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The fracture surfaces of fatigue crack propagation (FCP) test specimens of ASTM plate steel Grades A516-70, A533B-1, and A514F are characterized primarily by using the scanning electron microscope. The evaluation involved two plates of each grade with different inclusion structures. These had been produced by conventional steel-making techniques or by calcium treatment. The FCP tests were performed in the six possible testing orientations for each material and quality level. In each grade, the Type II manganese sulfide and alumina inclusions associated with the conventional quality plate resulted in faster FCP growth rates, particularly in the through-thickness testing orientations (S-T, S-L). Furthermore, the group nature of the occurrence of these inclusions was found to be the primary influence on FCP behavior. In addition, these inclusion formations were found to give greater anisotropy of FCP behavior for the steels of the higher yield strength, principally because of the fracture mode of the material between inclusions of a particular formation. For all steel grades, the calcium treatment led to improved levels and isotropy of FCP resistance.
fatigue crack propagation, inclusions, fractography, steels, anisotropy, calcium treatment, desulfurization, yield strength effects, materials, materials science
Senior research engineer, Lukens Steel Co., Coatesville, Pa.