Luis Carlos Méndez-González

Reliability Estimation for Products Subjected to Two-Stage Degradation Tests Based on a Gamma Convolution

Degradation models have received much attention in the area of reliability estimation, mostly because it is possible to obtain robust information about the lifetime of highly reliable products and systems. However, in the last years, multivariate models have received more attention. This is because the quality of many products is a function of various environmental conditions, and the effect of these conditions over a performance characteristic can cause a failure of the product, either marginally or jointly. A gamma process is considered in this study to marginally model the degradation of a performance characteristic through two degradation test phases performed sequentially. The degradation increments obtained during the first test and the second test are modeled jointly considering the convolution of two marginal gamma processes. In this way, it is possible to obtain a robust model to get reliability estimates considering the effect of two serial degradation test, which can consider multiple covariates. This modeling is illustrated with crack propagation data and important results are presented. Copyright

Reliability analysis for electronic devices using beta-Weibull distribution

Today, in reliability analysis, the most used distribution to describe the behavior of electronic products under voltage profiles is the Weibull distribution. Nevertheless, the Weibull distribution does not provide a good fit to lifetime datasets that exhibit bathtub-shaped or upside-down bathtub–shaped (unimodal) failure rates, which are often encountered in the reliability analysis of electronic devices. In this paper, a reliability model based on the beta-Weibull distribution and the inverse power law is proposed. This new model provides a better approach to model the performance and fit of the lifetimes of electronic devices. To estimate the parameters of the proposed model, a Bayesian analysis is used. A case study based on the lifetime of a surface mounted electrolytic capacitor is presented, the results showed that the estimation of the proposed model differs from the inverse power law–Weibull and that it affects directly the mean time to failure, the failure rate, the behavior, and the performance of the capacitor under analysis.

Reliability Analysis for Laptop Computer Under Electrical Harmonics

Because of electrical variations presented in the power line, the estimated reliability of electronic devices, such as a laptop computer, differs from the real reliability. The found sources of electrical variations in the power lines were the electrical harmonics in the order of frequencies of 3, 5, 7, 11, and 13. As a consequence of these harmonics, the voltage in the power line has a time varying behavior. And because the power law model used to estimate the reliability does not consider this varying behavior, the estimated reliability differs from the real one. Thus, in this paper, a model that incorporates the electrical harmonics in the power law model is presented. The model was developed according to the Cumulative Damage Model and the time varying voltage into the power law model was modeled by using the compact form of the Fourier series. A comparative study between constant and time varying voltage is presented to analyze the effect of the electrical harmonics in the laptop computer. The conclusions of the study showed that the laptop computer reduces its lifetime by 2000 h on average and the failure rate increases when the electrical harmonics are present in the power lines. Copyright

Bivariate degradation modelling with marginal heterogeneous stochastic processes

ABSTRACT In this paper, we consider that the degradation of two performance characteristics of a product can be modelled by stochastic processes and jointly by copula functions, but different stochastic processes govern the behaviour of each performance characteristic (PC) degradation. Different heterogeneous and homogeneous models are presented considering copula functions and different combinations of the most used stochastic processes in degradation analysis as marginal distributions. This is an important aspect to consider because the behaviour of the degradation of each PC may be different in its nature. As the joint distributions of the proposed models result in complex distributions, the estimation of the parameters of interest is performed via MCMC. A simulation study is performed to compare heterogeneous and homogeneous models. In addition, the proposed models are implemented to crack propagation data of two terminals of an electronic device, and some insights are provided about the product reliability under heterogeneous models.

Wood chile peppers stalks-plastic composite production.

Nowadays, most common wood fibers used to produce wood plastic composites are from Oak and pine trees, rice hull, recycled paper, pallets and post-industrial oak fiber, furniture waste, or cedar wood chips. This paper is focused on a new wood plastic composites based on pre-dried New Mexico red chile stems and leafs with high-density polyethylene. The specimens were manufactured following ASTM 638-9 standard using injection molding. A series of experiments were designed to investigate how wood fiber length, geometry, and water absorption affects the microcellular structure of the new sustainable material. The effects of proportions and particle size on the strength of the resulting blend were analyzed. It was found that larger particle sizes of wood fiber showed higher mechanical properties. The blending of additives in the new sustainable material has shown to increase proper fiber dispersion improving the strength of the material.

Analysis of the mechanical properties of wood-plastic composites based on agriculture Chili pepper waste

In this study, a new composite material conception based on polypropylene (PP) and wood particles obtained from agricultural waste was investigated. Specifically, stalks and leafs of chili plants of the region of New Mexico, Texas, USA were used as a plant-based reinforcing material. For this, a design of experiments (DOE) is proposed considering different percentages of PP, wood, fire retardant, coupling agent and UV stabilizer, different mesh sizes are also considered. Samples were obtained using an extrusion molding machine. A statistical analysis is proposed to obtain the optimal percentages of the components considered in the fabrication of the samples in order to enhance the mechanical and morphologic properties of this new composite. Results show that the yield stress, breaking stress and ultimate tensile stress can be maximized with a proportion of 3 to 1 (63,75% PP and 21,25% wood fiber) of PP and wood fiber.