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1.
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.  相似文献   

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3.
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 (Ra) and kerf taper (Kt). By generating a response surface model, the experimental values obtained for quality characteristics (Ra and Kt) 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.  相似文献   

4.
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.  相似文献   

5.
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.  相似文献   

6.
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 σ1i and σ2i 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 σ1i and σ2i 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 (μM) for a desired R(t) index and the R(t) index, which corresponds to a used μM value, are given.  相似文献   

7.
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).  相似文献   

8.
Prediction of the energy dissipation rate in ductile crack propagation   总被引:1,自引:0,他引:1  
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 Ra) which rises from Jc 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. JR is always greater than the crack tip Ra) and continues to rise even after Ra) 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 Ra) 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 Ra) may be the ‘transferable property’ for scaling problems in ELPL fracture. How it can be deduced from D vs. Δa curves (and by implication, JR vs. Δa curves) is established.  相似文献   

9.
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 (Ra) and the IES colour fidelity index (Rf) 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 Rf 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 Rf of 86 with LER of 380?lm/W.  相似文献   

10.
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.  相似文献   

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