Kyungmok Kim

Energy calibration and full-pattern refinement for strain analysis using energy-dispersive and monochromatic X-ray diffraction

Precise channel-to-energy conversion is very important in full-pattern refinement in energy-dispersive X-ray diffraction. Careful examination shows that the channel-to-energy conversion is not entirely linear, which presents an obstacle to obtaining accurate quantitative data for lattice strains by pattern refinement. In order to establish an accurate quadratic channel-to-energy conversion function, a Matlab program was written to find the best quadratic coefficient and hence the whole energy conversion function. Then this energy conversion function was used to perform a whole-pattern fitting of the energy-dispersive X-ray diffraction pattern of a Ti64 sample. The strain across the Ti64 bar calculated from the fitting results has been compared with values obtained by single-wavelength X-ray diffraction utilizing a Laue monochromator.

Two-dimensional finite element simulation of fracture and fatigue behaviours of alumina microstructures for hip prosthesis.

This paper describes a two-dimensional (2D) finite element simulation for fracture and fatigue behaviours of pure alumina microstructures such as those found at hip prostheses. Finite element models are developed using actual Al2O3 microstructures and a bilinear cohesive zone law. Simulation conditions are similar to those found at a slip zone in a dry contact between a femoral head and an acetabular cup of hip prosthesis. Contact stresses are imposed to generate cracks in the models. Magnitudes of imposed stresses are higher than those found at the microscopic scale. Effects of microstructures and contact stresses are investigated in terms of crack formation. In addition, fatigue behaviour of the microstructure is determined by performing simulations under cyclic loading conditions. It is shown that crack density observed in a microstructure increases with increasing magnitude of applied contact stress. Moreover, crack density increases linearly with respect to the number of fatigue cycles within a given contact stress range. Meanwhile, as applied contact stress increases, number of cycles to failure decreases gradually. Finally, this proposed finite element simulation offers an effective method for identifying fracture and fatigue behaviours of a microstructure provided that microstructure images are available.

High-cycle fatigue simulation for aluminium alloy using cohesive zone law

This article describes high-cycle fatigue simulation for 7050-T7451 aluminium alloy using cohesive zone law. A three-dimensional finite element model is developed for fatigue behaviour of aluminium alloy subjected to cyclic bending. A bilinear, cycle-dependent cohesive zone law is implemented with a help of experimental S-N (stress amplitude–number of cycles to failure) data. In the finite element model, a cycle jump strategy is used including stiffness degradation and reduction of fracture energy during cyclic loadings. Additionally, bending experiments are conducted with unnotched specimens and S-N curves are determined. Direct comparison of S-N curves between the simulation and the experiment is performed on bilogarithmic scale. Results show that the proposed method provides a good means of simulating high-cycle fatigue behaviour of aluminium alloys.

Fretting Damage of Ni—MoS2 Coatings: Friction Coefficient and Accumulated Dissipated Energy Evolutions

This article describes fretting wear damage of Ni—MoS2 coatings in terms of the friction coefficient (COF) and accumulated dissipated energy. Multi-layer dry lubricant coatings containing the elements Mo and S are known to maintain low friction under reciprocal relative displacement, and are considered for this purpose in fan blade attachments within aero-engines. The durability of these coatings for a dovetail connection needs to be investigated in order to help design calculations and overhaul scheduling. Experimental characterization of durability can be achieved by measuring COF as a function of the number of fretting cycles, or, alternatively, the accumulated sliding distance. Dissipated energy is also considered as an alternative indicator for characterizing fretting wear damage. In this article, fretting wear tests with Ni—MoS2 coatings and various surface treatments were performed using idealized contact geometries and loading conditions similar to those found in aero-engine components. Additionally, COF evolutions of Ni—MoS2 coatings were numerically determined as a function of accumulated dissipated energy. A direct comparison between a numerical evolution and a measured one was performed, and excellent agreement between the two evolutions was obtained.

Fretting corrosion processes and wear mechanisms in medical implants

Two materials (one is metal) under slight relative motion in a liquid medium are submitted to fretting corrosion. This chapter is dedicated on studying fretting corrosion of implants. After describing the most significant implants submitted to fretting, fretting corrosion is defined. Fretting corrosion is a particular mechanism of degradations; it highlights the key role of passive film, crevice corrosion, etc. For understanding the electrochemical effect of the fretting corrosion of metal, some investigations are presented at free corrosion potential and at applied potential in order to measure the specific current density. Moreover the role of proteins is investigated because they constitute the biological environment. Thus they play a significant role in the fretting corrosion processes. Finally results from Atomic Force Microscopy (AFM) show the particular debris, size about 100 nm. Problem about debris influence is discussed

Crack simulation of nano-bioceramic composite microstructures with cohesive failure law: effects of sintering, loads and time.

Crack behaviour of zirconia toughened alumina (ZTA) microstructures are simulated with a two-dimensional finite element simulation. Finite element models are developed using actual microstructure images of zirconia toughened alumina and a bilinear cohesive zone law. Simulation conditions are similar to those found at frictional contact between a femoral head and an acetabular cup of hip prosthesis. Effects of microstructures and contact stresses are investigated in terms of crack generation. Moreover, fatigue behaviour of a microstructure is determined by performing simulations under cyclic loading conditions. It is identified that total crack length observed in a microstructure increases with increasing the magnitude of applied contact stress. Cyclic simulation results show that progressive crack growth occurs with respect to number of fatigue cycles. In addition, it is demonstrated that zirconia grains resist crack growth in microstructures.

Residual Stress Analysis in Shot Peened and Fretting Fatigued Samples by the Eigenstrain Method

Residual stresses in titanium alloy samples that were subjected to shot peening followed by fretting fatigue loading were investigated using a combined experimental and numerical analysis procedure based on the concept of eigenstrain. Fretting fatigue loading was carried out in the pad – on-flat geometry using the Oxford in-line fretting rig. Flat-and-rounded pad shape was used to introduce the contact tractions and internal stress fields typical of the target application in aeroengine design. The specimens were in the shape of bars of 10mm square cross-section shotpeened on all sides. Both the pads and specimens were made from the Ti-6Al-4V alloy. Small remote displacement characteristic of fretting fatigue conditions was applied in the experiments. The residual elastic strains in the middle of the pad-to-sample contact and near the rounded pad edge were measured using synchrotron X-ray diffraction on Station 16.3 at SRS Daresbury. A combination of finite element analysis and the distributed eigenstrain method was used in the simulations. Commercial finite element analysis software, ABAQUS ver 6.41, was used to build the finite element model and to introduce the residual stresses into the model using eigenstrain distributions via a user-defined subroutine. In an unfretted shot peened sample an excellent agreement of residual stress profiles was obtained between the experimental data and model prediction by the variational eigenstrain procedure. In a fretted sample the residual stress change due to fretting was observed, and predicted numerically. A good correlation was found between the FE simulation prediction and the experimental data measured at contact edges.

Analysis of plastic deformation and residual elastic strain in a titanium alloy using synchrotron x-ray diffraction

This paper presents the results of a study of elastoplastic deformation of titanium alloy Ti-6Al-4V subjected to four point bending prior to residual elastic strain measurements by high energy synchrotron diffraction. Both white-beam and monochromatic x-rays were used at the SRS Station 16.3 in order to record diffraction patterns as a function of beam position across the sample. This allowed the comparison between the two techniques to be readily made. Residual (elastic) strain was calculated as a function of position across the sample, based on different reflections of hcp titanium. Inelastic bending analysis was used to extract the plastic strains. The results demonstrate that (i) the level of plastic deformation can be deduced from the x-ray diffraction profile, (ii) the asymmetry of the material response to plastic deformation in tension and compression can be identified and (iii) differences in the behaviour of different grain groups can be seen.

Effect of displacement on kinetic frictional behaviour between electro-deposited coating and AISI 52100 steel under fretting conditions:

This article investigates the effect of imposed displacement on the evolution of the kinetic friction coefficient between an epoxy-based cathodic electro-deposited coating and a steel ball. Small-amplitude reciprocal sliding is induced until the friction coefficient reaches a critical value. The kinetic friction coefficient is measured at various imposed displacements. Slip ratio, defined as the ratio of an actual sliding distance to imposed displacement, is determined to identify a slip regime. Experimental results show that a friction coefficient evolution varies in a gross slip regime according to imposed displacement amplitude. It is identified that the critical number of cycles to coating failure decreases with increased imposed displacement amplitude. The relation between the critical number of cycle and imposed displacement amplitude is found to be inverse power law.

Creep–rupture model of aluminum alloys: Cohesive zone approach

In this article, a creep–rupture model of aluminum alloys is developed using a time-dependent cohesive zone law. For long-term creep rupture, a time jump strategy is used in a cohesive zone law. Stress–rupture scatter of aluminum alloy 4032-T6 is fitted with a power law form. Then, change in the slope of a stress-rupture line is identified on a log–log scale. Implicit finite element analysis is employed with a model containing a cohesive zone. Stress–rupture curves at various given temperatures are calculated and compared with experimental ones. Results show that a proposed method allows predicting creep–rupture life of aluminum alloys.