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A specific objective of the European Mg-Engine project is to qualify at least two die cast Mg alloys with improved high temperature properties, in addition to satisfactory corrosion resistance, castability and costs. This paper discusses the selection criteria for high temperature alloys leading to four candidate alloys, AJ52A, AJ62A, AE44 and AE35. Tensile-, creep- and fatigue testing of standard die cast test specimens at different temperatures and conditions have led to a very large amount of material property data. Numerous examples are given to underline the potential for these alloys in high temperature automotive applications. The subsequent use of the basic property data in material models for design of automotive components is illustrated.

The paper presents a method for shorter evaluation of the fatigue damage done by an irregular sequence of loads. The method looks first for the largest overall range from highest peak to lowest valley, then for the next largest overall range that interrupts the first range, and so on, down until a suitable fraction (for example, 10%) of all reversals have been used. These few reversals form a short history, which will do substantially the same damage as the total history. The process is applied to three long histories selected by the SAE Fatigue Design and Evaluation Committee. The sensitivity of calculated damage to the omission of smaller ranges is computed for plain and for notched specimens. The error is compared with differences produced by different current rules for evaluating damage, by different cycle counting methods, and by smooth specimen simulation of notched parts.

This paper presents predictions of fatigue crack initiation life for three distinctly different, irregular load histories, each applied to keyhole-notched compact tension specimens at several maximum load levels and using two different structural steels. Work leading to this paper was done in conjunction with the cooperative research program of the SAE Fatigue Design and Evaluation Committee. Three computerized prediction methods (Landgraf, Wetzel, and a Nominal Stress Range approach) are used. All predictions are based on load histories condensed to 10% of their original number of reversals by the “Racetrack Method.” This method, which is described in detail, selects the most damaging overall ranges in an irregular load history while preserving the sequence of the original loading. Predictions are compared with test data for the two dozen combinations of loading type and level and steel used. Comments are made on the relative merits of the different prediction methods.

AM-SC1™ is a high temperature Mg alloy that was originally developed as a sand casting alloy for automotive powertrain applications. The alloy has been selected as the engine block material for both the AVL Genios LE and the USCAR lightweight magnesium engine projects. The present work assesses the potential of this alloy for permanent-mould die cast applications. Thermo-physical and mechanical properties of AM-SC1 were determined for material derived from a permanent-mould die casting process. The mechanical properties determined included: tensile, creep, bolt load retention/relaxation and both low and high cycle fatigue. To better assess the creep performance, a comparative analysis of the normalized creep properties was carried out using the Mukherjee-Dorn parameter, which confirmed the high viscoplastic performance of AM-SC1 compared with common creep resistant high pressure die cast (HPDC) Mg-alloys.

The Society of Automotive Engineers Fatigue Design & Evaluation Committee [SAE FD&E] is actively working on a total life project for weldments, in which the welding residual stress is a key contributor to an accurate assessment of fatigue life. Physics-based welding process simulation and various types of residual stress measurements were pursued to provide a representation of the residual stress field at the failure location in the fatigue samples. A well-controlled and documented robotic welding process was used for all sample fabrications to provide accurate inputs for the welding simulations. One destructive (contour method) residual stress measurement and several non-destructive residual stress measurements-surface X-ray diffraction (XRD), energy dispersive X-ray diffraction (EDXRD), and neutron diffraction (ND)-were performed on the same or similarly welded samples.

The bending fatigue performance of gears, cantilever beam specimens, and notched-axial specimens were evaluated and compared. Specimens were machined from a modified SAE-4118 steel, gas-carburized, direct-quenched and tempered. Bending fatigue specimens were characterized by light metallography to determine microstructure and prior austenite grain size, x-ray analysis for residual stress and retained austenite measurements, and scanning electron microscopy to evaluate fatigue crack initiation, propagation and overload. The case and core microstructures, prior austenite grain sizes and case hardness profiles from the various types of specimens were similar. Endurance limits were determined to be about 950 MPa for both the cantilever beam and notched-axial fatigue specimens, and 1310 MPa for the single gear tooth specimens.

In addition to the creep properties, the fatigue properties are essential for the design of a power-train component in Mg which is operated at elevated temperatures. In case of the new BMW I6 composite Mg/Al crankcase using the AJ alloy system, material testing focused on both subjects. The basic mechanical properties were determined from separately die cast samples and also from samples machined out from high-pressure die cast components. Tensile, high cycle fatigue properties, low cycle fatigue and crack propagation properties were established and analyzed within the technical context for power-train applications reflected in the temperature and load levels. The aspects of mean stress influence, notch sensitivity and crack propagation are evaluated to estimate the performances of the AJ62A alloy system.

This paper presents and validates a model of fatigue crack propagation in resistance spot welded joints; this model is called the “Structural Stress Model” (SSM). Important features of the SSM are that it is based on the physical evidence of fatigue in spot welded joints and on well-accepted descriptors of fatigue more generally. Furthermore, it is usable by designers evaluating fatigue response of structures containing multiple welds. Underlying assumptions of the SSM are also reviewed.