Accelerated degradation tests planning with competing failure modes

Accelerated degradation tests (ADT) have been widely used to assess the reliability of products with long lifetime. For many products, environmental stress not only accelerates their degradation rate but also elevates the probability of traumatic shocks. When random traumatic shocks occur during an ADT, it is possible that the degradation measurements cannot be taken afterward, which brings challenges to reliability assessment. In this paper, we propose an ADT optimization approach for products suffering from both degradation failures and random shock failures. The degradation path is modeled by a Wiener process. Under various stress levels, the arrival process of random shocks is assumed to follow a nonhomogeneous Poisson process. Parameters of acceleration models for both failure modes need to be estimated from the ADT. Three common optimality criteria based on the Fisher information are considered and compared to optimize the ADT plan under a given number of test units and a predetermined test duration. Optimal two- and three-level optimal ADT plans are obtained by numerical methods. We use the general equivalence theorems to verify the global optimality of ADT plans. A numerical example is presented to illustrate the proposed methods. The result shows that the optimal ADT plans in the presence of random shocks differ significantly from the traditional ADT plans. Sensitivity analysis is carried out to study the robustness of optimal ADT plans with respect to the changes in planning input

Reliability modeling and analysis of load-sharing systems with continuously degrading components

This paper presents a reliability modeling and analysis framework for load-sharing systems with identical components subject to continuous degradation. It is assumed that the components in the system suffer from degradation through an additive impact under increased workload caused by consecutive failures. A log-linear link function is used to describe the relationship between the degradation rate and load stress levels. By assuming that the component degradation is well modeled by a step-wise drifted Wiener process, we construct maximum likelihood estimates (MLEs) for unknown parameters and related reliability characteristics by combining analytical and numerical methods. Approximate initial guesses are proposed to lessen the computational burden in numerical estimation. The estimated distribution of MLE is given in the form of multivariate normal distribution with the aid of Fisher information. Alternative confidence intervals are provided by bootstrapping methods. A simulation study with various sample sizes and inspection intervals is presented to analyze the estimation accuracy. Finally, the proposed approach is illustrated by track degradation data from an application example

Biodegradation of Biomaterials from PLA/PHB Blends in Conditions in-vitro

Diplomová práce pojednává o problematice biodegradace biomateriálů v podmínkách in-vitro. Biomateriály jsou systémově a farmakologicky inertní substance určené k implantaci nebo začlenění do živého systému. V práci jsou popsány různé druhy biomateriálů, které se dají využít jako implantáty a také možnost využití 3D biotisku jako budoucí metody k urychlení jejich výroby a tím pádem zkrácení doby čekání pro pacienty. V experimentální části je testována schopnost směsí na bázi polymléčné kyseliny a polyhydroxybutyrátu s přídavkem změkčovadla triacetin biodegradovat v simulovaných podmínkách lidského těla, tedy roztocích o pH 4,5 – 7,2 při 37 °C. Experimenty byly provedeny s využitím akcelerované a přirozené biodegradace a na základě výsledků bylo potvrzeno, že daný biomateriál je v těchto podmínkách schopen biodegradovat.The thesis deals with the issue of biodegradation of biomaterials in conditions in-vitro. Biomaterials are systemically and pharmacologically inert substances intended for implantation or integration into the living system. The thesis describes various types of biomaterials that can be used as implants and the possibility of using 3D bioprint as a future method to accelerate their production and thus shorten the waiting time for patients. In the experimental part, the ability of blends based on polylactic acid and polyhydroxybutyrate with the addition of plasticizer triacetin is tested for biodegradation in simulated conditions of the human body, i.e. solutions with pH 4.5 – 7.2 at 37 °C. Experiments were performed using accelerated and natural biodegradation. Based on these results, it was confirmed that the biomaterial was biodegradable under these conditions.546 - Katedra environmentálního inženýrstvívýborn