Effects of loading modes on the fatigue behavior of Zr-based bulk-metallic glasses

The fatigue behavior of a Zr-based bulk-metallic glass (BMG), Zr₅₀Cu₃₇Al₁₀Pd₃ (in atomic percent), was systematically investigated under uniaxial tension–tension, uniaxial compression–compression, and bending loading. The fatigue results show that the fatigue limits of Zr₅₀Cu₃₇Al₁₀Pd₃ under tension–tension and compression–compression loading are comparable. However, the fatigue lifetimes above the fatigue limit under compression–compression loading are longer than that under tension–tension loading. Similarly, Zr₅₀Cu₃₇Al₁₀Pd₃ demonstrates the comparable fatigue limits and lifetimes under three- and four-point bending loading. In addition, the fatigue-endurance limits of Zr₅₀Cu₃₇Al₁₀Pd₃ under tension–tension and compression–compression loading were found to be greater than those under three- and four-point bend loading. The influence of the loading mode on the fatigue behavior of Zr-based BMGs is clarified.     

S-N fatigue behavior of Fe25Mn steel and its weld at 298 and 110 K

The S-N fatigue behavior of newly developed Fe25Mn steel, including base metal and butt-welded joint, was investigated at 298 and 110 K, and the results were compared to those of previously reported Fe16Mn2Al and STS304L steels. Fe25Mn steel has quite promising fatigue performance at 298 K and even at 110 K, showing comparable resistance to fatigue to STS304L. The S-N fatigue behavior of Fe25Mn steel was dependent on tensile strength at 298 and 110 K, the trend of which well agreed to that of other austenitic steels. The electron backscatter diffraction and micrographic analyses suggested that transformation induced plasticity and twinning induced plasticity effects did not occur in Fe25Mn steel under fatigue loading at room and cryogenic temperatures. The butt-welded Fe25Mn/Fe25Mn and Fe25Mn/STS304L specimens also showed a satisfactory fatigue behavior which was even comparable to that of STS304L/STS304L specimen at 110 K. The S-N fatigue behavior of Fe25Mn steel and its welds was discussed based on the fractographic and microscopic observations.     

基于缺口应力法的IIW多轴疲劳准则分析

焊接接头的多轴疲劳强度评估是领域内的复杂课题.结合缺口应力法,利用ⅡW多轴疲劳准则对已公开发表的文献中的多轴疲劳试验数据进行了重新评估.结果表明,缺口应力系统下的比例加载与非比例加载数据点重合性较好;ⅡW推荐的单轴疲劳评估S-N曲线不适用于多轴疲劳评估,循环次数在1×104 ~1×105范围内时,可能得到偏危险的评估结果;而高于1×105次时,评估结果过于保守.基于此,拟合得到了存活率P_s=97.7%的S-N评估曲线,其疲劳等级FAT=430 MPa,斜率m=5.8,同时适用于比例加载与非比例加载,可为工程焊接结构疲劳寿命预估提供参考.     

Effect of stress ratio on long life fatigue behavior of Ti-Al alloy under flexural loading

A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 10^10 cycles and associated fr＇dcture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 kHz with stress ratio R=0.5 and R=0.7 at ambient temperature in air. Three groups of specimens with different surface roughness were applied to investigate the effect of surface roughness on fatigue life. Furthermore, optical microscopy（OM） and scanning electron microscopy（SEM） were used for microstructure characteristic and fracture surface analysis. The S-N curves obtained show that fatigue failure occurs in the range of 10^5-10^10 cycles, and the asymptote of S-N curve inclines slightly in very high cycle regime, but is not horizontal for R=0.5. Fatigue limit appears after 10s cycles for R=0.7. Surface roughness （the maximum roughness is no more than 3 μm） has no influence on the fatigue properties in the high cycle regime. A detailed investigation on fatigue fracture surface shows that the Ti-Al alloy studied here is a binary alloy in the microstructure composed of α2-Ti3Al and γ-Ti-Al with fully lamellar microstructure. Fractography shows that fatigue failures are mostly initiated on the surface of specimens, also, in very high cycle regime, subsurface fatigue crack initiation can be found. Interlamellar fatigue crack initiation is predominant in the Ti-Al alloy with fully lamellar structure. Fatigue crack growth is mainly in transgranular mode.     

Effect of various heat and surface treatment conditions on the fatigue behavior of SKD11 steel

Effects of various heat treatment processes, including Q/L-T, Q/H-T, Q/L-T + Ti-nitriding, Q/H-T + Ti-nitriding, and Q/CT/H-T on the S-N fatigue behavior of SKD11 steel were investigated. Ti-nitriding is a modified nitriding process that entails a small addition of approximately 2 g to 20 g of electrolyzed fine, metallic titanium in a salt bath. In the present work, depending on the applied stress and surface conditions, different crack initiators were operative, resulting in three types of crack initiation. Multi-site crack initiation around the specimen surface, observed only for the Ti-nitrided specimens at a high stress range, decreased the resistance to S-N fatigue by reducing the crack initiation cycles, Ni. At low and intermediate stress ranges, the cracked carbide particles located near the surface or internally served as crack initiators for all the specimens, regardless of whether Ti-nitriding was applied, resulting in similar resistances to S-N fatigue. Different heat treatment parameters, such as tempering temperature and cryogenic treatment, did not affect the fatigue behavior of SKD11 steel to any considerable degree. The fatigue behavior of SKD11 specimens with different heat and surface treatment conditions is discussed based on micrographic and fractographic observations.     

Fatigue behavior of a Zr-based bulk metallic glass under uniaxial tension–tension and three-point bending loading mode

The fatigue behavior of as-cast Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass was systematically investigated under uniaxial tension–tension and three-point bending loading modes. To obtain the fatigue stress-life (S–N) diagram, stress-controlled experiments were conducted using a computer-controlled material test system electrohydraulic testing machine at 25 Hz with a 0.1 R ratio in air at room temperature. The fatigue endurance limit (630 MPa) in stress range terms under cyclic tensile load was almost three times higher than that under the three-point bending condition (225 MPa). Both fatigue resistances were similar at higher stress level. The fatigue fracture morphologies associated with the S–N curve indicated that the defects have little to no influence on the crack initiation and the fatigue life in low cycle fatigue range. However, at lower stress level, the most detrimental factor was the number of defects that resulted in fatigue endurance limit discrepancy.     

钢质蜂窝夹芯板的弯曲疲劳损伤模型

研究了钢质蜂窝夹芯板的室温四点弯曲疲劳行为,得到疲劳寿命(S-N)曲线.结果表明,在载荷比R=0.20时,钢蜂窝夹芯板疲劳行为体现明显的方向性.L向芯子排列试件较W向易于承受循环载荷,疲劳强度极限分别为1369和859 N.基于等效剪切模量退化理论,建立了寿命预测和损伤演化模型.结果发现,L向试件损伤开始萌生的循环次数约占总寿命的86%—90%,W向试件高载荷时为73%,较低载荷时退化为48%.在不区分芯子排列方向时,可分别用二阶多项式和指数模型描述高、低载荷水平的损伤演化规律,模型体现出较强的材料相关性.     

Ti-6Al-4V合金的超高周疲劳行为

采用超声疲劳实验分别确定了双态和网篮两种组织的Ti-6Al-4V合金的疲劳寿命(S-N)曲线,并用SEM观察疲劳断口.结果表明,两种组织合金的S-N曲线均保持下降趋势,在105-109cyc间不出现水平段,不存在传统意义的疲劳极限,断口形貌分析表明,随着应力幅的降低,二者的裂纹萌生位置都发生了由试样表面到内部的转变.与加载频率为25 Hz时的疲劳实验结果进行比较后发现,超声疲劳加载条件下,疲劳强度提高,疲劳寿命延长,且频率对网篮组织合金疲劳性能的影响大于对双态组织的影响.     

焊接结构模态结构应力法程序开发及工程应用

主S-N曲线法作为疲劳计算的新方法在焊接结构疲劳分析中被广泛采用.为了实现该方法在试验载荷下基于稳态动力学计算结果开展焊接结构疲劳寿命预测,首先引入台架模型作为边界条件,实现将试验载荷作为仿真分析的输入,基于模态叠加法的稳态动力学理论获得较准确的焊缝动态响应.其次在主S-N曲线法的准静态计算流程基础上,扩展其内涵,提出基于模态结构应力叠加的动态结构应力计算方法,该方法将稳态动力学计算的模态坐标与焊缝的模态结构应力进行叠加,实现动态结构应力计算及动态等效结构应力计算,再采用主S-N曲线进行寿命评估预测.进一步开发了焊接结构模态结构应力法疲劳评估软件,基于该软件开展了车体疲劳评估和疲劳试验对比.结果表明,该方法比传统方法更能有效地识别出动态加载下车体的疲劳破坏部位,验证了该方法在试验动态载荷加载下开展焊接结构疲劳评估的有效性和优越性,为研究焊接结构疲劳寿命评估理论和拓展主S-N曲线法提供了技术基础.     

Study the Fatigue-Wear Behavior of a Plasma Electrolytic Nitrocarburized (PEN/C) 316L Stainless Steel

In the present study, a suitable machine was developed in the laboratory to investigate the fatigue-wear behavior of the untreated 316L austenitic stainless steel and samples treated by plasma electrolytic nitrocarburizing process under different combinations of cyclic loading and contact pressure. The fracture cycles as a function of bending stress were recorded while a constant contact pressure was applied simultaneously. As a result, the PEN/C treated specimens exhibited a higher resistance (about 40% for 15.6 N contact load and about 60% for 25 N contact load) under the application of simultaneous cyclic stress and contact pressure. Also it was shown that under a range of combined fatigue and wear stresses, the specimens exhibit a better life than the conditions of performing wear or fatigue tests separately and this effect was much more observable for PEN/C-treated samples. The gravimetrical weight loss values in the fatigue-wear test were also measured at intervals 5000 to 300,000 cycles (with the contact stress = 6.25 MPa and the bending stress = 87 MPa). The results showed a better wear resistance for the treated surface at the first stage of the process.     

TC4钛合金三点弯曲超高周疲劳性能研究

针对航空发动机压气机叶片复杂载荷环境下的超高周疲劳问题,开展了TC4钛合金三点弯曲超高周疲劳试验,研究其在弯曲加载下的超高周疲劳破坏行为。疲劳试验结果表明：在两种应力比（R=0.3、0.5）下,当循环次数超过107次时,试件仍发生疲劳断裂,S-N曲线均呈现双线性特征;SEM断口分析表明,随着最大应力的降低,裂纹萌生位置由试件表面向次表面转移,疲劳裂纹萌生是表面滑移和内部解理断裂之间相互竞争的结果;基于疲劳寿命建立模型分析了应力比对2种裂纹萌生机制之间竞争行为的影响。采用红外热像仪监测试件表面的温度,高周疲劳试件的温度变化分为四个阶段：稳定升高、稳定降低、快速升高和快速降低阶段,而超高周疲劳试件的温度变化分为三个阶段：稳定升高、快速升高和降低阶段。最后,阐述了疲劳过程中热产生和传热的特点,并分析了温度变化与应力分布的关系。     

铝合金非承载十字接头疲劳特性

基于结构应力法,计算拉伸疲劳加载时不同几何尺寸的7N01铝合金非承载十字接头的结构应力,并结合疲劳试验数据建立其疲劳失效的主S-N曲线.结果表明,与名义应力法相比,基于结构应力法的主S-N曲线线性相关性更好,能够适应不同几何尺寸十字接头疲劳的计算.进一步分析相同名义应力下,承载板厚度对疲劳寿命的影响,结果表明,接头疲劳承载能力的增加与板厚的增加并非线性关系,随着承载板厚度增加,接头疲劳寿命降低.     

铜合金的疲劳寿命预测

采用平面弯曲疲劳试验,测定了C1100P-1／4H纯铜和C2801P-1／4H黄铜的S-N曲线。结果表明,实验铜合金没有疲劳极限。随外加交变应力增大,疲劳寿命缩短。在相同外加应力条件下,轧制态合金的疲劳寿命高于退火态合金的疲劳寿命。利用Manson—Coffin法则和塑性应变幅与疲劳循环次数的理论关系,通过计算得到了铜合金的理论S-N关系曲线,与实验结果吻合较好,表明该理论S-N关系曲线可用来预测铜合金的疲劳寿命。     

固溶氮原子对不锈钢单轴及多轴低周疲劳特性的影响

对３１６Ｌ和３１６ＬＮ不锈钢进行了单轴拉压及多轴非比例加载低周疲劳实验研究,分析了固溶的氮原子对单轴及多轴非比例加载低周疲劳特性及其微结构的影响。结果表明,固溶的氮原子增大了不锈钢铁的单轴循环软化程度及非比例循环附加强化程度,延长了不锈钢的单轴拉压低周疲劳寿命,但却缩短了不锈钢的多轴非比例加载低周疲劳寿命,固溶的氮原子对单轴及多轴低周疲劳密度位错结构的形成具有明显的抑制作用,利用Ｍｏｅｓｓｂａｕｅ     

2024锻造铝合金疲劳性能研究

本文着重研究了2024锻造铝合金经过T4及T6热处理后其单轴及多轴疲劳性能。采用了两种不同热处理方式制备出具有不同微结构的2024锻造铝合金,研究其在不同加载状态下的变形行为、疲劳机制及疲劳寿命。主要内容包括：对2024锻造铝合金进行单向拉伸、应力控制的单轴拉压疲劳及两种多轴拉扭疲劳试验,研究了加载状态与疲劳寿命的关系。结果表明,2024锻造铝合金材料的疲劳寿命对热处理和加载状态的敏感性很大,表现为在相同加载状态下2024-T4和2024-T6试样的寿命差别较大,以及同一热处理试样在不同应力幅值和不同加载路径下疲劳寿命差异大。     

回火温度对60Si2MnA弹簧钢高周疲劳性能的影响

利用旋转弯曲疲劳试验方法,研究了60Si2MnA弹簧钢在400℃和440℃回火后的高周疲劳破坏行为。结果表明,回火温度主要影响60Si2MnA弹簧钢的微观组织及强度从而影响其高周疲劳性能。当回火温度较高而强度水平较低时,疲劳破坏主要起源于试样表面,存在传统的疲劳极限;当回火温度较低而强度水平较高时,疲劳破坏主要起源于内部粗大的夹杂物,S-N曲线连续降低,传统的疲劳极限消失。尽管400℃回火试样的疲劳极限较高,但其疲劳强度比明显低于440℃回火试样。进一步细化钢中的夹杂物,则有望改善较低温度回火样的疲劳性能。     

单、多轴混合加载下GH4169合金的高温疲劳特性

利用拉扭薄壁管疲劳试样,在应变控制拉扭循环加载下对高温合金材料GH4169的多轴循环特性进行了实验研究,高温疲劳实验过程中,采用单、多轴混合加载路径对薄壁管疲劳试件进行加载,通过连续记录拉与扭的应力响应值研究了变幅高温多轴疲劳特性,结果表明,在高温低周单、多轴混合疲劳块载荷加载下,试件的疲劳寿命与应力响应特性不但取决于应变加载路径,而且与加载路径的排列顺序和加载参数的大小有关。     

ST12汽车钢板拉剪点焊接头的疲劳强度

针对车身使用的ST12低碳钢板,利用生产线上焊接设备制备了电阻点焊接头拉剪疲劳试件.对点焊接头区域进行了金相组织观测,测量了焊点附近的维氏硬度值分布.在MTS材料疲劳试验机上进行了恒幅疲劳加载,得到了ST12低碳钢板的电阻点焊接头的S-N曲线.基于所得的S-N曲线进行了两级疲劳加载试验,对两级加载下的线性累积疲劳损伤进行了分析.累积损伤结果分析表明,ST12低碳钢板电阻点焊件在两级加载下存在加载次序效应,高-低加载次序下加载次序效应更为明显.     

A CUMULATIVE FATIGUE DAMAGE RULE UNDER THE ALTERNATIVE OF CORROSION OR CYCLIC LOADING

Fatigue damage increases with the applied loading cycles in a cumulative manner and the material deteriorates with the corrosion time. A cumulative fatigue damage rule under the alternative of corrosion or cyclic loading was proposed. The specimens of aluminum alloy LY12-CZ soaked in corrosive liquid for different times were tested under the constant amplitude cyclic loading to obtain S-N curves. The test was carried out to verify the proposed cumulative fatigue damage rule under the different combinations among corrosion time, loading level, and the cycle numbers. It was shown that the predicted residual fatigue lives showed a good agreement with the experimental results and the proposed rule was simple and can be easily adopted.