Effects of Sample Size and Loading Rate on the Transition Behavior of a Ductile Iron (DI) Alloy

Abstract

The measurement and understanding of the fracture toughness of ductile irons (DI) are critical to the analysis of nuclear transportation casks made from these alloys. Cask containment must be assured for all loading events from normal handling to accidents during which high loads can be delivered at elevated rates. Cask walls are commonly in the range of 20 to 50 cm thick (or greater) in order to provide requisite nuclear shielding, and this requires that associated mechanical constraint effects must be considered. At elevated temperatures (that is, in the vicinity of ambient), DI behaves in an elastic-plastic manner, even for large section sizes (thickness > 20 cm) and moderately high loading rates. However, as the temperature is lowered or the loading rate is increased, ferritic DI alloys exhibit a relatively sharp transition to linear elastic behavior, with a significant decrease in the fracture toughness. The fracture toughness of a DI alloy has been measured using linear elastic and elastic-plastic experimental techniques. Measurements have been made as a function of temperature, loading rate, and section size. The loading rates span the range that a cask could experience during normal transport and handling, as well as accident events. Specifically, the stress intensity rate, K, was varied between 10-3 to > 10+5 MPa m1/2/s. The range in section size that was examined was restricted to moderate thicknesses (̃1 to ̃4 cm) because of the limitations of available hydraulic test frames. For static testing rates, it was found that increasing specimen thickness appears to shift the transition behavior to slightly higher temperatures. The effect of increased specimen thickness during elevated rate testing was more pronounced. As the thickness was increased from ̃1 to ̃2 cm, the fracture toughness was decreased by roughly 25% at 25°C, and 40% at -29°C (the fracture toughnesses of both thicknesses were the same at -50°C where fracture was essentially all cleavage). Initial fractographic results show that the amount of brittle cleavage (at initiation) was larger for the thicker specimens in the ductile-brittle transition region. The decrease in fracture toughness with increasing section size (in the transition region) occurred even though all specimens met the size criteria of the ASTM Test Method for JIc, A Measure of Fracture Toughness (E 813-87). Measurements also showed that the temperature range of the transition region for this DI alloy increased with increasing loading rate and extended up to room temperature. A consistent explanation for the measured behavior seems to be that increased constraint (from increased specimen thickness) can cause an increase in cleavage if the test temperature (for a specific loading rate) is close to the transition region. The extent of this effect has an obvious impact on the analysis of the fracture resistance of transportation containers.