疲劳与断裂第八章疲劳裂纹扩展

1,第八章疲劳裂纹扩展,8.1疲劳裂纹扩展速率,8.2疲劳裂纹扩展寿命预测,8.3影响疲劳裂纹扩展的若干因素,8.4疲劳裂纹扩展速率试验,返回主目录,2,前节回顾:,Theplotoflogda/dNversuslogDKisasigmoidal(S形)curve.Thiscurvemaybedividedintothreeregions.Atlowstressintensities,crackingbehaviorisassociatedwiththresholdeffects.Inthemid-region,thecurveisessentiallylinear.Finally,athighDKvalues,crackgrowthratesareextremelyhighandlittlefatiguelifeisinvolved.,3,MostofthecurrentapplicationofLEFMconceptstodescribecrackgrowthbehaviorareassociatedwithregion2.Inthisregionthelogda/dNversuslogDKcurveisapproximatelylinearandliesroughlybetween10-7and10-4mm/cycle.Manycurvefitstothisregionhavebeensuggested.TheParisequation,whichwasproposedintheearly1960s,isthemostwidelyaccepted.,大多数用线弹性断裂力学描述裂纹扩展的应用是与区域2相关的。在这一区域，logda/dN-logDK曲线近似线性且在10-7-10-4mm/c间。已有许多拟合曲线提出，60年代初的Paris公式是应用最广的。,4,8.3影响疲劳裂纹扩展的若干因素,K是控制da/dN的最主要因素。平均应力、加载频率、环境等的影响较次要，但有时也不可忽略。,同一材料,由不同形状、尺寸的试件所得到的da/dN-K曲线相同。,da/dN-K曲线可以描述疲劳裂纹扩展性能。,5,1.平均应力或应力比的影响,R0、R0时复杂得多。,有经验关系为：Kth=K0th(1-R)Koth是R=0时的基本门槛应力强度因子幅度。参数、由实验确定。图中钢材的下限为：Kth=7.03(1-0.85R),8,Formansequationisoftenusedtopredictstressratioeffects.AsRincreases,thecrackgrowthrateincreases.Thisisconsistentwithtestobservations.FormansequationisvalidonlywhenR0.Generally,itisbelievedthatwhenR0,nosignificantchangeingrowthrateoccurscomparedwiththeR=0.Againthisismaterialdependent,assomeresearchershaveobtaineddataforcertainmaterialswhichshowhighergrowthratesforR0时正确。一般认为与R=0相比，R0对da/dN没有显著影响。这仍与材料有关，对有些材料，也有研究者在R0,是K不断增加的试验方法。,18,3.K减小试验法,K减小试验法用于低速率区。,标准建议C-0.08mm-1。由此可计算不同a时的DK、DP。,19,小结,1）若arp,则线弹性断裂力学可用。应力强度因子为：K=f(a)；中心裂纹：f(a)=1.0；边裂纹：f=1.12。,2）疲劳裂纹扩展速率的主要控制参量是K，下限有Kth,上限有(1-R)Kc。K=Kmax-Kmin=f(a)R0,min0=KmaxR0,min0裂纹不扩展条件K=f(a0)Kth临界裂纹尺寸Kmax=Kcac=(1/)(Kc/fmax)2,20,3）Paris公式：da/dN=C(K)m在恒幅循环载荷作用下，积分后有：,4）初始裂纹尺寸a0对寿命影响很大，要控制a0。,21,6）基本疲劳分析方法的比较,22,23,24,25,summary,Fracturemechanicsapproachesprovideanestimateofthecrackpropagationfatiguelife.,Thefatiguecrackgrowthratecanberelatedtothestressintensityfactorrange.Fromthis,cyclestofailuremaybecalculated.,Thefatiguelifeestimateisstronglydependentontheinitialcracksize,ai,largechangesintheestimateofthefinalcracksize,ac,resultinonlysmallchangesinthelifeestimate.,26,问题2：变幅载荷作用次序，对da/dN有何影响；如何解释、预测其影响？,问题1：裂纹尖端的应力有奇异性。裂尖应力，至少也大于sys。那么，为什么会有Kth存在？,Schijve的实验结果:2024-T3铝,.施加了三次高载后，寿命延长4倍。,27,疲劳裂纹扩展速率的实验数据处理(第四次上机),目的：确定14MnNbq桥梁钢扩展速率曲线da/dN=C(DK)m。试样:中心裂纹板(B=10.02mm，W=59.88mm)。载荷条件：拉伸疲劳裂纹扩展试验R=0.05，Pmax=24kN=const.DP=0.95Pmax。,28,14MnNbq疲劳裂纹扩展试验结果（试样编号：SPA10R0001，试验日期2005.7.3）iaiNi(103)iaiNi(103)013.017001017.84230013.329001118.80240013.4611001219.76250013.9213001320.34255014.3015001421.14260014.8017001521.83264015.7019001622.33266016.1620001723.12268016.69210017.152200,29,说明：