不同应力比下U75V轨道钢疲劳裂纹扩展行为研究

对高速铁路常用轨道钢U75V进行了不同应力比下的疲劳裂纹扩展门槛值测试和疲劳裂纹扩展试验。试验结果表明,应力比对U75V轨道钢的裂纹扩展行为有着显著影响。利用分级降载法测得的疲劳裂纹扩展门槛值随应力比的增大而减小,裂纹扩展速率随应力比的增大而增大。对裂纹断口微观形貌进行了细致观察分析,发现整个疲劳断口分布有凸条纹棱线。疲劳门槛值附近出现沿晶体学平面的穿晶小刻面;低扩展速率区凸条纹棱线平行排列,棱线方向与珠光体片层方向一致;高扩展速率区的棱线出现弯曲及碎裂,同时伴有解理断面及二次裂纹。     

温度和应力比对高强高韧7055铝合金型材裂纹扩展行为的影响

在不同温度(150℃、室温、-70℃)、不同应力比(0.1,0.5)下对7055-T76511铝合金型材进行了裂纹扩展速率试验,研究了温度和应力比对其裂纹扩展速率和门槛值的影响。结果表明:随着温度的降低和应力比的减小,7055-T76511铝合金裂纹扩展门槛值逐渐增大,裂纹扩展速率逐渐降低;-70℃下合金断裂时对应的裂纹扩展速率较小,且裂纹扩展门槛值较大,抗裂纹扩展能力最好。     

蠕墨铸铁断裂韧度的试验研究

比较四种热处理方法下蠕墨铸铁试样的断裂韧度.试验结果表明,蠕墨铸铁的断裂韧度受不同基体组织的影响比较明显;受不同石墨形态的影响显著,与结构钢不同,蠕墨铸铁的断裂韧度随材料的屈服强度增大而增大,随石墨颗粒的长厚比增加会急剧下降.     

变幅载荷下的钢板弹簧疲劳裂纹扩展研究

钢板弹簧疲劳裂纹在变幅载荷作用下的扩展预测模型主要是Schütz疲劳裂纹扩展模型,但此模型未考虑裂纹闭合效应对疲劳裂纹扩展的影响,忽略了参数C和n随应力比的联系,疲劳裂纹门槛值△Kth也只考虑应力比为0的特殊工况.因此对Schütz疲劳裂纹扩展唯象模型进行修正,使修正后的模型各参数物理意义更加明确,适用于不同的工况.并将实车试验所测的应力幅值运用于修正后的模型,表征变幅载荷作用下钢板弹簧疲劳裂纹扩展进程.表明加载幅值增大,疲劳裂纹扩展速率加快,试验结果与模型预测结果良好吻合,为变幅载荷作用下疲劳裂纹扩展速率的计算提供了理论依据.     

航空发动机涡轮盘用GH4133B合金疲劳裂纹扩展行为研究

材料的疲劳寿命由裂纹形成寿命和扩展寿命两部分组成。针对航空发动机涡轮盘用GH4133B合金,进行室温下不同应力比的疲劳裂纹扩展试验,测试疲劳裂纹扩展门槛值。Paris公式回归分析结果表明,裂纹扩展速率随应力强度因子和应力比的增大而增大,含门槛值的修正Paris公式能精确描述疲劳裂纹扩展行为。利用光学显微镜在线观测裂纹扩展路径,并利用扫描电镜考察试样断口微观形貌。结果发现,随应力强度因子增大,裂纹扩展路径由平直变得曲折。在疲劳裂纹萌生区、稳定扩展区和快速扩展区,断裂表面依次呈现为解理断裂、疲劳条带和沿晶韧窝混合断裂模式。基于断口反推理论反推载荷和裂纹扩展方程,结果表明,利用反推方程预测疲劳裂纹的扩展,可有效防范疲劳断裂的发生。     

应力比对25Cr2Ni2MoV钢焊接接头近门槛值区疲劳裂纹扩展曲线转折点的影响

在常温下采用逐级降载法进行不同缺口位置、不同应力比下25Cr2Ni2Mo V钢焊接接头的疲劳裂纹扩展试验,研究疲劳裂纹扩展曲线上近门槛值区转折点的变化规律。结果表明,焊接接头的疲劳裂纹扩展曲线在近门槛值区内存在一个转折点,其对应的应力强度范围随应力比的增大而减小,其大小为疲劳门槛值的0.5~0.8。近门槛值区疲劳裂纹扩展转折点前后扩展曲线斜率发生变化表明裂纹扩展机理的改变,热影响区缺口处的转折行为对疲劳裂纹扩展影响较小。应力比R低于0.5时,循环塑性区尺寸总体接近于材料特征组织尺寸;应力比R高于0.5后,两者偏差加大。应力比低于0.5时转折点对应的循环塑性区尺寸同特征组织尺寸接近,驱动力由ΔK控制向Kmax主导的转变是引起转折点随应力比变化的原因。     

高产气井油管用13Cr-L80钢的腐蚀疲劳裂纹扩展行为

用复合磷酸盐完井液对取自13Cr-L80钢油管的紧凑拉伸试样进行预腐蚀处理,再进行疲劳裂纹扩展试验,通过分析疲劳裂纹扩展区形貌,研究了应力比(0.05～0.25)、加载载荷(10～14 kN)和载荷频率(0.6～1.8 Hz)对腐蚀疲劳裂纹扩展行为的影响。结果表明：随应力比增大,疲劳裂纹扩展区二次裂纹数量增多,13Cr-L80钢的裂纹扩展速率增大;随加载载荷增大,疲劳裂纹扩展区二次裂纹数量减少,裂纹扩展速率减小;载荷频率越高,疲劳裂纹扩展区二次裂纹数量越少,但裂纹尖端应变速率增大,裂尖变形更充分,导致裂纹扩展加快,试验中止时的循环次数减少。     

对拉压超载后裂纹扩展迟滞模型的改进

对已有的裂纹扩展迟滞模型进行改进,合理地考虑压缩载荷对裂纹扩展的影响,特别是拉伸超载后紧随的压缩载荷对有效塑性区的影响,以及拉伸超载后紧随多个压缩载荷的处理方法.此外,改进门槛值随应力比R变化的计算公式,使计算结果与实际情况更接近,提高模型参数—门槛值的精度.最后,将改进后模型的预测寿命与试验结果进行比较.从对比结果可...     

应力比影响下的铝合金全范围疲劳裂纹扩展速率表达式

通过试验测得T851处理的2124铝合金板TL取向与LT取向不同应力比时的裂纹扩展门槛值及裂纹扩展速率;将应力比影响下的有效应力强度因子ΔKeff、有效裂纹扩展门槛值ΔKeffth及裂纹在循环应力作用下的临界应力强度因子K′C引入全范围裂纹扩展速率表达式中,并用修正后的公式对不同应力作用下2124铝合金TL取向与LT取向的疲劳裂纹扩展速率进行了预测。结果表明:预测结果与试验结果基本吻合,说明修正后全范围裂纹扩展速率表达式可以用来描述2124铝合金裂纹扩展速率。     

LY12CZ铝合金在人工海水中的腐蚀疲劳裂纹扩展行为

采用腐蚀疲劳试验方法,研究了LY12CZ铝合金在人工海水(3.5%Na Cl溶液)中腐蚀疲劳裂纹的扩展规律,分析了载荷频率、应力比以及人工海水p H对裂纹扩展速率的影响,并借助扫描电镜对其断口形貌进行了观察。结果表明:随载荷频率、人工海水p H的降低,腐蚀疲劳裂纹扩展速率会显著加快,应力比则主要影响腐蚀疲劳近门槛值区的裂纹扩展;在中性人工海水中阳极溶解对裂纹加速扩展起主导作用;随人工海水p H的降低,裂尖氢置换反应加剧,裂尖材料氢损伤对裂纹扩展的加速效应逐渐显现。     

超声波检测技术在测定蠕墨铸铁蠕化率中的应用

介绍了超声波声速测定蠕墨铸铁蠕化率的原理,通过用金相和超声波两种方法对蠕墨铸铁蠕化率进行对比测量,验证了蠕化率与声速的对应关系。并通过一年多的实践进一步证明了用声速法代替金相法测蠕墨铸铁蠕化率是可行的、可靠的。     

铸铁中石墨形态和石墨结构的研究

采用扫描电子显微镜,通过深腐蚀和热腐蚀方法,研究了铸铁中各种典型石墨（片状石墨、蠕虫状石墨和球状石墨）的立体形貌和内部结构特征。对于片状石墨来说,A、C、D型均属同一种石墨形态,它们都是由分枝而又互相联系的石墨片组成,它们主要是沿着[1010]方向长大。对于球状石墨来说,用纯镁、纯钇、纯铈和工业球化剂均能得到球状石墨。热腐蚀结果表明,石墨球呈年轮状结构,它是由许多角锥体组成的多面体。这说明,石墨球的外表是（0001）面,石墨球的长大是由中心沿[0001]方向进行。蠕虫状石墨是介于片状和球状石墨之间的中间石墨形态。它的长大方式也介于片状和球状石墨之间。就石墨片的互相联系和彼此分枝情况来看,它与片状石墨相似,但就每个石墨片局部的长大方式来看,它又与球状石墨有某种程度的相似,即沿着[0001]方向长大较充分。     

The influence of surface oxidation on the wear resistance of cast iron

This paper reports a study of the influence of surface oxidation on the wear resistance of ductile iron, grey iron and vermicular iron during dry sliding friction. The mechanisms of wear are also reported. This study shows that the effect of surface oxidation (formed under normal atmospheric conditions) on the wear rate depends on the complex functions of graphite morphology and matrix structure. Generally the presence of surface oxidation decreases the wear rate of grey iron but increases the wear rate of ductile iron and vermicular iron when the cast iron has high hardness. This trend is reversed for low hardness cast iron.     

蠕虫状石墨铸铁的结构

本文对蠕墨铸铁进行深腐蚀和热氧腐蚀,并用扫描电子显微镜进行观察研究,揭示了蠕虫状石墨的三维立体形貌和内部晶面取向特征。通过共晶团腐蚀,观察了这种铸铁的共晶团数量和结构特点。结果表明,用各种含有稀土元素的变质剂处理均可得到蠕虫状石墨,其端部具有球状石墨的生长特征,在同一共晶团内部,各蠕虫石墨片间互相联系,这又类似于片状石墨。热氧腐蚀试验表明,蠕虫状石墨端部（0001）面沿圆周方向分布,呈年轮状排列。在枝干分部,则（0001）面沿着枝干方向分布,但其方向变化频繁,呈多晶体。据此,作者提出了蠕虫状石墨晶面取向的模型。蠕墨铸铁的共晶团数量介于灰铸铁和球墨铸铁之间,共晶团的结构也兼有片状石墨和球状石墨两者共晶团的特征。为此,作者也提出了相应的模型。     

Discrepancies of fatigue crack growth behaviour of API X65 steel

This paper discusses the fatigue life behaviour of the API X65 steel tested under constant amplitude loading in ambient temperature. The influence of repeated loading at a particular stress ratio would favour the initiation of fatigue cracks that would consequently affect the fatigue crack growth. The main objective of this paper is to evaluate the load ratio effects on fatigue crack growth rate, taking into account their statistical characteristics. The fatigue crack growth test was carried out using compact-tension specimen at different load ratios of 0.1, 0.4 and 0.7. The experimental results showed the crack growth rate was dependent on the applied load. It was observed that the load ratio effect was less significant in stable crack growth regions. The result was consistent with the multiple regression test obtained by the least square method at a significance level of 0.05. The empirical model of Paris and Walker was utilised to evaluate the effects of load ratio on the fatigue crack growth rates. The approximation of fatigue life lies between 10-25 % of error using conservative model and 6 % error using the Walker model. The experimental data was scattered within a factor-of-2 correlation line suggesting that the accuracy of the experimental data towards the estimated values was high.     

The unlubricated wear of flake graphite cast iron

The wear behaviour of flake graphite cast iron was correlated with the microstructural parameters of graphite volume fraction and flake size using a pin-on-ring specimen configuration. Pin specimens of cast iron were prepared under carefully controlled melting and casting conditions to provide microstructures with variation in either carbon content or flake size but with the same type A graphite structure and pearlitic matrix.Mild and severe modes of equilibrium wear were identified, the predominant effect of both microstructural parameters being in the severe wear regime. Decrease in flake size and increase in carbon content are detrimental to the wear behaviour resulting in a marked increase in the severe wear rate and a decrease in the mild-to-severe transition load.     

铈、硫在工业铸铁中的分布及其交互作用

本文采用电解相分离—放射性测量及显微自射线照相法,研究了球化元素铈及反球化元素硫在工业铸铁各相中的分布。结果表明：铈在石墨中富集;铸铁中铈含量的增加,使铸铁中的含硫量及石墨中的硫含量均下降,后者下降速度更快;发现即使球使石墨,其中仍有一定的含硫量;铈与硫彼此均阻碍对方在石墨中吸附;石墨中铈可能在晶体缺陷中吸附,而硫则以间隙固溶形式在石墨中固溶;硫的固溶量很低,硫主要以MnS形式存在,微量硫的存在有利于蠕墨铸铁的获得。     

Influence of load and speed on wear characteristics of grey cast iron in dry sliding — selection for minimum wear

The wear of grey cast iron in dry sliding conditions has been studied with the aims of (1) finding the influence of working conditions on the wear rate, and (2) determining the region of speed and load where low wear is accomplished. Grey cast iron with flake and nodular graphite was submitted to investigation using a pin-on-disc machine. The results indicate that the flake graphite cast iron is more suitable for applications at speeds greater than 4 m s⁻¹ and lower loads, while nodular cast iron has greater wear resistance at lower speeds in the range of loads investigated - from 50 N cm⁻² to 200 N cm⁻²     

The crack growth behavior of Incoloy 800H under fatigue and dwell-fatigue conditions at elevated temperature

Fatigue crack growth rate (FCGR) tests with different load ratios and dwell-fatigue crack growth rate (DFCGR) tests with different dwell times were conducted at 750°C for Incoloy 800H. As the load ratio increases from 0.1 to 0.5, the crack growth rate increased and the transition ??K value from region I to region II (Paris regime) shifted leftward. In DFCGR tests with dwell time of 10 and 30 seconds, the Paris regime started at relatively lower ??K level and the crack grew much faster than in FCGR tests. However, the crack growth rates between the 10 sec and 30 sec dwell times were relatively similar. The higher crack growth rates in the DFCGR tests compared to FCGR tests was associated with the reduction of the M₂₃C₆ precipitates in the vicinity of the advancing crack by the Cr depletion, suggesting the crack propagation in DFCGR conditions was environmentally assisted. The crack growth rate was controlled by trans-granular mode regardless of the dwell time because the dwell time was not enough to cause creep damage.