Study on effect of mean stress on fatigue life prediction of thin film structure

This paper describes the effect of mean stress on fatigue life prediction of structure made with thin film. It is well known that the mean stress influences fatigue life prediction of mechanical structure. We investigated a reasonable method for considering mean stress when fatigue strength assessment of micro structure of thin film should be performed. Fatigue tests of smooth specimen of beryllium-copper (BeCu) thin film were performed in ambient air at R = 0.1 with 5 Hz. A micro probe was designed and made with BeCu thin film by the precision press process. Fatigue tests of micro structure were performed with 5 Hz frequency, in ambient air to verify the fatigue life predicted by computer simulation through FE analysis. The fatigue life predicted by the S _a -N curve modified by Goodman method with principal stress through FE analysis shows a more reasonable result than other methods.     

Fatigue crack growth behavior of Mod.9Cr-1Mo steel at elevated temperatures: Effect of temperature,loading frequency and R ratio

The fatigue crack growth behavior of Mod.9Cr-1Mo steel was investigated as a function of temperature, loading frequency and R ratios in the Paris regime. The relationship between fatigue crack growth rate and stress intensity factor range was acquired for each test condition. The results revealed that crack growth rate was accelerated with increasing temperature and decreasing loading frequency. The influence of the R ratio on crack growth rate was only pronounced at the low loading frequency condition. In order to understand the crack growth mechanism, activation energy analysis and normalized ΔK analysis were performed. This study suggests that oxidation and the degradation of mechanical properties promote crack growth behavior.     

Creep crack growth analysis using <Emphasis Type="Italic">C</Emphasis><Subscript><Emphasis Type="Italic">t</Emphasis></Subscript>-parameter for internal circumferential and external axial surface cracks in a pressurized cylinder

Creep crack growth at elevated temperatures is a critical consideration in estimating the remaining life of high temperature structural components and in deciding their inspection interval. In this study, creep crack growth analyses for external radial-axial and internal radial-circumferential surface cracks in a pressurized cylinder were conducted by an analytical method. The effect of crack depth and crack length on the variations in C _t and remaining life predictions were investigated for surface cracks with various initial aspect ratios. It was observed that the remaining life of an internal radial-circumferential surface crack was approximately 53 times longer than that of an external radial-axial surface crack for the same crack size and loading conditions with 316 stainless steel material. It was also observed that the variations in remaining life, crack propagations, and the C _t values were considerably sensitive to the crack location and crack depth. Convergence of crack aspect ratio was not observed when the crack depth ratio was increased. Since the method is independent of material properties and location of the crack geometries, it can be extended to various material properties and various locations of the surface crack geometries.     

Big data analysis of a mini three-axis CNC machine tool based on the tuning operation of controller parameters

The characteristic responses of a mini three-axis computer numerical control (CNC) machine tool based on the controller tuning operation were investigated for big data estimation. The major tuning parameters included the position control gains K _p, the position feed-forward control gains K _f, the speed control gains K _v, and the gain ratios K _g of the position and speed control values in manufacturing industries. K _p gains of 10, 30, 50, 80, 100, 200, 300, and 400 rad/s, K _f gains of 0, 30, 50, 60, 80, and 100 %, K _v gains of 30, 50, 70, 100, 300, 900, 2000, and 3000 rad/s, and K _g ratios of (1:1), (3:1), (5:1), and (7:1) were analyzed for smart productivity. The results show that the settling times at different K _p values were almost constant when the K _p gain was over 200 rad/s. The maximum overshoots, when the feed-forward gain is over 60 %, almost increased with increasing feed-forward gains. However, the overshoot of the three-axis CNC machine tool decreased as the K _v gain increased until the K _v gain reached 70 rad/s. The settling times at a constant K _g ratio decreased with an increase in the K _p and K _v gains. The characteristic responses of the tuning operations were enabled with connectivity to a cloud network to share the big data, to support decision making, and to adjust operations in real time.     

A new approach to study the influence of the weld bead morphology on the fatigue behaviour of Ti–6Al–4V laser beam-welded butt joints

Ti–6Al–4V is an alloy increasingly used in aeronautics due to its high mechanical properties coupled with lightness. An effective technology used to manufacture titanium components with a reduced buy-to-fly ratio is laser beam welding. Previous studies showed that the key factor that rules the mechanical properties and the fatigue life of the joint is its morphology. The aims of this paper were to investigate the influence of the geometrical features of the joints (height of the top and root reinforcement, depth and radius of the underfill, and the valley–valley underfill distance) on their mechanical properties and also to conduct a finite element (FE) analysis on the real geometry of the welded joints. Ti–6Al–4V rolled sheets 3.2 mm thick were welded in butt joint configuration using a laser source and their performance was studied in terms of weld morphology, microstructure, Vickers microhardness and fatigue life. A full factorial plan, designed varying the welding speed and laser power, was carried out. The real geometry and then the joint morphology were studied through an innovative approach: for each specimen, both the total weld face and the total root surface were acquired using a confocal microscope. Finally, through these acquisitions, the clouds of points of the scanned surfaces were used in order to carry out a FE analysis capable of providing a stress concentration factor, K _t , value for each detected joint. The main results are the realization of a reliable FE model by an experimental agreement and the relationship found amongst the fatigue performances and some noticeable metallurgical and geometrical features, such as the underfill depth and the aspect ratio defined as the ratio between the maximum height of the joint and the valley–valley underfill distance.     

Crack-tip constraints of through-wall cracked pipes and its similarity to curved wide plates

In the present study, the effects of pipe geometries, material properties and loading conditions on crack-tip constraints of pipes with circumferential Through-wall crack (TWC) were investigated via systematic 3-dimensional (3-D) Finite element (FE) analyses. The crack-tip constraints were quantified by Q-stress, and to characterize the elastic-plastic strain hardening material behavior, Ramberg-Osgood (R-O) material was employed. Based on the FE results, it was observed that crack-tip constraints of pipes with TWC were dependent on crack length and thickness of pipe, however, the effects of each variables decreased as either thickness of pipe becomes thinner or crack length becomes longer. Moreover, the effects loading modes on Q-stresses for thin-walled pipes with TWC are negligible. Finally, the present Q-stresses of pipes were compared with those of Curved wide plate (CWP) in tension to address the similarity of crack-tip constraints between pipe and CWP, which could be used to produce the CWP to measure the fracture toughness of pipes accurately.     

Physical interpretation of the <Emphasis Type="Italic">J</Emphasis><Subscript>2</Subscript> integral by identifying the associated crack translation

The physical interpretation of the J integral (i.e., J ₁ integral) is clear. J ₁ integral is the energy release rate associated with crack extension (i.e., translation of the crack in the x ₁ direction). However, the physical interpretation of the J ₂ integral remains unclear. In this study, different crack translations in the x ₂ direction were selected and tested by calculating the energy release rates associated with the crack translations and comparing them with the theoretical value of the J ₂ integral.     

Probabilistic fatigue-creep life reliability assessment of aircraft turbine disk

Life consumption is one of the serious concerns for turbine disk performance and maintenance cost. The low cycle fatigue of turbine disk coming from the creep and their interaction has a direct influence on life and reliability of an aero engine. So, in this paper, the finite element analysis fatigue creep on Nickel base super alloy GH4133 turbine disk at 650 °C is computed based on the material properties, load and performance parameter of the low cycle fatigue as a random factor. The LCF Manson coffin model for cycles to failure (N_f ) and Larson Miller (N_c) are calculated. Damage fraction summation is used to calculate the low cycle fatigue creep life and fitted by two parameter Weibull distribution. Therefore, Monte Carlo simulation is used in random sampling from two parameter Weibull distribution to estimate the reliability assessment of turbine disk.     

Discretized Markov chain in damage assessment using Rainflow cycle with effects of mean stress on an automobile crankshaft

We studied the effect of mean stress correction factor using the Rainflow counting technique to assess the fatigue damage of an automobile crankshaft under service loading by considering the stochastic process of the Markov chain. The failure of the crankshaft will cause serious damage to the engine and also to other connecting subcomponents. The service loading is computationally generated from the Discrete Markov chain model and the fatigue cycle is counted using the Rainflow counting technique with the consideration of the local minima and maxima load. To quantify the fatigue damage, the strain-life curve using the fatigue mean stresses was used to model the fatigue failure of the material used in for the crankshaft at N _f = 10 ⁶ . The fatigue mean stresses were used to estimate the effects of the mean stress on the fatigue strength of the component under service loading condition. Statistical verification with the boundary condition of the 90% confidence level was performed to observe the difference between the stochastic algorithms when compared towards the fatigue life behavior of the ductile cast iron material. We concluded that for the practical application, the proposed stochastic model provides a highly accurate assessment of fatigue damage prediction for improving the safety and controlling the risk factors in terms of structural health monitoring.     

Influence of combined impact and cyclic loading on the overall fatigue life of forged steel,EA4T

The performance of forged steel, EA4T, used in rail industry, under simulated in service conditions, i.e. combined impact - cyclic loading, was investigated through a comprehensive experimental programme. The standard Paris-Erdogan fatigue design curve parameters, m and C, were calibrated to account for the effect of the impact component of loading. A minimum threshold for impact load component, identified in the experiments, was also incorporated in the proposed empirical model. Comparison with experimental findings indicated that this “modified” Fatigue design curve could predict the fatigue life of pre impact loaded specimens with sufficient accuracy. It was therefore suggested that the modified model may be used as a novel design tool for predicting the overall fatigue life of components made of this material under the specified combined impact and fatigue loading conditions.     

Examining the Experience of Revealing Delayed Deformation Corrosion Cracking

The problem of skipping iron-oxide–filled damages in the welding junctions between the collector and Dn1200 branch pipe of the PGV-1000 steam generator in a VVÉR-1000 reactor facility is considered. The mechanism of crack nucleation and development is described. A hypothesis that cracks filled with corrosion deposits exhibit minimum reflectivity has been proposed and confirmed. Results of theoretical research into transmission and reflection of ultrasonic waves in the presence of a thin layer of corrosion products surrounded by pearlitic steel are provided. The reflectances of longitudinal and transverse waves from corrosion damages have been calculated and proved the viability of the hypothesis.     

Manufacturing and mechanical characterization of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/β-TCP/TiO<Subscript>2</Subscript> biocomposite as a potential bone substitute

This paper focuses on the mechanical characterization of a bioceramic based on commercial alumina (Al₂O₃) mixed with synthesized tricalcium phosphate (β-TCP) and commercial titania powder (TiO₂). The effect of β-TCP and TiO₂ addition on the mechanical performance was investigated. After a sintering process at 1600 °C for 1 h, various mechanical properties of the samples have been studied, such as compressive strength, flexural strength, tensile strength, elastic modulus, and fracture toughness. The measurements of the elastic modulus (E) and the tensile strength (σ _t ) were conducted using the modified Brazilian test while the compressive strength (σ _c ) was determined through a compression test. Also, semi-circular bending (SCB) specimens were used to evaluate the flexural strength (σ _f ) and the opening mode fracture toughness (K _IC). From the main results, it was found that the best mechanical performance is obtained with the addition of 10 wt.% TCP and 5 wt.% TiO₂. Alumina/10 wt.% tricalcium phosphate/5 wt.% titania composites displayed the highest values of mechanical properties and a good combination of compressive strength (σ _c ?≈?352 MPa), flexural strength (σ _f ?≈?98 MPa), tensile strength (σ _t ?≈?86.65 MPa), and fracture toughness (K _IC?≈?13 MPa m^1/2).     

Calculation of negative-pressure chip in suction-type internal chip removal system and analysis of influencing factors

Compacted graphite iron (CGI) is considered as the ideal material to make modern fuel-efficient diesel engine. Due to the vermicular or worm-like graphite distributed among the ferrite/pearlite matrix, CGI behaves better physical and mechanical properties in comparison with gray cast iron (GCI) and spherical graphite spheroidal cast iron (SGI). However, these good properties bring about the machining challenges. So it is important to appropriately select cutting parameters to machine this material with economy and efficiency. The present study investigated the influence of cutting parameters, such as cutting speed V, feed rate f, and exit angle Ψ, on workpiece material removal volume Q and cutting burr height on the entrance side H₁ and on the exit side H₂ during high-speed milling of CGI by the coated carbide tools_. On this basis, the relatively optimum high-speed cutting parameters were selected under the research condition. Cutting tool failure mechanism was also investigated with the aid of scanning electronic microscope (SEM) and energy-dispersive system (EDS) (SUPRA55, Germany) analysis. The results showed that Q, H₁, H₂, and the type of cutting burr on the exit side of the machined surface could be influenced by the cutting parameters. And the relatively optimum cutting parameters are V = 800 m/min, f = 0.25 mm/rev, and Ψ = 60°. Adhesive wear and thermal cracks which were perpendicular to the cutting edge were common wear mechanisms during the cutting process. However, with an increase in feed rate, mechanical cracks which were parallel to the cutting edge could be found on the flank face of the cutting tool.     

Lubricity of High Water Content Aqueous Gels

Aqueous gels such as biopolymer gels, mucus, and high water content hydrogels are often qualitatively described as lubricious. In hydrogels, mesh size, ξ, has been found to be a controlling parameter in friction coefficient. In the tribology of aqueous gels, we suggest that the Weissenberg number (Wi) is a useful parameter to define different regimes, and following the original formulations in rheology, Wi is given by the polymer relaxation time (ηξ³/k_BT) multiplied by the shear rate due to fluid shear through a single mesh (V/ξ): Wi?=?ηVξ²/k_BT. At sliding speeds below a Weissenberg number of approximately 0.1, Wi?<?0.1, the friction coefficient is velocity-independent and scales with mesh size to the ??1 power, µ ∝ ξ^?1. De Gennes’ scaling concepts for elastic modulus, E, give a dependence on polymer mesh size to the ??3 power, E ∝ ξ^?3, and following Hertzian contact analysis, the contact area is found to depend on the mesh size squared, A ∝ ξ². Combining these concepts, the shear stress, τ, and therefore the lubricity of aqueous gels, is predicted to be highly dependent on the mesh size, τ ∝ ξ^?3. Studies aimed at elucidating the fundamental mechanism of lubricity in biopolymer gels, mucus, and hydrogels have wrestled with comparisons across mesh size, which can be extremely difficult to accurately quantify. Using scaling concepts relating polymer mesh size to water content reveals that shear stress decreases rapidly with increasing water content, and plots of shear stress as a function of swollen water content are suggested as a useful method to compare aqueous gels of unknown mesh size. As a lower bound, these data are compared against estimates of fluid shear stress for free and bound water flowing through a mesh size estimated by the water content of the gels. The results indicate that the strong dependence on lubricity is likely due to a synergistic combination of a low viscosity solvent (water) coupled to a system that has a decreasing friction coefficient, modulus, and the resulting contact pressure with increasing water content. Although the permeability, K, of aqueous gels increases dramatically with water content (and mesh size), K ? ξ²/η, the stronger decrease of the elastic modulus and subsequent decrease in contact pressure due to an increase in the contact length, predicts that the draining time under contact, t, actually increases strongly with increasing water content and mesh size, t ∝ ξ². Consistent with the finding of extremely high water content aqueous gels on the surfaces of biological tissues, these high water content gels are predicted to be optimal for lubrication as they are both highly lubricious and robust at resisting draining and sustaining hydration.     

Parametric study on the fatigue life of railways under rolling contact fatigue by three-dimensional numerical analysis

The current paper presents the results of parametric analyses on the stress intensity factor (SIF) of railways with inclined cracks under rolling contact fatigue (RCF). A 3D finite element (FE) model was proposed to demonstrate the shear mechanism in RCF. The feasibility of the suggested numerical model was verified through the SIF (K) obtained from advanced 3D FE analysis compared with existing 2D FE results. Based on the series of FE analyses, the sensitivity analysis on the cracked depth, surface/crack friction coefficients, and inclined angle, which mainly affected SIF history at the cracked tip, was examined. SIF distributions for various locations of the wheel along the cracked tip were also presented.     

A new approach to measure the elasticity modulus for ceramics using the deformation energy method

This paper presents an alternative method to measure the modulus of elasticity to traction, E, for relatively limited sample sizes. We constructed a measurement system with a Force sensor (FS) and a Rotation movement sensor (RMS) to obtain a relationship between force (F) and bending (ΔL). It was possible by calculating the strain energy and the work of a constant force to establish a relationship between these quantities; the constant of proportionality in this relationship depends on E, I and L. I and L are the moment of inertia of the uniform cross-section in relation to an oriented axis and length, respectively, of the sample for bending. An expression that could achieve the value of E was deduced to study samples of Y-TZP ceramics. The advantages of this system compared to traditional systems are its low cost and practicality in determining E.     

Nonparametric estimation of a probability density function and its derivatives. <Emphasis Type="Italic">C</Emphasis>-approach

Nonparametric (kernel) estimation of a probability density function f(x) for a sample of finite size is considered using the C-approach. The smoothness parameter β of the estimated probability density is introduced. For the case β > 2, it is shown that the convergence of the density estimate f _n (x) to the function f(x) can be improved by using alternating-sign weight functions (higher-order weight functions). Estimation of the derivatives of a function is briefly considered using the same approach.     

Soft computing models and intelligent optimization system in electro-discharge machining of SiC/Al composites

In this paper, a multi-variable regression model, a back propagation neural network (BPNN) and a radial basis neural network (RBNN) have been utilized to correlate the cutting parameters and the performance while electro-discharge machining (EDM) of SiC/Al composites. The four cutting parameters are peak current (I_p), pulse-on time (T_on), pulse-off time (T_off), and servo voltage (S_v); the performance measures are material remove rate (MRR) and surface roughness (Ra). By testing a large number of BPNN architectures, 4-5-1 and 4-7-1 have been found to be the optimal one for MRR and Ra, respectively; and it can predict them with 10.61 % overall mean prediction error. As for RBNN architectures, it can predict them with 12.77 % overall mean prediction error. The multivariable regression model yields an overall mean prediction error of 13.93 %. All of these three models have been used to study the effect of input parameters on the material remove rate and surface roughness, and finally to optimize them with genetic algorithm (GA) and desirability function. Then, an intelligent optimization system with graphical user interface (GUI) has been built based on these multi-optimization techniques, in which users can obtain the optimized cutting parameters under the desired surface roughness (Ra).     

Determining electrophysical characteristics of metal-oxide-semiconductor structures from the data of voltage-capacitance analysis of the depletion region of a semiconductor surface

The oxide layer in nanotransistors with metal-oxide-semiconductor (MOS) structures may be as thin as 20Å. The physical diagnostics of such structures via conventional methods of voltage-capacitance characteristics (VFCs) is impossible without taking into account the usually disregarded effects of degeneracy and dimensional quantization of the electron gas. However, as the oxide-layer thickness decreases, these effects make an increasingly substantial contribution to capacitance C of the MOS structure not only at C?C _i (where C _i is the “oxide capacitance”) but also at C < C _i . In this study, we have developed a general method for determining the principal characteristics of MOS structures from the data of analysis of the VFCs in the region of the Schottky depletion layer. The doping level, the surface potential, the semiconductor surface charge, the voltage of “flat bands,” oxide capacitance C _i , the voltage drop across the oxide, and the sign and density of the charge fixed in it can be found at an accuracy of ?0.1% within the framework of a single experiment regardless of the oxide-layer thickness and without using fitting parameters and a priori assumptions concerning the properties of the electron gas in the accumulation and inversion layers. The stages and results of the implementation of this method are demonstrated by the results of experiments performed on an n-Si-based MOS structure with a 171.2 Å-thick oxide layer.     

Measurements of the solidification point of the GKGh-136 silicone liquid

Two methods for measuring solidification point T _S of the GKGh-136 silicone liquid are described. T _S ≌ 125 K is determined by the first method from specific features of the temperature dependence of the resistance of an organic quasi-2D conductor in a GKGh-136 droplet with a size of ～1 mm. T _S ≌ 130 K is assessed by the second method from the occurrence of a specific feature in the temperature dependence of the resistance of the GKGh-136 and fine-dispersed graphite mixture, which is caused by desorption of helium in the sample volume during warming-up >T _S .