Calibration approach for fluorescence lifetime determination for applications using time-gated detection and finite pulse width excitation

Time-gated techniques are useful for the rapid sampling of excited-state (fluorescence) emission decays in the time domain. Gated detectors coupled with bright, economical, nanosecond-pulsed light sources like flashlamps and nitrogen lasers are an attractive combination for bioanalytical and biomedical applications. Here we present a calibration approach for lifetime determination that is noniterative and that does not assume a negligible instrument response function (i.e., a negligible excitation pulse width) as does most current rapid lifetime determination approaches. Analogous to a transducer-based sensor, signals from fluorophores of known lifetime (0.5-12 ns) serve as calibration references. A fast avalanche photodiode and a GHz-bandwidth digital oscilloscope is used to detect transient emission from reference samples excited using a nitrogen laser. We find that the normalized time-integrated emission signal is proportional to the lifetime, which can be determined with good reproducibility (typically <100 ps) even for data with poor signal-to-noise ratios ( approximately 20). Results are in good agreement with simulations. Additionally, a new time-gating scheme for fluorescence lifetime imaging applications is proposed. In conclusion, a calibration-based approach is a valuable analysis tool for the rapid determination of lifetime in applications using time-gated detection and finite pulse width excitation.     

Control-limit preventive maintenance policies for components subject to imperfect preventive maintenance and variable operational conditions

This paper develops two component-level control-limit preventive maintenance (PM) policies for systems subject to the joint effect of partial recovery PM acts (imperfect PM acts) and variable operational conditions, and investigates the properties of the proposed policies. The extended proportional hazards model (EPHM) is used to model the system failure likelihood influenced by both factors. Several numerical experiments are conducted for policy property analysis, using real lifetime and operational condition data and typical characterization of imperfect PM acts and maintenance durations. The experimental results demonstrate the necessity of considering both factors when they do exist, characterize the joint effect of the two factors on the performance of an optimized PM policy, and explore the influence of the loading sequence of time-varying operational conditions on the performance of an optimized PM policy. The proposed policies extend the applicability of PM optimization techniques.     

Die Integration der Lebensdauerberechnung in den Fahrzeugentwicklungsprozess am Beispiel MIG‐geschweißter Bauteile aus Aluminium im Fahrwerksbereich

Integration of the lifetime calculation in the automobile development process based on MIG‐welded aluminium chassis components The lifetime prediction results for metallic components depend significantly on the required calculation inputs. These inputs include the finite‐element‐meshing of the component geometry, the material properties and the load time histories for the component. In the present study the influence of the finite‐element‐modelling on the lifetime prediction is investigated, based on a line‐welded aluminium chassis component. Of special interest is the modelling of the weldline. For the lifetime calculation the software tool FEMFAT from “Engineering Center Steyr” is used. The evaluation of the lifetime prediction results for weldlines is based on a local stress approach. The qualification of the calculation for the product development process is described, based on typical BMW weldline S‐N‐curves.     

Modelling of fatigue lifetime under non-proportional multiaxial alternating loading

This paper presents two approaches to lifetime prediction under non‐proportional multiaxial alternating loading; a phenomenological approach using the Manson–Coffin relation and a microstructural approach. Both models have in common the use of a new multiaxiality factor. The data sets for the adaptation and validation of both models are taken from the authors' own experiments. In these tests, both the load paths and the phase shift are varied. The biaxial test apparatus allows for an application of fixed principal stress or strain directions even under non‐proportional loading. A fairly good agreement with our multiaxial lifetime results is obtained with both models. For an advanced assessment of the quality of the approaches used, the models are compared with several other well‐known models from the literature. An additive yardstick is used for the comparison of the different models.     

Sharp inequalities for quantiles of system lifetime distributions from failure-dependent proportional hazard model

We consider coherent systems with components whose exchangeable lifetime distributions come from the failure-dependent proportional hazard model, i.e., the consecutive failures satisfy the assumptions of the generalized order statistics model. For a fixed system and given failure rate proportion jumps, we provide sharp bounds on the deviations of system lifetime distribution quantiles from the respective quantiles of single component nominal and actual lifetime distributions. The bounds are expressed in the scale units generated by the absolute moments of various orders of the component lifetime centered about the median of its distribution.     

Application of the weibull proportional hazards model to aircraft and marine engine failure data

The environment in which an equipment operates and associated diagnostic variables, such as metal particle level in engine oil, are factors that can influence an equipment's failure time. These factors can be incorporated into concomitant variable models such as the proportional hazards model (PHM), which has been widely used in medical research but not in engineering reliability. A Weibull PHM is applied to both aircraft engine failure data and marine gas turbine failure data. Examination of the residuals shows a good fit of the Weibull proportional hazards model to the data.     

Robust parameter design optimization for type‐I right censored data

Robust parameter design (RPD) aims to build product quality in the early design phase of product development by optimizing operating conditions of process parameters. A vast majority of the current RPD studies are based on an uncensored random sample from a process distribution. In reality, censoring schemes are widely implemented in lifetime testing, survival analysis, and reliability studies in which the value of a measurement is only partially known. However, there has been little work on the development of RPD when censored data are under study. To fill in the research gaps given practical needs, this paper proposes response surface–based RPD models that focus on survival times and hazard rate. Primary tools used in this paper include the Kaplan‐Meier estimator, Greenwood's formula, the Cox proportional hazards regression method, and a nonlinear programming method. The experimental modeling and optimization procedures are demonstrated through a numerical example. Various response surface–based RPD optimization models are proposed, and their RPD solutions are compared.     

Estimation of vaccine efficacy in a repeated measures study under heterogeneity of exposure or susceptibility to infection

Vaccine efficacy (VE) is commonly estimated through proportional hazards modelling of the time to first infection or disease, even when the event of interest can recur. These methods can result in biased estimates when VE is heterogeneous across levels of exposure and susceptibility in subjects. These two factors are important sources of unmeasured heterogeneity, since they vary within and across areas, and often cannot be individually quantified. We propose an estimator of VE per exposure that accounts for heterogeneous susceptibility and exposure for a repeated measures study with binary recurrent outcomes. The estimator requires only information about the probability distribution of environmental exposures. Through simulation studies, we compare the properties of this estimator with proportional hazards estimation under the heterogeneity of exposure. The methods are applied to a reanalysis of a malaria vaccine trial in Brazil.     

Predicting remnant lifetime of high-temperature materials by a resistance degradation model

Abstract

Lifetime and remnant life of engineering materials at high temperature has been analyzed based on a resistance degradation model. It can be demonstrated that the lifetime includes two time processes: resistance degradation process before crack initiation and crack growth process after the crack initiation. Traditional lifetime prediction, via the crack growth model, was found to involve the paradox that the lifetime is in proportion to the initial crack size. Whereas, experiments of static fatigue using glass sheet specimens did not confirm this proportional relationship. For a smooth sample, fracture resistance depends on the strength of the material, so a strength degradation model was used to estimate the lifetime zone between an upper and lower bound. It is defined that the material fails when the residual strength decline to the working stress or deformation reaches a designed limit. It is concluded that the quantity of lifetime mainly depends on the rate of resistance degradation for a brittle component under applied load. Thus, lifetime prediction is simulated as a simple relationship between distance, rate and time, in which the distance is known, the rate can be obtained from experiments and then the lifetime can be calculated.     

Accelerated failure time models for reliability data analysis

Despite the popularity of the proportional hazards model (PHM) in analysing many kinds of reliability data, there are situations in which it is not appropriate. The accelerated failure time model (AFT) then provides an alternative. In this paper, a unified treatment of the accelerated failure time model is outlined for the standard reliability distributions (Weibull, log-normal, inverse Gaussian, gamma). The problem of choosing between the accelerated failure time models and proportional hazard models is discussed and effects of misspecification are reported. The techniques are illustrated in the analysis of data from a fatigue crack growth experiment.     

Proportional hazards modelling in reliability analysis—an application to brake discs on high speed trains

There has been much recent interest in the reliability field in the application of proportional hazards modelling. A number of practitioners and academics have recently been advocating its use and reviewing the basic theory. The technique is, however, in the early stages of application in reliability studies, and apparently no satisfactory examples of its application to reliability data are to be found in the open literature. An implication is that the enormous potential of this approach for reliability analysis, as well as the particular shortcomings and difficulties of applying it in this field are not well understood in the reliability community as a whole. In this article we discuss the application of the method to the reliability analysis of the brake discs on British Railways high speed trains. The need for such an approach arose owing to the paucity of current reliability analysis methods for the problem in hand. The proportional hazards methodology provided an effective means of exploratory analysis and the application of the method provided an explanation for a previously unresolved discrepancy between the statistical and metallurgical analyses. As a result of its use, the effects of the underlying failure mechanism and the various design and operating variables and their interactions are better understood, and British Rail's maintenance policy has been changed accordingly. On the basis of experience of this and similar applications, the potential for application of proportional hazards modelling in reliability, and the likely benefits to be obtained, are discussed.     

具有有效期易变质品的最优补货策略研究

易变质物品库存管理策略一直是实务界与学术界高度关注的问题,其中具有有效期易变质物品的最优补货策略是研究中的难点。本文研究了有限销售时域内具有有效期易变质物品的库存补货策略,在物品的需求率依赖于物品的库存水平,且物品有效期已知的假设下,本文建立了一个确定易变质物品最优补货策略的优化模型,并给出了具体的求解算法。文章最后给出了数值算例,并对模型中的相关参数作了敏感性分析。     

Semi-parametric additive risk models: Application to injury duration study

In survival analysis, the Cox model is a multiplicative model and widely used in survival analysis. However, the assumption of proportional hazards in the Cox multiplicative model is a crucial one that needs to be fulfilled for the results to be meaningful. When proportionality is a questionable assumption, an alternative but less widely used method is additive model. The additive hazards model assumes that covariates act in an additive manner on an unknown baseline hazard rate.Using the emergency department (ED) visits data, we demonstrated the additive hazards regression models and showed the differences in estimates obtained by the additive hazards models and the Cox model. In our study, the Cox model gave a higher estimate than the additive hazards model. However, both models revealed similar results with regard to covariates selected to remain in the model and the estimated survival functions based on the Cox and additive hazards models were almost identical. Since Cox and additive hazards models give different aspects of the association between risk factors and the study outcome, it seems desirable to use together to give a more comprehensive understanding of data.     

Effect of finite lifetime for the electrons in nearly magnetic metals

The RPA dynamic susceptibility for the interacting electrons in a nearly magnetic metal is calculated, taking a finite lifetime for these electrons. By decreasing the lifetime, one observes a shift of the imaginary part of the dynamic susceptibility toward low frequencies. The physical consequences on the spin-fluctuation electrical resistivity are discussed.Laboratoire associé au CNRS.     

Fluorescence from airborne microparticles: dependence on size, concentration of fluorophores, and illumination intensity

We measured fluorescence from spherical water droplets containing tryptophan and from aggregates of bacterial cells and compared these measurements with calculations of fluorescence of dielectric spheres. The measured dependence of fluorescence on size, from both droplets and dry-particle aggregates of bacteria, is proportional to the absorption cross section calculated for homogeneous spheres containing the appropriate percentage of tryptophan. However, as the tryptophan concentration of the water droplets is increased, the measured fluorescence from droplets increases less than predicted, probably because of concentration quenching. We model the dependence of the fluorescence on input intensity by assuming that the average time between fluorescence emission events is the sum of the fluorescence lifetime and the excitation lifetime (the average time it takes for an illuminated molecule to be excited), which we calculated assuming that the intensity inside the particle is uniform. Even though the intensity inside the particles spatially varies, this assumption of uniform intensity still leads to results consistent with the measured intensity dependence.     

Quantitative kinetic analysis in a microfluidic device using frequency-domain fluorescence lifetime imaging

A novel microfluidic approach for the quantification of reaction kinetics is presented. A three-dimensional finite difference numerical simulation was developed in order to extract quantitative kinetic information from fluorescence lifetime imaging experimental data. This approach was first utilized for the study of a fluorescence quenching reaction within a microchannel; the lifetime of a fluorophore was used to map the diffusion of a quencher across the microchannel. The approach was then applied to a more complex chemical reaction between a fluorescent amine and an acid chloride, via numerical simulation the bimolecular rate constant for this reaction was obtained.     

Non‐proportional size scaling of strength of concrete in uniaxial and biaxial loading conditions

A procedure for non‐proportional size scaling of the strength of concrete based on the weakest‐link statistics is proposed to synchronize strength data from specimens of different geometries and different loading modes. The procedure relies on proportional size scaling of strength to determine the parameters of the statistical model and often on finite element analysis to calculate the coefficient of the equivalent strength. The approach for non‐proportional size scaling is capable to synchronize the uniaxial strength data of concrete from uniaxial tensile specimens and 3‐point bending specimens, or the biaxial tensile strength data of circular plates in different loading mode. The non‐transference of the uniaxial strength data to the biaxial strength data is unclear in its mechanism but possibly due to the variation of statistical distribution of microcracks with stress states in different specimens.     

Proportional hazards model: a review

The proportional hazards model was introduced in 1972 by D. R. Cox in order to estimate the effects of different covariates influencing the times to the failures of a system. The model has been used rather extensively in biomedicine and, recently, interest in its application in reliability engineering has increased. The main purpose of this expository paper is to review the existing literature on the proportional hazards model. At first, the characteristics of the method are explained and its importance in reliability analysis is presented. Subsequently, methods for estimating parameters, along with the small and large sample properties of the estimators, are briefly discussed. Afterwards, work carried out so far on topics such as the effects of interaction, omission, measurement error, misclassification, monotonicity, multicolinearity and time dependency of covariates on the estimator are summarized. Some goodness-of-fit tests, especially those based on graphical methods, are described. We also describe some possible extensions of this model considered so far and available computer programs and packages for estimating the parameters of this model. Finally, some areas for further research are also discussed.     

刚性固定对接裂纹试验的拘束度有限元计算分析

本文采用了一种通过中间的验证性试验来保证刚性固定对接结构拘束度有限元计算的有效性,刚性固定对接结构的拘束度计算结果表明,从焊缝中心到焊缝两端拘束度逐渐变大,在焊缝中间位置处,试板板厚与拘束度近乎成正比例线性关系.     

Recombination dynamics in CdTe/CdSe type-II quantum dots

Recombination dynamics in CdTe/CdSe core-shell type-II quantum dots (QDs) has been investigated by time-resolved photoluminescence (PL) spectroscopy. A very long PL decay time of several hundred nanoseconds has been found at low temperature, which can be rationalized by the spatially separated electrons and holes occurring in a type-II heterostructure. For the temperature dependence of the radiative lifetime, the linewidth and the peak energy of PL spectra show that the recombination of carriers is dominated by delocalized excitons at temperatures below 150?K, while the mixture of delocalized excitons, electrons and holes overwhelms the process at higher temperature. The binding energy of delocalized excitons obtained from the temperature dependence of the non-radiative lifetime is consistent with the theoretical value. The energy dependence of lifetime measurements reveals a third power relationship between the radiative lifetime and the radius of QDs, the light of which can be shed by the quantum confinement effect. In addition, the radiative decay rate is found to be proportional to the square root of excitation power, arising from the change of wavefunction overlap of electrons and holes due to the band bending effect, which is an inherent character of a type-II band alignment.