Life estimation of Ti-6Al-4V specimens subjected to fretting fatigue and effect of surface treatments

Suitability of different multi-axial parameters in predicting fretting fatigue life of Ti-6Al-4V specimens has been investigated. Ameliorating effect of surface treatments on fretting fatigue has been studied. In simple uni-axial/multi-axial fatigue tests, nucleation as well as propagation of cracks occur under the influence of identical stresses. Hence nucleation accounts for most of the total life. Fretting fatigue crack nucleation occurs due to very large contact stresses, effect of which is felt only close to the surface (due to steep gradients). Propagation mostly occurs due to lower stresses in the bulk of the material (negligible influence of contact tractions) and forms a significant portion of total life. Total life has to be taken as sum of initiation life calculated from different multi-axial fatigue parameters and propagation life from conventional fracture mechanics approach. Steep stress gradients necessitate the adoption of a statistics based approach to predict the crack initiation life, based on an assumed distribution of flaws. The quality of comparison between predicted and experimentally observed failure lives provides confidence in the notion that conventional fatigue life prediction tools can be used to assess fretting fatigue failure. Effect of surface treatments like shot-peening with or without additional surface coatings on total life of the specimen and on friction coefficient has been studied.     

Development of a contact compliance method to detect the crack propagation under fretting

Partial slip fretting conditions are classically known to favor contact crack nucleation and crack propagation. Considered as a plague for modern industries, numerous theoretical researches have been conducted during the past two decades to predict such fretting damage. However, a review of last few years critically outlines the need of precise and in-situ experiments to qualify and quantify the given models. To palliate such aspect, an original approach which consists in following the contact stiffness evolution as an indicator of the fretting cracking phenomena, has been developed. Applied for an aluminium/steel contact, it demonstrates that the incipient crack propagation is related to a discontinuous decrease of the contact stiffness. Based on this online analysis, a fretting cracking endurance parameter has been extrapolated to develop fast and low cost fretting cracking endurance chart. A FEM analysis has been performed in an attempt to formalize the given experiments.     

Coalescence and breakup of contact areas: Effects on surface temperatures

In a study of the mechanisms by which thin polymeric films can prevent or delay the onset of fretting corrosion, experimental observations were made of the apparent real area of contact and temperatures generated by friction in a dry-sliding system consisting of stationary polymer-coated steel balls loaded against a vibrating sapphire disk. Five different polymers were used in the original study at vibrating frequencies ranging from 100 to 200 Hz and amplitudes from 20 to 100 μm; but this paper focuses on only one of these—polystyrene coated steel balls in contact with sapphire. Surface temperatures generated by friction were measured using an infrared microscope system. A photomacro/video technique was developed to view the fretting contact interface and to measure the size and distribution of the real areas of contact. The experiments revealed several complex patterns and unusual phenomena. In one example of behavior, a number of small contact patches would suddenly coalesce into one larger contact patch and then break up again into a similar collection of separate patches. This coalescence and breakup occurred at a regular frequency which was much lower than the oscillating frequency. In addition, significant surface temperature spikes corresponding to the occurrence of coalesced areas were observed.A general thermal model previously developed was used to theoretically predict the temperatures corresponding to the experimental conditions [Furey MJ, Vick B, Foo SJ, Weick BL. A theoretical and experimental study of surface temperatures generated during fretting. In: Proceedings of Japan international tribology conference, Nagoya, Japan, 29 October 1990–1 November 1990, vol. II, pp. 809–14; Vick B, Furey MJ. A basic theoretical study of the temperature rise in sliding contact with multiple contacts. Tribology International 2001;34:823–29]. The thermal model consists of a sliding pair of any material combination with three-dimensional and transient conditions. The key feature is the contact area, which is modeled as a collection of rectangular patches in which each patch can have any specified size, shape, position, and time duration. In this way, each contact has a unique start and finish time and the entire collection of contacts can evolve with time in any specified manner. This provides the flexibility to model everything from hard, brittle surfaces such as ceramics to softer, deformable surfaces such as polymers.Using the changes in the apparent real area of contact as observed in the experiments, the theoretical model predicted surface temperatures in close agreement with experimental values. The results of this study show not only that the area of contact is complex and dynamically changing, but that the surface temperatures produced are extremely sensitive to the real area of contact. Although the fundamental mechanisms for the observed phenomena of breakup, coalescence and motion of contact areas are unknown, the study is important since it illustrates the connection between areas of contact and surface temperature—a key unknown which influences physical and chemical behavior in tribological processes.     

Evolution of matt surface topography in aluminium pack rolling. Part II: effect of material properties

A model has been presented in a companion paper [1] to predict the generation of roughness on the matt surface in pack rolling of aluminium foil. The model is based on the finite element method using isotropic plasticity. This model is used in the current paper to investigate the effect of material properties on the generation of surface roughness. There is a large inhomogeneity of strain during deformation, with harder grains generally deforming less than softer ones. It is found that the roughness amplitude is roughly proportional to the standard deviation of the initial grain yield stress distribution, normalised by the initial mean yield stress, so that a wider distribution of the initial yield stress results in greater surface roughness. It is shown that a suitable linear hardening law can be used to approximate the roughening behaviour for real material flow stress curves.     

The fretting wear-resistant properties of steel with ion-sulphuration+shop-peening and shot-peening+ion-sulphuration duplex treatments

The fretting wear-resistant properties of ion-sulphuration+shot-peening and shot-peening+ion-sulphuration duplex-treated coatings on 1045 steel substrate were investigated. The results show that the fretting wear-resistance of shot-peening+ion-sulphuration coating was better than that of the ion-sulphuration+shot-peening coating. The fretting wear process of the duplex-treated coatings in the gross slip regime could be described as the sulphuration coatings playing an important lubricating role in the incubation period. Then a series of changes took place continuously and alternatively on the contact surfaces, including adhesion, material transfer, oxidation, delamination of the oxide layer and degradation. In the last stage, the trapping, pulverization and elimination of debris restrained the adhesion between the contact surfaces and led to a decrease in the friction coefficient.     

Electrical discharge machining of the AISI D6 tool steel: Prediction and modeling of the material removal rate and tool wear ratio

In this investigation, response surface method was used to predict and optimize the material removal rate and tool wear ratio during electrical discharge machining of AISI D6 tool steel. Pulse on time, pulse current, and voltage were considered as input process parameters. Furthermore, the analysis of variance was employed for checking the developed model results. The results revealed that higher values of pulse on time resulted in higher values of material removal rate and lower amounts of tool wear ratio. In addition, increasing the pulse current caused to higher amounts of both material removal rate and tool wear ratio. Moreover, the higher the input voltage, the lower the both material removal rate and tool wear ratio. The optimal condition to obtain a maximum of material removal rate and a minimum of tool wear rate was 40 μs, 14 A and 150 V, respectively for the pulse on time, pulse current and input voltage.     

Process mapping and functional correlation in surface metrology: a sheet metal case study

This paper presents a case study on process mapping and functional correlation in surface metrology. Stainless steel sheets are used for home and industrial applications in a variety of products. Visual characteristics form the dominant functional aspect in these sheets. Therefore, monitoring and controlling surface finish is critical. Because the process is in the development stage, there are no specifications for surface texture parameters. Therefore, quality control involves visual inspection by human inspectors. There is a need for mapping the process to develop a preliminary specification and researching tools for automating the process of quality assessment. While surface texture is traditionally measured only for tolerance compliance in most industrial situations, the case study developed in this paper captures the larger scope of surface texture measurement and analysis. Also, in developing the case study, novel techniques for functional correlation in surface metrology are explored .     

The effect of surface texturing on seizure resistance of a steel-bronze assembly

The tribosystem consisted of a stationary block pressed at a required load P against a ring rotating at a defined speed. Sliding was unidirectional. Block samples, made from bronze CuSn10P with 138-HB hardness, were modified using a burnishing technique to obtain surfaces with circular oil pockets. Rings were made from 42CrMo4 steel, of hardness 40 HRC, which was obtained after heat treatment. Tests were conducted at a constant speed of 0.27 m/s. Before the test, an oil drop was added to lubricate sliding surfaces. A seizure resistance test was carried out at constant normal load of 2700 N. Tribotests were automatically stopped when the coefficient of friction reached a limit value 0.15. Selected textured samples clearly exhibited a lifetime longer than untextured reference specimens.     

The use of notch and short crack approaches to fretting fatigue threshold prediction: Theory and experimental validation

In this paper the similarities between fretted and notched components in terms of stress gradient and the consequent “size effect” are discussed. Critical distance and short crack arrest approaches for the prediction of fretting fatigue thresholds are then presented and the predictions are compared with experimental results. Two geometry and alloy combinations are considered in order to validate the prediction by the means of experimental results. In particular, both Hertzian fretting tests performed on Al4%Cu alloys and experiments carried out employing ‘flat and rounded’ contact pads, made of Ti6Al4V alloy, are used for the comparison. It is shown that both criteria provide good predictive capabilities. However, the short crack arrest method is less empirical and can be adapted to a wider range of applications (e.g. surface treated components).     

Analysis of surface roughness and cutting force components in hard turning with CBN tool: Prediction model and cutting conditions optimization

In this study, the effects of cutting speed, feed rate, workpiece hardness and depth of cut on surface roughness and cutting force components in the hard turning were experimentally investigated. AISI H11 steel was hardened to (40; 45 and 50) HRC, machined using cubic boron nitride (CBN 7020 from Sandvik Company) which is essentially made of 57% CBN and 35% TiCN. Four-factor (cutting speed, feed rate, hardness and depth of cut) and three-level fractional experiment designs completed with a statistical analysis of variance (ANOVA) were performed. Mathematical models for surface roughness and cutting force components were developed using the response surface methodology (RSM). Results show that the cutting force components are influenced principally by the depth of cut and workpiece hardness; on the other hand, both feed rate and workpiece hardness have statistical significance on surface roughness. Finally, the ranges for best cutting conditions are proposed for serial industrial production.     

The effect of surface texturing in soft elasto-hydrodynamic lubrication

A theoretical model is developed to study the potential use of laser surface texturing (LST) in the form of spherical micro-dimples for soft elasto-hydrodynamic lubrication (SEHL). The model consists of mutual smooth elastomeric and LST rigid surfaces moving relatively to each other in the presence of viscous lubricant. The pressure distribution in the fluid film and the elastic deformations of the elastomer are obtained from a simultaneous solution of the Reynolds equation and the equation of elasticity for the elastomer. An extensive parametric investigation is performed to identify the main important parameters of the problem, which are the aspect ratio and area density of the dimples. The parametric analysis provides optimum parameters of the surface texturing and shows that LST effectively increases load capacity and reduces friction in SEHL.     

机械零件表面粗糙度的影响与选择

从摩擦学的角度出发 ,阐明对于配合表面只取Ra值时的不足 ,建议应当再增加一个表面纹理加工方向 ,以此确保配合表面的润滑效果 ,减少磨损 ,提高使用寿命。     

A study of the integrity of a burnisher’s working surface manufactured from a Diamond-composition thermostable material

This paper investigates the effect of the microstructure of a new diamond-composition thermostable material (DCTM) on the workability of the working surface of a diamond burnishing instrument with improved mechanical and physical characteristics, developed at the N.V. Bakul Institute for Superhard Materials of the National Academy of Ukraine. Examination of the surface integrity of the working part of burnisher, made of DCTM and traditional synthetic polycrystalline diamonds of the carbonado and ballas components, revealed that the workability of the former is similar and, in some cases, better than the performance of the latter. The DCTM material, therefore, is suitable for a wide range of material removal operations.     

青年与老年手指甲表面形貌、成分与力学性能的实验分析

应用红外光谱仪、场发射扫描电镜、差示扫描热量仪、原位纳米力学测试系统对青年(20~25岁)与老年(70~75岁)指甲的表面结构、形貌、水含量及力学性能进行了实验分析。结果表明:与青年组成员指甲相比,老年组成员指甲的PO 2、C-O、CH 2与CONH基团的峰位向高波数移动;老年组成员指甲表面比青年组成员指甲粗糙并表现出明显的纵向纹理;尽管其总含水量与青年组成员指甲相当,但老年组成员指甲结合水的含量比青年组成员指甲的低;老年组成员指甲的硬度与约化弹性模量均高于青年组成员指甲相应对比量,因此,老年组成员指甲在划痕实验中显示出更好的耐磨性,但在较大载荷时出现明显的裂纹。     

The effect of variable amplitude loading on stress distribution within a cylindrical contact subjected to fretting fatigue

A finite element model of a cylindrical Hertzian contact on a test sample subjected to alternating shear loading has been developed. The model has been used to investigate shear stress distributions at the contact during variable amplitude fretting fatigue for a load configuration in which the sample cyclic stress is applied in phase with shear force on the cylindrical contact. It has been found that during constant amplitude cyclic loading, shear stress distributions and positions of the stick-slip boundary at load maxima and minima remain fixed. Application of overloads changes the stress distribution and the position of the stick-slip boundary attained by loading of subsequent cycles. The largest cycle maximum stress determines the position of the stick-slip boundary adopted by subsequent smaller amplitude cycles. In general variable amplitude fretting fatigue the position of the stick-slip boundary will be changing with each load cycle. Hence fatigue initiation processes will occur at locations dispersed over an extended region over the contact. The implications of this behaviour for models for fretting fatigue life calculation are explored.     

The degradation of EBSD-patterns as a tool to investigate surface crystallized glasses and to identify glassy surface layers

Surface crystallized samples of glass-ceramics containing cordierite, rhombohedral BaAl₂B₂O₇ and fresnoite were analyzed using electron backscatter diffraction (EBSD). The first two materials were chosen because surface crystallized samples of these materials have previously been shown to contain crystals covered by a very thin layer of glass. In all materials, EBSD pattern degradation occurs if the step size of a scan is chosen to be small. It is shown that the minimum step size enabling an evaluable EBSD-scan increases notably, if the crystals are covered by a thin layer of glass. It is also shown that pattern degradation may be utilized to prove the existence of such a thin glass or otherwise thermally sensitive layer. This provides significant information concerning the nucleation process of glasses also with respect to nucleation theory of glass-ceramics. It is also possible to describe the quantity of crystalline surface covered by the thermally sensitive layer.     

Elastic recovery of a scratch in a polymeric surface: experiments and analysis

A scratch may be regarded as a tangential indentation. Hence standard indentation laws can be used to analyse the geometry of the scratches left by a moving tip on the surface of a viscoelastic viscoplastic body such as a commercial grade of cast polymethylmethacrylate (PMMA). This paper presents experimental results and an analysis of the elastic recovery of a scratch after contact with a tip. The experimental data were obtained with a new scratch apparatus fitted with a built-in microscope, which allows in situ analysis of the contact area and the groove left on the surface. The elastic plastic total penetration depth h_ep is split into its plastic part h_p and elastic part h_e. In the case of full plasticity around the tip during scratching, which for an elastic plastic material implies a sufficiently high value of the contact strain, the elastic law describes the depth relaxation and experimental data agree with the analysis. In the case of a purely elastic response of the material, corresponding to low values of the contact strain, the rear contact radius is equal to the front contact radius. At intermediate levels of strain, an analysis of the elastic recovery must take into account the contribution of the plastic term to the elastic plastic response of the material.     

Cu含量对耐海水腐蚀不锈钢组织与性能的影响

借助XRD、SEM等研究了0％～0．8％的Cu对20Cr-18Ni-6Mo-0．2Mn-0．8V-0．1Si耐海水腐蚀不锈试验钢力学性能的影响。结果表明：当Cu为0．3％时,抗拉强度（σb）为642Mpa;度（HRB）为93．28,延伸率（δ）为27．1％,其综合力学性能最优。     

NURBS曲线曲面重构的方法

CAD/CAM中用有理多项式函数表示曲面越来越广泛。由于非均匀有理B样条(NURBS)可以精确表示解析形状和自由曲线曲面,国际标准组织(ISO)与1991年把NURBS作为表示工业产品几何形状的工业标准。这里主要讨论了NURBS曲线曲面重构的方法。