The effect of changing loads affecting the martensite steel on its structure and the Barkhausen noise level

The intended aim of the paper is to demonstrate how changes in the Barkhausen noise (BN) intensity depend on changes in the microstructure of rotor blades of an axial compressor, ones made of the martensite steel exposed to changing loads. What has been explained is the non-linear dependence of the BN on the number of steel-deformation cycles applied on a test stand under service-loading conditions. The modification of the dislocation structure emerging in the course of steel fatigue has been presented. It has also been shown that any change in the dislocation distribution and the accompanying increase in the misorientation of sub-grains as well as the modification of retained austenite precipitates both considerably affect the BN intensity. The dependence of the BN on the blade's operation time has been compared with that of the BN on the number of service-loading cycles. It has been found that results of both the stand tests and metallographic examination can be used to determine the level of blade material fatigue in the course of operational use thereof.     

The evaluation of surface residual stress in aeronautic bearings using the Barkhausen noise effect

Bearings in aeronautic engines are subject to heavy mechanical demands. The bearing raceways withstand levels of mechanical stress capable of causing metal fatigue that can lead to bearing malfunction, which in turn may cause engine failure mid-flight. For this reason, regular verifications of engine bearings to gauge the degree of metal fatigue are essential. Such verifications require knowledge of the pre-stress state of the bearing raceways through use of surface residual stress (SRS) estimates. In this paper, we present a non-destructive method for estimating SRS, based on the Barkhausen noise (BN) effect. This method was validated on several different batches of bearings. Our investigations have shown this method to be rapid, well suited to industrial imperatives connected to on-line measurement and easily adapted to the circular geometries of the bearings rings. In addition, we have shown the efficiency of the BN effect for estimating the SRS of bearing raceways after engine operation, in order to perform necessary bearing maintenance.     

Application of a novel type Barkhausen noise sensor to continuous fatigue monitoring

A novel Barkhausen noise (BN) sensor with no need for external magnetization was tested and applied to continuous fatigue monitoring of mild steel and high strength steel specimens. This new type of sensor indicated an increase in the BN rms value under maximum tensile stress during one cycle in cyclic bending tests with increasing stress amplitude. The BN rms value under maximum compressive stress stayed, however, approximately constant. The reason for this behaviour was the stress-induced anisotropy of the BN. Bending fatigue experiments with constant stress amplitude and R=−1 were conducted at different stress levels. In addition to the BN also the acoustic emission of the specimen was measured. In the mild steel specimens the BN amplitude stayed constant after the initial saturation period, but just prior to the failure of the specimen the amplitude increased meaningfully. This increase occurred at the same time as the increase in the acoustic emission signal indicating the beginning of crack initiation and growth. In the high strength steel specimens the BN amplitude decreased after the initial saturation period. The increase of the BN signal started well before the failure of the specimen and even before the increase in the acoustic emission signal.     

10CrNi3MoV钢低匹配对接接头的拉伸疲劳性能

通过余高削平的10CrNi3MoV钢低匹配对接接头的拉伸和脉动拉伸疲劳试验,研究了匹配比对高强钢对接接头力学性能的影响,给出了匹配比与削平对接接头疲劳强度及寿命的关系方程.结果表明,随匹配比降低,削平对接接头抗拉强度、断后伸长率和疲劳强度降低,但匹配比对接头抗拉强度影响较小,对断后伸长率和疲劳强度影响较大;由于焊缝冶金强化和力学强化效应的作用,接头抗拉强度和疲劳强度远高于焊缝熔敷金属本身.匹配比过低的焊接结构缺少满足需要的塑性储备,以接头断裂模式为标准,进行了匹配比下限值的初步界定.     

延长辊锻热成形模寿命的综合技术措施

辊锻模具在工作时不仅受到高的弯曲及挤压应力的作用,而且还受到重复剧烈的急热急冷的作用,往往呈现有机械疲劳断裂和脆性断裂、磨损、塌陷等多种失效形式。因工作条件的不同,辊锻模寿命相差也极为悬殊。本文介绍了提高辊锻热成形模寿命的综合技术措施。     

Fatigue Life Prediction of Metallic Materials Based on the Combined Nonlinear Ultrasonic Parameter

The fatigue life prediction of metallic materials is always a tough problem that needs to be solved in the mechanical engineering field because it is very important for the secure service of mechanical components. In this paper, a combined nonlinear ultrasonic parameter based on the collinear wave mixing technique is applied for fatigue life prediction of a metallic material. Sweep experiments are first conducted to explore the influence of driving frequency on the interaction of two driving signals and the fatigue damage of specimens, and the amplitudes of sidebands at the difference frequency and sum frequency are tracked when the driving frequency changes. Then, collinear wave mixing tests are carried out on a pair of cylindrically notched specimens with different fatigue damage to explore the relationship between the fatigue damage and the relative nonlinear parameters. The experimental results show when the fatigue degree is below 65% the relative nonlinear parameter increases quickly, and the growth rate is approximately 130%. If the fatigue degree is above 65%, the increase in the relative nonlinear parameter is slow, which has a close relationship with the microstructure evolution of specimens. A combined nonlinear ultrasonic parameter is proposed to highlight the relationship of the relative nonlinear parameter and fatigue degree of specimens; the fatigue life prediction model is built based on the relationship, and the prediction error is below 3%, which is below the prediction error based on the relative nonlinear parameters at the difference and sum frequencies. Therefore, the combined nonlinear ultrasonic parameter using the collinear wave mixing method can effectively estimate the fatigue degree of specimens, which provides a fast and convenient method for fatigue life prediction.     

Temperature evolution and fatigue life evaluation of AZ31B magnesium alloy based on infrared thermography

The surface temperature of extruded AZ31B alloy plate was measured by infrared thermograph in air during tension and high-cycle fatigue tests. The mechanism of heat production was discussed and the value of critical fatigue damage temperature was calculated according to the P—ΔT curve. Results show that the variation trend of temperature is different between tension and fatigue tests. The temperature evolution in tension test consists of four stages: linear decrease, reverse linear increase, abrupt increase, and final drop. The initial decrease of temperature is caused by thermal elastic effect, which is corresponding to the elastic deformation in tension progress. When cyclic loading is above the fatigue limit, the temperature evolution mainly undergoes five stages: initial increase, steep reduction, steady state, abrupt increase, and final drop. The peak temperature in fatigue test is caused by strain hardening that can be used to evaluate the fatigue life of magnesium alloy. The critical temperature variation that causes the fatigue failure is 3.63 K. When ΔT≤3.63 K, the material is safe under cyclic loading. When ΔT>3.63 K, the fatigue life is determined by cycle index and peak temperature.     

新型22Cr-15Ni奥氏体耐热钢高温低周疲劳行为

22Cr15Ni3.5CuNbN新型奥氏体耐热钢是为620 ~ 650 ℃的超(超)临界电站锅炉管道制造而研发的新型奥氏体耐热钢,其高温性能的优劣对机组的安全可靠运行具有重要意义. 文中通过22Cr15Ni3.5CuNbN钢在650 ℃下的低周疲劳试验,研究了其在不同应变幅条件下的应力?应变关系及疲劳寿命. 通过对断口形貌的分析研究了其断裂机理. 结果表明,22Cr15Ni3.5CuNbN钢在高温下表现出明显的循环硬化行为,且没有明显的应力饱和现象出现. 其硬化行为与材料内部位错密度的增加有关. 采用基于塑性应变能密度对其疲劳寿命进行了预测,取得了良好的预测效果. 疲劳断口可以分为3个区域:裂纹源区、裂纹扩展区以及瞬断区. 在较高的应变幅条件下,在断口处可观察到多个裂纹源. 多个裂纹源的形成和二次裂纹的产生是导致其疲劳寿命下降的重要原因.     

EFFECT OF Mg AND δ-PHASE ON MECHANICAL PROPERTIES OF ALLOY GH169(INCONEL718)

Micro-alloying of Mg,ranged from 30 to 70 ppm,may improve the high temperatureductility,stress rupture,especially,certain important mechanical properties,such as cyclicstress rupture,creep/fatigue interaction and crack propagation rate.The quantity,morphology and distribution of δ-phase may obviously influence the notched cyclic stress rup-ture life.The relevant microstructure shows that the Mg may also influence to certain extenton the feature of δ-phase.Thus,to keep the Mg and δ-phase under control may further im-prove the mechanical properties of alloy GH169.     

航空铝合金疲劳参数的理论估算方法分析

基于12种航空铝合金的力学性能及疲劳试验结果,通过绘制疲劳寿命图的方法,将预测寿命与试验寿命进行对比,得到5种常用疲劳参数估算方法预测疲劳寿命的精度.结果表明:5种方法中,四点关联法和修正的四点关联法在中、低周疲劳范围内寿命预测结果比较理想,而在高周疲劳范围内各方法的寿命预测结果均不理想;硬度法不适用于航空铝合金的疲劳...     

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对断裂失效的螺栓件进行宏观分析、化学成分分析、力学性能试验、断口分析、金相分析和硬度测试,结果表明,该螺栓属于起裂螺纹根部表面的单向弯曲疲劳断裂,断裂螺栓起裂区断口为穿晶和少量沿晶混合的疲劳断口,扩展区断口为穿晶疲劳断口,终断区很窄,为韧窝断口,说明螺栓起裂过程除受到弯曲疲劳应力外,还受到氢的作用,螺栓终断时弯曲应力并不大。螺栓镀锌后残留氢偏高和工作中存在振动是导致螺栓发生疲劳断裂的主要原因。     

Damage evolution in compacted graphite iron during thermomechanical fatigue testing

Thermomechanical fatigue properties of a compacted graphite iron in an out of phase configuration are investigated for different maximum temperatures and mechanical strain ranges. Furthermore, the stress–strain hysteresis loops are analysed, and, in particular, the unloading modulus, i.e. the elastic modulus measured during specimen unloading, is obtained from each cycle. This material parameter has earlier been explicitly related to the amount of microcracking in cast irons. The results show that the unloading modulus linearly declines with the numbers of cycles in all tests performed. In addition, the rate of change of the unloading modulus is closely related to the number of cycles to failure. Accordingly, it is concluded that microcracks are independently propagated by fatigue until a point of rapid crack linking resulting in ultimate failure. This is supported by microstructural analyses consisting of optical microscope images taken at different stages throughout the life of a specimen.     

Mechanical deterioration of polyethylene greenhouses covering under arid conditions

One of the most common greenhouse covering materials in Saudi Arabia is polyethylene film. However, polyethylene films are susceptible to mechanical failure due to harsh conditions of high temperature, solar radiation, and wind as occurs in Riyadh, Saudi Arabia. This study examined effects of ambient conditions on the deterioration of mechanical properties of polyethylene films over 14 months, using an experimentally cooled empty greenhouse (3.6 m length, 2.4 m width, and 3.6 m height) covered with a single layer of 200-μm thick polyethylene.Three mechanical tests were conducted on the polyethylene samples: penetration, shear, and tension utilizing a SMS texture analyzer. The force–distance curves produced were characterized by two stages, the elastic and plastic regions. Mechanical properties were determined, including modulus of elasticity, rupture point, and total work for each stage of the three tests. Generally, mechanical resistance of the samples decreased with increased exposure time. The results of the tension tests were preferable to penetration and shear tests. For tension tests, work decreased from 21,693 N mm for new samples, to 6658 N mm after 14 months. Based on elongation at break data, the shelf life of polyethylene covers was 12 months under the tested environmental conditions. This illustrates the effect of arid conditions and age on the mechanical deterioration of polyethylene films. The presented data can be utilized to predict the deformation and mechanical behavior of greenhouse polyethylene covers at different exposure times under arid conditions.     

低温卷取热轧双相钢的显微组织及疲劳性能

采用扫描电镜、拉-拉疲劳试验机等研究了低温卷取热轧双相钢的显微组织及疲劳性能.结果表明:热轧双相试验钢的疲劳极限约为530 MPa;低温卷取工艺生产的热轧双相试验钢夹杂物平均尺寸多在5 μm以下,晶粒比较细小,马氏体组织较细小且弥散均匀分布,具有良好的综合力学性能.热轧双相试验钢疲劳裂纹源位于样品表面的棱角处,疲劳裂纹...     

ZG35CrMoRE曲轴断裂原因分析

材质为ZG35CrMoRE的空心曲轴铸件,在装配使用1年后发生断裂,为了分析造成断裂的原因,进行了低倍、高倍、断口等一系列理化试验。结果表明,曲轴断裂的原因是该空心曲轴的轴承部位即轴颈的堆焊和局部二次补焊造成的初始焊接裂纹,在偏心载荷低周高应力作用下引发为疲劳断裂。     

变速器机械封闭试验台自动化改造设计

变速器机械封闭疲劳寿命试验台由于结构简单,能耗小,成本低,因此应用广泛。随着企业人工成本的上升,对疲劳寿命试验系统提出了自动化需求。以现有的机械封闭式变速器疲劳寿命试验台为基础进行自动化改造,包括变频电气系统、液压加载系统、测控系统等,并设计开发了一套自动化程度较高的测控软件,可实现变速器试验过程中的试验流程设置、转速转矩自动控制、自动启停、自动采集存储试验数据等功能。疲劳寿命试验结果表明,该系统能具有较高的测控精度及自动化水平,具有广阔的应用前景。     

Application of the wavelet transform in mechanical spectroscopy and in Barkhausen noise analysis

In this work, the wavelet transform and the Fast Fourier Transform FFT are introduced to analyze the quality of the strain response signal in order to improve the accuracy of computation of both the logarithmic decrement and the mechanical loss angle. The wavelet transform of the strain response signal yields a three-dimensional representation (the time-scale joint representation), i.e. space (time), frequency (scale) and amplitude. To emphasize that not only time but also frequency content of the strain response signal collected from a mechanical spectrometer is identified, the author suggests that the time-scale joint representation of the strain response signal should be called ‘Identified Strain Response Signal — ISRS’. The wavelet transform was shown to be an excellent tool to test the quality of the strain and/or of the stress signal in a mechanical spectrometer working in a resonant or subresonant mode. A new approach to the non-stationary Barkhausen noise (BN) signal based on the wavelet transform is also presented. The BN level is usually expressed with the magnetoelastic parameter MP which is a relative number proportional to the root mean square level of the BN. It was found that the MP value is not a reliable parameter for the identification of stress concentrations in ferromagnetic materials. Indeed, a stress concentration resulting from internal stresses can be successfully revealed by the time-scale joint representation of the BN signal. The author suggests that the wavelet transform of the non-stationary RN signal should be called the ‘Identified Barkhausen Noise — IBN’. This approach produces a physically reliable relationship between the IBN and either the level of internal stresses or fine variations in the microstructure.     

Fatigue-life prediction of SiC aluminum composite using a Weibull model

The feasibility of the acoustic emission technique in predicting the residual fatigue life of 6061-T6 aluminum matrix composite reinforced with 15 vol.% SiC particulates (SiC_p) is presented. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were induced at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. The number of cumulative AE events increased exponentially with the increase in strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion. Cumulative events during post fatigue tensile tests reduced with a decrease in the residual fatigue life. Based on the high cycle fatigue damage accumulation model, a Weibull probability distribution model is developed to explain the post fatigue AE activity of specimens during tensile tests. Using the model, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change significantly with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.     

Grain size based low cycle fatigue life prediction model for nickel-based superalloy

Nickel-based superalloys are easy to produce low cycle fatigue (LCF) damage when they are subjected to high temperature and mechanical stresses. Fatigue life prediction of nickel-based superalloys is of great importance for their reliable practical application. To investigate the effects of total strain and grain size on LCF behavior, the high temperature LCF tests were carried out for a nickel-based superalloy. The results show that the fatigue lives decreased with the increase of strain amplitude and grain size. A new LCF life prediction model was established considering the effect of grain size on fatigue life. Error analyses indicate that the prediction accuracy of the new LCF life model is higher than those of Manson-Coffin relationship and Ostergren energy method.     

金属材料大气环境静态腐蚀老化的疲劳特性

对ＬＣ４ＣＳＡｌ合金和３０ＣｒＭｎＳｉＮｉ２Ａ钢的疲劳件在大气环境预腐蚀其疲劳寿命性能。检验了疲劳寿命的分布特性。分析了材料在大气环境预腐蚀后疲劳寿命分布的数字特征,寿命分散系数,疲劳Ｓ－Ｎ曲线等的变化。以及冷挤压强化 疲劳增寿效应与大气环境腐蚀日历时间的关系。