Very High Cycle Fatigue Fracture Behavior of High Strength Spring Steel

进行超声波疲劳和疲劳裂纹扩展速率实验, 研究了3种60Si2CrVA弹簧钢的超高周疲劳破坏行为.结果表明, 60Si2CrVA弹簧钢的超高周疲劳性能主要与其中夹杂物的尺寸有关,即随着夹杂物尺寸的减小, 钢的疲劳寿命和疲劳强度均逐渐提高.对于内部夹杂物引起的疲劳破坏, 在低应力幅、高循环周次(约大于106 cyc)条件下,在夹杂物周围的鱼眼处往往存在粗糙的粒状区域(GBF). 对于A-60钢,随着疲劳源夹杂物处应力场强度因子幅的减小, 疲劳寿命增加;而GBF处的应力场强度因子幅并不随疲劳寿命变化而变化,基本为一常数(平均值为4.6 MPa×m1/2),与疲劳裂纹扩展门槛值(4.3 MPa×m1/2)接近.     

热处理工艺对50CrVA高强弹簧钢超高周疲劳损伤机制的影响

研究了50CrVA高强弹簧钢在不同热处理状态下(淬火+中温回火和退火)的超高周疲劳破坏行为及其裂纹萌生机理。结果表明,50CrVA高强弹簧钢在107~109循环周次内发生疲劳破坏,两种热处理状态的S-N曲线下降形态不同,均未出现疲劳极限。热处理工艺改变50CrVA的微观组织,从而影响超高周疲劳阶段(寿命107周次)的疲劳破坏损伤机制:经淬火+中温回火处理材料破坏多起源于内部夹杂物,夹杂物周围存在的应力场与溶质原子发生弹性交互作用,吸引间隙原子向其周边扩散、富集,使间隙原子富集区材料性能下降,导致裂纹在内部夹杂物处萌生;退火热处理后材料微观组织对间隙原子向材料内部夹杂物扩散起到阻碍作用,所以超高周疲劳裂纹易于在材料表面萌生。     

SUJ2轴承钢超长寿命疲劳行为研究

采用超声疲劳试验机研究SUJ2轴承钢的超长寿命疲劳。结果表明:对于复合氧化物和TiCN裂纹源,裂纹从夹杂物与基体界面处萌生;铁、铬合金碳化物裂纹源则为夹杂物本身开裂。颗粒状亮面(GBF)相对尺寸正比于裂纹源处夹杂物边缘的应力强度因子范围1/ΔK2inc,对于本实验的SUJ2材料,当ΔKinc8MPa·m1/2时GBF不再形成。通过数据拟合得到了GBF内裂纹扩展规律:area~(1/2)_(GBF)/area~(1/2)_(inc)=(m_1+m_2N_f)~(m_0),证实了Paris公式可以用来描述GBF内的裂纹扩展。     

高压气相热充氢对SUJ2轴承钢超高周疲劳行为的影响

研究了JIS-SUJ2轴承钢的超高周疲劳行为及高压气相热充氢对疲劳性能的影响。结果表明,高压气相热充氢后疲劳性能明显降低,裂纹源周围"GBF"区的颗粒状特征变浅甚至消失。断口上裂纹源处缺陷尺寸及分布对疲劳寿命没有影响,疲劳寿命随着"GBF"与夹杂物尺寸比的增加而增加。充氢前后裂纹源边缘的应力强度因子范围均近似正比于裂纹尺寸的1/3次方,"GBF"裂纹扩展的门槛值正比于"GBF"尺寸的1/6次方。高压气相热充氢明显提高了氢致附加应力强度因子,估算的"GBF"裂纹尺寸的极限值与实验值能够较好地吻合。     

淬火冷却速率对60Si2CrVAT弹簧钢高周疲劳性能的影响

采用13%PAG(聚烷撑乙二醇)和油对60Si2Cr VAT弹簧钢进行淬火热处理,研究了不同淬火冷却速率对其轴向高周疲劳(r=-1)性能的影响。利用SEM、TEM和EBSD等方法对疲劳断口形貌、源区成分、显微组织进行表征。结果表明,13%PAG淬火后的疲劳极限(781.5 MPa)比油淬高67.5 MPa(714.0 MPa)。疲劳断口分析表明,疲劳破坏大部分起源于试样内部夹杂物和碳化物,形成"鱼眼"型撕裂的粒状亮区(GBF)。随裂纹源夹杂物处应力强度因子幅?Kinc的减小,疲劳寿命Nf增加,而GBF区边界的应力场强度因子幅?KGBF并不随Nf变化而改变,且13%PAG淬火试样的?KGBF大于油淬试样。实验钢13%PAG淬火试样组织中分布更多的纳米孪晶;马氏体板条块和板条更加细化,碳化物呈均匀细小弥散分布;而油淬碳化物较粗大,沿马氏体板条界和原奥氏体晶界分布。这些因素是PAG淬火后疲劳性能优于油淬的主要原因。     

汽轮机转子钢常温与600℃超高周疲劳行为研究

利用自主研发的高温超声疲劳实验系统,开展Cr Mo W转子钢常温及600℃下的超高周疲劳实验。为新型超超临界汽轮机转子提供高温超高周疲劳数据,实验结果表明高温会极大降低转子钢的疲劳强度。S-N曲线在常温及600℃下均呈现连续下降型,且600℃下S-N曲线在整个疲劳寿命周次内保持一定下降趋势。断口分析发现,常温下疲劳寿命107周次试件的疲劳裂纹以内部萌生为主,600℃下疲劳破坏的内部夹杂萌生方式与表面萌生方式均分布于整个疲劳寿命。夹杂物尺寸分析表明,高温降低疲劳裂纹内部萌生夹杂物的临界尺寸。     

某液压系统大型弹簧断裂原因

某液压系统使用的大型圆柱螺旋压缩弹簧在运行20万次后发生断裂,采用断口形貌、显微组织观察,化学成分、硬度测试等方法对弹簧的断裂原因进行了分析。结果表明:弹簧的断裂为低周疲劳断裂,裂纹源区位于弹簧钢丝近表面位置的大尺寸夹杂物处;夹杂物的存在降低了弹簧钢内部的连续性,弹簧钢受到拉应力和扭转作用力时萌生裂纹,在后续服役过程中裂纹扩展,最终断裂;弹簧喷丸处理形成的硬化层深度不够,不足以抵消弹簧钢丝表面脱碳层对疲劳寿命的不利影响,导致疲劳寿命降低。     

镍基粉末高温合金中夹杂物导致裂纹萌生和扩展行为的研究

采用扫描电镜原位拉伸和原位疲劳的方法,跟踪观察了人工植入Al2O3夹杂物的镍基粉末高温合金P/M Rene95中夹杂物导致裂纹萌生、扩展乃至断裂的过程,结果表明,无论是在单轴拉伸还是低周疲劳实验中,裂纹均首先萌生于脆性非金属夹杂物Al2O3处,大于一定尺寸的夹杂物,还会使该裂纹扩展成为导致合金断裂的主裂纹,从而大大降低合金的屈服强度、断裂强度及低周疲劳寿命.     

CrMoW转子钢的高温超高周疲劳性能

使用自主搭建的高温超声疲劳实验系统在常温和600℃进行Cr Mo W转子钢1×1010周次超高周疲劳实验,研究了Cr Mo W转子钢在工作温度下的超高周疲劳性能。结果表明:在600℃和常温下转子钢的S-N曲线均呈连续下降型,试件在109周次后仍发生疲劳断裂。在常温实验条件下疲劳寿命小于1×107周次试件的裂纹多萌生在表面,而高于1×107周次的多以内部萌生为主。在高温实验环境下裂纹也在内部夹杂处萌生,且裂纹萌生方式对疲劳寿命没有决定性作用。鱼眼定量分析结果表明,Cr Mo W转子钢在600℃和常温下裂纹扩展的应力强度因子门槛值分别为3.4 MPa·m1/2和1.0 MPa·m1/2。     

42CrMo钢疲劳裂纹扩展剩余寿命评估

以高频三点弯曲疲劳试验机为平台,进行42CrMo钢疲劳裂纹扩展试验研究,通过建立裂纹扩展剩余寿命评估模型,实现对存在裂纹的工程机械零部件剩余寿命的评估。采用显微成像测试系统实时采集并测量疲劳扩展裂纹,使用声发射系统监测整个疲劳裂纹扩展过程。结果表明:声发射幅值、能量等特征参数可以实时反应疲劳裂纹萌生、稳定扩展和失稳扩展等各个损伤阶段,并在疲劳断裂时产生急剧的突变;裂纹扩展速率的对数值与应力强度因子幅的对数值具有较高的线性相关性,建立了不同应力工况条件下裂纹扩展剩余寿命评估模型,以双排链轮轴为例进行裂纹扩展剩余寿命评估;随着疲劳应力的增加,裂纹扩展剩余寿命减小。     

Piezoelectric composites with high sensitivity and high capacitance for use at high pressures

A type of piezoelectric composite has been developed for oceanographic applications. The composites have a large figure of merit (d(h)xg(h) or d(h )xg(h)/tan delta, where d(h ) is the hydrostatic piezoelectric voltage coefficient), a large dielectric constant (K) and low dielectric loss, and great mechanical strength. A shallow cavity between the PZT ceramics and thick metallic electrode is designed to convert a portion of the z-direction stress into a large radial and tangential stress of opposite sign. thereby causing the d(33) and d(31) contributions to d(h) to add rather than subtract, and raising the figure of merit. Theoretical stress analysis was carried out using an axisymmetric finite element method. Experimental results show that the d(h)x g(h), K, and withstandable pressure are extremely high.     

Fracture of high carbon high strength anchor rods

During a heavy windstorm, there was failure of anchors supporting transmission towers. We were given the mandate to examine the reasons for the failure of the anchor rods and suggest measures to prevent such failures from occurring. The study involved the in-depth examination of the chemical composition, microstructures, mechanical properties and fracture morphologies of specimens from failed anchors. Results indicated that the chemical composition, yield and tensile strengths of the anchor rods are within the norm of ASTM standard A722. The chemical composition corresponds to that of AISI 1070 with slightly higher manganese and silicon levels. Microstructure is ferrite–pearlite with dominance of pearlite due to the relatively high carbon content. Fracture surfaces of failed anchor rods exhibited pronounced cleavage facets (cleavage steps and river pattern) indicating that failure mode is brittle and the mechanism of fracture is cleavage. There were no signs for the occurrence of fatigue – any beach mark or striations. Final area of fracture showed traces of ductility (small shallow dimples). To prevent brittle failure of anchor rods, their toughness has to be improved. To achieve this, the potentiality of applying HSLA steel technology of micro alloying and TMCP (thermo mechanical control process) has been suggested.     

Repetitive waveform high frequency, high precision digitizer

A digitizing technique for measuring high-speed phenomena with very high precision is described. A commercially available waveform digitizer using this method is shown to exhibit settling times of less than 5 ns to 0.1% and less than 10 ns to 0.01% after a voltage transition. Also, RMS measurements of sine waves up to 100 MHz show errors on the order of that obtained using a thermal transfer standard     

Intermittent bonding for high toughness/ high strength composites

High strength and high toughness are usually mutually exclusive in brittle filament/brittle matrix composites. The high tensile strength characteristic of strong interfacial filament/matrix bonding can, however, be combined with the high fracture toughness of weak interfacial bonding, when the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibres. The strong regions ensure that the filament strength is picked up; weak areas randomly in the path of running cracks serve to blunt them by the Cook/Gordon mechanism which, in turn, produces long pull-out lengths with an associated large contribution to toughness. Boron-epoxy composites of volume fraction 0.20 to 0.25 have been made in this way which have fracture toughnesses of over 200 kJ m^?2, whilst retaining rule of mixtures tensile strengths (～ 650 MN m^?2). At the volume fractions used, this apparently representsK _IC values greater than 100 MN m^?3/2. An analysis is presented for toughness and strength which demonstrates, in broad terms, the effects of varying the coating parameters of concern. Results show that the “toughness” of interfaces is an important parameter, differences in which may not be shown up in terms of interfacial “strength”. The choice of coating material is crucial in getting the desired effect. Some observations are made upon methods of measuring the components of toughness in composites.     

高刚度高阻尼结构试验研究

利用预置变形碳纤维梁、螺旋弹簧、碟簧和橡胶块构成一种高刚度高阻尼结构,其承载刚度和阻尼均较大。为了便于对高刚度高阻尼结构的力学性能进行评价,提出技术指标,即:等效弹性模量、等效阻尼系数和等效刚度系数。在此基础上,设计静动态力学试验,验证高刚度高阻尼结构设计方法的有效性并评价其力学性能。试验结果表明,高刚度高阻尼结构的等效弹性模量显著大于单个预置变形碳纤维梁;等效阻尼系数随着频率增大而减小,等效刚度系数随着频率增大而增大,分别大于104 Ns/m和107 N/m。由此证明,高刚度高阻尼结构的设计方法有效,且实现了输出刚度和阻尼均较大的设计目标。     

Behaviour of B-N-Si under high pressure and high temperature

The phase relations of the B-N-Si system have been studied using a quenching method up to 10GPa and 2000 °C using a high-pressure apparatus of the octahedral anvil type. Pressure-temperature conditions for obtaining z-BN (diamond analogue of boron nitride) were delineated for turbostratic BN (t-BN), t-BN/amorphous Si₃N₄ and t-BN/-Si₃N₄. These conditions shift toward higher regimes of temperature as amorphous Si₃N₄ or -Si₃N₄ is incorporated into t-BN. Spontaneous sintering occurringin situ at high pressure yields z-BN-based composite compacts.     

Hot workability of a high carbon high chromium tool steel

In this work, hot tension tests were conducted on as cast and wrought samples of a high carbon high chromium tool steel to study the hot workability under the rolling conditions. The flow curves illustrate the classical shape of dynamic recrystallization (DRX). It is observed that broken carbide nets in the wrought samples result in lower deformation activation energy 398 kJ/mol in comparison to the as cast samples 432 kJ/mol. Necking strains were calculated using the inflection point of the work-hardening (θ) vs. stress curves. Wrought samples show higher hot ductility and lower maximum stress than the as cast ones. It is shown that lower activation energy of deformation and lower stress concentration around the smaller carbides in the wrought samples is responsible for their higher hot workability.     

Polyimide survival in high temperature high pressure dilute aqueous solutions

Measurements of the gross properties of polyimides at high temperature and pressures in dilute aqueous environments are reported. The polyimide polymer solution is spun onto SiO₂ substrates prepared with an aluminum chelate adhesion promoter. The cured films are subjected to temperatures exceeding 150 °C and pressures of more than 1000 lbf in^-2 in dilute aqueous solutions ranging in pH from 3 to 10 for 2–4 h. Film quality, adhesion and water adsorption determined by IR measurements are reported.