【作者】 聂德福；

【导师】 赵杰；

【作者基本信息】 大连理工大学 ， 材料学， 2008， 博士

【摘要】 随着测量技术的发展和要求精度的提高,人们发现即使在较低的同系温度(T/T_m＜0.2)下,许多工程结构钢如不锈钢、管线钢、高强钢等在承受恒定载荷作用时,变形量也会随着时间的延长而增加,该现象被称为室温蠕变。而且这种时间相关的变形已经引发了一些技术问题,相应的研究工作逐渐被重视起来。尤其是对于存在应力集中的情况,即使在名义应力较低的条件下,在某些严重应力集中区域也可能会累积较大室温蠕变变形,而且室温蠕变的发生势必会改变材料原有的组织状态,进而会对其后的性能产生影响。许多工程结构部件常常处于波动载荷和恒定载荷随机交替的复杂载荷状态下服役,载荷和环境等因素共同作用诱发的裂纹萌生、扩展是最终导致构件失效的主要形式之一。如果在恒定载荷过程中裂纹尖端区域发生时间相关的变形,将会改变该区域内的应力应变状态,从而会对随后波动载荷作用下的疲劳裂纹扩展行为产生影响。研究室温蠕变对疲劳裂纹扩展行为的影响对建立能够处理复杂服役条件的疲劳可靠寿命预测模型具有重要意义。本文选用X70管线钢(体心立方结构)和SUS304不锈钢(面心立方结构)两种工程结构钢,利用光滑试样、缺口试样和紧凑拉伸试样,以液压伺服疲劳试验机为主要设备研究了有无应力集中条件下的室温蠕变现象、影响因素,以及室温蠕变和单峰过载对疲劳裂纹扩展的影响,并结合光学显微镜、扫描电镜、X射线仪和透射电镜的试验结果,对微观机制进行了分析、讨论。1.在有无应力集中的条件下,X70管线钢和SUS304不锈钢在各种应力水平下,均存在室温蠕变现象。X70管线钢光滑试样和缺口试样中的室温蠕变呈现出较强的应力敏感性,在远低于抗拉强度的应力水平下只存在减速蠕变阶段,当接近抗拉强度时室温蠕变由减速的第一阶段蠕变逐渐向加速的第三阶段蠕变过渡,并伴随着试样的颈缩,最终导致材料发生断裂。对于SUS304不锈钢光滑试样,在各种应力水平下只存在减速的室温蠕变阶段,且这种时间相关的变形不会导致材料发生断裂。SUS304不锈钢缺口试样与X70管线钢相似,室温蠕变明显地依赖于应力水平,随着应力水平的提高蠕变变形逐渐由减速阶段向加速阶段过渡。对于存在严重应力集中的裂纹尖端区域,X70管线钢和SUS304不锈钢的室温蠕变明显依赖于应力强度因子的大小,且在各种应力强度因子水平下均呈现为速率递减的蠕变特征。2.当以材料的抗拉强度作为标准来衡量室温蠕变时,随着应力速率的增加X70管线钢和SUS304不锈钢中的室温蠕变量均明显增大:正火态较轧制态的X70管线钢具有更加明显的室温蠕变现象;SUS304不锈钢的室温蠕变量明显大于X70管线钢,此外室温蠕变也受加载历史条件的影响。由于缺口的存在限制了材料的变形,因此与光滑试样相比,缺口试样的室温蠕变量明显减小,且缺口角度对室温蠕变也有一定的影响,随着缺口角度的增加室温蠕变略有增加,但影响并不十分明显。3.X70管线钢和SUS304不锈钢减速阶段的蠕变呈现为典型的低温对数蠕变,变形量与时间的关系可用包含两个回归参数α和β的对数方程ε=αlog(βt+1)来描述。X70管线钢和SUS304不锈钢光滑试样和缺口试样室温蠕变回归参数α与应变硬化系数的倒数呈现相似的变化规律,基于室温蠕变变形的连续性和拉伸试验中应力应变行为分析,建立了能够定量描述室温蠕变的模型。对于紧凑拉伸试样,回归参数α与d(V_g/α)/dK的变化规律相似,仍可用上述模型对裂纹尖端的室温蠕变进行描述。另外,回归参数α主要受应力水平的影响,应力速率对其影响较小,而β则显示出了较强的应力速率敏感性,对应力水平并不十分敏感。4.X70管线钢和SUS304不锈钢中的室温蠕变明显提高了屈服后材料的流变应力,并在一定程度上降低了屈服前材料的比例极限。基于已有的可动位错密度理论和应力帮助下的热激活理论,提出了局部可动位错密度理论,并据此解释了上述现象和屈服前的室温蠕变行为以及室温蠕变导致随后应变速率-应力曲线出现的瞬态突变。另外,在各种应力水平下,SUS304不锈钢光滑试样只存在减速阶段的蠕变行为是由于材料中发生了形变诱发马氏体相变。5.与单峰过载效应相似,X70管线钢和SUS304不锈钢裂纹尖端室温蠕变也会导致随后疲劳裂纹扩展出现延滞,只是在相同过载比下室温蠕变后的延滞现象更为明显,且提出了分段描述室温蠕变和单峰过载后疲劳裂纹扩展出现延滞的经验公式。通过与单峰过载效应的比较,并结合裂纹闭合效应的测试结果和疲劳裂纹及断口形貌的观察,分析了室温蠕变对疲劳裂纹扩展的影响机理,主要是由于裂纹尖端时间相关的变形加剧了塑性诱发的裂纹闭合所致。更多还原

【Abstract】 With the development of measuring technique and the improvement of required accuracy, it is found that the deformation under constant load increases with time even at low homologous temperature(T/T_m＜0.2) in many structural steels(such as stainless steels, pipeline steels,high strength steels and so on),which is called room temperature creep(RTC). Moreover,such a time dependent deformation has brought about some technical problems and received more attention.Particularly in the case of stress concentration,significant RTC can occur due to stress concentration even under low nominal stress.The occurrence of RTC may influence the original microstructure of materials and hence subsequent properties.Many structural components in service usually experience constant and variable load by turns and may fail due to the crack initiation and growth caused by the combined influence of stress and environment.If the time dependent deformation takes place at crack tips under constant load, it may influence the stress-strain behavior in this region and play an important role on the subsequent fatigue crack growth under variable load.The investigation of effect of RTC on fatigue crack growth is of advantage to developing reliable life prediction models which are capable of handling complex service conditions.This paper investigates RTC,its influencing factors and the effect of RTC and single wave overload(SWOL) on fatigue crack growth with and without stress concentration on a servo-hydraulic fatigue machine using smooth,notched and compact tension(CT) specimens of X70 pipeline steel(BCC) and SUS304 stainless steel (FCC).And based on observation by optical microscopy(OM),scanning electron microscopy (SEM),X-ray diffraction(XRD) and transmitted electron microscopy(TEM),the fundamental mechanisms are discussed.1.At various stress levels,X70 pipeline steel and SUS304 stainless steel with and without stress concentration show RTC behavior.For the smooth and notched specimens of X70 pipeline steel,RTC is strongly dependent on the stress level.RTC is primary at stress levels far lower than the ultimate strength but becomes tertiary as the ultimate strength is approached,during which the fracture following necking occurs.However,the smooth specimens of SUS304 stainless steel only shows the primary RTC at various stress levels,and RTC alone may not cause facture.Similar to X70 pipeline steel,RTC strongly depends on the stress level in the notched specimens of SUS304 stainless steel,and with increasing stress level,it gradually transits from the primary to the tertiary type.RTC at crack tip is significantly influenced by stress intensity factor and the primary creep is the dominant deformation mode at various stress intensity factor levels. 2.At the same percentage of the ultimate strength,RTC in both X70 pipeline steel and SUS304 stainless steel becomes significant with increasing loading stress rate,RTC strain in the normalized X70 pipeline steel is larger than that in the as-received X70 pipeline steel and SUS304 stainless steel show more RTC deformation relative to X70 pipeline steel.Moreover, RTC is also dependent on loading history.Since the deformation in the notched specimen is confined in a localized region,RTC strain markedly decreases compared with the case of the smooth specimen.RTC strain slightly increases with increasing the angle of notch.3.The primary RTC of X70 pipeline steel and SUS304 stainless steel agrees well with logarithmic creep lawε=αlog(βt +1),which includes two regression parametersαandβ.As to the smooth specimens and the notched specimens of X70 pipeline steel and SUS 304 stainless steel,the regression parameterαmonotonously develops with the inverse strain hardening coefficient.Based on the continuity of RTC deformation and the analysis of stress-strain behavior in the tensile test,a model is developed to quantitatively describe RTC strain.Since the evolution ofαis similar to that of d(V_g/α)/dK,this model can also be applied to the case of the CT specimen.In addition,the regression parameterαis strongly dependent on the stress level and is hardly influenced by the loading stress rate,butβis strongly dependent on the loading stress rate and is hardly influenced by the stress level.4.The post-yield RTC markedly increases the flow stress of X70 pipeline steel and SUS304 stainless steel while the pre-yield RTC reduces the elastic limit to some extent. Based on the theory of mobile dislocation density and the theory of stress dependent thermal activation,a theory of localized mobile dislocation density is presented to interpret the aforementioned behaviors,the pre-yield RTC and the transient burst of strain rate-stress curves caused by RTC.In addition,due to strain induced martensite transformation in SUS304 stainless steel,RTC at various stress levels is of primary nature and occurs at continuously falling rate.5.Similar to the SOWL effect,RTC at crack tips can retard the subsequent fatigue crack growth in X70 pipeline steel and SUS304 stainless steel except that the retardation following RTC is more significant at the same overload ratio.Some empirical equations are presented to describe different fatigue crack growth behaviors following RTC and SWOL.Compared with the SOWL effect,the retardation mechanism due to RTC is discussed on the basis of measurement of crack closure and observation of fatigue crack and fracture surface.The serious retardation of fatigue crack growth following RTC is attributed to the plasticity-induced crack closure owing to the time dependent deformation at crack tips.更多还原