Verification of Thin-Wall Ductile Iron Test Methodology

摘要

Modifications need to be made to standard test procedures to determine the actual properties of thin-walled ductile iron castings. A series of experiments was performed on test bars to examine the effects of surface finish, elongation measurement techniques, and the amount of material removed during grinding. A paired t-test was used to show that strength and ductility increased as a result of removing the as-received (as-cast) surface. This is attributed to inaccuracies in measuring the cross section area of the rough surface as well as an actual increase in strain through removal of surface effects. A nontraditional method of measuring elongation (to retain the fracture surface) was compared to the standard ASTM method. The nontraditional method was found to be a conservative measure of elongation that is highly correlated with plastic strain as measured by the stress-strain curve. The amount of surface removed during grinding was found not to affect the mechanical properties with the exception of hardness, a surface measurement. It is suggested that thin-walled castings be tested in the condition the casting will be used, i.e., if the casting will be used with the as-cast surface, testing with the as-cast surface will accurately reproduce the decrease in strain caused by the surface effects. If the casting will be ground before use, any amount of surface can be removed to make the testing convenient.