挤压AZ31B镁合金圆棒不同取向的疲劳裂纹扩展

使用紧凑拉伸(CT)试样,研究了挤压AZ31B镁合金圆棒三个方向的组织及疲劳裂纹扩展性能。结果表明,疲劳裂纹以穿晶为主的混合方式沿滑移带扩展。T-L方向裂纹呈直线扩展,扩展速率最高,T-R方向孪生和滑移协同塑性变形,裂纹局部偏转,呈波浪形沿径向扩展,扩展速率较低,强织构和不均匀组织引起L-T方向裂纹分叉和偏离,降低了裂纹尖端有效驱动应力强度因子幅,裂纹扩展速率最低,在疲劳裂纹扩展速率(da/dN)与应力强度因子幅(ΔK)的关系曲线上出现水平段。在ΔK较小时加载频率对裂纹扩展速率的影响不大,ΔK>3 MPa m时裂纹扩展速率随着加载频率的提高而减小。     

预制疲劳裂纹应力强度因子幅对超高强度钢断裂韧度的影响

采用多试样法对D406A超高强度钢进行了准静态断裂韧度KⅠC试验,分析了不同应力强度因子幅预制疲劳裂纹对疲劳预裂纹扩展周期、疲劳预裂纹扩展速率、试样断口形貌以及最终断裂韧度试验结果的影响。结果表明:疲劳预裂纹扩展周期和扩展速率均与应力强度因子幅呈指数变化规律,断口上的疲劳裂纹间距及最终断裂韧度试验结果均随应力强度因子幅的增大而增大,在材料断裂韧度KⅠC的20%~30%选择最大应力强度因子进行KⅠC试验结果较为稳定。     

800MPa级钢悬臂弯曲加载低周腐蚀疲劳表面裂纹扩展特性研究

在悬臂弯曲加载方式下,研究了800MPa级10CrNiMo结构钢在3.5%NaCl水溶液中的低周腐蚀疲劳表面裂纹扩展特性,采用金相显微镜、扫描电子显微镜及X射线能谱仪观察并分析了腐蚀疲劳试样断口,与空气中相同加载方式下该材料的低周疲劳表面裂纹扩展速率进行了比较,结果表明,在表面裂纹扩展初期,在3.5%NaCl水溶液中的表面裂纹扩展速率较空气中的快,但随着总应变范围的增加,这种快的趋势在逐渐减小,当总应变范围达到约1%时,在两种环境中的表面裂纹扩展速率基本相当;在表面裂纹扩展的后期,裂纹内部聚集了大量的腐蚀产物,该腐蚀产物主要为铁的氧化物,氧化物的存在增加了裂纹扩展的闭合效应,减缓了表面裂纹的扩展。     

冷却剂加锌对316LN应力腐蚀性能影响

目的 在模拟压水堆一回路水化学环境中开展加锌对316LN应力腐蚀开裂行为研究，获得加锌对316LN腐蚀影响特性。方法 通过直流电位降方法获得316LN在不加锌和加锌水化学中的裂纹扩展速率，对比分析不同应力强度因子下加锌对316LN裂纹扩展速率影响，利用扫描电镜、透射电镜分别对316LN应力腐蚀断口形貌以及裂纹尖端元素分布进行观察和分析。结果 在不加锌水溶液中，应力强度因子K=38.5MPa·m1/2，裂纹扩展速率为2.80×10-8 mm/s，应力强度因子K=45.5MPa·m1/2，裂纹扩展速率为3.51×10-8 mm/s；在加锌水溶液中，应力强度因子K=38.5MPa·m1/2，裂纹扩展速率为1.29×10-8 mm/s，应力强度因子K=45.5 MPa·m1/2，裂纹扩展速率为1.74×10-8 mm/s；在加锌或不加锌水溶液中，应力强度因子对316LN裂纹扩展速率有促进作用，但在相同应力强度因子下，随锌的加入，316LN裂纹扩展速率降低46.0%～49.5%，使316LN生成保护性更好、韧性更高的氧化膜。结论 冷却剂加锌有利于提高316LN抗应力腐蚀开裂的能力。     

HY-130钢在3.5%NaCl溶液中的短疲劳裂纹扩展

使用带有长度为0.4～2.5mm单侧裂纹的短裂纹试样在空气和3.5％NaCl溶液中进行了HY-130钢的疲劳裂纹扩展试验。发展了用直流电位差法连续检测短裂纹扩展。通过对长裂纹(长度大于10mm)的裂纹扩展速率与应力强度因子幅值(△K)之关系的比较,研究了在疲劳裂纹扩展中裂纹尺寸的化学诱发效应。一个长度为0.4mm的裂纹被认为是微观组织及力学意义上的长度,它在空气中呈现出与长裂纹相同的da/dN-△K关系,在3.5％NaCl溶液中,这种长度的裂纹比长裂纹扩展约快一倍。裂纹长度达1.1mm时,扩展速率与长裂纹的da/dN-△K曲线吻合,且根本不受应力幅值及应力比的影响。根据介质种类的输送方式,讨论了电化学裂纹模型。     

沉积颗粒对7N01-T6铝合金疲劳裂纹扩展行为的影响

利用自主设计的实验方法,结合疲劳裂纹扩展速率测试以及断口微观形貌观察研究了R=0.1和R=0.5两种应力比下,石墨和氧化铝沉积颗粒对7N01-T6铝合金疲劳裂纹扩展行为的影响。结果表明:在两种应力比条件下,裂纹扩展Ⅰ、Ⅱ阶段中,合金在石墨颗粒环境下的疲劳裂纹扩展速率均最快。当R=0.5时,在裂纹扩展Ⅰ、Ⅱ阶段,合金在氧化铝颗粒环境下的疲劳裂纹扩展速率最慢。当R=0.1时,在应力强度因子ΔK15 MPa·m~(1/2)阶段,合金在氧化铝颗粒环境下的疲劳裂纹扩展速率最慢,在ΔK=15～30 MPa·m~(1/2)阶段,合金在氧化铝颗粒环境和大气环境下的疲劳裂纹扩展速率相当。石墨颗粒环境下合金疲劳裂纹扩展速率的增加是由于石墨颗粒的润滑作用降低了疲劳卸载过程中的裂纹闭合效应。氧化铝颗粒环境下合金疲劳裂纹扩展速率的降低是由于氧化铝颗粒在断面的沉积增强了疲劳卸载过程中的裂纹闭合效应。     

16MnR钢在液氨环境中的应力腐蚀裂纹扩展研究

采用T形楔形张开加载(T-WOL)恒位移试样,进行16MnR钢在无水液氨环境的应力腐蚀实验.实验中采用了8只预裂纹试样,分别加载到不同应力强度因子K1值,放入盛有液氨的实验容器中.试样定期取出测量裂纹的长度,经总共3450h应力腐蚀实验,测得各试样的裂纹扩展速率在0.08×10-8～3.54×10-8 mm/s之间.结果表明:16MnR材料在无水液氨环境中裂纹基本没有扩展,其应力强度因子临界值KISCC大于90 MPa√m.同时分析了应力腐蚀开裂机理和影响其应力腐蚀行为的介质与材料因素.     

EFFECT OF ENVIRONMENT ON THE BEHAVIOR OF FATIGUE CRACK GROWTH IN AN AS-ROLLED DUAL-PHASE STEEL

本文研究了一种热轧双相钢在空气和3.5%NaCl 溶液中的疲劳裂纹扩展行为,这种双相钢在空气中具有较好的疲劳裂纹扩展阻力。在3.5%NaCl 溶液中的门槛值比空气中高,这是裂纹内腐蚀产物的楔子作用所致。随着应力比的增加,门槛值降低,并符合应力比对门槛值影响的竞争模型。在较高△K 下,扩展速率符合Paris 公式。在3.5%NaCl 溶液中的腐蚀为活性腐蚀。     

环境对热轧双相钢疲劳裂纹扩展行为的影响

本文研究了一种热轧双相钢在空气和3.5%NaCl 溶液中的疲劳裂纹扩展行为,这种双相钢在空气中具有较好的疲劳裂纹扩展阻力。在3.5%NaCl 溶液中的门槛值比空气中高,这是裂纹内腐蚀产物的楔子作用所致。随着应力比的增加,门槛值降低,并符合应力比对门槛值影响的竞争模型。在较高△K 下,扩展速率符合 Paris 公式。在3.5%NaCl 溶液中的腐蚀为活性腐蚀。     

30CrMnSiA钢在盐雾腐蚀条件下疲劳裂纹扩展的初步研究

本文用断裂力学的方法对30CrMnSiA结构钢在盐雾介质和室温空气下的疲劳裂纹扩展特性进行了初步研究。 用紧凑拉伸试样,在应力强度因子△K=70～200kg/mm~3/~2范围内,其试验结果表明:试验初期,盐雾介质中的裂纹扩展速率比室温空气中的要高,并随着试验频率的降低而急剧增加,这种情况通常是氢脆机制造成的。当盐雾介质中的试验进行到20～40小时以后,由于裂纹表面腐蚀产物的堆积,裂纹扩展速率逐渐减缓,甚至低于室温空气下的da/dN曲线。在试验的最后阶段,由于应力强度因子幅值的增大,上述两种介质中的裂纹扩展速率趋于接近。     

3.5%NaCl腐蚀环境下2种航空铝合金材料疲劳性能试验研究

腐蚀环境下的疲劳性能是航空金属结构疲劳寿命设计的重要前提,为此,试验测定了2种航空铝合金材料（2E12-T3、7050-T7451）的光滑试样和缺口试样在干燥大气和3.5%NaCl腐蚀环境下的疲劳性能,在试验数据的基础上进行性能对比,并对试样断口进行扫描电镜（SEM）分析,研究了3.5%NaCl腐蚀环境与载荷联合作用对腐蚀疲劳性能的影响机理,研究结果表明：3.5%NaCl腐蚀环境对2种铝合金材料的疲劳性能均产生不利影响,且腐蚀与疲劳载荷的交互作用随着应力水平的降低而增强,疲劳性能下降更明显;与光滑试样相比,腐蚀环境对铝合金2E12-T3缺口试样疲劳性能的影响更大,但对铝合金7050-T7451缺口试样疲劳性能的影响却变小;在腐蚀环境下,裂纹尖端易发生电化学反应产生腐蚀产物和[H]离子,腐蚀产物的存在会阻碍裂纹闭合,同时,[H]离子导致裂纹尖端的氢脆效应,加快裂纹扩展,使疲劳性能降低。     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

结构钢S355N及S355N-Z25的低温CTOD断裂韧性及其da/dN研究

分别采用三点弯曲SE(B)与紧凑拉伸C(T)试样,通过-20℃的低温裂纹尖端张开位移CTOD断裂韧性试验及疲劳裂纹扩展速率da/dN试验,研究了S355N及S355N-Z25结构钢的裂纹启裂与扩展的抗力.研究结果表明:S355N钢抵抗裂纹启裂及前期扩展的能力比S355N-Z25钢强,但前者抵抗裂纹后期扩展的能力比后者弱...     

The fatigue crack growth of a ship steel in seawater under spectrum loading

Fatigue crack growth of ABS EH36 steel under spectrum loading intended to simulate sea loading of offshore structures in the North Sea was studied using fracture mechanics. A digital simulation technique was used to generate samples of load/time histories from a power spectrum characteristic of the North Sea environment. In constant load-amplitude tests, the effects of specimen orientation and stress ratio on fatigue crack growth rates were found to be negligible in the range 2 × 10^?5 to 10^?3 mm/cycle. Fatigue crack growth rates in a 3.5% NaCl solution were two to five times faster than those observed in air in the stress intensity range 25 to 60 MPa √m. The average fatigue crack growth rates under spectrum loading and constant-amplitude loading were in excellent agreement when the fatigue crack growth rate was plotted as a function of the appropriately defined equivalent stress intensity range. This procedure is equivalent to applying Miner's summation rule in fatigue life calculations.     

A model to predict S–N curves for surface and subsurface crack initiations in different environmental media

The influence of environmental media on crack propagation of a structural steel at high-cycle and very-high-cycle fatigue (VHCF) regimes was investigated with the fatigue tests in air, water and 3.5% NaCl aqueous solution. The fatigue strength in water and 3.5% NaCl solution is significantly decreased and the cracking morphology due to different driving forces is presented. A model is proposed to explain the influence of environmental media on fatigue life, which reflects the variation of fatigue life with applied stress, grain size, inclusion size and material yield stress. The model prediction is in good agreement with experimental observations.     

CORROSION FATIGUE FRACTURE BEHAVIOUR OF A SiC WHISKERALUMINIUM MATRIX COMPOSITE UNDER COMBINED TENSIONTORSION LOADING

We describe an investigation into the fatigue fracture behaviour under combined tension–torsion loading of a SiC whisker-reinforced A6061 aluminium alloy fabricated by a squeeze casting process. Special attention was paid to the environmental effects on fatigue fracture behaviour. Tests were conducted on both the composite and its unreinforced matrix material, A6061-T6, under load-controlled conditions with a constant value of the combined stress ratio, α = τ_max /σ_max in laboratory air or in a 3.5% NaCl solution at the free corrosion potential. The corrosion fatigue strength of both the matrix and composite was less in the solution than in air. The dominating mechanical factor that determined the fatigue strength in air was either the maximum principal stress or the von Mises-type equivalent stress, depending on the combined stress ratio. However, in the 3.5% NaCl solution, the corrosion fatigue strength of both materials was determined by the maximum principal stress, irrespective of the combined stress ratio. In the case of the matrix material, crack initiation occurred by a brittle facet normal to the principal stress due to hydrogen embrittlement. However, in the composite material, the crack was initiated not at the brittle facet, but at a corrosion pit formed on the specimen surface. At the bottom of the pit, a crack normal to the principal stress was nucleated and propagated, resulting in final failure. Pitting corrosion was nucleated at an early stage of fatigue life, i.e. about 1% of total fatigue life. However, crack initiation at the bottom of a pit was close to the terminal stage, i.e. about 70% or more of total fatigue life. The dominating factor which determined crack initiation at a pit was the Mode I stress intensity factor obtained by assuming the pit to be a sharp crack. Initiation and propagation due to pitting corrosion and crack growth were closely examined, and the fatigue fracture mechanisms and influence of the 3.5% NaCl solution on fatigue strength of the composite and matrix under combined tension–torsion loading were examined in detail.     

Acoustic Emission Monitoring the Fatigue Crack Growth in SM50B-Zc Steel

The acoustic emission(AE)characteristicsfrom the fatigue crack propagation in SM50B-Zcsteel were strongly affected by the environment me-dium.The AE feature from the fatigue process bothin air and in 3.5% NaCl aqueous solution mani-fested a periodicity which indicated the fatigue ex-tension was discontinuous.The fatigue striationand secondary cracking were the main AE sourcesduring the fatigue crack growth in air.In contrast,AE during the fatigue crack propagation in 3.5%NaCl solution was released from the intergranularfracture and quasicleavage cracking.     

金属材料疲劳裂纹扩展速率Paris模型中材料常数的相关性

金属材料疲劳裂纹扩展速率是工程应用中进行损伤容限设计、疲劳寿命评估的一项重要力学性能指标。Paris模型是疲劳裂纹扩展速率最常用的表达形式,该模型认为疲劳裂纹扩展速率随裂纹前缘应力强度因子范围的变化呈幂函数关系,涉及到两个材料常数C和m。本研究借助部分文献中的试验数据,分析合金钢、铜合金、钛合金、铝合金等不同金属材料疲劳裂纹扩展速率Paris模型中材料常数C和m的关系。结果表明:Paris模型中材料常数C和m的关系可用m=alnC+b线性模型来描述,且不受试样取样方向、焊缝位置、试验环境等因素影响;但不同应力比对该模型的线性度有一定影响,尤其是当应力比为负值时,影响较为显著;不同金属材料常数C和m线性模型的斜率并不相等,其影响因素有待于进一步系统研究。分析结果可为金属材料在工程中的疲劳设计与应用提供参考。     

CRO试样的疲劳裂纹扩展行为试验方法研究

考虑到如紧凑拉伸和三点弯曲等标准试样的大尺寸要求以及高试验成本等突出问题,该文发展了含外侧径向裂纹C形环小试样（C-ring with an outer radial crack,CRO）的疲劳裂纹扩展行为试验方法。利用有限元分析建立了CRO小试样的高精度应力强度因子算式以及基于柔度法理论的裂纹长度预测公式。采用5083-H112铝合金分别完成了CRO试样和标准CT试样的疲劳裂纹扩展速率试验,获得了相应的Paris方程中的幂指数。通过对比发现,CRO和CT试样的疲劳裂纹扩展规律基本一致,验证了新方法的有效性。基于上述方法对C250钢两种厚度CRO试样的疲劳裂纹扩展行为进行了应用研究。