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The approaches of obtaining both fatigue strength distribution and fatigue life distribution for a given set of fatigue S-N data are reviewed in this paper. A new fatigue S-N curve transformation technique, which is based on the fundamental statistics definition and some reasonable assumptions, is specifically developed in this paper to transform a fatigue life distribution to a fatigue strength distribution. The procedures of applying the technique to multiple-stress level, two-stress level, and one-stress level fatigue S-N data are presented.

This paper summarizes some recent results on fatigue evaluation of welded joints. A mesh-insensitive structural stress procedure was discussed and employed to characterize geometric stress raiser effects at welded joints. Existing weld fatigue data published in the open literature were analyzed using the structural stress parameter.

A special spot weld element (SWE) is presented for simplified representation of spot joints in complex structures for structural durability evaluation using the mesh-insensitive structural stress method. The SWE is formulated using rigorous linear four-node Mindlin shell elements with consideration of weld region kinematic constraints and force/moments equilibrium conditions. The SWEs are capable of capturing all major deformation modes around weld region such that rather coarse finite element mesh can be used in durability modeling of complex vehicle structures without losing any accuracy. With the SWEs, all relevant traction structural stress components around a spot weld nugget can be fully captured in a mesh-insensitive manner for evaluation of multiaxial fatigue failure.

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.

Engineering components and systems are usually subjected to mixed-mode and multiaxial fatigue loadings, and these conditions should be considered in product durability and reliability design and the maintenance of aging equipment, especially mission-critical components and systems. However, modeling the damage and degradation processes under these complex loading conditions is difficult and challenging task because not only the concepts, such as range, mean, peak, valley etc., developed for uniaxial loading usually cannot be directly transferred to mixed-mode and multiaxial loadings, but also some very unique phenomena related to these complex loading conditions. One such a phenomenon is the loading path effect that can be simply described as: out-of-phase loading is more damaging than in-phase loading for some ductile materials.

Durability and reliability performance is one of the most important concerns for vehicle components and systems, which experience cyclic fatigue loadings and may eventually fail over time. Durability and reliability assessment and associated product validation require effective and robust testing methods. Several testing methods are available and among them, three basic testing methods are widely used: life testing, binomial testing (bogey testing), and degradation testing. In fact, their commonalities, differences, and relationships have not been clearly defined and fully understood. Therefore, the maximum potential of these testing methods to generate efficient, optimized, and cost-effective testing plans, consistent results, and meaningful results interpretation have been significantly limited. In this paper, a unified framework for representing these testing methods and conducting reliability analysis in a single damage-cycle (D-N) diagram is provided.

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.

Great efforts have been made to develop the ability to accurately and quickly predict the durability and reliability of vehicles in the early development stage, especially for welded joints, which are usually the weakest locations in a vehicle system. A reliable and validated life assessment method is needed to accurately predict how and where a welded part fails, while iterative testing is expensive and time consuming. Recently, structural stress methods based on nodal force/moment are becoming widely accepted in fatigue life assessment of welded structures. There are several variants of structural stress approaches available and two of the most popular methods being used in automotive industry are the Volvo method and the Verity method. Both methods are available in commercial software and some concepts and procedures related the nodal force/moment have already been included in several engineering codes.

Road vibrations cause fatigue failures in vehicle components and systems. Therefore, reliable and accurate damage and life assessment is crucial to the durability and reliability performances of vehicles, especially at early design stages. However, durability and reliability assessment is difficult not only because of the unknown underlying damage mechanisms, such as crack initiation and crack growth, but also due to the large uncertainties introduced by many factors during operation. How to effectively and accurately assess the damage status and quantitatively measure the uncertainties in a damage evolution process is an important but still unsolved task in engineering probabilistic analysis. In this paper, a new procedure is developed to assess the durability and reliability performance, and characterize the uncertainties of damage evolution of components under constant amplitude loadings.

In this paper, an advanced computational framework is presented for integrated simulation of hydroforming effects and welding assembly operations. The finite element procedures take advantages of existing commercial finite element codes such as ABAQUS by employing a series of user-developed interface modules and a unified material constitutive model formulated with internal state variables that are used to track stress/strain histories induced during forming and welding operations. Its applications in design and welding assembly of hydrofomed components are demonstrated with a series of selected case studies. Based on the detailed finite element simulations described in the above, the following important observations can be made: Weld placements are extremely important in order to mitigate the significant cold work effects in hydroforming.

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.

This paper presents a numerical analysis of the effect of weld induced residual stress on the fatigue behavior of a T-joint. The thick-section T-joint contained 18 individual weld passes and was subjected to fully-reversed, zero-maximum, and zero-minimum fatigue cycling. The effect of the residual stress was demonstrated by comparing the result with and without residual stress. It was concluded that the local fatigue parameters (mean stress, alternating stress, and stress ratio) at the suspected crack initiation site were changed significantly by the residual stresses when the applied stress were other than fully reversed. In addition, the effect of the stress concentration at the weld was more significant that the effect of the residual stress for the applied fatigue loads levels that were considered. The analysis method presented can be used to assess weldment design and process variables.

Time-domain and frequency domain methods are two common methods for fatigue damage and life assessment. The frequency domain fatigue assessment methods are becoming increasingly popular recently because of their unique advantages over the traditional time-domain methods. Recently, a series of moment of load path based multiaxial fatigue life assessment approaches have been developed. Among them, the most recently developed effective second moment of load path (ESMLP) approach demonstrates its potentials of conducting fatigue damage and life assessment accurately and efficiently. ESMLP can be used for fatigue analysis even without resorting to cycle counting because of its unique mathematical and physical properties, such as quadratic form in the kernel of the moment integral, rotationally invariant, and being proportional to damage. Developing a better parameter for frequency-domain analysis is the driving force behind the development of ESMLP as a new fatigue damage parameter.

This paper summarizes the results of a recent study on a fatigue predictive technique for spot-welded automotive structures. The technique makes use of an equivalent stress intensity factor (Keq) as fatigue parameter for life predictions. A series of fatigue tests were conducted by using different types of fatigue specimens and weld arrangements. Using the raw test data collected, fatigue properties were processed in the form of ΔKeq versus fatigue life by a fracture mechanics based stress intensity factor technique. It is demonstrated that the fatigue properties are consistent among all the specimens tested and relatively geometry-independent. With the stress intensity factor based fatigue properties, the predictive technique was applied to more complex specimens with non-symmetric weld configurations and non-uniform loading conditions (resulting in mixed-mode loading on each weld). The results indicate good correlation between life predictions and test data.

Vehicle exhaust components and systems under fatigue loading often show multiple failure modes, which should be treated, at least theoretically, with rigorous advanced bi-modal and multi-modal statistical theories and approaches. These advanced methods are usually applied to mission-critical engineering applications such as nuclear and aerospace, in which large amounts of test data are often available. In the automotive industry, however, the sample size adopted in the product validation is usually small, thus the bi-modal and multi-modal phenomena cannot be distinguished with certainty.

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 testing and related fatigue life assessment are essential parts of the design and validation processes of vehicle components and systems. Fatigue bench test is one of the most important testing methods for durability and reliability assessment, and its primary function is to construct design curves based on a certain amount of repeated tests, with which recommendations on product design can be advised. How to increase the accuracy of predictions from test results, the associated life assessment, and to reduce the cost through reducing test sample size is an active and continuous effort. In this paper the current engineering practices on constructing design curves for fatigue test data are reviewed first.

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.

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.