Modeling rolling contact fatigue lives of composite materials based on the dual beam FIB/SEM technique

Rolling contact experiments of (TiB + TiC)/Ti–6Al–4V composites reveal that inhomogeneous reinforcements reduce the materials’ rolling-contact fatigue (RCF) lives. The composite microstructures are examined and reconstructed based on the dual-beam focus ion beam scanning electron microscopy (FIB/SEM) technique. Integration of the microstructure-based stress analysis approach and Zaretsky’s RCF life prediction methodology leads to a new numerical RCF life modeling method. The newly developed mesh differential refinement scheme, together with the fast Fourier Transform techniques, is utilized to improve the efficiency and versatility of the proposed method. The RCF lives of the composites predicted by the new model are then compared with the experimental data, illustrating the effectiveness of the proposed method.     

The frictionless rolling contact of a rigid circular cylinder on a semi-infinite granular material

The resulting flow and deformation of a semi-infinite granular material under a rolling, smooth rigid circular cylinder is investigated using a perturbation method. Based on the double-shearing theory of granular flow, complete stress and velocity fields, resistance to rolling and the permanent displacement of surface particles are determined to first order; when the internal friction angle is zero, the solutions reduce to those obtained in the corresponding analysis for Tresca or von-Mises materials. The solution scheme and the double-shearing model for granular flow both find their origins in the work of A.J.M. Spencer.     

On the limitations of low-rank approximations in contact mechanics problems

Typical strategies for reducing the computational cost of contact mechanics models use low-rank approximations. The underlying hypothesis is the existence of a low-dimensional subspace for the displacement field and a non-negative low-dimensional subcone for the contact pressure. However, given the local nature of contact, it seems natural to wonder whether low-rank approximations are a good fit for contact mechanics or not. In this article, we investigate some of their limitations and provide numerical evidence showing that contact pressure is linearly inseparable in many practical cases. To this end, we consider various mechanical problems involving nonadhesive frictionless contacts and analyze the performance of the low-rank models in terms of three different criteria, namely, compactness, generalization, and specificity.     

Influence of inclusion content on rolling contact fatigue in a gear steel: Experimental analysis and predictive modelling

Rolling contact fatigue tests were carried out on ring specimens made of quenched and tempered SAE 5135 gear steel with three different steel-production processes, through a bi-disc machine under pure rolling condition and water lubrication. Early formation of micro-pits then coalescing into macro-pits was observed on the rolling surface, while the final failure was caused by subsurface originated spalling phenomena. Microscope analysis of specimens section highlighted the complex surface and subsurface crack layout, and permitted to recognise sulphides as preferential sites for cracks initiation. The inclusion content was analysed throughout the extreme value statistics and the maximum expected inclusion in the Hertzian contact zone was introduced in a failure assessment diagram recently proposed, which resulted effective in predicting the specimen failures.     

A fundamental study on the impact of surface integrity by hard turning on rolling contact fatigue

Hard turning has the potential to produce favorable surface integrity that would improve component life in rolling contact. However, the effects of the process-induced residual stress profile and the white layer on rolling contact fatigue (RCF) are poorly understood. This study aims to answer the long-standing question of how residual stress and the white layer affect RCF. Based on the developed real-time RCF testing system, a series of RCF tests were conducted for hard turned AISI 52100 steel components. The test results have shown that the acoustic emission amplitude is most consistent and sensitive to fatigue damage than other AE parameters. A white layer induced by hard turning is very detrimental to RCF. A component free of a white layer can have a life six times that of a white layer component. As the white layer increases in thickness, the fatigue life decreases. Surface residual stresses and near-surface residual stress profiles are significant factors for RCF, while the depth of maximum compressive residual stress in subsurface is not critical. Surface integrity affects RCF through the mechanism of near surface damage rather than subsurface damage.     

The impact of surface integrity by hard turning versus grinding on rolling contact fatigue – Part I: Comparison of fatigue life and acoustic emission signals

Hard turning and grinding are finishing processes for the manufacture of precision components, such as bearings, gears and cams. However, the effects of distinct surface integrity by hard turning versus grinding on rolling contact life are poorly understood. Four representative surface types were prepared: as-turned, as-ground, turned and polished and ground and polished. Surface integrity was characterized by surface topography, microstructure and micro/nanohardness. Fatigue tests were performed with an acoustic emission sensor and the signal processing software. The surface topographies show that skewness of the as-ground surface is much more negative than the as-tuned one while other surface parameters are equivalent. The turned surface has a thicker strain hardened zone and a thinner thermal affected zone than those of the ground one. The ground surface has higher micro- and nanohardness on surface and in the subsurface than the turned one. The amplitude of acoustic emission signal is the most stable and sensitive signal to fatigue failure. The turned surface may have a longer life (> 84%) than the ground one with equivalent surface finish. The fatigue lives of the bearing assembly are nearly identical for the turned surface versus the polished surface and the turned polished surface versus the ground and polished surface. In addition, polishing may not necessarily improve fatigue life of the machined surface, but increase bearing assembly life as much as 40%.     

On the interaction of channeling/segmentation cracks of coating: Study on the critical spacing

Channeling/segmentation cracks may arise in the coating subjected to in-plane tensile stress. The interaction between these multiple cracks, say the effect of the spacing between two adjacent cracks on the behaviors of channels themselves and the interface around the interface corners, attracts wide interest. However, if the spacing is greater than a specific magnitude, namely the Critical Spacing (CS), there should be no interaction between such channeling/segmentation cracks. In this study, the mechanism of the effect of the crack spacing on the interfacial stress around the interface corner will be interpreted firstly. Then the existence of the CS will be verified and the relationship between the CS and the so-called stress transfer length in coating will be established for plane strain condition. Finally the dependence of the stress transfer length, simultaneously of the CS, on the sensitive parameters will be investigated with finite element method and expressed with a simple empirical formula.     

On the free vibration of sandwich panels with a transversely flexible and temperature-dependent core material – Part I: Mathematical formulation

The free vibration analysis of sandwich panels with a core that is flexible and compliant in the vertical direction and with temperature-dependent mechanical properties is presented in two parts. The first part presents the mathematical formulation while the second deals numerically with the effects of the degrading properties of the core on the free vibration response. The analysis is based on the high-order sandwich panel theory approach (HSAPT), and the equations of motions along with the appropriate boundary conditions are derived using the Hamilton’s principle. The study investigates the role of increasing temperature, through the degradation of the mechanical properties of the core, on the free vibration response of structural sandwich panels. The mathematical formulation uses two types of computational models. At first, following the HSAPT approach, the unknowns include the displacements of the face sheets as well as the shear stress in the core. Secondly, it is assumed that the through-thickness distributions of the vertical and horizontal core displacements can be represented as polynomials, following the results of the HSAPT static case, and the effect of the variable mechanical properties are implemented directly.     

Study of the densification of a nanostructured composite powder: Part 1: Effect of compaction pressure and reinforcement addition

Al/Al₂O₃ composite coating was prepared by plasma spraying and characterized by XRD and SEM. Some thermal–mechanical properties of the composite coating including thermal diffusivity, microhardness, fracture toughness and sliding wear rate were measured. The results showed that the Al/Al₂O₃ composite coating, compared with Al₂O₃ coating, exhibits denser structure and developed splat interface. The coexistence of Al metal phase and Al₂O₃ ceramic phase effectively increased the fracture toughness and thermal diffusivity of composite coating, in spite of the slight decrease in microhardness. Furthermore, the wear resistance of Al/Al₂O₃ composite coating is superior to that of Al₂O₃ coating.     

Approximate Analytical Determination of Vibrodiagnostic Parameters of Non-Linearity of Elastic Bodies Due to the Presence of a Closing Crack. Part 1. Available and Proposed Methods of Solution

The paper discusses the available analytical methods and the results of investigation of vibrations of elastic bodies with a bilinear asymmetric characteristic of restoring force that simulates the behavior of a local discontinuity in a material in the form of a closing fatigue crack. An approximate analytical method is put forward for the determination of vibrodiagnostic parameters of the vibration process of the nonlinear system under study in the range of weak superharmonic resonances.     

On the overall elastic moduli of polymer-clay nanocomposite materials using a self-consistent approach. Part I: Theory

Although few investigations recently proposed to describe the overall elastic response of polymer-clay nanocomposite materials using micromechanical-based models, the applicability of such models for nanocomposites is far from being fully established. The main point of criticism to mention is the shelving of crucial physical phenomena, such as interactions and length scale effects, generally associated by material scientists, in addition to the nanofiller aspect ratio, to the remarkable mechanical property enhancement of polymer-clay nanocomposites. In this Part I of two-part paper, we present a micromechanical approach for the prediction of the overall moduli of polymer-clay nanocomposites using a self-consistent scheme based on the double-inclusion model. This approach is used to account for the inter-inclusion and inclusion-matrix interactions. Although neglected in the models presented in the literature, the active interaction between the nanofillers should play a key role in the reinforcing effect of nano-objects dispersed in a polymer matrix. The present micromechanical model incorporates the nanostructure of clay stacks, modeled as transversely isotropic spheroids, and the so-called constrained region, modeled as an interphase around reinforcements. This latter is linked to the interfacial interaction between matrix and reinforcements that forms a region where the polymer chain mobility is reduced. To account for length scale effects, interphase thickness and particle dimensions are taken as explicit model parameters. Instead of solving iteratively the basic homogenization equation of the self-consistent scheme, our formulation yields to a pair of equations that can be solved simultaneously for the overall elastic moduli of composite materials. When the interphase is disregarded for spheroids with zero aspect ratio, our formulation coincides with the Walpole solution (J Mech Phys Solids 1969;17:235-251). Using the proposed general form, a parametric study is presented to analyze the respective influence of aspect ratio, number of silicate layers, interlayer spacing and nanoscopic size of the transversely isotropic spheroids on the overall elastic moduli of nanocomposite materials.     

On the overall elastic moduli of polymer-clay nanocomposite materials using a self-consistent approach. Part II: Experimental verification

Polyamide-6 (PA6) based nanocomposites were prepared using a modified montmorillonite (MMT) Cloisite 20A as nanofillers. The silicate weight fraction of the prepared nanocomposites, determined by burning off the PA6 matrix, was ranged from 0.2 wt% up to 7.5 wt%. The thermomechanical properties of both the neat PA6 and the PA6 filled with MMT nanoclay were measured by means of uniaxial tension tests and dynamic mechanical thermoanalysis, their crystallinity analyzed by differential scanning calorimetry and their morphology observed by transmission electron microscopy. The elastic stiffness of PA6-clay nanocomposites was examined under two moisture levels and was analyzed with the theory formulated in the Part I of this work. Predicted results are found in good agreement with our experiments. The model capabilities are also critically discussed by comparisons with both experiments issued from the literature and the Mori-Tanaka approach widely used in recent literature. It is demonstrated that the proposed micromechanical model is more efficient than the Mori-Tanaka approach. Moreover, the obtained results support the idea that the elastic stiffness of polymer-clay nanocomposites is governed by the same mechanisms as microcomposites, the effects of particle dimension or constrained region being of a second order.     

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 1: Round and Angular Monosize and Binary Mixtures

The bulk mechanical properties of two different types of rootzone sands (round and angular) were measured using a cubical triaxial tester. Two monosize sands (d 50 = 0.375 mm and 0.675 mm) and their 50:50 binary mixtures (d 50 = 0.500 mm) were studied. The compression, shear, and failure responses of the above-mentioned six compositions were analyzed, compared, and modeled. Two elastic parameters (bulk and shear moduli) and two elastoplastic parameters (swelling and consolidation indices) of the six sand compositions were also calculated and compared. The angular sand was more compressible than round sand during isotropic compression. In addition, the angular sands tended to have lower initial bulk density and high porosity values. Among the three different size fractions, the 0.375 mm mixture was least compressible for both sand shapes. The failure strength and shear modulus of the angular sand were higher than the round sands. In addition, due to their simplicity, phenomenological models were developed to predict the compression and shear behavior of the sands. The prediction models were validated using subangular and subround sands. Average relative difference values were calculated to determine the effectiveness of the prediction models. The mean average relative difference values for compression profiles, i.e., volumetric stress vs. volumetric strain, were from 16 % to 39 %, except for the initial load-response portion (< 1 % volumetric strain). The predictive models were effective in reproducing the failure responses: at 17.2 kPa confining pressure, the mean of average relative difference was 23 %; at 34.5 kPa , the mean difference was 24 %.     

On the use of conformal mapping for the computation of hydrodynamic forces acting on bodies of arbitrary shape in viscous flow. Part 1: simply connected body

Some aspects of the force and moment computations in incompressible and viscous flows are revisited. The basic idea was developed in Quartapelle and Napolitano (AIAA J. 21:991–913, 1983). They formulated the way to compute the force and moment without explicitly calculating the pressure. The principle is to project Navier–Stokes equations on a set of functions. Surprisingly these functions have a meaning in potential theory. They are precisely the solutions which give the added masses and added moment of inertia for potential flow. By revisiting this problem for two-dimensional flows in unbounded liquid, a general identity giving the added masses and added moment of inertia is formulated. To this end a conformal-mapping technique is used to transform the fluid domain. Once the potential solution has been obtained, the projection method by Quartapelle and Napolitano is implemented. In addition an a posteriori computation of the pressure is described. Applications illustrate the present study.     

Accelerated testing of biological stain growth on external concrete walls. Part 1: Development of the growth tests

The development of biological stains has a great influence on the durability of building materials. For common concrete structures, the main and most rapid disorder linked with this development is aesthetic. In recent years, architects have been increasingly using formwork surfaces for external walls, so the search for aesthetic quality and durability has become as important as the search for mechanical quality and durability. Hence, there is a demand from industry for the qualification of concrete wall surface behaviour toward biological growths. This paper aims to itemize the various biological stains affecting concrete and to put forward two accelerated tests for the growth of algae, the organisms responsible for the first visible stains. These tests enable a wall surface to be qualified with respect to biological stains.     

Measurement of Bulk Mechanical Properties and Modeling the Load-Response of Rootzone Sands. Part 2: Effect of Moisture on Continuous Sand Mixtures

The compression and failure responses of four rootzone sand mixtures (with different types of particle shapes) were analyzed, compared, and modeled at two different moisture states (air dried and 30 cm tension). Differences in particle packing characteristics arising from particle shape and moisture were quantified. The air-dried and moist samples of the sand mixtures had initial bulk density (IBD) values ranging from 1.55 to 1.67g/cc and 1.23 to 1.48g/cc, respectively. The low IBD values observed for moist mixtures were attributed to the particle-particle agglomeration effects that take place in the presence of moisture. In addition, it was observed that the sand mixture's porosity increased with decreasing particle sphericity. During compression testing, moist samples underwent a greater volumetric deformation compared to the air-dried samples for the same pressure levels, e.g., at 69kPa, the volumetric strain of moist round sand mixtures was 8% higher than that of the air-dried round sand mixtures. Therefore, moisture acted as lubricant during volumetric compression of sand mixtures. Also, the bulk modulus values decreased with increasing moisture content and decreasing particle sphericity. During shear testing, the moist samples underwent a larger amount of strain deformation compared to the air-dried samples for the same stress difference values. This suggests that the presence of moisture makes the sand mixtures ductile during shear testing, unlike the usual brittle response in air-dried state. Shear modulus values linearly increased with the increase in mean pressure for the air-dried samples, whereas, for moist samples, the shear modulus values increased gradually or remained practically constant. The effect of pressure, moisture, and particle shape was also quantified for two elastoplastic parameters (consolidation and swelling indices). It was generally observed that the average consolidation index values decreased with pressure but increased with moisture and particle angularity. On the other hand, average swelling index values increased with pressure, moisture, and particle angularity. Overall, it was concluded that the moisture and particle shape had a decisive influence on the compression and shear profiles of continuous rootzone sand mixtures.     

Effect of heat treatment in air on the thermal properties of SiC fibre-reinforced composite. Part 1: a barium osumilite (BMAS) matrix glass ceramic composite

The thermal properties have been studied on a glass ceramic composite comprised of a barium osumilite (BMAS) matrix reinforced with SiC (Tyranno) fibres which has been subjected to a heat treatment in air in the range of 700–1,200 °C. Microstructural studies were carried out especially on of the interface between fibre and matrix. The presence of a carbon thin layer in the interface is a typical observation in SiC fibre-reinforced glass ceramic matrix composite systems. The microstructural evaluation and thermal properties showed a degradation of interfacial layer occurred at low heat treatment temperatures, (700–800 °C) this was attributed to the fact that, at those heat treatment temperatures the carbon rich layer formed during processing was oxidised away leaving voids between fibre and matrix, which were linked by isolated silicon-rich bridges. After heat treatment at higher temperatures of 1,000–1,200 °C, the thermal properties were retained or even enhanced by leaving a thick interfacial layer.     

On the changes in a structure of water due its contact with a solid phase. III. Determination of relations T1/T1 0=f1(m/M), Me=f2(α). Estimation of variation effect of the valence angle (α) in a water molecule on water solubility of substances

Calculated and experimental methods have been suggested to determine the valence angle in a water molecule in a volume with the solid phase mass variation in a system. The dependences of the dipole moment in H₂O and of static dielectric water permeability on the valence angle value have been established and the changes in water solubility of different substances in the presence of a solid phase have been substantiated.Central Research Institute for Complex Utilization of Water Resources, Minsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 1, pp. 80–87, July, 1992.     

On the use of conformal mapping for the computation of hydrodynamic forces acting on bodies of arbitrary shape in viscous flow. Part 2: multi-body configuration

Two-dimensional viscous flows around obstacles are considered in an unbounded liquid. The basic idea developed in Part 1 is further extended from a single body to multi-body configurations. This idea follows from the formulation by Quartapelle and Napolitano (AIAA J. 21:991–913, 1983) who proposed computation of the force and moment in incompressible viscous flow without explicitly calculating the pressure. The principle is the projection of Navier–Stokes equations on a set of functions. Surprisingly, these functions have a precise meaning in potential theory. They are the solutions which lead to the added masses and added moment of inertia for the potential flow around the studied arrangement of obstacles. By revisiting this problem, a general identity of the full coupled matrix of added masses and added moment of inertia is formulated. To this end conformal mappings for multi-body configurations are used. Robustness of the proposed algorithms is tested and illustrated. The obtained potential solution is merely a mathematical solution and it does not allow to describe the actual potential flow since the circulation is not accounted for. However, its interest is crucial for implementing the projection technique developed by Quartapelle and Napolitano. The interest of such a method is two-fold. Firstly, it provides a way of computing the force without explicitly calculating the pressure. Consequently and secondly, it offers an alternate way to validate the computation of the loads. In effect, these loads are always available from a direct integration of the Cauchy stress tensor (pressure plus friction). It is worth mentioning that the present technique allows an a posteriori computation of the pressure.