Fretting fatigue behavior of Al7075-T6 at sub-zero temperature

In some fretting fatigue applications, such as aero industry, the temperature may drop well below −50 °C Fretting fatigue behavior of aluminum alloy Al7075-T6 is investigated at temperatures of 24, 0, −25 and −50 °C in this work. The results show that (i) normal fatigue life increases considerably at sub-zero temperatures up to around 85% for low working stresses and reduces to about 40% for higher working stresses; (ii) fretting fatigue life at sub-zero temperatures rises significantly up to around 220% for low working stresses and reduces to about 50% for higher working stresses; (iii) ultimate strength of material changes from −15% to 15% under the fretting fatigue test conditions; and finally (iv) some parameters such as mechanical properties and fatigue behavior of material at low temperatures, contact load relaxation, crack closure, oxidation and some unknown sources can be thought to be responsible for fretting fatigue behavior of Al7075-T6 at sub-zero temperatures.     

Numerical Investigation into the Effect of Contact Geometry on Fretting Fatigue Crack Propagation Lifetime

Fretting fatigue is a phenomenon in which two contact surfaces undergo a small relative oscillatory motion due to cyclic loading. There is a need to analyze the effects of contact geometry on crack propagation under fretting fatigue conditions. In this investigation, a finite element modeling method was used to study the effects of different contact geometries along with crack–contact interaction on crack propagation lifetime. Different contacts geometries—that is, cylindrical on flat and flat on flat—along with different contact span widths were analyzed. In addition, the effects of different contact spans on stress distribution at the contact interface were investigated. The computed crack propagation life was compared with experimental results. It was found that the crack initiated near the contact trailing edge for all contact geometries, which agreed with experimental observations. In terms of crack propagation for different contact spans, the fretting fatigue life for a two-based cylindrical pad was shorter than that for a two-based flat pad. By increasing the contact span width for both flat and cylindrical pads, the crack propagation lifetime increased. A comparison between the experimental and numerical results demonstrated a difference of about 18% in crack propagation lifetime.     

Modification of fretting fatigue behavior of AL7075-T6 alloy by the application of titanium coating using IBED technique and shot peening

In this work, improvement in fretting fatigue life of AL7075-T6 has been investigated by titanium surface coating using ion-beam-enhanced deposition (IBED) technique and shot peening. From the experiments, the following conclusions were derived: (i) Shot peening increased the fretting fatigue life up to 350%. (ii) Titanium coating increased the fatigue life up to 100% with respect to virgin specimens for low working stresses, while it reduced the fatigue life at higher working stresses significantly. (iii) Titanium coating+shot peening increased the fatigue life up to 130% with respect to the virgin specimens for low working stresses, while it reduced the fatigue life at higher working stresses significantly. The highest and the lowest increase in coefficient of friction are obtained for virgin and shot-peened+titanium-coated specimens, respectively. IBED surface-modification technique is not successful in reducing fretting fatigue, except at low stresses.     

Characterisation of fretting fatigue damage using synchrotron X-ray micro-tomography

The fretting fatigue damage of mechanical joints is studied experimentally by considering the following scenario: first, the crack nucleation in the joint is investigated via fretting tests. Second, the propagation of the fretting cracks under cyclic uniaxial tensile loading is carried out and analysed. Synchrotron radiation X-ray micro-tomography is used to image for the very first time the three-dimensional shape of the initial fretting cracks as well as that of the propagating cracks at different stages of the fatigue life, thanks to a dedicated set-up. This technique brings new experimental data on the influence of the local microstructure on fretting fatigue damage, initiation and growth.     

A fuzzy logic predictive model for better surface roughness of Ti–TiN coating on AL7075-T6 alloy for longer fretting fatigue life

In this study, the fretting fatigue resistance of AL7075-T6 alloy is investigated using surface treatment Ti–TiN multilayer coating by physical vapor deposition (PVD) magnetron sputtering technique. A fuzzy logic model was established to forecast the surface roughness of Ti–TiN coating on AL7075-T6 with respect to changes in the input process parameters of DC power, temperature, DC bias voltage, and nitrogen flow rate. The results indicate an agreement between the fuzzy model and experimental results with 95.349% accuracy. The fretting fatigue lives of Ti–TiN-coated specimens with the lowest surface roughness resulting from fuzzy logic were enhanced by 18% at low cyclic fatigue, while at high cyclic fatigue the result was reversed.     

Propagation in fretting fatigue from a surface defect

This paper analyses the growth of cracks in fretting fatigue from an initial flaw at the surface. Different crack growth laws are used in order to take into consideration the particular behaviour of short cracks. This methodology is applied to estimate life in various fretting fatigue tests with spherical contact characterized by two different geometries. The material used in the experiments is Al7075. The two geometries present significant differences in the evolution of the stresses, crack growth, etc. which are discussed. The approaches used to model short crack growth give different results, some of them being in good agreement with the experiments.     

Effects of humidity and contact material on fretting fatigue behavior of an extruded AZ61 magnesium alloy

Fretting fatigue tests of the extruded AZ61 magnesium alloy with the same contact material under low and high humidity were carried out to investigate basic fretting fatigue characteristics and effect of humidity on fretting fatigue behavior. Influence of contact material was also studied by using JIS S45C carbon steel contact material. Degradation of fatigue strength due to fretting was much more significant than that due to corrosion under high humidity condition. Therefore, no effect of humidity on fretting fatigue strength was found. Reduction rate of fatigue strength due to fretting for the magnesium alloy was between those of aluminum alloys and titanium alloys. Tangential force coefficient of the magnesium alloy was rather low compared to other materials such as steels, aluminum alloys and titanium alloys. Fretting fatigue strength with the S45C contact material was inferior compared to that with the same contact material. This is mainly due to higher tangential force in AZ61/S45C contact. Fretting fatigue cracks at the edge of fretting contact region were observed to nucleate in the very early stage of fatigue life, similar to other structural materials.     

紧固孔的加工工艺对其表面硬化的影响

研究 5种制孔工艺对高强度材料 70 75铝合金飞机装配紧固孔表面加工硬化的影响 ,分析加工硬化的主要影响因素 ,并提出降低加工硬化的措施     

Rolling contact fatigue in lubricated contacts

This work presents and discusses the results of rolling contact fatigue tests (IP-300) with six different lubricants using a four-ball E.P. lubricant tester. These results tended to confirm the mechanism for rolling fatigue proposed by Jin and Kang. The lubricants tested were two mineral oils (SN 350 and SN 600) and four synthetic oils (PAG-9, PAG-12, PAO 6, and PAO X), the test machine used was a Four-Ball E.P., and the IP 300/87 standard was applied. The results indicated that: 1) lubrication has an important influence on the rolling fatigue life of mechanical components; 2) normally, in oils of the same family, the higher the viscosity is, the higher the rolling fatigue life is; 3) besides viscosity, other lubricant properties such as the pressure–viscosity coefficient (α), compressibility (B), and the EHL friction coefficient (γ), should all be taken into account for lubricated contact design.     

Coatings on Ti6Al4V as palliatives for fretting fatigue in cryogenic environment

Fretting, also known as small oscillatory sliding motion, can lead to catastrophic failure in industrial applications. To reduce the damage caused by fretting increasing use has been made of surfaces treatments. These treatments result in multilayer solids (coating, diffusion layer…). The understandings of fretting fatigue have enabled us to evaluate the fretting resistance of homogeneous and coated materials. The experimental part is associated to a numerical one to obtain cracking threshold of the coated materials to obtain lifetime evaluation. This present study seeks to compare the behaviour of bare substrate and coated substrate submitted to fretting and evaluates the improvement of the fretting fatigue strength.     

A New Investigation on the Effect of Re-shot Peening on Fretting Fatigue Behavior of A17075-T6

Fretting fatigue life of materials can be improved by surface treatment such as shot peening. In this investigation, the effect of multiple re-shot peening on the fretting fatigue behavior of A17075-T6 is studied. After each re-shot peening, the specimen was subjected to 60% of its expected fretting fatigue life. The process of re-shot peening continued until the effect of any further re-shot peening became insignificant. The results showed an increase of 60–70% for the first re-shot peening depending on stress level. The increase, however, was sharply reduced for the next re-shot peenings such that for the third re-shot peening the increase dropped below 10%, which was not as significant. On the whole, the fretting fatigue life increased by 390–410% with respect to the life of virgin specimens depending on the stress level. The results indicated that fretting fatigue life improvement using the 60% of prior life consumption was considerably lower than that obtained for the 80% of expected life as used in previous investigations. An artificial neural network can be employed for estimation of fretting fatigue life at the stress levels not considered in the investigation.     

Radial fretting behaviours of dental ceramics

Radial fretting tests on a Si₃N₄ ceramics ball opposite to the two dental ceramics flats (Vita VMK95 and Cerec Vita Mark II) have been carried out. The test apparatus was developed from a tension-compression hydraulic machine. Maximum normal load (F_max) was varied from 100 to 800 N with a speed of 6 mm/min, and the number of cycles from 1 to 10⁵. The fretting scars were examined by optical microscope and laser confocal scanning microscope (LCSM). The results of kinetics behaviours showed that all loading and unloading curves of load (F)–displacement (D) curves were almost superposed in the whole fretting process for two dental ceramics under a lower normal load (), and all F–D curves opened, correspondingly some micro-cracks initiated and developed in contact area, when the normal load increased to a higher level (). Under lower normal load, the fretting scars displayed the worn zones in shape of annulus. The cracks in shape of homocentric circularity can be observed for Vita VMK95 at and for Cerec Vita Mark II at . Therefore, there is better radial fretting damage resistance for Cerec Vita Mark II. In the microslip zone, the microscopic analysis showed that the particles detached by the mechanism of delamination. It the meantime, the competing mechanisms of fatigue cracks and wear also were discussed in this paper.     

Identification of fretting fatigue crack propagation mechanisms using acoustic emission

In this study, a fretting fatigue test has been equipped with an acoustic emission (AE) device in order to identify the successive crack propagation mechanisms. The fretting fatigue crack nucleation and propagation is a complicated process. Cracks initiate and propagate firstly due to shearing (mode II) and then by tension (mode I). The crack propagation generates mechanical energy emission. Elastic waves appear and can be detected through AE. A complete analysis of the AE signals (multi-parameter analysis, location of the AE in the loading cycle and a statistical analysis) led to an identification of three different steps in the crack propagation process. The evolution of the shearing and the tension influences in the crack propagation process is recognizable separately. Therefore, the three crack propagation steps have been identified as (a) crack propagation in mode II, (b) mixed mode crack propagation and (c) pure mode I crack propagation.     

Fracture mechanics approach to fretting fatigue and problems to be solved

A large number of research works have been devoted to fretting fatigue from both mechanical and metallurgical viewpoints. In the present paper, fracture mechanical approaches for evaluating fretting fatigue life and strength have been briefly reviewed. Furthermore, a new approach based on a singular stress field near the contact edge and on fracture mechanics has been proposed. The directions of crack initiation and propagation as well as fretting fatigue life, which have coincided with the experimental results, could be estimated according to the new approach, in which singular stress near the contact edge and mixed mode crack growth have been taken into consideration. In the application of the new method to predict the fretting fatigue behavior, there are still several problems to be clarified, which have also been discussed in detail.     

Improvement of the fretting damage resistance of Ti-811 alloy by Cu/Ni multilayer films

The Cu/Ni multilayer films were prepared on the titanium alloy surface by ion-assisted magnetron sputtering deposition (IAD) technique. The Cu/Ni multilayer films could significantly improve the resistance of fretting wear and fretting fatigue (FF) of Ti-811 alloy at room temperature. The FF resistance of the titanium alloy substrate did not increase monotonically with increase in the modulation period thickness of the Cu/Ni multilayer films. The Cu/Ni multilayer films with modulation period thickness of 200 nm had the highest FF resistance among the prepared Cu/Ni multilayer films for its comprehensive properties with high toughness, high strength and good lubricating action.     

Modelling and evaluation of the fretting fatigue cracking risk in smooth spherical contacts

A numerical model for the calculation of fretting fatigue crack initiation is presented and compared with experiments. The model is focused on smooth sphere-on-plane contact in partial and gross slip conditions. It is based on Hamilton’s explicit stress equations and the multi-axial Dang Van and Findley fatigue criteria enhanced with a statistical size factor concept. Promising correlation was found between the model and the experimental results with quenched and tempered steel 34CrNiMo6. The model assumptions, limitations and general application are also discussed.     

Optimization of end mill tool geometry parameters for Al7075-T6 machining operations based on vibration amplitude by response surface methodology

This paper aims at developing a statistical model to envisage vibration amplitude in terms of geometrical parameters such as radial rake angle, nose radius of cutting tool and machining parameters such as cutting speed, cutting feed and axial depth of cut. Experiments were conducted through response surface methodology experimental design. The material chosen is Aluminum (Al 7075-T6) and the tool used was high speed steel end mill cutter with different tool geometry. Two channels piezoelectric accelerometers were used to measure the vibration amplitude. The second order mathematical model in terms of machining parameters was built up to predict the vibration amplitude and ANOVA was used to verify the competency of the model. Further investigation on the direct and interactive effect of the process parameter with vibration amplitude was carried out for the selection of process parameter so that the vibration amplitude was maintained at the minimum which ensures the stability of end milling process. The optimum values obtained from end milling process are Radial rake angle-12°, Nose radius-0.8 mm, Cutting speed-115 m/min, Cutting feed rate-0.04 mm/tooth, axial depth of cut-2.5 mm. The vibration amplitude exhibited negative relationship with radial rake angle and nose radius. The dominant factors on the vibration amplitude are feed rate and depth of cut. Thus it is envisaged that the predictive models in this study could produce values of the vibration amplitude close to the experimental readings with a 95% confidence interval.     

应力与腐蚀因素对连接件腐蚀疲劳寿命交互作用的研究

在应力控制下分别研究了不同应力,预腐蚀时间对3．5％NaCl溶液中有涂层的LY12CZ铝合金铆接连接件的腐蚀疲劳性能影响的规律。试样结构是根据飞机蒙皮的真实结构设计制成的单剪切连接件,按实际工艺过程进行表面处理。试验结果表明,提高应力和延长预腐蚀时间均会缩短连接件的腐蚀疲劳寿命。应力与环境腐蚀因素对连接件的腐蚀疲劳寿命的影响作用不是简单的叠加关系,而是相互影响的,结果还表明,在无预腐蚀的条件下有涂层连接件在pH值为6的NaCl溶液中的腐蚀疲劳寿命比空气中的寿命要高,并且在高低应力范围内均有此现象,在30MPa下预腐蚀对连接件的疲劳寿命的影响作用比50MPa下预腐蚀对连接件疲劳寿命的作用要明显。     

7075-T6铝合金的放电等离子烧结连接工艺优化

采用放电等离子烧结技术对7075-T6铝合金进行连接,通过单因素法研究了母材表面预处理（400^#砂纸打磨、化学浸蚀、400^#砂纸打磨+化学浸蚀、2000^#砂纸打磨+化学浸蚀）以及升温速率（10,30,50 ℃·min⁻¹）、连接温度（450,470,490,510 ℃）、连接压力（4.5,5.0,5.5,6.0,7.0 MPa）和保温时间（45,60,75,90,120 min）对接头连接质量、剪切性能和变薄率的影响,确定了合适的表面预处理工艺以及最优的工艺参数。结果表明：合适的表面预处理工艺为400^#砂纸打磨,获得7075-T6铝合金接头的连接质量最好,接头界面未闭合孔洞很少,焊合率最大,为71.7%;随着升温速率的升高,接头的连接质量变差,但其焊合率均高于76%,接头的抗剪强度和变薄率均降低;随着连接温度的升高或连接压力的增加,接头的连接质量变好,接头的焊合率、抗剪强度和变薄率均增大,但当连接压力大于6.0 MPa时,焊合率和抗剪强度增加幅度减小,而变薄率大幅增加;随着保温时间的延长,接头的焊合率、抗剪强度和变薄率的变化幅度均较小。400^#砂纸打磨后7075-T6铝合金的最优连接工艺为升温速率50 ℃·min⁻¹、连接压力6.0 MPa、连接温度490 ℃、保温时间45 min,此时接头的焊合率为81.3%,抗剪强度为132.9 MPa,变薄率为1.91%,剪切断裂方式为韧性断裂。