Schwingungsverschleißfestigkeit von plasmanitriertem, 34 CrAlMo 5

Fretting Fatigue Strength of Plasmanitrided 34 CrAlMo 5 Fretting fatigue load here means oscillating sliding movements of very small amplitude (1–3 μm) and synchronous fatigue load. Characteristic of this is the appearance of frettings. These surface damages lead to the formation of microcracks, which spread and finally lead to the premature fatigue failure of the component. To simulate such processes of damage, which are known of the bearing surfaces of roller bearing races, of turbine blades and of shaft-hub-combinations, a fretting bridge apparatus is used. The plasmanitriding and plasmanitrocarburizing of metals are thermochemical heat treatments, which change the microstructure of near to surface areas. By means of this the mechanical and technological properties of components, e.g. wear and corrosive resistance as well as fatigue strength, are improved. Dur to the advantage sof the process, plasmanitriding is already used in many regions of production. Investigating the fretting fatigue strength of plasmanitrided respectively plasmanitrocarburized 34 CrAlMo 5 shows, that these treatments improve the fatigue strength values to a level 14% higher, the fretting fatigue strength increases about 50%.     

微动疲劳中的应力状态参数和微动磨损参数的研究

本文对微动疲劳中的力学参数作出了研究。微动接触面上的力学参数可分为应力状态参数（SSP）和微动磨损参数（FWP）两类，并将应力状态参数综合为当量应力σ-1E,而将微动磨损参数用摩擦功W来表示．对桥式微动疲劳试件和燕尾型榫联接试件的数值分析表明，在微动接触面上疲劳断裂处的σ-1E和W值较大。因此，有可能使用了σ-1E和W值作为预测微动疲劳失效的两个基本参数。     

激光冲击强化技术的研究与应用

本文介绍了激光冲击强化技术这种新型的金属材料表面强化技术,该技术可大幅度提高金属材料的疲劳寿命,为提高传统材料的综合力学性能和服役行为开辟了新路;文章重点论述了该技术在美国的工程应用情况以及国内的研究进展,最后简述了我国首条激光冲击强化生产线相关情况。     

Influence of coatings and hot corrosion on the fatigue behaviour of nickel-based superalloys

The fatigue behaviour of materials and components is strongly affected by the surface conditions, i.e. the surface finish and the materials properties in the near- surface region. Therefore any change in these conditions during fabrication or service may change the fatigue properties.It is common practice to coat turbine blades with materials that are resistant to hot corrosion. However, this treatment involves chemical and mechanical changes in the materials surface properties which could also change the fatigue properties of these components. The recommended heat treatment for the coating normally differs from that which produces the best mechanical or creep properties of the base metal and hence may cause additional changes in the fatigue properties. Chemical and mechanical changes in the surface conditions of an uncoated component may also take place during operation. Additionally, a large amount of notching may occur since oxidation and hot corrosion are not homogeneously distributed but take place preferentially at sites such as grain boundaries.In this work we investigate the effect of Pt-Al coatings on the high cycle fatigue (HCF) behaviour of the cast nickel-based alloys IN 738LC and IN 939 which are commonly used in large industrial gas turbines. A reduction in the fatigue life due to the coating was observed. However, the simultaneous occurence of hot corrosion attack and cyclic loading was much more detrimental. Fractographic and metallographic investigations showed that, in the as-coated condition, the crack initiation sites in the Pt-Al-coated alloys were internal pores situated just below the surface of the substrate. After aging or hot corrosion cracks initiate at the surface probably as a result of notch development by the attack. When HCF and hot corrosion are acting concurrently the coatings are expected to give a beneficial effect by protecting the surface from accelerated crack initiation.     

An assessment of the role of near-threshold crack growth in high-cycle-fatigue life prediction of aerospace titanium alloys under turbine engine spectra

Increasingly accurate life prediction models are required to utilize the full capability of current and future advanced materials in gas turbine engines. Of particular recent interest are predictions of the lifetimes of engine airfoil materials that experience significant intervals of high-frequency, high-cycle fatigue (HCF). Conventional life management practices for HCF in the turbine engine industry have been based principally on a total-life approach. There is a growing need to develop damage tolerance methods capable of predicting the evolution and growth of HCF damage in the presence of foreign object damage (FOD), low cycle fatigue (LCF), and surface fretting fatigue. To help identify key aspects of the HCF life prediction problem for turbine engine components, a review is pressented of the extensive results of an Air Force research contract with Pratt & Whitney on the high strength titanium alloy Ti-8Al-1Mo-1V. Data from this representative turbine-airfoil material are used to examine the applicability of linear elastic fracture mechanics methods for prediction of service lifetimes under load spectra that include high cycle fatigue. The roles of fatigue crack initiation and growth are examined for materials that are nominally-defect-free, as well for materials that have experienced significant prior structural damage. An assessment is presented of the potential utility of the conventional threshold stress intensity factor range, ΔK _th, defined by testing specimens containing large cracks. Although the general utility of a large-crack-ΔK _th approach is questionable due to the potentially rapid growth of small fatigue cracks, the low allowable stresses involved in turbine engine high cycle fatigue appear to limit and simplify the small-crack problem. An examination is also presented of the potential effects of high-cycle fatigue and low-cycle fatigue (HCF/LCF) interactions.     

Root Cause Analysis of Cracking in Shaft End and Coupling of a High-Power Generator

Failure analyses and root cause determination were carried out on the rotor of a high-power generator of gas–diesel dual fuel which presented cracking due to torsional fatigue in its end (region of section change and coupling), after 30,000 h of service. The generator of 307 MW–3000 rpm has a rotor (shaft of 400 mm Ø) manufactured in a proprietary steel grade equivalent to ASTM A470 type, Class 7 of high hardenability. It was reported that the equipment control system showed, in service, a high level of vibrations, not admissible for continuing the operation. First, and during the equipment shutdown for inspection, the presence of cracks, slant to the rotor shaft, was detected by means of visual inspection and dye penetrant test. The failure region corresponds to the zone of coupling–shaft joint, linked by means of fixation by interference, whereas the cracking spread on two fracture planes at 45° with respect to the rotor shaft. On this zone, where cracking started, a severe fretting corrosion damage was evidenced. The characterization and identification of present damage mechanisms were conducted through macrographic, fractographic, SEM, EDS, chemical analyses, and mechanical tests. It was recognized that from the damage by fretting corrosion, fatigue micro-cracks were produced that spread due to service tensions by a mechanism of fretting fatigue. The fatigue fracture propagation was developed into two orthogonal planes at 45° from the longitudinal shaft, which reveals an inversion in the loading condition, only justifiable by torsional vibrations that were assigned to a torsional resonance typical of the system dynamics. It was considered that the torsional vibrations cause micro-movements between components, promoting fretting corrosion and the subsequent fretting fatigue that finally induced the failure by high-cycle torsional fatigue with low-stress amplitude.     

Failure of connecting pins of a compressor disc in an aero engine

This paper describes the failure analysis of pins connecting the second and third stage compressor discs in an aero-engine that had failed after 46,000 h of service. The investigation included metallography, microscopy (SEM), hardness and fractographic examination. Analysis revealed that failure was due to overloading by bending stresses and fretting owing to improper riveting of the pins. There was no indication of fatigue failure in these pins.     

The strengthening mechanism of a nickel-based alloy after laser shock processing at high temperatures

Abstract

We investigated the strengthening mechanism of laser shock processing (LSP) at high temperatures in the K417 nickel-based alloy. Using a laser-induced shock wave, residual compressive stresses and nanocrystals with a length of 30–200 nm and a thickness of 1 μm are produced on the surface of the nickel-based alloy K417. When the K417 alloy is subjected to heat treatment at 900 °C after LSP, most of the residual compressive stress relaxes while the microhardness retains good thermal stability; the nanocrystalline surface has not obviously grown after the 900 °C per 10 h heat treatment, which shows a comparatively good thermal stability. There are several reasons for the good thermal stability of the nanocrystalline surface, such as the low value of cold hardening of LSP, extreme high-density defects and the grain boundary pinning of an impure element. The results of the vibration fatigue experiments show that the fatigue strength of K417 alloy is enhanced and improved from 110 to 285 MPa after LSP. After the 900 °C per 10 h heat treatment, the fatigue strength is 225 MPa; the heat treatment has not significantly reduced the reinforcement effect. The feature of the LSP strengthening mechanism of nickel-based alloy at a high temperature is the co-working effect of the nanocrystalline surface and the residual compressive stress after thermal relaxation.     

A comparative study of the fretting fatigue behavior of 4340 steel and PH 13-8 Mo stainless steel

The fretting fatigue behavior of two high strength structural steels, PH 13-8 Mo stainless steel and quenched and tempered 4340 steel, is investigated. Both were heat treated to a similar hardness (43-44 HRC), comparable to the condition used in structural components. Both materials experienced significant reductions in fatigue strength due to fretting, with PH 13-8 Mo stainless steel exhibiting a greater susceptibility to fretting than 4340 steel, when operating in the mixed fretting regime. The use of fretting pads with different surface profiles showed that contact geometry did not significantly influence the fretting fatigue behavior of either steel for the range of loading conditions considered. The fretting fatigue lives are discussed in light of the low cycle fatigue and crack growth rate behavior of these steels. The life trends in fretting fatigue correlate more closely to the low cycle fatigue behavior.     

Shot peening of plasma nitrided steel for fretting fatigue strength enhancement

Abstract

The potential of fretting fatigue strength enhancement by a duplex surface engineering process involving shot peening of plasma nitrided steel, termed duplex SP/PN, is demonstrated. Specimens of 709M40 steel were individually plasma nitrided, shot peened, or duplex SP/PN treated. Fretting fatigue properties of the surface engineered specimens were evaluated. Surface roughness, residual stress, and hardening effect following the various surface treatments were examined and compared. It has been found that the duplex treatment can significantly improve the fretting fatigue strength of the investigated low alloy steel. Under the present testing conditions, the duplex SP/PN treatment increased the fretting fatigue strength (at 10⁷ cycles) of 709M40 steel by more than 70% relative to the nitrided, 120% to the shot peened, and 500% to the untreated steel. The improvement has been explained in terms of the significantly increased surface hardness and compressive residual stress in the near surface region following the duplex SP/PN treatment. By analysing the stress distributions in a shot peened surface, the influence of surface roughness on fretting fatigue strength is also discussed.     

Dislocation density-based modeling of subsurface grain refinement with laser-induced shock compression

Laser shock peening (LSP) is an innovative surface treatment technique applied to improve the mechanical properties and surface microstructures of metallic components. This paper is concerned with prediction of the microstructural evolution of metallic components subjected to single or multiple LSP impacts. A numerical framework is developed to model the evolution of dislocation density and dislocation cell size using a dislocation density-based material model. It is shown that the developed model captures the essential features of the material mechanical behaviors and predicts that the total dislocation density reaches the order of 10¹⁴ m^?2 and a minimum dislocation cell size is below 250 nm for LSP of monocrystalline coppers using the laser energy density on the order of 500 GW/cm². It is further shown that the model is cable of predicting the material strengthening mechanism in terms of residual stress and microhardness of the LY2 aluminum alloy due to grain refinement in a LSP process with less laser energy densities on the order of several GW/cm².     

The effect of laser power density on the fatigue life of laser-shock-peened 7050 aluminium alloy

Laser shock peening (LSP) is an innovative surface treatment method that can result in significant improvement in the fatigue life of many metallic components. The process produces very little or no surface profile modification while producing a considerably deeper compressive residual stress layer than traditional shot peening operations. The work discussed here was designed to: (a) quantify the fatigue life improvement achieved by LSP in a typical high strength aircraft aluminium alloy and (b) identify any technological risks associated with its use. It is shown that when LSP conditions are optimal for the material and specimen configuration, a —three to four times increase in fatigue life over the as-machined specimens could be achieved for a representative fighter aircraft loading spectrum when applied at a representative load level. However, if the process parameters are not optimal for the material investigated here, fatigue lives of LSP treated specimens may be reduced instead of increased due to the occurrence of internal cracking. This paper details the effect of laser power density on fatigue life of 7050-T7451 aluminium alloy by experimental and numerical analysis.     

椭圆形路径载荷下的微动疲劳失效特征

为了解微动疲劳失效机理,通过柱面对柱面的接触方式,研究了60Si2Mn钢在椭圆形路径、拉扭耦合作用下的多轴低周微动疲劳特性,深入分析讨论了不同轴向循环拉伸应力幅值对摩擦磨损表面和断口形貌的影响.结果认为：磨损区产生的氧化物磨屑对微动区磨擦损伤行为具有显著影响;微动摩擦磨损对试样表面的影响深度只有数十微米;微动疲劳裂纹源...     

Influence of pulse sequence and edge material effect on fatigue life of Al2024-T351 specimens treated by laser shock processing

Laser shock processing (LSP) is being considered as a competitive alternative technology to classical treatments for improving fatigue, corrosion cracking and wear resistance of metallic materials. The purpose of this paper is to present a fully 3D finite element model for predicting the residual stresses that result from the LSP of aluminum alloy Al2024-T351 samples of interest for aeronautic industry in order to optimize the laser treatment to increase the fatigue life of the material. In order to correlate the simulation results with experimental data, three different laser shock processing strategies (pulse sequences) were performed on fatigue specimens and their fatigue life were compared. The starting points of cracks were identified by means of optical and scanning electron microscope examinations and a correlation with the maximum tensile stress regions predicted by the numerical model has been established.     

Influence of contact pressure on fretting fatigue behaviour of AA 2014 alloy with dissimilar mating material

The effect of contact pressure on the fretting fatigue behaviour of 2014 Al alloy which has been solution heat treated and age hardened (T6 heat treatment) with dissimilar mating materials, was investigated. The fretting fatigue configuration involved bridge‐type contact pads on a flat fatigue specimen. Specimens were made of 2014 Al alloy and bridge‐type pads were made AISI 4140 steel. All the fatigue tests were conducted at a rotational speed of 5000 rpm with a rotating bending fatigue machine (R=?1), using S–N curves to evaluate the fatigue and fretting fatigue properties. The fretting fatigue strength of the material subject to a T6 heat treatment condition at 1 × 10⁷ cycles was dramatically reduced, as compared to that without fretting and with as‐cast. The fretting fatigue life exhibited a variable behaviour with an increase in the contact pressure. A scanning electron microscope was employed to observe the fretting scars and fracture surfaces of the specimens. This analysis showed that cracks originated at the contact surface and crack orientations were approximately ±56 ° from perpendicular to the loading direction.     

陶瓷基复合材料表面环境障涂层材料研究进展

随着新一代航空发动机推重比的不断提升,其内部燃气温度已经远远超过传统高温合金的承温极限,因此亟待研发新材料来满足先进航空发动机的发展需求.SiCf/SiC陶瓷基复合材料具有优异的高温力学性能和高温抗氧化性能,且其密度较低,是下一代高性能航空发动机热端部件的理想结构材料.但是,SiCf/SiC材料在实际服役环境中同样面临...     

A Critical Review of Laser Shock Peening of Aircraft Engine Components

Many aviation accidents are caused by the failure of aircraft engine components, and engine blades are especially vulnerable to high-cycle fatigue fracture in severe working environments as well as to impact damage caused by foreign objects. To address this problem, the United States took the lead and has been successful in implementing laser shock peening (LSP) as a surface treatment for aircraft engine components to enhance their fatigue performance. This review provides an overview of the development of LSP for use in treating aircraft engine components over the past three decades, with a brief introduction to the development of high-energy pulsed lasers for LSP. A particular focus of this review is on the limitations and challenges associated with the application of LSP for treating critical aircraft engine components. It is hoped that this review serves as a reference for future research and development that can lead to better performance of these components.     

Nondestructive Characterization of Fretting Fatigue Damage

Fretting fatigue has been the cause of many premature failures in aerospace components. There is a growing need of nondestructive evaluation techniques to characterize damage and detect cracks due to fretting fatigue. This paper presents a methodology to characterize the fretting fatigue damage by analyzing the surface topography and to detect cracks under fretting fatigued surface by imaging heat generation due to high amplitude acoustic excitation. The White Light Interference Microscopy (WLIM) was used to obtain three-dimensional surface profilometry data of fretted and non-fretted regions of titanium alloy (Ti-6A1-4V) specimens subjected to different number of fretting fatigue cycles. Surface topography measurements were analyzed in terms of the Power Spectral Density (PSD) and Fretting Fatigue Damage Parameter (FFDP). The FFDP showed an increasing trend in magnitude with increasing numbers of fretting fatigue cycles, when the fretting fatigue damage occurred through stick-slip condition. When the fretting fatigue damage occurred due to gross sliding, the FFDP did not show enough change. Thus, it appears that FFDP may be used as an indicator of the degradation of fretted surface under stick-slip condition. Cracks in presence of fretting fatigue damage were imaged using Sonic infrared technique. This technique appears to have a capability to detect cracks with a resolution of at least 200 m. The benefits and limitations of thes two NDE techniques for fretting fatigue damage evaluation and crack detection are discussed.     

Applications of crack analogy to fretting fatigue

In literature the most common approach to investigate fretting fatigue is based on contact mechanics. Crack initiation parameters of fretting fatigue are developed using elastic solution of two contacting bodies. Even though contact based parameters has been used extensively, they could not fully capture crack initiation mechanism due to the complexities of the fretting fatigue damage process, which depends on pad geometries, surface properties, material properties, and mechanical loading conditions. This has instigated fretting fatigue researcher to investigate other approaches. Recently, taking advantage of the similarities between contact mechanics and fracture mechanics lead to the development of crack analogy methodology (CAM), which defines the stress intensity factor as a fretting fatigue crack initiation parameter. CAM has shown a great potential investigating fretting fatigue. However, it has not been applied to wide range of fretting fatigue scenarios. The scope of this paper is not to focus on analytical development of CAM as much as validating its ability to analyze various fretting fatigue scenarios. Based on CAM, the present study introduces the crack analogy fretting parameter (CAF-parameter) to investigate crack initiation of fretting fatigue, which is equivalent to the change of mode II stress intensity factor at the contact surface, since the change in the stress intensity factor reflects the cyclic mechanism of fatigue. Further, a modification to the CAM is adopted to include various indenter-substrate geometries. Also, CAF-parameter-life curve, similar to the stress-life S-N curve, will be developed as a prediction tool to crack initiation for various geometric configurations using experimental data. This is consistent with presenting fatigue data. The results show similar pattern to plain fatigue with lower damage tolerance. It also shows scatter and dependency on the pad configuration as expected. Finally, the CAF-parameter shows potentials in effectively analyzing/predicting the complex mechanism of fretting fatigue.     

Experimental results in fretting fatigue with shot and laser peened Al 7075-T651 specimens

This work focuses on determining the effect of shot and laser peening on fretting fatigue in the Al 7075-T651 alloy. These surface treatments generate a residual compressive stress field near the treated surface where contact under fretting fatigue produces high stress levels. The fretting fatigue resistance of shot and laser peened specimens was assessed in a series of tests involving measurements of the residual stress field, residual stress relaxation under the action of cyclic loads, the friction coefficient, surface roughness and material hardness. The obtained results are compared with those for untreated specimens. The tests show the beneficial effect of the compressive residual stresses and the improvement that surface roughness causes in fretting fatigue life, especially in shot peened specimens. Another important effect observed, is the partial residual stress relaxation produced during the fretting fatigue tests.