表面再结晶对定向凝固DZ4合金疲劳行为的影响

对表面通过喷丸预变形后的定向凝固DZ4板材试样进行1220℃/4h的热处理,以在板材试样表面形成再结晶层.在760℃下进行疲劳试验,以研究表面再结晶层对定向凝固DZ4合金板材的疲劳性能的影响.结果表明,表面有再结晶层的定向凝固DZ4板材试样的疲劳性能大大降低,且疲劳寿命随着再结晶层深度的增大而降低;断口观察表明,疲劳裂纹均起源于表面的再结晶区或其与基体的界面.通过有限元模拟对含再结晶区的板材试样的损伤演化行为进行模拟,结果表明再结晶区的存在改变了试样表面的应力场分布,再结晶区与基体的界面上的应力最大,裂纹一般从此处萌生,并向表面和基体扩展.     

喷丸处理对6151-T6铝合金旋转弯曲疲劳性能的影响

对经过电解抛光、喷丸处理和喷丸处理再抛光的三种6151-T6铝合金试样的表面粗糙度及喷丸处理后试样表面硬化层的硬度进行测试。对三种表面状态不同的铝合金材料进行旋转弯曲疲劳实验。研究喷丸处理试样表面残余应力随疲劳实验的变化情况,以及喷丸处理对该铝合金疲劳性能的影响。结果表明:喷丸处理对材料疲劳性能的影响与加载的压应力水平有关。当加载的压应力与表面残余压应力之和不超过材料表面硬化层的循环屈服强度时,硬化层中的残余压应力在疲劳过程中不发生应力松弛,疲劳寿命得到大幅度提高;反之,将发生应力松弛现象,疲劳寿命的提高程度受残余压应力松弛程度的影响。此外,喷丸处理造成的材料表面粗糙度的增加,在全应力幅范围,使材料的疲劳寿命略有降低。     

残余应力和粗糙度对叶片振动疲劳性能的影响

为了研究叶片表面完整性对其振动疲劳性能的影响,本文模拟分析了某型高温合金叶片在振动疲劳实验过程中的动力学应力响应,获得叶片共振时应力幅值随时间的变化规律,分析了残余应力和粗糙度对叶片振动疲劳寿命和疲劳极限的影响规律.结果表明:叶片共振过程中的应力响应幅值先增大后减小呈周期性变化,属于"拍"现象,满足关系σ=1 046sin(242.83t)sin(5 828t);叶片的振动疲劳极限和疲劳寿命均随残余应力和粗糙度的增大而减小,振动疲劳极限和残余应力之间的关系满足σfat=510.9-0.31-70.93σrest;而疲劳极限和粗糙度之间的关系则满足σfat=9.67R2roughness-70.93Rroughness+713.23.     

钛合金疲劳试样加工残余应力的表征及其对低周疲劳寿命的影响

目的 探究钛合金TC4低周疲劳试样加工过程中残余应力的变化及残余应力对低周疲劳寿命的影响.方法 采用X射线衍射法分别对车、磨、抛后的试样进行表面残余应力的表征,分析加工工艺对其的影响,利用液压伺服万能试验机进行低周疲劳试验,分析其表面残余应力对寿命的影响.结果 经过粗车、精车、磨削、纵抛,试样加工表面的残余应力不断减小...     

喷丸强化对2XXX铝合金疲劳寿命的影响

研究喷丸对2XXX铝合金拉-拉疲劳性能的影响。对未喷丸试样和喷丸强化试样的微观组织、显微硬度、残余应力和拉-拉疲劳性能进行对比分析。结果表明:喷丸处理后,试样的组织和微结构未发现明显变化,但其粗糙度、残余压应力和显微硬度有所提高,分别是未喷丸试样的6.25倍,3.85倍和1.12倍;拉-拉疲劳性能显著提高,其中值疲劳寿命是未喷丸的1.67倍。在99.9%存活率下,喷丸试样的安全寿命是未喷丸试样的1.45倍。且表面喷丸强化后疲劳裂纹源由多个变为一个。     

冷挤压GH4169合金孔结构疲劳性能与断口分析EI北大核心CSCD

对G H4169合金中心孔板材进行冷挤压强化,研究其挤压前后825MPa/600℃/R=0.1疲劳寿命,分析挤压前后表面粗糙度变化和疲劳过程中的残余应力场演化,并细致观察两件挤压试样不同寿命(分别为25105周次和10719周次)断口以分析表面完整性对疲劳过程的作用.结果表明:相比原始试样,冷挤压强化后试样中值疲劳寿命估计量提高了1倍,挤压后较低的表面粗糙度和疲劳过程中稳定的残余应力场是疲劳寿命提高的主要原因.同时,挤压后疲劳寿命标准差增大.由断口定量分析可知,两件试样距疲劳源区0.1mm之后的扩展寿命相当,而萌生寿命(分别为18786周次和5915周次)却相差巨大.造成孔挤压后寿命分散性大的原因是0.1mm以内的裂纹萌生寿命差别.为提高孔结构疲劳性能稳定性,挤压时应注意近表层表面完整性的控制.     

冷挤压GH4169合金孔结构疲劳性能与断口分析

对GH4169合金中心孔板材进行冷挤压强化,研究其挤压前后825 MPa/600℃/R=0.1疲劳寿命,分析挤压前后表面粗糙度变化和疲劳过程中的残余应力场演化,并细致观察两件挤压试样不同寿命(分别为25105周次和10719周次)断口以分析表面完整性对疲劳过程的作用。结果表明:相比原始试样,冷挤压强化后试样中值疲劳寿命估计量提高了1倍,挤压后较低的表面粗糙度和疲劳过程中稳定的残余应力场是疲劳寿命提高的主要原因。同时,挤压后疲劳寿命标准差增大。由断口定量分析可知,两件试样距疲劳源区0.1 mm之后的扩展寿命相当,而萌生寿命(分别为18786周次和5915周次)却相差巨大。造成孔挤压后寿命分散性大的原因是0.1 mm以内的裂纹萌生寿命差别。为提高孔结构疲劳性能稳定性,挤压时应注意近表层表面完整性的控制。     

GH169合金孔内表面层的挤压强化对孔边疲劳性能的影响

研究了孔内表面层的挤压强化对ＧＨ１６９高温合金高温疲劳性能的影响。结果表明，孔内表面层的挤压强化对ＧＨ１６９高温合金的孔边疲劳性能具有明显的强化效果，在６５０℃下的疲劳寿命提高近三倍。表面强化层内宏观残余压应力的产生和位错密度的增加、孔壁表面粗糙度的降低是提高疲劳寿命的主要因素。     

GH169合金孔内表面层的挤压强化对孔边疲劳性能的影响

研究了孔内表面层的挤压强化对ＧＨ１６９高温合金高温疲劳性能的影响，结果表明，孔内表面层的挤压强化对ＧＨ１６９高温合金的孔边疲劳性能具有明显的强化效果，在６５０℃下的疲劳寿命提高近三倍，表面强化层内宏观残余应力的产生和位错密度的增加，孔壁表面粗糙度的降低是提高疲劳寿命的主要因素。     

超声深滚处理改善预腐蚀7A52-CZ铝合金疲劳性能机理

研究了超声深滚(UDR)处理对预腐蚀7A52-CZ铝合金疲劳性能的作用.7A52铝合金试样在剥蚀腐蚀溶液中浸泡不同时间后进行了超声深滚处理.分别对未腐蚀试样、腐蚀试样和腐蚀+UDR处理试样进行了疲劳试验,用XRD应力测试和扫描电镜等方法分析了UDR处理前后试样的残余应力和断口形貌,并对疲劳断口进行了分析.结果表明:UDR处理在铝合金中引入超过1mm深的残余压应力层,延长了7A52的预腐蚀疲劳寿命.对于腐蚀较轻的试样,UDR处理使裂纹源在表层下残余压应力和拉应力过渡区产生,延长了疲劳裂纹萌生寿命;对腐蚀较重试样,疲劳裂纹仍从晶间腐蚀处形核,但由于引入残余压应力及腐蚀裂纹的部分愈合效应,仍在很大程度上改善了7A52的预腐蚀疲劳寿命.     

Fatigue life prediction of corrosion-damaged high-strength steel using an equivalent stress riser (ESR) model: Part I: Test development and results

The fatigue life of metallic aircraft structural components can be significantly reduced by environmentally induced corrosion. As part of a NAVAIR High Strength Steel Corrosion–Fatigue Assessment Program, methods were studied to predict the impact that corrosion-induced surface roughness has on the fatigue life of high-strength steel aircraft components. In order to adequately capture the corrosion damage features that cause fatigue cracking, a representative set of well-characterized corrosion–fatigue test results were generated to be used for model development. The test specimens fabricated for this program consisted of bare, unnotched AF1410 steel flat plates with a 25.4 mm diameter corrosion patch on one side. Two sets of test specimens were fabricated and tested, with one set abrasive blasted after heat treatment, and the other set hand polished after heat treatment. A method of growing corrosion in the laboratory was developed that consisted of filter paper soaked in a 3.5% NaCl solution and placed at the center of the test plate gage section, with a voltage applied across the filter paper to accelerate corrosion growth. High-resolution 3D surface topography data was collected from the corroded region on each test plate prior to fatigue testing using a commercial white-light interference microscope. Constant-amplitude fatigue tests were performed on corroded and uncorroded test plates at several different stress levels, for three different corrosion exposure levels. Post-test fractographic analysis of the corroded specimens indicate that all of the critical cracks originated from small corrosion notches on the order of 10–200 μm in width, 10–120 μm in height and 2–100 μm in depth. These notches were not considered to be pits in that the depth dimension was less than the surface dimensions. The repeatability of the fatigue initiating mechanism for corrosion damaged surfaces in this material indicates that it should be possible to develop a single modeling approach that reasonably captures the effects of corrosion notches in reducing fatigue life.     

Corrosion induced fatigue failure of railway wheels

Two AAR class B rolled wheels for locomotives failed after about two years of service. The fracture surfaces of the failed railway wheels were examined. The examination showed that there were corrosion pits on the back plate surface of the failed wheels. All of the fracture originated from corrosion pits at the wheel plate surface and fatigue propagated to a length and then expanded rapidly by cleavage. Fatigue specimens cut from the wheel plate were corroded with different time duration in an artificial corrosion environment to simulate the corrosion states of the wheels. The fatigue properties of the un-corroded specimens and the specimens corroded with different times were tested in air. Finite element method (FEM) and Sines' criterion were used to evaluate the safety of the wheels. The results showed that the wheel plates without corrosion pits exhibited an excellent resistance to failure. The corrosion pits could promote the initiation of fatigue cracks and drastically lower the fatigue limits of corroded specimens. The real root cause of the failure of the subject wheels was due to the corrosion pits at the wheel plate surfaces. A critical depth of the corrosion pit on the wheel plate 300 μm was recommended. Protection of the wheel plate was important to ensure the safety of wheels and the rust prevention oil was recommended to be applied on the wheel plate regularly.     

Effects of atmospheric corrosion on fatigue properties of a medium carbon steel

The effects of atmospheric corrosion on fatigue properties were examined using a medium carbon steel, corroded in various atmospheres. Three different places, having various atmospheric conditions, were selected for the corrosion tests: (i) an industrial area, (ii) near the ocean, and (iii) beside a river in a hot spring region. A water and/or air electrochemical cell corroded the carbon steel to rust that had several forms, depending on the atmosphere. The form of the corrosion was distinguished visually and by spectroscopy. Strong oxidation occurred in all samples with the formation of rust. In addition, a more severe chemical reaction with chlorine was detected near the ocean although carbon was obtained in the industrial area. On the other hand, a high level of sulfur reacted with the sample near the river. Such chemical reactions gave rise to different corrosion mechanisms leading to different corrosion surfaces. A rough corrosion face with corrosion pits was obtained in two of the samples (industrial area and near the ocean), while a smooth surface was produced for the sample near the river. The change of the surface morphology clearly affected the fatigue strength, e.g., the rougher the sample surface, the lower the fatigue strength. On the basis of the corrosion system, details of the fracture and fatigue characteristics are discussed in the present work.     

The effect of pitting corrosion on the safe-life prediction of the Royal Australian Air Force P-3C Orion aircraft

This paper investigates if corrosion pitting invalidates the fatigue lifeing method used by the Royal Australian Air Force to manage the structural integrity of its P-3C Orion aircraft. Fatigue life predictions made using the P-3C Service Life Assessment Program method were compared with those from an Equivalent Crack Size model for pitting corrosion. The data for this comparison came from a program of variable amplitude fatigue tests of high-k_t specimens of 7075-T6. This testing was performed using two stress levels on both corroded and uncorroded specimens. Analysis of the results showed that the safe-life estimates made using the P-3 life assessment method were conservative in the presence of corrosion as they were above the 99.9th percentile of fatigue life predicted by the equivalent crack size method.     

EIFS-based crack growth fatigue life prediction of pitting-corroded test specimens

Corrosive environment causes corrosion pits at material surface and reduces the fatigue strength significantly. Fatigue crack usually initiates at and propagates from these locations. In this paper, a general methodology for fatigue life prediction for corroded specimens is proposed. The proposed methodology combines an asymptotic stress intensity factor solution and a power law corrosion pit growth function for fatigue life prediction of corroded specimens. First, a previously developed asymptotic interpolation method is proposed to calculate the stress intensity factor (SIF) for the crack at notch roots. Next, a growing semi-circular notch is assumed to exist on the specimen’s surface under corrosive environments. The notch growth rate is different under different corrosion conditions and is assumed to be a power function. Fatigue life can be predicted using the crack growth analysis assuming a crack propagating from the notch root. Plasticity correction is included into the proposed methodology for medium-to-low cycle fatigue analysis. The proposed methodology is validated using experimental fatigue life testing data of aluminum alloys and steels. Very good agreement is observed between experimental observations and model predictions.     

Evidence on the corrosion-induced hydrogen embrittlement of the 2024 aluminium alloy

The present work aims to provide evidence of corrosion‐induced hydrogen embrittlement of the aircraft aluminium alloy 2024. An extensive experimental investigation involving metallographic and fractographic analyses as well as mechanical testing was performed. The corrosion exposure led to a moderate reduction in yield and ultimate tensile stress and a dramatic reduction in tensile ductility. Metallographic investigation of the specimens revealed a hydrogen‐rich embrittled zone just below the corrosion layer. Furthermore, fractographic analyses showed an intergranular fracture at the specimen surface followed by a zone of quasi‐cleavage fracture and further below an entirely ductile fracture. Mechanical removal of the corroded layers restored the yield and ultimate stress almost to their initial values but not the tensile ductility. The tensile ductility was restored to the level of the uncorroded material only after heat treatment at 495°C. Measurement of hydrogen evolution with temperature showed that by heating the corroded alloy at 495°C, the trapped hydrogen is released.     

Impact of corrosion on low-cycle fatigue degradation of reinforcing bars with the effect of inelastic buckling

The combined effect of inelastic buckling and chloride induced corrosion damage on low-cycle high amplitude fatigue life of embedded reinforcing bars in concrete is investigated experimentally. A total of forty-eight low-cycle fatigue tests on corroded reinforcing bars varied in percentage mass loss, strain amplitudes and buckling lengths are conducted. The failure modes and crack propagation are investigated by fractography of fracture surfaces using scanning electron microscope. The results show that the inelastic buckling, percentage mass loss and nonuniform corrosion pattern are the main parameters affecting the low-cycle fatigue life of reinforcing bars. It was found that the fatigue life of corroded reinforcing bars combined with inelastic buckling has a significant path dependency. The results show that in some cases the number of cycles to failure of corroded bars under constant amplitude fatigue test is increased.     

Corrosion fatigue and electrochemical behaviour of steel wires used in bridge cables

In‐service bridge wires often fail prior to the design life subjected to alternating stresses and environmental erosion. In this paper, a novel corrosion fatigue test device, integrating fatigue testing machine and electrochemical accelerated corrosion assemblies, was developed to characterize the corrosion fatigue and electrochemical behaviour of the wires. Using the developed device, corrosion fatigue tests of corroded bridge wires under different corrosion and loading conditions were conducted. Electrochemical characteristics, corrosion fatigue behaviour, failure mechanism, and so forth were investigated according to electrochemical measurements, fracture morphologies and the lifetime of wires. Results evidence the synchronization of corrosion and fatigue and show the accelerated corrosion due to static and fatigue stresses. Additionally, cracking and fracture induced by multiple crack initiation was dominant in corrosion fatigue of corroded wires, and the coexistence of multiple corrosion pits decreased the lifetime significantly.     

Investigation on low cycle fatigue of nickel‐based single crystal turbine blade in different regions

Low cycle fatigue experiments of nickel‐based single crystal superalloy miniature specimens were carried out at 760 °C/1000 MPa and 980 °C/750 MPa. According to testing results, low cycle fatigue life is dependent on sampling position of turbine blade under same test conditions. Fracture surface morphology and longitudinal profile microstructure indicated that the fracture mechanism transformed from cleavage fracture to ductile fracture with the changing of medium temperature to high temperature due to the particle cutting at yield stress intensity. The scanning electron microscopy observation of original material demonstrated that the smaller precipitate size of samples have a shorter fatigue life. Meanwhile, the constitutive model considering size effect was built based on the crystal plastic theory. The finite element analysis demonstrated that the smaller precipitate size could dramatically reduce the plastic deformation suffering the same cycle loading.     

Fatigue life prediction of corrosion-damaged high-strength steel using an equivalent stress riser (ESR) model. Part II: Model development and results

The fatigue life of metallic aircraft structural components can be significantly reduced by environmentally induced corrosion. However, there have historically been no analytical methods to quantify the specific fatigue life reduction of individual unfailed corroded components with any reasonable degree of confidence. As part of a NAVAIR high-strength steel corrosion–fatigue assessment program, methods were studied to predict the impact that corrosion-induced surface roughness has on the fatigue life of high-strength steel aircraft components. The steels of interest produce general corrosion in patches as well as localized material loss similar to pitting. In addition, this type of corrosion has characteristic features over a wide range of scales. Consequently, traditional finite element analysis approaches are not well suited to this problem, since the mesh required to accurately reflect the fine details distributed over the entire corrosion patch make computation unrealistic. Therefore, approximate methods were developed that allow localized regions of interest of high stress to be identified. Subsequently, a simple notch metric formula is employed to approximate the stress riser in these regions of interest. Finally, an extension of Peterson’s fatigue notch sensitivity theory is applied to these small “notches” that has the result of suppressing the effect of smaller notches compared to larger notches in the prediction of life. Each region of interest is assigned a probability of crack initiation as a function of fatigue cycles, based on a probabilistic strain–life analysis using the predicted notch factor. The net life (to crack initiation) for the component is then the product of the survivabilities of all of the individual regions of interest on the component surface. Tests on corroded fatigue specimens have been conducted to both calibrate the parameters in the Peterson model as well as to test the life prediction capability of the approach. Predictions from the resulting model have demonstrated that an empirical approach to corrosion surface damage can be utilized to generate probabilistic life predictions that have substantial engineering value in assessing the residual fatigue life of corroded AF1410 steel components, and that the modeling technique can capture the significant corrosion features that cause fatigue cracking in most cases, especially for more severely corroded surfaces.