返回料添加比例对K44合金热疲劳性能的影响

研究了返回料添加比例对新型抗热腐蚀高温合金K44热疲劳性能的影响.结果表明:新料和返回料合金试样V型缺口尖端主裂纹扩展长度与热循环次数之间遵循L=bNa规律.新料合金热疲劳裂纹萌生和扩展速率最低,随着合金中返回料比例的增大,热疲劳裂纹萌生速率和扩展速率也增大.热疲劳裂纹萌生于V型缺口尖端附近区域,沿枝晶间、晶界和开裂的碳化物扩展,主裂纹扩展以裂纹尖端连续开裂的形式进行.返回料合金由于氮含量增加导致共晶和夹杂物增多,碳化物聚集块化,加速了热疲劳裂纹的萌生与扩展.合金经热疲劳实验后,裂纹两侧产生氧化带和γ'相贫化带.     

超声疲劳试验方法对S06钢疲劳性能及裂纹萌生机制的影响

采用超声疲劳试验方法和试样尺寸相近的高频疲劳试验方法对S06钢进行疲劳性能试验,将试验数据和试样断口形貌进行对比,研究超声疲劳试验方法对S06钢疲劳性能和裂纹萌生机制的影响.结果表明:在相同的应力水平下,超声加载频率下S06钢的疲劳寿命高于高频疲劳试验测得的疲劳寿命;超声疲劳试验中裂纹全部从表面萌生,而高频疲劳试样裂纹有表面萌生和内部萌生两种机制.分析了超声加载频率对S06钢疲劳性能和裂纹萌生机制产生影响的原因:对疲劳性能的影响与金属材料的晶体结构和裂纹尖端的化学反应有关,对裂纹萌生机制的影响与试样表层残余应力松弛有关.     

TC4钛合金复合加载下超高周疲劳性能研究EI北大核心CSCD

针对航空发动机压气机叶片在实际工况下的超高周疲劳断裂问题,研究了三种锻造温度下TC4钛合金三点弯曲–轴向拉伸复合加载的疲劳破坏行为。试验结果表明,S-N曲线呈直线下降型和双平台型,采用985℃近锻造时疲劳性能最好。随着应力幅值降低,裂纹由表面萌生向次表面萌生转变,断口形貌呈现准解理断裂特征。表面裂纹萌生于α晶界或α-β相界,由位错滑移堆积导致;而次表面裂纹萌生于刻面,由初生α相解理导致。疲劳寿命由裂纹萌生阶段主导,且所占比例随总寿命的增加而变大。双态组织中初生α含量和尺寸均小于等轴组织,且β转变组织含量更高,从而具备更好的疲劳性能。轴向拉伸改变了试件的轴向应力分布,有利于提高裂纹萌生于次表面的概率,使裂纹起源点向内部迁移。     

1Cr18Ni9Ti油管裂纹分析

某发动机在工作过程中连续发生3起油管漏油事故,通过对其中一根油管上的裂纹进行宏观观察、金相检验、化学成分分析以及应力测定,并打开裂纹进行断口电镜观察,确定了裂纹的性质及油管开裂的原因。结果表明：油管上的裂纹性质为起始应力较大的疲劳裂纹,疲劳裂纹起源于油管转接嘴的R外表面,油管的装配应力较大是导致疲劳裂纹萌生的主要原因,高压燃油冲击和油管本身的振动对裂纹的萌生也有一定的影响,管接嘴处焊料局部未焊合对裂纹的萌生有一定的促进作用。     

应力幅对退火态Mg-3Al-2Sc合金疲劳行为的影响

在不同应力幅下对退火处理后的Mg-3Al-2Sc合金进行应力控制的疲劳实验,研究应力幅对合金的裂纹扩展行为和断裂机制的影响。结果表明:在12～75MPa应力范围内,当控制应力比为0.2,随着应力幅增加,裂纹扩展速率增加,Mg-3Al-2Sc合金的疲劳断裂寿命降低。在裂纹萌生区合金主要表现为解理断裂机制,进入裂纹稳定扩展阶段的疲劳断裂主要为准解理断裂。而在瞬断区,低应力幅时主要表现为准解理断裂,高应力幅时主要表现为微孔聚合断裂。     

铝合金钻杆旋转弯曲疲劳断口特征

首先对铝合金钻杆进行了不同应力幅值的旋转弯曲疲劳试验,然后采用扫描电镜观察其断口微观形貌特征。结果表明:在170 MPa低应力下,铝合金钻杆的疲劳断口可分为疲劳源区、裂纹扩展区及瞬断区3个区域,微裂纹在试样次表面相界处萌生,裂纹稳态扩展区面积较大,占60%以上;在290 MPa高应力下,铝合金钻杆的疲劳断口上有多个疲劳源区,微裂纹于试样表面或近表面位置形核,裂纹扩展区面积仅占15%~30%。对于同一试样,随着裂纹的扩展,断口上疲劳滑移台阶变宽,疲劳条带逐渐清晰;对于不同试样,随着应力的提高,裂纹扩展速率增大,疲劳条带间距变宽,更易出现二次疲劳裂纹,断口上裂纹扩展区面积明显减小,相应的疲劳寿命也大幅降低。     

表面压应力对10Ni5CrMoV钢疲劳性能的影响

分别采用精磨和冲击加工在IONi5CrMoV钢试样表面形成不同的残余压应力,对具有不同表面压应力的试样进行高周和低周疲劳试验,采用扫描电子显微镜对疲劳断口进行分析,并测试经不同周次高应变幅循环加载后的试样表面应力。结果表明,较高的表面压应力能有效抑制小应力幅下疲劳裂纹在表面的起裂,从而有效的提高了高周疲劳性能;在高应变幅作用下,初始的表面压应力迅速发生松弛而失去对疲劳裂纹在表面起裂的抑制作用,因而对低周疲劳性能无明显影响。     

轧机减速箱重载齿轮断齿分析

对机减速箱重载斜齿轮断齿从化学成分、金相、断口等方面做了综合分析.)断口分析表明,裂纹萌生于应力最大的齿接触表面和次表面,断口呈现疲劳辉纹特征.金相检验证实裂纹源区存在硫化物夹杂.综合分析表明,夹杂物尖端的应力集中导致齿轮裂纹源的萌生,而后在接触应力作用下裂纹逐渐扩展导致接触疲劳断裂.     

18CrNiMo7-6合金钢的弯曲微动疲劳特性

对18CrNiMo7-6合金钢进行弯曲微动疲劳实验,建立弯曲微动疲劳S-N曲线,并对实验结果进行分析。结果表明:该合金钢的弯曲微动疲劳S-N曲线不同于中碳钢材料,也不同于常规弯曲疲劳,而是呈"ε"型曲线特征。随着弯曲疲劳应力的增加,微动运行区域由部分滑移区向混合区和滑移区转变,损伤区的磨损机制以剥层、磨粒磨损和氧化磨损为主。在混合区内,裂纹最易萌生和扩展,且裂纹均萌生于材料接触区次表面。受接触应力和弯曲疲劳应力影响,弯曲微动疲劳裂纹的萌生和扩展可分为三个阶段:初期,在接触应力控制下,裂纹萌生于次表面;随后,裂纹受接触应力和弯曲疲劳应力共同控制,转向更大角度方向扩展;最后,裂纹完全受弯曲疲劳应力控制而垂直于接触表面扩展,直至断裂失效。     

汽轮机低压次末级叶片开裂原因分析

某汽轮机组低压次末级叶片在运行过程中发生开裂,通过化学成分分析、力学性能测试和金相检验等方法对叶片开裂原因进行了分析。结果表明:叶片开裂性质为疲劳开裂,裂纹萌生于叶根最大受力面的应力集中处;叶片型线过渡区的尖角和缺口是形成疲劳裂纹源的主要原因,叶片近表面的夹杂物对裂纹萌生也有一定的影响。     

3D modeling of subsurface fatigue crack nucleation potency of primary inclusions in heat treated and shot peened martensitic gear steels

A computational strategy is developed to characterize the driving force for fatigue crack nucleation at subsurface primary inclusions in carburized and shot peened C61^® martensitic gear steels. Experimental investigation revealed minimum fatigue strength to be controlled by subsurface fatigue crack nucleation at inclusion clusters under cyclic bending. An algorithm is presented to simulate residual stress distribution induced through the shot peening process following carburization and tempering. A methodology is developed to analyze potency of fatigue crack nucleation at subsurface inclusions. Rate-independent 3D finite element analyses are performed to evaluate plastic deformation during processing and service. The specimen is subjected to reversed bending stress cycles with R = 0.05, representative of loading on a gear tooth. The matrix is modeled as an elastic–plastic material with pure nonlinear kinematic hardening. The inclusions are modeled as isotropic, linear elastic. Idealized inclusion geometries (ellipsoidal) are considered to study the fatigue crack nucleation potency at various subsurface depths. Three distinct types of second-phase particles (perfectly bonded, partially debonded, and cracked) are analyzed. Parametric studies quantify the effects of inclusion size, orientation and clustering on subsurface crack nucleation in the high cycle fatigue (HCF) or very high cycle fatigue (VHCF) regimes. The nonlocal average values of maximum plastic shear strain amplitude and Fatemi–Socie (FS) parameter calculated in the proximity of the inclusions are considered as the primary driving force parameters for fatigue crack nucleation and microstructurally small crack growth. The simulations indicate a strong propensity for crack nucleation at subsurface depths in agreement with experiments in which fatigue cracks nucleated at inclusion clusters, still in the compressive residual stress field. It is observed that the gradient from the surface of residual stress distribution, bending stress, and carburized material properties play a pivotal role in fatigue crack nucleation and small crack growth at subsurface primary inclusions. The fatigue potency of inclusion clusters is greatly increased by prior interfacial damage during processing.     

A study of crack growth retardation due to artificially induced crack surface contact

The contact of the cracked surfaces during a part of a loading cycle generally results in a reduced crack growth rate. A critical experiment was designed to evaluate the influence of the crack surface contact on crack growth. A round compact specimen made of 1070 steel with a round hole at the wake of the fatigue crack was designed. Two mating wedges were inserted into the hole of the specimen while the external load was kept at its maximum in a loading cycle. In this way, the wedges and the hole in the specimen were in firm contact during the entire loading cycle in the subsequent loading. Experiments showed that the addition of the wedges resulted in a reduction of crack growth rate in the subsequent constant amplitude loading. However, crack growth did not arrest. With the increase in the subsequent loading cycles, crack growth rate increased. The traditional crack closure concept cannot explain the experimental phenomenon because the effective stress intensity factor range was zero after the insertion of the wedges. The detailed stress–strain responses of the material near the crack tip were analyzed by using the finite element method with the implementation of a robust cyclic plasticity theory. A multiaxial fatigue criterion was used to determine the fatigue damage based upon the detailed stresses and strains. The crack growth was simulated and the predicted results were in good agreement with the experimental observations. It was confirmed that the stresses and strains near the crack tip governed cracking behavior. Crack surface contact reduced the crack tip cyclic plasticity and the result was the observed retardation in crack growth.     

SPECIMEN SIZE DEPENDENCE OF LOW FREQUENCY FATIGUE TESTS IN HIGH-PRESSURE HYDROGEN

Abstract— Fatigue data required for estimates of cracked component lifetimes are conventionally obtained by cyclic loading of specimens manufactured to a specific geometry. Crack growth in the specimen results in an increase in the stress intensity factor range and crack growth curves are calculated from the variation of crack length with time. An environmental fatigue study of the effect of high pressure hydrogen on the low cycle fatigue of a medium strength steel has shown that, due to effects of elapsed time in the environment and effects of specimen size, in certain circumstances this procedure may not yield geometry-independent results which can be applied with confidence to cracked components. It is concluded that to obtain useful crack growth data in cases where fracture is influenced by diffusion or other strongly time dependent processes might require a modified approach to fracture mechanics testing procedures.     

Mechanisms and modeling of low cycle fatigue crack propagation in a pressure vessel steel Q345

The fatigue process near crack is governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic–plastic fracture mechanics, we explore the cyclic J-integral as breakthrough point, an analytical model is presented in this paper to determine the CTOD for cracked component subjected to cyclic axial in-plane loading. A simple fracture mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack tip opening displacement (ΔCTOD) and the crack growth rate (da/dN). In order to validate the model and to calibrate the model parameters, the low cycle fatigue crack propagation experiment was carried out for CT specimen made of Q345 steel. The effects of stress ratio and crack closure on fatigue crack growth were investigated by elastic–plastic finite element stress–strain analysis of a cracked component. A good comparison has been found between predictions and experimental results, which shows that the crack opening displacement is able to characterize the crack tip state at large scale yielding constant amplitude fatigue crack growth.     

SHEAR FATIGUE CRACK GROWTH: A LITERATURE SURVEY

A fatigue crack is often initiated by a localized cyclic plastic deformation in a crystal where the active slip plane coincides with the plane of maximum shear stress. Once a crack is initiated, the crack will propagate on the maximum shear plane for a while and, in the majority of the cases, will eventually change to the plane of the applied tensile stress. The “shear” and “tensile” modes of fatigue crack propagation are termed stage I and stage II fatigue crack growth. They are also known as mode II and mode I fatigue crack growth. However, the mechanism of the tensile mode fatigue crack propagation is shear in nature. Considerable progress has been made recently in the understanding of mode II fatigue crack growth. This paper reviews the various test methods and related data analyses. The combined mode I and mode II elastic crack tip stress field is reviewed. The development and the design of the compact shear specimen are described and the results of fatigue crack growth tests using the compact shear specimens are reviewed. The fatigue crack growth tests and the results of inclined cracks in tensile panels, center cracks in plates under biaxial loading, cracked beam specimens with combined bending and shear loading, center cracked panels and the double edge cracked plates under cyclic shear loading are reviewed and analyzed in detail.     

Gigacycle fatigue behavior of a high chromium alloyed cold work tool steel

The influence of carbides and the effect of surface residual stresses (RS), resulting from heat treatment or from the grinding/polishing process, on the fatigue behavior in the gigacycle regime of ingot metallurgy produced D2 type tool steel was examined. RS were found to be responsible for the occurrence of two failure modes: Internal cracks initiating at large primary carbides (clusters) were observed in the cycle number range of 10⁵–10⁶, while in the gigacycle regime near-surface cracks originating at primary carbides caused failure, which was related to degradation of the RS by cyclic loading. Simple models were employed estimating the RS degradation process and the local fatigue strength along the specimen cross section as a function of active RS. In absence of considerable RS predominantly near-surface crack initiation was obtained.     

NUCLEATION OF INTERGRANULAR CRACKS DURING HIGH TEMPERATURE LOW CYCLE FATIGUE OF SUS316 STAINLESS STEEL

Fundamental behaviour of intergranular cracking of SUS316 stainless steel which characterises the high temperature low cycle fatigue process has been studied with a special emphasis on the interaction between oxidation and a grain boundary sliding. Two types of specimens were prepared for fatigue experiment to extract a sole effect of surface oxidation on crack nucleation. One was heat-treated in air and the other was in vacuum so that the specimens had the same history of heat treatment except oxidation to the surface. Results of fatigue tests of these specimens well explain the relationship between oxidation and surface cracking as follows. The morphology of the oxidised surface of the specimen subjected to low-cycle fatigue at 700°C is quite different from that of the oxidised surface caused by simply holding at the same temperature in air for several hours with no applied stress. Localised oxidation along the grain boundary is a characteristic feature for the specimen fatigued at 700°C, while no localised oxidation was observed when the specimens were simply held at the same 700°C, i.e. with no fatigue loading. Accordingly, intergranular cracking in high temperature low cycle fatigue in air occurs when grain boundary sliding due to cyclic loading is accelerated by localised oxidation along the grain boundary.     

Influence of inclusion size on S-N curve characteristics of high-strength steels in the giga-cycle fatigue regime

Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10⁶ cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusion-induced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.     

Inertial effects on fatigue crack growth

In this paper, a cohesive zone model is used to study the influence of inertial effects on crack growth considering cyclic loading in homogenous rate‐independent materials. Quasi‐static and dynamic solutions are compared in order to establish the conditions in which the inertial effects become important in the analysis. It is discussed how speed and frequency of the loading and specimen sizes modify crack growth characteristics. In general, an increase in the loading frequency leads to a higher propagation velocity. Very high loading frequencies may lead to the formation of microcracks ahead of the crack tip and may change the failure mode of the cracked structure from crack propagation to uniform debonding. This work shows that inertial effects are specially noticeable for frequencies in the kHz range. However, applied frequencies close to natural frequencies of the cracked specimen can give rise to strong inertial effects and then a substantial reduction of fatigue life for much lower frequencies. This work also shows that critical frequencies depend on the specimen size.     

Mechanisms of Fatigue in Cemented Carbides at Elevated Temperatures

Cemented carbides are mainly used as cutting tools in various fields of application. In nearly all industrial uses they have to resist wear and concurrent complex thermomechanical loading, often including alternating mechanical loads at high temperatures. It has been shown that cemented carbides suffer lifetime‐limiting fatigue under cyclic loads at 25 °C caused by subcritical crack growth. Present investigations show that strong, temperature‐dependent fatigue effects occur in cemented carbides at elevated temperatures. TEM investigations revealed temperature‐dependent processes that are responsible for the changes in the fatigue behavior: at low temperatures a phase transformation of the Co‐binder phase and at higher temperatures oxidation in the cracks and brittle–ductile transitions of cubic carbides.