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Technical Paper

Characterization and Ranking of Materials for Exhaust Systems Under Thermal-Cycling Condition

There is a broad range of material choices for on-road and off-road exhaust systems. The final selection of the materials depends on the balance of engineering performance of the materials and the cost. Thermal-cycling resistance of exhaust materials is an extremely important criterion for the long-term durability and reliability performance of very high temperature exhaust components and systems. To optimize the thermal-cycling resistance and cost of those materials, a selection matrix must be established. Several material evaluation and selection matrices are already available, however, these are not sufficient to meet the industry needs. The current procedure of material selection is essentially based on the trial-and-error approach, which is not efficient in the current market environment. In this paper, a general rational approach for thermal-cycling resistance characterization and ranking is demonstrated.
Technical Paper

A Thermal-Fatigue Life Assessment Procedure for Components under Combined Temperature and Load Cycling

High-temperature thermal-mechanical systems are considered as an indispensable solution to modern vehicle emission control. Such systems include advanced engines, manifolds, thermal regeneration systems, and many other systems. Creep, fatigue, oxidation, or their combinations are the fundamental underlying material degradation and failure mechanisms in these systems subjected to combined thermal and mechanical loadings. Therefore, the basic understanding and modeling of these mechanisms are crucial in engineering designs. In this paper, the state-of-the-art methods of damage/failure modeling and life assessment for components under thermal-fatigue loading, are reviewed first. Subsequently, a new general life assessment procedure is developed for components subjected to variable amplitude thermal- and mechanical- loadings, with an emphasis on hold-time effect and cycle counting.
Technical Paper

Design Curve Construction Based on Two-Stress Level Test Data

A design curve, such as a fatigue design S-N curve, is required in engineering design processes. The design curve is usually constructed by analyzing test data, which often exhibit relatively large scatter. For assumed linear test data, two-stress level test plan is commonly used for accelerated life testing (ALT) and subsequent design curve construction. In this paper, based on the two-stress level test plan, a tolerance limit approach is adopted to develop a simple design curve construction procedure. The predicted results from the new method are compared with that of other methods. The advantage of the new method is demonstrated by analyzing the fatigue S-N test data of exhaust components. The determination of minimum sample size is also discussed with a worked table and a graph.
Journal Article

Fatigue Life Assessment of Welded Structures with the Linear Traction Stress Analysis Approach

Structural stress methods are now widely used in fatigue life assessment of welded structures and structures with stress concentrations. The structural stress concept is based on the assumption of a global stress distribution at critical locations such as weld toes or weld throats, and there are several variants of structural stress approaches available. In this paper, the linear traction stress approach, a nodal force based structural stress approach, is reviewed first. The linear traction stress approach offers a robust procedure for extracting linear traction stress components by post-processing the finite element analysis results at any given hypothetical crack location of interest. Pertinent concepts such as mesh-insensitivity, master S-N curve, fatigue crack initiation and growth mechanisms are also discussed.