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A round-robin low cycle fatigue test program was conducted by the SAEFDE Committee using A356-T6 cast aluminum alloy. Three different microstructures representative of three solidification rates were sought, but only two significantly different secondary dendrite arm spacings, DAS, resulted. The smaller DAS had slightly greater monotonic yieid and ultimate strengths, greater permanent deformation at fracture and better low cycle fatigue resistance. Under strain-controlled axial low cycle fatigue conditions. A356-T6 was observed to cyclicaiiy strain harden and hysteresis loops were skewed toward the compressive stress from about 1 to 10 percent. Fatigue failures usually initiated at surface or near surface porosity. About 25 percent of the 173 test specimens that were considered valid failed between the strain gage knife edges and the specimen fillet radius.

The objective of this research was to predict the fatigue life of a member at 10 percentile levels for SAE-1045 (260HB) steel and 7075-T73 aluminum in aqueous and saline environments. Consideration of environmental effects as rate dependent phenomena promoted the usage of controlled strain rate testing on axial loaded, smooth specimens of each material. It was determined primarily that long-life fatigue resistance is more affected by aqueous and saline environments, and that a simple modification of the fatigue strength exponent adequately described strain-life behavior. The statistical aspects of scatter in fatigue lives for the 7075-T73 aluminum were evaluated by testing several samples at each of four different strain amplitudes. Agreement between predicted and laboratory test results appears encouraging.

A technique which treats the free graphite in gray and nodular cast iron, gas pores and microshrinkage cavities in cast steels and inclusions in wrought, high hardness steels as notches in a steel matrix results in quantitative predictions of the fatigue resistance of these ferous-based systems.

A case study describes a fatigue damage analysis of a light truck frame. The objective of the analysis is to determine whether an existing frame design can safely accept a ten percent increase in load. The analysis, completed in less than a month, incorporates an experimental stress-strain analysis, proving ground test data and experimentally determined properties of the frame material. Three common methods of damage analysis and a relatively new procedure are compared and the advantages of the new method are demonstrated.

The fatigue resistance of gray cast iron is shown to be strongly dependent on graphite morphology and the strength of the steel-like matrix. Considering graphite flakes in gray iron as internal notches, a comparison is made of the fatigue resistance of gray irons and steels of comparable composition, hardness, and microstructure. Application of a Neuber analysis, previously employed in geometrically notched members to relate nominal stresses and strains to local stresses and strains at notch roots, produces quantitative values of the fatigue notch factor, Kf, for various graphite morphologies, matrix structures, and hardnesses. Fatigue resistance of gray irons is enhanced by decreasing graphite flake size. Matrix hardness is of greater importance than structure in determining the fatigue resistance.