Conventional Life Tests Research Articles

A life extrapolation model using accelerated equivalent luminance and its application

Aiming at predicting the life of vacuum fluorescent display (VFD) accurately, four groups of constant-stress accelerated degradation tests (ADTs) were conducted, and the life extrapolation model of accelerated equivalent luminance (LEMAEL) based on each test sample was proposed. In this model, firstly, Weibull function was employed to fit luminance degradation data of each sample at each stress. Secondly, Logistic function was determined by Anderson-Darling test to respectively describe the distributions of shape and scale parameters, and the degradation formula of accelerated equivalent luminance was obtained to estimate accelerated life. Finally, Power function was selected by comparing determination coefficients to fit the data points formed by accelerated life and stress, and thereby VFD life was extrapolated. The results indicate that the collected test data of each sample objectively reflect VFD luminance characteristics; the degradation formula intuitively exhibits the luminance variation law at each accelerated stress, and Power function accurately reveals the relationship between life and stress; LEMAEL is characterized by high precision and large amount of data information, which not only actualizes the accurate life estimation for light-emitting devices without conventional life tests, but also provides entirely new modeling ideas and thoughts for reliability evaluation and life prediction of products.

Two life prediction models for optoelectronic displays without conventional life tests.

To precisely predict the life of optoelectronic displays over a short time, two life prediction models were established based on the three-parameter Weibull right approximation method (TPWRAM). In Model I, the acceleration life under each stress was obtained using TPWRAM, the data points formed by acceleration life and acceleration stress were, respectively, fitted by three extrapolation functions and the optimal extrapolation function was determined by comparing the fitting determination coefficient and root-mean-square error. In Model II, after introducing an acceleration parameter, the luminance attenuation data under conventional stress were calculated directly by combining the ones obtained using TPWRAM under each acceleration stress. The luminance attenuation test data from the vacuum fluorescent display (VFD) were collected through four groups of constant-stress accelerated degradation tests (ADT), and the two models were applied to the life prediction of VFD. The results indicated that the designed ADT scheme was feasible, Model I revealed the changing law of life with stress and simplified the process of life prediction, and Model II made it possible to obtain the luminance attenuation formula at conventional stress without conducting a conventional life test, overcoming the shortcomings of long time-consuming traditional life tests. It was verified by comparing the life prediction values that the two models had very high precision, and both of these not only achieve the accurate estimation of optoelectronic product life without resorting to conventional life test, but also improved the method of life prediction and perfect its theoretical system.

Two optimized models of life prediction based on luminance degradation for VFD under a conventional life test

AbstractFocusing on improving the accuracy of existing life prediction models for optoelectronic products, the three‐parameter Weibull right approximation method (TPWRAM) was employed to substitute exponential function based on the least square method in the analysis and two‐staged methods. Two optimized models were established (Model I and Model II), based on maximum likelihood estimation and the Monte Carlo method, respectively. One group of conventional life tests (CLTs) of vacuum fluorescent display (VFD) were conducted to collect luminance degradation data for each sample, and the two optimized models were applied to achieve VFD life prediction and obtain mean time to failure, median life, and confidence intervals. The results indicate that the CLT test design is correct and feasible, the amount of data on luminance degradation is large, and the test data selection method is reasonable. Model I and Model II optimized by TPWRAM both reflect the VFD luminance variation law well, and the predicted life approaches VFD service life from user feedback, proving that the two models are precise, and thus, can provide technical references for researchers and engineers regarding aspects of life prediction.

A life estimation model applied to organic light-emitting diodes

For predicting life for light-emitting devices quickly and accurately, a novel life prediction model, namely an extrapolation model of accelerated life and stress, has been proposed. In this model, a Weibull function is employed to fit luminance decay data under multiple groups of accelerated stresses, and the corresponding accelerated life is obtained. By determination coefficients and root mean square errors, a power function is determined as an extrapolated function to describe the relationship between accelerated life and stress and the life of the light-emitting devices. For organic light-emitting diodes, three groups of constant-stress accelerated degradation tests were conducted by increasing current stress. An extrapolation model of accelerated life and stress was applied to process the collected luminance decay data and was evaluated by a careful comparison with organic light-emitting diode life. The results indicate that the self-designed experimental scheme for organic light-emitting diode is feasible and versatile; the predicted life is 17,113 hours, which is close to the service life derived from user feedback, and the relative error is only 2.2%. This shows that the extrapolation model of accelerated life and stress has high precision; the model reveals the expected law of luminance changing with time and intuitively depicts the life characteristics under accelerated stresses without conventional life tests. This will pave the way for a new method to predict and evaluate the life of modern light-emitting devices.

Accelerated Life Test for Reliability Evaluation of Pneumatic Cylinders

Pneumatic cylinders are intended to be designed for high reliability and long lifetimes. Evaluating such designs through conventional life tests is difficult as the affordable test time is too short to generate a meaningful amount of failure data. In this paper, we design and implement an accelerated life test (ALT) for evaluating the reliability of pneumatic cylinders. From a detailed analysis of failed samples from field operation, the major failure mechanisms that affect the reliability of pneumatic cylinders are identified, e.g., wear and tear of sealing rings. For these failure mechanisms, temperature and frequency are found out to be the significant stresses of the pneumatic cylinders. A two-factor-four-level full-factor design of ALT is, then, performed to set up an ALT plan for the pneumatic cylinders. A total number of 95 samples are tested according to the ALT plan, and a mixed Arrhenius-inverse-power-law model is used to fit the test data and predict the reliability under normal operation conditions. In addition, a normal stress test of eight samples is performed to validate the reliability predicted by the ALT. The results show that using the proposed ALT, the reliability of the pneumatic cylinder can be predicted with satisfactory accuracy. It is also shown that the designed ALT plan requires only 38.77% of the testing effort of other conventional life tests.

A Bayesian Predictive Analysis of Step‐Stress Accelerated Tests in Gamma Degradation‐Based Processes

Degradation modeling might be an alternative to the conventional life test in reliability assessment for high quality products. This paper develops a Bayesian approach to the step‐stress accelerated degradation test. Reliability inference of the population is made based on the posterior distribution of the underlying parameters with the aid of Markov chain Monte Carlo method. Further sequential reliability inference on individual product under normal condition is also proposed. Simulation study and an illustrative example are presented to show the appropriateness of the proposed method. Copyright © 2017 John Wiley & Sons, Ltd.

气缸双应力交叉步降加速寿命试验研究

本文设计了一组双应力交叉步降的气缸加速寿命试验并进行试验。文中讨论了双应力交叉步降试验相比于常规寿命试验和双应力恒定试验的效率优势，使用了累计效率模型和GLL-Weibull模型对试验数据进行了分析。最终通过该方法获得的寿命数据有较高的精度，而且试验效率大大提高。该试验方法有着很大的发展空间。 A Double-Crossed Step-Down-Stress Accelerated Life Testing (DCSDS-ALT) for pneumatic cylinder is designed and carried out in this paper. The paper also discusses the superiority on efficiency of DCSDS-ALT compared with conventional life testing and Twin-Stress Constant Life Testing. The Cumulative Effect Model and GLL-Weibull Model are used in the conversion and analysis of the statistic. The result of the test got satisfied precision, and DCSDS-ALT promotes efficiency signifi-cantly. This method has huge potential on development.

Temperature Index of Electrical Insulations by mTGA

Abstract Temperature index (TI) is a common measure for the temperature classification of electrical insulation materials (EIMs) used in electric machines and apparatus. TI is generally obtained through accelerated life testing lasting >7–8 months at certified labs. This paper deals with a new life-testing method to determine TI using thermogravimetric analysis (TGA), more particularly, modulated TGA (mTGA), The method of estimating precision of the TI is also given. The testing time may be reduced to <15 h per test. The new concept simplifies the life equation for practicability. The new method is not a replacement of conventional life testing, but may be of particular utility for screening new EIMs and evaluating materials with a known TI, after minor changes in composition have been introduced to improve physical properties. It can be a recommended method once it has been verified over a suitable range of EIMs.

Design of Sudden Death Tests for Estimation of a Weibull Percentile

Abstract Sudden Death Testing is a strategy for conducting life tests in which n specimens are divided into g groups each of size m (n=gm). Testing continues simultaneously on the specimens in each group until the first failure occurs in each. The testing thus results in g failures among the n specimens. It is shown how to determine the group size m so that the pth percentile of the Weibull distribution of life may be estimated with greater precision than in a conventional life test wherein n specimens are tested until the occurrence of the gth failure. Comparisons of the expected duration of the life tests in both cases are given for two combinations of g and m for four values of the Weibull shape parameter.

Thermal endurance evaluation of isotactic poly(propylene) based nanocomposites by short-term analytical methods

The thermal endurance of nanocomposites based on isotactic poly(propylene) filled with nanosilicates, is evaluated by a short-term analytical procedure. The procedure is based on oxidative stability measurements by isothermal thermogravimetric analysis in a pure oxygen flow, over the temperature range form 120 to 150 degC, and provides the activation energy of the thermo-oxidative degradation process. The activation energy values thus obtained were compared with those derived by scanning thermogravimetric analysis at different heating rates. The thermal endurance lines for the investigated materials can be drawn with the aid of a conventional life test, based on weight loss measurement as a function of aging time, according to the IEC 60216 procedure. A noticeable increase of the thermal endurance of the isotactic poly(propylene) based nanocomposite with respect to the unmodified polymer is found out.

Thermal endurance evaluation of poly(butylene terephthalate) by conventional and analytical methods

The thermal endurance of poly(butylene terephthalate) was evaluated by both consolidated long-term conventional life tests and a short-term analytical procedure. The latter is based on oxidative stability measurements by isothermal differential calorimetry in a pure oxygen flow. The capability of the analytical technique to provide an activation energy for the thermo-oxidation process was verified. The results obtained by the two methods were compared and discussed. It is shown that the temperature index derived by the analytical procedure can be close to that obtained by the conventional tests, provided that a suitable selection of the test temperatures and failure criteria is made; however, benefits in term of test time shortening are lower than those previously obtained for polyolefin-based materials.

MODELING AND RELIABILITY PREDICTION FOR THE STEP-STRESS DEGRADATION MEASUREMENTS USING NEURAL NETWORKS METHODOLOGY

Rapidly evolving accelerated degradation testing (ADT) has provided an efficient channel for reliability assessment without the labor of conventional life testing that observations are complete or incomplete data. In step-stress loading, a specimen is subjected to successively higher level stress until an appropriate termination time. All specimens go through the same specified pattern of stress levels and testing time. A step-stress degradation testing (SSDT) is corresponding to collect the degradation measurements over time from a step-stress life testing before any specimen fail. The advantages of SSDT may shorten the testing time and reduce the test units compared to ADT. Conventional statistical approach for modeling the step-stress degradation process may be complicated. In this paper, we address a nonparametric approach by applying the methodology of neural networks to calibrate the step-stress degradation measurements and conducting the reliability prediction at use condition. A light emitting diode (LED) example is given to illustrate the implementation procedure.

Evaluation of the United Kingdom ion thruster

A test and evaluation study examining the United Kingdom's 10-cm-diam xenon ion thruster system, known as the UK-10, has been performed at The Aerospace Corporation. The purpose of the study was to determine whether the UK-10 is a suitable replacement on U.S. Air Force satellites for presently employed propulsion devices. The most essential aspects of the UK-10 pertaining to its potential use were evaluated, but a conventional life test was beyond the scope. An examination of performance, lifetime, spacecraft integration issues, mission analysis, and computational modeling and a comparative technology assessment were performed. Emphasis was placed on the creation of a database that will be useful for the integration and operation of satellites using the UK-10 or other gridded electrostatic ion thrusters. Advanced electric-thruster characterization methods produced detailed measurements, in real time, of many fundamental properties of the thruster. The test program concluded that a UK-10 ion propulsion system will be acceptable for use on suitable spacecraft, subject to the considerations discussed, and of considerable potential benefit.

Semiconductor device reliability vs process quality

This paper describes a method for defining a quantitative model relating “quality” expressed in terms of parameter distributions and “reliability” expressed in terms of failure rates. This model makes it possible to generate a more realistic failure rate estimate for semiconductor devices. With this model, failure rate predictions based on conventional life test data and on process parameter distributions, can be defined.

Thermal endurance evaluation of XLPE insulated cables

Thermal endurance characterization of XLPE cable models is realized by applying conventional procedures and an analytical method. The latter method is based on oxidation induction time measurements to estimate the activation energy of the degradation reaction and then the slope of the thermal endurance line. The conventional life test at an intermediate temperature is used to determine the ordinate intercept of the same line. The estimates of temperature indices, obtained by different diagnostic properties and end-point selections on the basis of both conventional and analytical techniques, are fairly close, thus revealing the ability of appropriate analytical procedures to characterize reliably the thermal endurance of XLPE cables, with the advantage of shortening test times with respect to conventional methods. A discussion is presented on the choice of diagnostic properties and end points, in order to achieve thermal endurance lines and indices which provide actual information on cable performance under service stresses. Analysis of the possible choices is performed with reference to the results of multiple-stress tests which simulate service stresses and by investigating the aging mechanisms. Finally, considerations of the compensation effect applied to life lines, with reference to both Eyring and Arrhenius models, are made. Checks relevant to different property end points reveal very good agreement of the experimental results with the aging compensation effect.

Reliability problems in TTL-LS devices

An analysis of the possible failure modes of TTL-LS was carried out, with particular emphasis on long term thermal stability of Schottky diodes, all realized by PtSi - Ti/W - Al metallization system. Only from one supplier were there diodes in which changes in the characteristics, due to metallization defects, were observed during accelerated thermal cycles. On the contrary, once its uniformity is guaranteed, the Ti/W barrier layer inhibits the Si-Al interdiffusion up to 550 °C, 1 hr thermal annealing. Sensitivity to negative input pulses and electrical overstresses were also investigated. Scanning electron microscope and microprobe, and voltage contrast techniques seem to be more suitable instruments to investigate the reliability of such devices than the conventional life tests. The technology employed today seems provide suitable reliability for TTL Low-Power Schottky devices.

The use of thermal analysis to predict the thermal life of organic electrical insulating materials

Three different thermoanalytical methods are introduced as significant time-saving techniques in conventional life tests of some organic electrical insulating materials. DTA in cyclically alternated atmospheres of nitrogen and oxygen according to Randino and Andreotti, and TG analysis methods at different heating rates according to Flynn and Wall and to Broido were applied to two electrical insulations based on aromatic poly-imide and epoxy resin. The activation energies obtained are compared with those derived from the slopes of the life-lines produced via conventional life tests. The assumptions necessary for applying the thermal analyses in these cases are discussed.

Application of Cumulative Degradation Model to Acceleration Life Test

A general degradation model which includes conventional acceleration tests such as fixed, progressive, and step stress experiments is derived from the reaction theory under the assumption of linear degradation accumulation. Its application to the acceleration test is discussed. According to the reaction theory, degradation of the characteristic parameter, μ is connected to reaction rate K and time t by a linear transformation function f(μ) = Kt. The total degradation is determined by the linear accumulation of the Kiti product such as f(μn)= # Kiti. This relationship is also expressed as a generalized Miner's equation #(ti/Li) = 1 in which ti and Li are the actual stressing time and expected life under the ith stress condition, respectively, and # ti is the life expectancy of the component under successive stress conditions from i = 1 to n. Validity of this linear accumulation assumption is investigated under various stress application paths. For monotonic step-up, cyclic, and random stress, the rule almost holds as a whole, but monotonic step-down stress sometimes causes erroneous recovering effects of the parameter. The degradation accumulation principle is effectively applicable to integrate degradation and failure pattern information and also to avoid some shortcomings of conventional life test methods. 1) For example, in order to find efficiently the life vs. stress plot, we could combine the degradation (failure fraction) vs. stress diagram, obtained by one step stress experiment, and the knowledge of the degradation pattern obtained by a constant stress test.

Accelerated life testing and over-stress testing of transistors

The advantages and disadvantages of accelerated life tests compared with conventional life tests are discussed and the methods by which accelerated life tests are used are examined. The physical basis of accelerated life tests in reaction kinetics is described and the methods by which the Arrhenius equation and the Eyring equation can be used to relate time to failure and temperature are discussed. On the basis of the Arrhenius equation and using the log normal failure distribution the results of temperature storage accelerated life tests on germanium alloy transistors are used to make predictions about life behaviour at lower temperatures. These predictions are compared with the results of long-term life tests on several hundred transistors extending to 20,000 hr. The results are then used to compare constant stress and step stress accelerated life tests. Electrical over-stress tests are described in which germanium alloy transistors are subjected to very high power pulses of short duration and silicon planar transistors have high currents passed through their emitter junctions in the reverse direction. Mechanical over-stress tests are exemplified by centrifuge tests on germanium alloy transistors. It is shown for one transistor type that the failure distribution, although in all cases normal, is markedly dependent upon the direction of stress.