Testing for the validity of the assumptions in the exponential step-stress accelerated life-testing model
ABSTRACT In the application of the exponential step-stress accelerated life-testing model, there are usually three assumptions required: (1) for any stress level, the lifetime distribution of a test unit is exponential; (2) for any stress level, the mean life of a test unit is a log-linear function of stress; (3) a cumulative exposure model holds. This paper explores the validity of assumptions 1 and 3. It is proved that assumption 3 is unnecessary to the exponential step-stress accelerated life-testing model. A test statistic is proposed to test the validity of the assumptions 1. The null distribution of the test statistic is derived. A Monte Carlo simulation is given to study the power of the proposed test procedure. Finally, an example is given to illustrate the proposed test procedure.
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ABSTRACT: Lifetime performance assessment has been crucial to the manufacturing industry. In practice, a lifetime performance index CL is used to measure the larger-the-better type quality characteristics. Accelerated life test (ALT) has often been used to yield information quickly so that the life distribution of products can be estimated. This study constructs a maximum likelihood estimator (MLE) of CL for exponential products based on type II right censored data from the step-stress accelerated life test (SSALT). The MLE of CL is then utilized to develop the hypothesis testing procedure with the given lower specification limit L . This new testing procedure can be easily applied to assess whether the lifetime of products meets the requirements. Finally, we give two examples to explicate the proposed testing procedures.IEEE Transactions on Reliability 03/2013; 62(1):296-304. DOI:10.1109/TR.2013.2241197 · 1.66 Impact Factor
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ABSTRACT: Constant-stress procedures based on parametric lifetime distributions and models are often used for accelerated life testing in product reliability experiments. Maximum likelihood estimation (MLE) is the typical statistical inference method. This paper presents a new inference method, named the random variable transformation (RVT) method, for Weibull constant-stress accelerated life tests with progressively Type-II right censoring (including ordinary Type-II right censoring). A two-parameter Weibull life distribution with a scale parameter that is a log-linear function of stress is used. RVT inference life distribution parameters and the log-linear function coefficients are provided. Exact confidence intervals for these parameters are also explored. Numerical comparisons of RVT-based estimates to MLE show that the proposed RVT inference is promising, in particular for small sample sizes.IEEE Transactions on Reliability 09/2014; 63(3):807-815. DOI:10.1109/TR.2014.2313804 · 1.66 Impact Factor
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ABSTRACT: Copper-graphite composite is an important tribological material used in electrical sliding contact applications like electrical brushes in motors and generators. The electrical sliding contact experiences multiple stresses such as mechanical pressure and temperature. Traditional life tests under normal operating condition would be a time-consuming process due to the longer expected life of the composite. Accelerated wear testing was carried out to evaluate the life characteristics of the composite. This work focuses on evaluation of tribological performance of microwave-sintered copper-graphite composite using accelerated wear testing methodology using high temperature pin-on-disc tribometer. Microstructural studies of worn out surfaces were carried out using SEM with EDAX. Reliability and analysis on life characteristics were performed on the time-to-failure data using temperature-nonthermal-accelerated life-stress model. The obtained times-to-failure data from the accelerated wear testing was extrapolated to normal usage condition. Temperature and pressure are significantly affecting the wear performance. Self-lubricating action of graphite and improvement in wear resistance is helpful in extending the life of copper graphite composite. The life of the composite obtained through testing at mean and 99% reliability are 18,725 and 16,950 h, respectively.Journal of Materials Engineering and Performance 11/2012; 21(11). DOI:10.1007/s11665-012-0161-z · 0.98 Impact Factor