Dezhen Yang

An Alternative Lifetime Model for White Light Emitting Diodes under Thermal–Electrical Stresses

The lifetime prediction using accelerated degradation test (ADT) method has become a main issue for white light emitting diodes applications. This paper proposes a novel lifetime model for light emitting diodes (LEDs) under thermal and electrical stresses, where the junction temperature and driving current are deemed the input parameters for lifetime prediction. The features of LEDs’ lifetime and the law of lumen depreciation under dual stresses are combined to build the lifetime model. The adoption of thermal and electrical stresses overcomes the limitation of single stress, and junction temperature in accelerated degradation test as thermal stress is more reliable than ambient temperature in conventional ADT. Furthermore, verifying applications and cases studies are discussed to prove the practicability and generality of the proposed lifetime model. In addition, the lifetime model reveals that electrical stress is equally significant to the thermal stress in the degradation of LEDs, and therefore should not be ignored in the investigation on lumen decay of LEDs products.

A Time-Variant Reliability Model for Copper Bending Pipe under Seawater-Active Corrosion Based on the Stochastic Degradation Process

In the degradation process, the randomness and multiplicity of variables are difficult to describe by mathematical models. However, they are common in engineering and cannot be neglected, so it is necessary to study this issue in depth. In this paper, the copper bending pipe in seawater piping systems is taken as the analysis object, and the time-variant reliability is calculated by solving the interference of limit strength and maximum stress. We did degradation experiments and tensile experiments on copper material, and obtained the limit strength at each time. In addition, degradation experiments on copper bending pipe were done and the thickness at each time has been obtained, then the response of maximum stress was calculated by simulation. Further, with the help of one kind of Monte Carlo method we propose, the time-variant reliability of copper bending pipe was calculated based on the stochastic degradation process and interference theory. Compared with traditional methods and verified by maintenance records, the results show that the time-variant reliability model based on the stochastic degradation process proposed in this paper has better applicability in the reliability analysis, and it can be more convenient and accurate to predict the replacement cycle of copper bending pipe under seawater-active corrosion.

Fleet-level selective maintenance problem under a phased mission scheme with short breaks: A heuristic sequential game approach

Abstract Selective maintenance is the most widely used strategy for identifying and performing the maintenance actions necessary for fleet mission success. A fleet of equipment is usually required to perform phased missions with short scheduled breaks. In this case, a selective maintenance model should be extended for frequency-based maintenance optimization. We research the problem considering the application of condition-based maintenance (CBM). The problem is formulated with the objective of reducing the repair frequency and cost. The constraint is the reliability of the phased mission, and the variables are the remaining useful lifetimes (RUL) of all the key subsystems. The equipment can be classified into three echelons based on the health status before each wave of a mission, and a heuristic game framework with state backtracking is proposed for the three echelons to solve the problem. The flowchart and heuristic rules of the game framework are given, and the game algorithms for the second and third echelons are presented. The second echelon algorithm aims to select the dispatched equipment for the current wave and minimize maintenance, and the third echelon algorithm aims to ensure that sufficient equipment is available for the next wave by performing necessary maintenance. Finally, we present two types of strategy adjustment methods with state backtracking to turn infeasible solutions into feasible solutions and to optimize feasible solutions. To verify the capacity of the proposed method, a case involving a fleet of 12 aircraft is analyzed for a three-mission scheme, and the aircraft repair times and costs are reduced by the method.

Enhanced GO methodology to support failure mode, effects and criticality analysis

Failure mode, effects and criticality analysis (FMECA) is a prerequisite and significant task for reliability analysis of a safety critical complex system; meanwhile, it is complicated, error-prone and time-consuming due to the gap between a dedicated process for system functional design and fault propagation analysis. Moreover, traditional empiricism analysis usually omits potential fault propagations, and the Risk Priority Number cannot display the risk of failure modes that attribute to the entire system effectively and precisely. In this article, GO methodology, a wide-spread intuitive reliability and safety model technique in practical engineering systems, is enhanced to support FMECA in a qualitative and quantitative way. Function operators in the GO model are extended to support the function hierarchy, failure modes definition, parameters, failure computation and the connectivity with other components. Based on the information above, the failure propagation algorithm is presented, utilizing the relationship signified by signal flows in the GO model. Next, depending on the extended GO model and automatic FMEA synthesis algorithm, all of the existent and potential failure effects are obtained automatically without artificial experience. With Bayesian Network, the reliability of the system and the risk assessment of each failure mode are obtained simultaneously. Finally, a case study of a mobile platform of robot is introduced to verify and validates the applicability of the proposed method in reliability analysis domains. The proposed method is compared with conventional FMECA approach. It is shown that the proposed method has a better performance in qualitative FMEA analysis and quantitative analysis in risk assessment and sensitivity analysis.