Accelerated Life Testing for a Class of Linear Hazard Rate Type Distributions

Accelerated life testing for distributions with hazard rate functions of the form r(t) = Ag(t) + Bh(t) are considered. Let V ₁, …, V _k be stress levels larger than V ₀—the stress level under normal conditions [V ₀ > 0]—and let a(v) be a nondecreasing function on (0, ∞). We discuss a generalization of the common accelerated models (the power rule model and the Arrhenius model) by assuming that the hazard rate under the stress level V, is of the form (a(V _t )) ^P (Ag(t) + Bh(t)). The maximum likelihood estimators of A, B and P for complete and censored samples are studied. The estimation procedure reduces to a solution of one equation with one unknown parameter. The estimation procedure under the assumption of aging is also described. The asymptotic variance-covariance matrix is given.     

Investigation on performance of abrasive water jet in machining hybrid composites

For machining of composites, abrasive water jet machining is widely employed. For assembly of the machine tool structure, production of slots is essential. In this paper, abrasive water jet machining of composite laminates was experimentally investigated for various cutting parameters in terms of average surface roughness (R_a) and kerf taper (K_t). By generating a response surface model, the experimental values obtained for quality characteristics (R_a and K_t) were empirically related to cutting parameters. The effects of cutting parameters on quality characteristics were analyzed by utilizing empirical models and also optimized within the tested range based on desirability approach. The optimum parameter levels were also validated by confirmation test. From this investigation, it is evident that for obtaining a minimum kerf taper, traverse speed, water pressure, and abrasive mass flow rate are significant parameters and for obtaining less surface roughness traverse speed is the significant parameter.     

Weibull analysis for normal/accelerated and fatigue random vibration test

In this paper, the formula to estimate the sample size n to perform a random vibration test is derived only from the desired reliability (R(t)). Then, the addressed n value is used to design the ISO16750‐3 random vibration test IV for both normal and accelerated conditions. For the normal case, the applied random vibration stress (S) is modeled by using the Weibull stress distribution [W(s)]. Similarly, for the testing time (t), the Weibull time distribution [W(t)] is used to model its random behavior. For the accelerated case, by using the over‐stress factor fitted from the W(t) and W(s) distributions, four accelerated scenarios are formulated with their corresponding testing's profiles. Additionally, from the W(s) analysis, the stress formulation to perform the fatigue and Mohr stress analysis is given. Since the given Weibull/fatigue formulation is general, then the formulas to determine the W(s) parameters, which correspond to any principal stresses values and/or vice versa, are given. Although the application is performed to demonstrate R(t) = 0.97 by testing only n₂ = 6 parts, the guidelines to use the values given in columns n, S, and t of the Weibull analysis table to generate several accelerated testing plans are given.     

The surface topography of retrieved femoral heads

Excessive wear of UHMWPE sockets is considered detrimental to the long-term performance of total hip replacement procedures. Although many factors contribute to the wear process, laboratory experiments have indicated that one of the most important factors affecting the wear rate is surface topography and in particular, the arithmetic mean surface roughness, R _a, of the hard counterface. Surface roughness values were therefore obtained from the heads of 37 explanted and five new Charnley prostheses. The surface topography was measured using a Rodenstock RM 600 non-contacting profilometer. Five parameters were used to give a quantitative characterization of the surface texture; arithmetic mean surface roughness, R _a, root mean square (RMS) surface roughness, R _q, peak to valley height, R _t, average single peak to valley height from five adjoining sample lengths, R _z, and the skewness of the height distribution, Sk. Further, qualitative investigations were undertaken using a Joel-JSM-IC848 scanning electron microscope (SEM). The median values of R _a, R _q, R _t and R _z for explanted heads showed statistically significant increases when compared with values from new prostheses. No significant difference was found between any of these parameters measured in the anterior-posterior and the medial-lateral directions. This result may have important implications for the design of joint simulators. No correlation was found between any of the parameters and implant period.     

Weibull stress distribution for static mechanical stress and its stress/strength analysis

The method's steps to estimate the Weibull shape (β) and scale (η) parameters, based only on the ratio of the maximal and minimal principal stresses (σ₁/σ₂) and on the designed reliability (R(t)) are given in Section 4.1 . The method's efficiency is based on the following facts: (1) The square root of σ₁/σ₂ represents the base life on which the Weibull lifetimes are estimated (see Equation  61 ). (2) The mean of the logarithms of the expected lifetimes (g(x)) is completely determined by the determinant of the analyzed stress matrix (see Equation  13 ). (3) The Weibull distribution is a circle centered on the arithmetic mean (μ), and it covers the whole principal stresses' span (see Figure  5 ). (4) σ₁/σ₂ and g(x) completely determine the σ_1i and σ_2i values, which correspond to any lifetime in the Weibull analysis (see Equation  54 ). And (5) σ₁/σ₂ and η completely determine the minimal and maximal lifetime, which corresponds to any σ_1i and σ_2i values (see Equation  57 ). Additionally, by using the addressed stress β and η parameters, when the stress is either constant or variable, the formulation to estimate the designed R(t) index is given. The steps to determine both the material's strength average (μ_M) for a desired R(t) index and the R(t) index, which corresponds to a used μ_M value, are given.     

Prediction of the energy dissipation rate in ductile crack propagation

In this paper, energy dissipation rate D vs. Δa curves in ductile fracture are predicted using a ‘conversion’ between loads, load‐point displacements and crack lengths predicted by NLEFM and those found in real ELPL propagation. The NLEFM/ELPL link was recently discovered for the DCB testpiece, and we believe it applies to other cracked geometries. The predictions for D agree with experimental results. The model permits a crack tip toughness R(Δa) which rises from J_c and saturates out when (if) steady state propagation is reached after a transient stage in which all tunnelling, crack tip necking and shear lip formation is established. J_R is always greater than the crack tip R(Δa) and continues to rise even after R(Δa) levels off. The analysis is capable of predicting the usual D vs. Δa curves in the literature which have high initial values and fall monotonically to a plateau at large Δa. It also predicts that D curves for CCT testpieces should be higher than those for SENB/CT, as found in practice. The possibility that D curves at some intermediate Δa may dip to a minimum below the levelled‐off value at large Δa is predicted and confirmed by experiment. Recently reported D curves that have smaller initial D than the D‐values after extensive propagation can also be predicted. The testpiece geometry and crack tip R(Δa) conditions required to produce these different‐shaped D vs. Δa curves are established and confirmed by comparison with experiment. The energy dissipation rate D vs. Δa is not a transferable property as it depends on geometry. The material characteristic R(Δa) may be the ‘transferable property’ for scaling problems in ELPL fracture. How it can be deduced from D vs. Δa curves (and by implication, J_R vs. Δa curves) is established.     

Multilayer Co/Pd films with nanocrystalline and amorphous Co layers: Coercive force, random anisotropy, and exchange coupling of grains

The values of saturation magnetization M _s, exchange coupling constant A, local magnetic anisotropy field H _a, random anisotropy correlation radius R _c, and coercive force H _c were independently measured for multilayer Co/Pd films with nanocrystalline and amorphous Co layers. It is shown that variation of the coercive force H _c(t _Co) as a function of the cobalt layer thickness t _Co is related to changes in characteristics of the magnetic microstructure. The main factor determining changes in the ferromagnetic correlation radius R _f and the average anisotropy 〈K〉 of a magnetic block in multilayer Co/Pd films is variation of exchange coupling constant A(t ^Co).     

Prospects for 4-laser white-light sources

This paper describes a series of simulation experiments designed to demonstrate the feasibility of using solid-state lasers in white-light mixtures. The mixed-laser sources are evaluated in terms of the luminous efficacy of the radiation (LER) as well as different measures of colour performance. The latter include the CIE colour rendering index (R_a) and the IES colour fidelity index (R_f) as well as a selection of additional parameters in the two systems. Optimization of the mixtures is achieved by the use of purpose-designed differential evolution algorithms. The best results to date (with four real laser wavelengths) are R_f of 84 with LER 364?lm/W, which indicate the feasibility of the mixed-laser approach to provide highly efficient, energy-saving light sources. These prospects will be further enhanced by potential future developments in semiconductor lasers, with the possibility of producing R_f of 86 with LER of 380?lm/W.     

On Methods of Constructing Sets of Mutually Orthogonal Latin Squares Using a Computer. I

This paper deals with the problem of finding sets of mutually orthogonal Latin squares of order 4t (where 4t – 1 is a prime power) based on orthogonal mappings of a group. For the group G we take the module G(2, 2t) whose elements are vectors (a ₁, a ₂,) where a ₁ is a residue class (mod 2) and a ₂ is a residue class (mod 2t), the addition being defined by (a ₁, a ₂) + (b ₁, b ₂) = (c ₁, c ₂) where a ₁ + b ₁ = C ₁ (mod 2) and a ₂ + b ₂ = c ₂ (mod 2t). Then the search for orthogonal mappings is materially simplified by using a configuration based on the balanced incomplete block design with parameters v = b = 4t – 1, r = k = 2t – 1, λ = t – 1. Using this method, two sets of five mutually orthogonal Latin squares of order 12 were obtained.     

Physical properties of pulsed current copper electrodeposits

Square-wave unipolar pulse electroplating and bipolar pulse reverse electroplating of copper using an acid copper electrolyte of the type used for printed circuit board production has been studied. The effects of pulse-plated coatings on cathode efficiency and on the coatings properties, such as surface roughness, microhardness, stress-strain characteristics and elongation, are reported. The results show that microhardness, tensile strength and elongation are directly affected by the pulse technique used.Nomenclature A Cross-sectional area (m²) - CE Relative cathode efficiency - i Current density (A dm^–2) - i _m,i _p Mean, peak current density (A dm^–2) - M _a Actual deposit mass (g) - M _t Theoretical mass (g) - R _a Average roughness - R _tm Mean roughness - t Coating thickness (m) - t _off Off-time (ms) - t _on On-time (ms) - duty cycle (%) - density (g m^–3)     

Parametrizing ductile tearing resistance by four parameters

The energy dissipation rate, R, is considered as a measure of resistance to crack extension in elasto-plastic fracture mechanics. It can be re-evaluated from J_R test records of bend and tensile specimens. Three types of R(Δa)-curves are identified. If crack initiation occurs close to or at maximum load, the energy dissipation rate is decreasing with crack extension and approaches a stationary value. This type of R(Δa)-curves can be described by an exponential function with three parameters, namely the initial value, the final stationary value, and a transition length. The cumulative J_R-curves for different specimen geometries can be derived by integration. The three parameters of the R(Δa)-fit together with an integration constant, the initiation value, J_i, characterize ductile fracture resistance both quantitatively and physically interpretable. Constraint effects on R-curves can be quantified in terms of these parameters. A procedure for transferring J_R-curves from one geometry to another is proposed.     

Adaptive Distribution-free Regression Methods and their Applications

We consider the model Y = βx + Z, where the random variable Z has a continuoustype distribution that can be badly skewed, contaminated, or censored. To test the hypothesis H ₀ : β = β₀, we use the distribution-free statistic K(β₀) = Σc(Q _i )a(R _i ), where c(·) and a(·) are increasing score functions and Q _i and R _i are the respective ranks of x _i and y _i – β₀ x _i . The score functions c(·) and a(·) can be adapted or chosen after observing the data without destroying the distribution-free nature of the test. A Monte Carlo study is presented which illustrates the excellent performance of an adaptive test when a wide range of distributions is considered for the residuals. Interval and point estimates of sβ can be found by employing the “inverse” of the testing procedure. These results are used to find estimates of the percentile lines. Two examples are given which involve lifetimes of electric motor insulation and grade point averages of beginning university students, respectively.     

Assessing abrasion performance of self-consolidating concrete containing synthetic fibers using acoustic emission analysis

The key objective of this investigation was to evaluate the abrasion resistance of self-consolidating concrete (SCC) with and without synthetic fibers (SynFs). The abrasion resistance of normal concrete was also investigated in this study for comparison. The abrasion test was performed on concrete specimens according to the rotating-cutter method along with continuous monitoring of acoustic emission (AE) using attached AE sensors. The effects of changing concrete type and incorporating various types (flexible and semi-rigid) and lengths of SynFs on the abrasion behaviour were investigated with the aid of AE analysis. AE signal characteristics such as amplitude, signal strength, number of hits, and duration were gathered during testing. Furthermore, the collected AE data was used to complete b-value analysis as well as intensity analysis resulting in three additional parameters: b-value, severity (S_r), and historic index (H(t)). The results showed that the AE parameters were directly correlated with the abrasion damage in all tested mixtures. Adding SynFs to all SCC mixtures enhanced their abrasion resistance. The flexible fibers variety exhibited better abrasion performance on average than the semi-rigid fibers. Meanwhile, longer fibers showed lower abrasion resistance than the shorter ones with the same type. The results also indicated that AE intensity analysis was able to determine the ranges for H(t) and S_r that identify the extent of damage due to abrasion of SynF-reinforced SCC.     

AWJM Performance of jute/polyester composite using MOORA and analytical models

This work addresses the machinability performance of jute/polyester composites with variable laminate thickness using Abrasive water jet machining (AWJM) process. A hybrid objective function was developed using surface roughness (R_a) and kerf taper angle (T_a) and studied using a cost-effective Multi Objective Optimization by Ratio Analysis named as MOORA. The influence of machining parameters such as hydraulic pressure (P), feed rate (V_f) and standoff distance (S_d) on quality characteristics were considered for this analysis. Among all, V_f was found to be a strong influencing factor on T_a and R_a. The deviation in the magnitude of T_a and R_a was observed in the case of varying laminate thicknesses without affecting the optimum condition. Besides, a mathematical regression model was developed for both T_a and R_a based on the correlation between the dependent variables. Furthermore, two other models of R_a available in the literature were considered for comparison with experimental results. The results revealed the suitability of these models for the polymer-based fiber-reinforced composite materials, but limited to the maximum thickness of 3?mm. The good agreement of the models with two different sets of experimental values was also found.     

Crystal structure of synthesized CuGa0.5In0.5Te2 determined by X-ray powder diffraction using the Rietveld method

A full profile X-ray powder diffraction structure refinement has been carried out on a sample of synthesized CuGa_0.5In_0.5Te₂ using graphite monochromatized CuK step-scan data and a profile shape of the Pearson VII type. The most satisfactory convergence was achieved atR _P=0.0483,R _WP=0.0641,R _B=0.0208 andR _F=0.0320, where,R is the Rietveld refinement. The derived structural parameters at 26 °C area=0.610 09(2) nm,c=1.219 79(4) nm andx [Te]=0.2279(3). The ratio between lattice parameters, =c/2a=0.9997 (0) differed very slightly from 1, while the non-ideal anion displacements,x [Te] 1/4, was manifested by the existence of bond alternation of Cu-Te, Ga-Te and In-Te.     

A concept for scaling J R-curves by plastic constraint factors

Ductile tearing resistance can be characterized in a physically meaningful way by the energy dissipation rate, R. The respective R(a)-curves can be re-evaluated from experimental J _R-curves, and the latter can in reverse be obtained from the former by integration. For fully yielded specimens the geometry dependence of R(a)-curves can be scaled by plastic limit-load factors. This normalization opens a possibility for transferring a J _R-curve from one specimen geometry to another.     

Optimizing Single Point Diamond Turning for Mono-Crystalline Germanium Using Grey Relational Analysis

In this paper the results of an experimental study conducted for precision machining of mono-crystalline germanium with single point diamond turning (SPDT) have been reported. The input parameters include the top rake angle, tool overhang, depth of cut, tool feed rate, and rotational speed of the workpiece. The flat profile is generated on a disk of mono-crystalline germanium possessing three performance characteristics: surface roughness (R_a), profile error (P_t), and waviness error (W_a). The process parameters are optimized to obtain the best surface finish with minimum profile and waviness errors by using the Taguchi method. The grey relational analysis is employed for carrying out multiresponse optimization of performance parameters. The best value of surface finish obtained after multiresponse optimization is 10.7 nm having a profile error and a waviness error of 0.202 µm and 0.046 µm, respectively.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.