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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.

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.

Durability and reliability performance is one of the most important concerns of a recently developed Thermal Regeneration Unit for Exhaust (T.R.U.E-Clean®) for exhaust emission control. Like other ground vehicle systems, the T.R.U.E-Clean® system experiences cyclic loadings due to road vibrations leading to fatigue failure over time. Creep and oxidation cause damage at high temperature conditions which further shortens the life of the system and makes fatigue life assessment even more complex. Great efforts have been made to develop the ability to accurately and quickly assess the durability/reliability of the system in the early development stage. However, reliable and validated simplified engineering methods with rigorous mathematical and physical bases are still urgently needed to accurately manage the margin of safety and decrease the cost, whereas iterative testing is expensive and time consuming.

Active regeneration systems for cleaning diesel exhaust can operate at extremely high temperatures up to 1000°C. The extremely high temperatures create a unique challenge for the design of regeneration structural components near their melting temperatures. In this paper, the preparation of the sheet specimens and the test set-up based on induction heating for sheet specimens are first presented. Tensile test data at room temperature, 500, 700, 900 and 1100°C are then presented. The yield strength and tensile strength were observed to decrease with decreasing strain rate in tests conducted at 900 and 1100°C but no strain rate dependence was observed in the elastic properties for tests conducted below 900°C. The stress-life relations for under cyclic loading at 700 and 1100°C with and without hold time are then investigated. The fatigue test data show that the hold time at the maximum stress strongly affects the stress-life relation at high temperatures.

Fatigue, creep, oxidation, or their combinations have long been recognized as the principal failure mechanisms in many high-temperature applications such as exhaust manifolds and thermal regeneration units used in commercial vehicle aftertreatment systems. Depending on the specific materials, loading, and temperature levels, the role of each damage mechanism may change significantly, ranging from independent development to competing and combined creep-fatigue, fatigue-oxidation, creep-fatigue-oxidation. Several multiple failure mechanisms based material damage models have been developed, and products to resist these failure mechanisms have been designed and produced. However, one of the key challenges posed to design engineers is to find a way to accelerate the durability and reliability tests of auto exhaust in component and system levels and to validate the product design within development cycle to satisfy customer and market's requirements.