点:断口黑暗断口黑暗,鹅毛装或纤维状鹅毛装或纤维状,断裂面与拉伸断裂面与拉伸轴成一定角度轴成一定角度,断裂发生前断裂发生前,

38、发生了明显的塑性变形发生了明显的塑性变形.5%5%k断裂的工程学分类断裂的工程学分类 脆性断裂与韧性断裂脆性断裂与韧性断裂韧性材料:缺口试件在冲击载荷下破断试验得到的缺口韧性值来衡量脆性材料:脆性断裂韧性断裂判断断裂韧性的指标脆性断裂:平齐而光亮,且与正应力垂直.断口上呈现人字纹或放射花样,基本上没有塑性变形.韧性断裂:断裂前发生明显的宏观塑性变形,灰暗色纤维状.1.1.线弹性断裂力学线弹性断裂力学 裂纹扩展前裂纹尖端无塑性区裂纹扩展前裂纹尖端无塑性区,或塑性区尺寸远远或塑性区尺寸远远小于裂纹长度小于裂纹长度.K K准则和准则和G G准则准则.发展成熟发展成熟,理论简单理论简单,基础较牢固基础

39、较牢固.研究内容研究内容3.3.断裂动力学断裂动力学裂纹快速传播过程中裂纹快速传播过程中,必须考虑材料的惯性效应必须考虑材料的惯性效应.2.2.弹塑性断裂力学弹塑性断裂力学弹塑性断裂力学应用弹性力学、塑性力学研究物弹塑性断裂力学应用弹性力学、塑性力学研究物体裂纹扩展规律和断裂准则，适用于断裂前断裂体裂纹扩展规律和断裂准则，适用于断裂前断裂尖端处的塑性区足够大尖端处的塑性区足够大.目前多采用目前多采用J J积分法、裂纹张开位移（积分法、裂纹张开位移（CODCOD）法、）法、R R曲线法等近似或试验方法进行分析曲线法等近似或试验方法进行分析在工程上已应用在工程上已应用,但完善程度不够但完善程度不够.Shanghai University