Limin Luo

Design Curve Construction Based on Monte Carlo Simulation

Good durability/reliability performance of products can be achieved by properly constructing and implementing design curves, which are usually obtained by analyzing test data, such as fatigue S-N data. A good design curve construction approach should consider sample size, failure probability and confidence level, and these features are especially critical when test sample size is small. The authors have developed a design S-N curve construction method based on the tolerance limit concept. However, recent studies have shown that the analytical solutions based on the tolerance limit approach may not be accurate for very small sample size because of the assumptions and approximations introduced to the analytical approach.In this paper a Monte Carlo simulation approach is used to construct design curves for test data with an assumed underlining normal (or lognormal) distribution. The difference of factor K, which measures the confidence level of the test data, between the analytical solution and the Monte Carlo simulation solutions is compared. Finally, the design curves constructed based on these methods are demonstrated and compared using fatigue S-N data with small sample size.Copyright

A New Linear Superposition Theory and its Application in Creep Fatigue-Oxidation Crack Growth Modeling

Fatigue, creep, oxidation or their combinations have long been recognized as the principal mechanisms in many high-temperature failures in power plant components, turbine engines, and exhaust systems in vehicles. Depending on the specific materials and loading conditions and temperature, the role of each damage mechanism may change significantly, ranging from independent development to competing and combined creep-fatigue, fatigue-oxidation, and creep-fatigue-oxidation. In this paper a new linear superposition theory is proposed to model the cycle-dependent and time-dependent creep-fatigue-oxidation crack growth phenomena. The model can be reduced to creep-fatigue and fatigue-oxidation crack growth models previously developed by the authors as well as, under some assumptions, the current widely used linear superposition theory. The limits of the current superposition theory and the advantages of the new theory are clearly demonstrated with several worked examples. A general probabilistic analysis procedure is also proposed by introducing the uncertainties of parameters in fatigue, creep, and oxidation crack growth laws with the help of the Monte Carlo simulation.Copyright

Hold-Time Effect on Thermo-Mechanical Fatigue Life and its Implications in Durability Analysis of Components and Systems

Thermo-mechanical fatigue (TMF) resistance of engineering materials is extremely important for the durability and reliability of components and systems subjected to combined thermal and mechanical loadings. However, TMF testing, modeling, simulation, validation, and the subsequent implementation of the findings into product design are challenging tasks because of the difficulties not only in testing but also in results interpretation and in the identification of the deformation and failure mechanisms. Under combined high-temperature and severe mechanical loading conditions, creep and oxidation mechanisms are activated and time-dependent failure mechanisms are superimposed to cycle-dependent fatigue, making the life assessment very complex. In this paper, the testing procedures and results for high-temperature fatigue testing using flat specimens and thermal-fatigue testing using V-shape specimens are reported; emphasis is given to hold-time effects and the possible underlying mechanisms. The uncertainty nature and the probabilistic characteristics of the V-shape specimen test data are also presented. Finally, the impact of hold-time effect on current product design and validation procedure is discussed in terms of virtual life assessment.

A Bayesian Statistics Based Design Curve Construction Method for Test Data With Extremely Small Sample Sizes

Fatigue design curve construction is commonly used for durability and reliability assessment of engineering components subjected to cyclic loading. A wide variety of design curve construction methods have been developed over the last decades. Some of the methods have been adopted by engineering codes and widely used in industry. However, the traditional design curve construction methods usually require significant amounts of test data in order for the constructed design curves to be consistently and reliably used in product design and validation. In order to reduce the test sample size and associated testing time and cost, several Bayesian statistics based design curve construction methods have been recently successfully developed by several research groups. Among all of these methods, an efficient Monte Carlo simulation based resampling method developed by the authors of this paper is of particular importance. The method is based on a large amount of reliable historical fatigue test data, the associated probabilistic distributions of the mean and standard deviation of the failure cycles, and an advanced acceptance-rejection resampling algorithm. However, finite element analysis (FEA) methods and a special stress recovery technique are required to process the test data, which is usually a time-consuming process. A more straightforward approach that does not require these intermediate processes is strongly preferred.This study presents such an approach, in which the only historical information needed is the distribution of the standard deviation of the cycles to failure. The distribution of the mean is directly calculated from the current tested data and the Central Limit Theorem. Neither FEA nor stress recovery technique is required for this approach, and the effort put into design curve construction can be significantly reduced. This method can be used to complement the previously developed Bayesian methods.Copyright

ACCELERATED DURABILITY TESTING AND DATA ANALYSIS FOR PRODUCTS WITH MULTIPLE FAILURE MECHANISMS

Durability and reliability performance is one of the most important concerns of ground vehicle systems, which usually experience cyclic fatigue loadings and eventually fail over time. Creep and oxidation caused damages at elevated temperature conditions further shorten the life of a system and make the life assessment even more complex. One of the key challenges posed to design engineers is to find a way to accelerate the durability tests for products with multiple failure modes and to validate designs within development cycle to satisfy customers and markets requirements. The accelerated testing procedures for products with single failure modes have been studied for several decades and essentially well established even though some fundamental issues are still unsolved. By contrast, much is needed to do for the accelerated testing procedure of products with multiple failure modes and their interactions. In this paper, a new accelerated testing and data analysis procedure suitable for products with linear homoscedastic data pattern is proposed. Examples related to durability life of high temperature components with single failure modes such as fatigue, creep, and oxidation are provided to demonstrate the procedure developed. Durability life assessment of components with multiple failure modes is also investigated and demonstrated with creep-fatigue and fatigue-oxidation damage analyses.

Failure mechanisms and modes analysis of vehicle exhaust components and systems

Abstract The basic functions of vehicle exhaust components and systems are to control emissions, noise, and manage thermal energy. Increasingly stringent government regulations on emissions and fuel economy challenge the exhaust designers and manufacturers to a degree that they have not yet experienced. In order to meet the regulations and customer requirements, vehicle exhaust systems are becoming more complex with increased service life. Failure mechanisms and modes and related durability and reliability performance assessment of vehicle exhausts become a more important concern to product design and validation engineers. In this chapter, the trend of emission control products and the challenges related to materials failure mechanisms and modes, such as fatigue, creep, oxidation, corrosion, erosion or their interactions, are reviewed in detail. Probabilistic failure data analysis, failure cause determination, effective failure prevention strategies, materials screening, and material ranking are also provided and discussed.

Product Durability/Reliability Design and Validation Based on Test Data Analysis

Better quality leads to less waste, improved competitiveness, higher customer satisfaction, higher sales and revenues, and eventually higher profitability. Meeting the quality and performance goals requires that decisions be based on reliable tests and quantitative test data analysis. Statistical process control (SPC) is such a fundamental quantitative approach to quality control and improvement. Walter Shewhart in 1920s and 1930s pioneered the use of statistical methods as a tool to manage and control production.

A Probabilistic Life, Damage, and Uncertainty Assessment Method for Engineering Components and Systems

Fatigue life and damage assessment is crucial to the durability and reliability performances of engineering components and systems, especially at early design and validation stages. However, durability and reliability assessment of engineering components and systems is difficult due to, partially, the large uncertainties introduced from many sources. How to quantitatively measure the uncertainties is an important task in engineering probabilistic analysis. Recently, based on fatigue failure data, a new procedure has been developed to assess the durability, reliability and uncertainty of components under constant amplitude loadings. An uncertainty measure, which is very similar to those used in computational complexity, classical statistical physics and information theory, has been proposed. In this paper, two new developments are reported: (1) the uncertainty measure is modified in such a way that the uncertainty measure can be normalized, (2) the durability, reliability and uncertainty analysis procedure is extended to system level. Several examples are provided to demonstrate the effectiveness of the new probabilistic life, damage, and uncertainty assessment approach.Copyright