Degradation Process Model Research Articles

Reliability analysis of direct drive electrohydraulic servo valves based on a wear degradation process and individual differences

Electrohydraulic servo valves play critical roles in modern servo control systems, which require high reliability and high safety. The reliability analysis of a direct drive electrohydraulic servo valve is conducted in this article. First, the failure mechanism of the direct drive electrohydraulic servo valve is investigated by analyzing the structure and the working principle of the direct drive electrohydraulic servo valve. It shows that clamping stagnation, internal leakages and spring fatigue are the main failure modes of direct drive electrohydraulic servo valve. The structure degradation caused by wear enlarges the clearance and results in the increase in null leakages. Then, a gamma process is adopted to describe the internal structure degradation based on the failure mechanism analysis. Heterogeneity among different samples of direct drive electrohydraulic servo valves is studied and handled by introducing unit-specific random effects into the gamma process degradation model. Additionally, in this article, a Bayesian method is used to facilitate the degradation analysis and reliability estimation. The reliability models of sealing, springs and spool valves are presented. Finally, a brief introduction of the experiment of the direct drive electrohydraulic servo valves and an illustrative example of reliability analysis are presented to demonstrate the introduced failure mechanism analysis and the proposed reliability analysis method for direct drive electrohydraulic servo valves.

Understanding Membrane Degradation Mechanisms Under Heavy Duty Fuel Cell Conditions: A Multi-Disciplinary Approach

The Automotive Partnership of Canada (APC) project on Next-Gen Heavy Duty Fuel Cell Buses is a government supported three-year project dedicated to research and product development of next generation heavy duty fuel cell buses in Canada (www.apc-hdfc.ca). The project represents a collaborative effort between Ballard Power Systems, Simon Fraser University and University of Victoria in British Columbia, Canada. The overarching objective is fundamental understanding of membrane degradation mechanisms, degradation rates, and failure modes under drive cycles and conditions that are typical for heavy duty vehicle operation. The aim is to help develop next generation fuel cell technology that is equivalent to or surpasses incumbent diesel engines in terms of durability and reliability, while reducing capital and warranty costs.In order to develop new durable membrane technologies and devise mitigation strategies to reach desired membrane lifetime, empirical and physical models need to be developed and employed. In this context, understanding the relationship between operation mode and membrane degradation under heavy duty fuel cell conditions is of vital importance. Apart from mechanical degradation such as thinning and pinhole formations, chemical and electrochemical degradation could also take place in perfluorosulfonated acid (PFSA) ionomer membranes. Due to complexity of the underlying processes, degradation mechanisms and their dependence on relevant operating conditions are not generally well established.We present results from an extensive effort that integrates multi-scale, multi-disciplinary modeling with large–scale accelerated degradation testing data. Results are analysed in view of the relative importance of various membrane degradation mechanisms via various chemical and mechanical processes. The versatile multi-scale modeling framework includes structure formation at molecular/meso-scales, water sorption characteristics, kinetics, as well as continuum modelling of chemical and mechanical degradation processes.

Modeling of degradation processes in historical mortars

The aim of presented paper is modeling of degradation processes in historical mortars exposed to moisture impact during freezing. Internal damage caused by ice crystallization in pores is one of the most important factors limiting the service life of historical structures. Coupling the transport processes with the mechanical part will allow us to address the impact of moisture on the durability, strength and stiffness of mortars. This should be accomplished with the help of a complex thermo-hygro-mechanical model representing one of the prime objectives of this work. The proposed formulation is based on the extension of the classical poroelasticity models with the damage mechanics. An example of two-dimensional moisture transport in the environment with temperature below freezing point is presented to support the theoretical derivations.

Accelerated Degradation Tests Modeling Based on the Nonlinear Wiener Process with Random Effects

Accelerated degradation tests (ADT) modeling is an important issue in lifetime assessment to the products with high reliability and long lifetime. Among the literature about the accelerated nonlinear degradation process modeling, the current methods did not consider the product-to-product variation of the products with the same type. Therefore, this paper proposes an accelerated degradation process modeling method with random effects for the nonlinear Wiener process. Firstly, we derive the lifetime distribution of the nonlinear Wiener process with random effects. Secondly, the nonlinear Wiener process is used to model the degradation process of a single stress, and the drift coefficient is considered as a random variable to describe the product-to-product variation. Using the random acceleration model, the random effects are incorporated into the constant stress ADT models and the step stress ADT models. Then, a two-step maximum likelihood estimation (MLE) method is presented to estimate the unknown parameters in the degradation models. Finally, a simulation study and a case study are provided to demonstrate the application and superiority of the proposed model.

Step Stress Accelerated Degradation Process Modeling and Remaining Useful Life Estimation

Accelerated degradation process modeling and remaining useful life(RUL) estimation based on Accelerated degradation testing(ADT) are the essential parts of condition based maintenance and health management.Among the literature about the accelerated degradation process modeling,the current methods didn't consider the unit-to-unit variation of the equipment with the same type.Therefore,we present an accelerated degradation process modeling and RUL estimation method based on Wiener process.The Wiener process is used to model the degradation process of a single stress,and the drift coefficient is considered as a random variable to describe the unit-to-unit variation of different equipment.Based on the presented model,the information of reliability and RUL can be obtained.The parameter of the traditional acceleration model is considered as a random variable to describe the unit-to-unit variation of different equipment,and the traditional method turns out to be its special case.Through the step stress ADT data,a two-step maximum likelihood estimation method is presented to estimate the unknown parameters of the presented model.The results of the simulation about the laser degradation show that the presented method performs better than the traditional method and thus can be potentially applied in practice.

A coupled chemo mechanical model for polymer degradation and erosion processes

AbstractIn nowadays polymer materials are used in a multitude of different applications. A promising class of smart materials are biodegradable polymers. The present work proposes a micromechanical motivated description of degradation. Since degradation leads finally to an eroding material both cases of erosion, namely bulk and surface erosion, are considered. Degradation is modelled in a strong coupled chemo mechanical framework for an arbitrary spatial motion problem. Deformations arising due to surface erosion are described as material motion. Finally, the modelling concept is embedded in a finite element method. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Kinetic modeling of native Cassava starch thermo‐oxidative degradation using Weibull and Weibull‐derived models

A new approach in kinetic modeling of thermo-oxidative degradation process of starch granules extracted from the Cassava roots was developed. Based on the thermoanalytical measurements, three reaction stages were detected. Using Weibull and Weibull-derived (inverse) models, it was found that the first two reaction stages could be described with the change of apparent activation energy (Ea) on conversion fraction (α(T)) (using "Model-free" analysis). It was found that first reaction stage, which involves dehydration and evaporation of lower molecular mass fractions, can be described with an inverse Weibull model. This model with its distribution of Ea values and derived distribution parameters includes the occurrence of three-dimensional diffusion mechanism. The second reaction stage is very complex, and it was found to contain the system of simultaneous reactions (where depolymerization occurs), and can be described with standard Weibull model. Identified statistical model with its distribution of Ea values and derived distribution parameters includes the kinetic model that gives the variable reaction order values. Based on the established models, shelf-life studies for first two stages were carried out. Shelf-life testing has shown that optimal dehydration time is achieved by a programmed heating at medium heating rate, whereas optimal time of degradation is achieved at highest heating rate.

A State-Space-Based Prognostic Model for Hidden and Age-Dependent Nonlinear Degradation Process

Hidden or partially observable degradation state of the equipment is frequently encountered in many engineering practices. This may encourage the state space modeling technique as a feasible way to estimate equipment's remaining useful life (RUL). However, most of the existing state space models falling into this category are based on the assumptions that the degradation process is linear or can be linearized. Therefore, modeling the hidden degradation process under a general nonlinear function and deriving the corresponding analytical form of the RUL distribution are still challenging and have not been well solved in literature. In this paper, we present a state-space-based prognostic model to address the above issues, in which the nonlinearity is characterized by an age-dependent general nonlinear function. Specifically, we model the degradation process as the unobservable nonlinear drift-based Brownian motion (BM) and apply extended Kalman filter (EKF) and expectation-maximization (EM) algorithm to estimate and update the degradation state and the unknown parameters of the established model jointly. Furthermore, we derive the analytical form of the RUL distribution approximately which incorporates the uncertainty of the estimation for hidden state and can be real-time updated based on the available observations. For verifying our approach, a numerical example and a case study for a NASA battery are provided, and the results show that both the parameters and the RUL are estimated accurately. We also consider several different nonlinear functions and compare them with the linear model. The comparative results demonstrate our approach is better than the results in the linear case.

Electrocatalytic degradation kinetic of 4-chlorophenol by the Pd/C gas-diffusion electrode system

A Pd/C gas-diffusion cathode which generated H2O2 through a two-electron reduction process of fed oxygen molecule was used to degrade 4-chlorophenol in an undivided electrolysis device. The kinetics of 4-chlorophenol degradation has been investigated by the electrochemical oxidation processes. By inspecting the relationship between the rate constants (k) and influencing factors, using first-order kinetics to describe the electrochemical oxidation process of 4-chlorophenol, a kinetic model of 4-chlorophenol degradation process was proposed to calculate the 4-chlorophenol effluent concentration: C = C0 exp( -3:76 × 10(-6) C(-0.5)0 J(2) M(-0.7) Q(0.17) Dt). It was found that the electrocatalytic degradation rate of 4-chlorophenol was affected by current density, electrode distance, air-feeding rate, electrolyte concentration and initial 4-chlorophenol concentration. The kinetics obtained from the experiments under corresponding electrochemical conditions could provide an accurate estimation of 4-chlorophenol effluent concentration and lead to better design of the electrochemical reactor.

Stochastic model for the geometrical rail track degradation process in the Portuguese railway Northern Line

The geometrical track degradation is characterized by the evolution over time (or tonnage) of several parameters such as the longitudinal level, the alignment, the gauge, the twist and the cross level. Dynamic track inspections allow monitoring the track geometrical quality which is essential to ensure track availability and reliability, passenger safety and comfort and also energy efficiency. The track geometrical quality is guaranteed by performing condition-based maintenance and renewal actions during the life of the track and for that it is crucial to understand the geometrical track degradation process.In this paper, a stochastic model for characterizing the geometrical track degradation process over time is presented. The Portuguese railway Northern Line is adopted as a case-study and a statistic analysis is performed for different vehicle speed groups, in accordance with CEN [1].The new contribution of this research is that the Dagum distribution, usually adopted for representing the income distribution, may represent the geometrical track degradation process in terms of the longitudinal level.

A Multistate Physics Model of Component Degradation Based on Stochastic Petri Nets and Simulation

Multistate physics modeling (MSPM) of degradation processes is an approach proposed for estimating the failure probability of components and systems. This approach integrates multistate modeling, which describes the degradation process through transitions among discrete states (e.g., initial, microcrack, rupture, etc.), and physics modeling by (physics) equations that describe the degradation process within the states. In reality, the degradation process is non-Markovian, its transition rates are time-dependent, and the degradation is possibly influenced by uncertain external factors such as temperature and stress. Under these conditions, it is in general difficult to derive the state probabilities analytically. In this paper, we overcome this difficulty by building a simulation model supported by a stochastic Petri net representing the multistate degradation process. The proposed modeling approach is applied to the problem of a nuclear component undergoing stress corrosion cracking. The results are compared with those derived from the state-space enrichment Markov chain approximation method applied in a previous work of literature.

Unscented Kalman Filter with Gaussian Process Degradation Model for Bearing Fault Prognosis

The degradation of rolling-element bearings is mainly stochastic due to unforeseeable influences like short term overstraining, which hampers the prediction of the remaining useful lifetime. This stochastic behaviour is hardly describable with parametric degradation models, as it has been done in the past. Therefore, the two prognostic concepts presented and examined in this paper introduce a nonparametric approach by the application of a dynamic Gaussian Process (GP). The GP offers the opportunity to reproduce a damage course according to a set of training data and thereby also estimates the uncertainties of this approach by means of the GP’s covariance. The training data is generated by a stochastic degradation model that simulates the aforementioned highly stochastic degradation of a bearing fault. For prediction and state estimation of the feature, the trained dynamic GP is combined with the Unscented Kalman Filter (UKF) and evaluatedin the context of a case study. Since this prognostic approach has shown drawbacks during the evaluation, a multiple model approach based on GP-UKF is introduced and evaluated. It is shown that this combination offers an increased prognostic performance for bearing fault prediction.

Mathematical Model of Harmful Substance Amount in Soil

There are brought into soil various chemical substances as a result of human activity with a positive or negative impact on the soil characteristics. Concentration of the added substances is changing over time and it is important due to many reasons to know the relation between the substance concentration and the time. One of important factors with an impact on the substance concentration in the soil is the natural degradation of materials. That is why there is presented in this paper a mathematical model of substance degradation process during a simple and repeated application of the given substance.

Thermal Stability and Kinetics of Thermal Decomposition of Salvianolic Acid B

The processes of thermal decomposition of salvianolic acid B were studied by TG-DTG. The TG-DTG curves showed that the process of mass loss of the substance proceeded in two steps. The first step could be attributed to the expulsion of water in the temperature range of 305 similar to 373 K. The second step was corresponding to the large scale degradation of salvianolic acid B in the temperature range of 413 similar to 864 K. The activation energies E of thermal degradation process of three stages were obtained by Friedman method and Ozawa-Flynn-Wall method. According to 15 mechanism functions, the possible kinetic model of thermal degradation process was estimated to be Fn-F2-F1 through the multiple linear regression method.

Research on the Fiber Breakage Damage in Fiber Reinforced Polymer Laminate

In this paper, the single ply fiber breakage model is established based on the distribution function of fiber static strength, and on this basis, the acting process of fiber breakage in FRP laminate is analyzed in detail. According to the experiment data, a method is devised to identify the parameters of the static strength distribution function. Through the references to the classical laminate theory, a computation model of the laminate stiffness degradation process is developed which takes the fiber breakage damage into consideration. The analysis of the results shows that this model can employ only several parameters to describe very well the laminate stiffness degradation process under different stresses.

DEGRADATION AND RECOVERY PROCESSES IN SEMI‐ARID PATCHY RANGELANDS OF NORTHERN PATAGONIA, ARGENTINA

ABSTRACTVegetation in many arid and semi‐arid shrublands frequently occurs in patches with high plant cover (shrub patches) interspersed in a low‐cover herbaceous matrix (inter‐shrub areas). We hypothesized that (a) livestock grazing is an important determinant of such spatial patterns of vegetation, and (b) redistribution of soil resources associated with shrub patches helps in the recovery of vegetation in inter‐shrub areas. To test these hypotheses, we (a) used line transects to compare spatial variations in vegetation, soil microtopography, and soil physicochemical properties in grazed areas and areas protected from grazing since 1970, (b) added sediment and seeds to inter‐shrub areas, and (c) measured resource redistribution after a wildfire. Results were consistent with the hypotheses. They indicated greater spatial heterogeneity in vegetation, soil microtopography and soil physicochemical properties in grazed areas than in protected areas, and that addition of sediment and seeds or redistribution of soil resources from shrub patches after a wildfire enhanced re‐establishment of vegetation in degraded inter‐shrub areas. As a synthesis, a conceptual model of degradation and recovery processes in semi‐arid shrublands of Northern Patagonia is presented. Copyright © 2011 John Wiley & Sons, Ltd.

An age- and state-dependent Markov model for degradation processes

Many technological units are subjected during their operating life to a gradual deterioration process that progressively degrades their characteristics until a failure occurs. Statisticians and engineers have almost always modeled degradation phenomena using independent increments processes, which imply that the degradation growth depends, at most, on the unit age. Only a few models have been proposed in which the degradation growth is assumed to depend on the current unit state. In many cases, however, both the current age and the current state of a unit can affect the degradation process. As such, this article proposes a degradation model in which the transition probabilities between unit states depend on both the current age and the current degradation level. Two applications based on real data sets are analyzed and discussed.

Hydrolytic and photochemical aging studies of a Kevlar ®-PBI blend

The focus of this work is the study of the hydrolytic and photochemical aging behavior of a Kevlar ®-PBI blend fabric. Tensile tests carried out on yarns extracted from this fabric after either irradiation with UV light or exposure to high humidity indicated a continuous decrease of the breaking force with exposure time. ATR-FTIR analyses of photo-chemically aged samples showed evidence of a photo-oxidative reaction initiated by the cleavage of the amide bond of Kevlar. The overlapping of the breaking-force curves that was observed as the irradiance level was increased at constant temperature is believed to be caused by a “screen” effect produced by Photo-Fries products. The fact that at constant temperature the breaking force was unaffected by the variation of the relative humidity suggests that the absorption of water is not the rate-controlling step in the degradation kinetics. ATR-FTIR analyses revealed the presence of a new absorption band ascribed to carboxylic acid end groups produced during the hydrolysis of the amide linkage that occurred after humidity aging. The relative intensity of the –COOH band tended to a constant value as exposure times increased, suggesting that in addition to the hydrolysis, a competing recombination reaction takes place during degradation. A kinetic model for the hydrolytic degradation process was formulated and solved.

An integrated method for safety pre-warning of complex system

In large-scale and complex industrial systems, unplanned outages and hazardous accidents cause huge economic losses, environmental contamination, and human injuries, due to component degradation, exogenous changes, and operational mistakes. In order to ensure safety and increase operational performance and reliability of complex system, this study proposes an integrated method for safety pre-warning to analyze the current safety state of each component and the whole system indicating hidden hazards and potential consequence, and furthermore predict future degradation trends in the long term. The work presented here describes the rationale and implementation of the integrated method incorporating HAZOP study, degradation process modeling, dynamic Bayesian network construction, condition monitoring, safety assessment and prognosis steps, taking advantage of the priori knowledge of the interactions and dependencies among components and the environment, the relationships between hazard causes and effects, and the use of historical failure data and online real-time data from condition monitoring. The application of the integrated safety pre-warning approach described here to the specific example of the gas turbine compressor system demonstrates how each phase of the presented method contributes to completion of the safety pre-warning system development in a systematic way.

Nanoscale degradation of polypyrrole films under oxidative stress: An atomic force microscopy study and review

Atomic force microscopy (AFM) is employed to monitor the surface morphology of polypyrrole (PPy) films grown on vitreous carbon substrates during the catalytic reduction of Cr(VI) to Cr(III). The morphology of freshly-prepared films depends on substrate characteristics. Upon reaction, uniform nodules of aggregated PPy clusters appear. No significant differences in surface morphology are found between its oxidized and reduced forms. Loss of catalytic activity after 8–9 oxidation/reduction cycles of exposure to the chromate solution (oxidation) and electrochemical recharging of the film at negative potentials (reduction) correlates well with the observed polymer film dissolution/detachment from the carbon substrate. Formation of well-defined circular features (PPy rings) at different stages leads to a model for the film degradation process that includes formation of Cl 2 gas inside the polymer matrix. In the final stages, the bulk of the film typically fractures and detaches from the electrode. A catalytically inactive, ultrathin PPy layer remains on the substrate even after prolonged exposure to the target solution. A review of techniques for the study of PPy aging/degradation is given.