Published: Jan 1950
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Commercial gray iron castings are often designed in complex shapes which may integrate a number of simple geometrical figures into a final form. Plotting the flow of stress in a complex casting under load may become a jigsaw puzzle. One of the most aggravating cases occurs when a surface that should develop a tensional stress actually registers a compressional stress. Stress patterns that are further complicated by the cross-movements of static loads, dynamic loads, and thermal loads, combine to confuse the designer. In early designs, it is common to plan, build, test, and load the job to destruction, then redesign or “beef up” the weakened zones, and finally proceed with casting production. Because cast iron will break with a non-ductile fracture under simple tensile stress, its application has occasionally been avoided in favor of more ductile material which will deform and thus sound a warning, or it will malfunction in a dynamic loading system and thereby give notice of impending failure. Non-ductile fractures are known to develop in ductile materials like steel, as in the case of the fatigue failure. Methods for exploring the stress distribution in castings have been provided by various means and specifically in the strain-resistance wire gage system. Where equipment for stress-strain analysis is available, it has invariably been found possible to significantly improve the physical configuration of a given casting so as to meet more efficiently the stresses involved and also to materially decrease the weight of the casting, with an advantageous effect on economy. Equally important is the application of stress-strain measurements for the purpose of increasing confidence of the designer in gray iron as an engineering material.
Vanick, J. S.
Chairman of Committee A-3 on Cast Iron; Metallurgist, The International Nickel Co., New York, N. Y.