Correlations of Crack Branching Properties with Mott Fragmentation Parameters in Hypereutectoid Steels

When brittles metals are subjected to blast loads, they tend to shatter into a large number of very small particles. The number and size of the particles generated by such an explosive process is well characterized by the semi-empirical cumulative distribution function of Mott. This function, however, contains no information regarding either the manner of break-up or the reason for a particular distribution. The present experimental investigation was undertaken to clarify the role played by dynamic material properties in the ultimate distribution of particle sizes when brittle metals fracture under explosive loads. The phenomenology of brittle fragmentation was examined previously, and a simple model was developed utilizing the concept of crack branching. Using statistical arguments to establish relationships between the energies associated with crack branching and the Mott parameters that characterize the particle distribution, that model predicts that the average mass of a particle from the shattered body is proportional to the fourth power of the crack branching stress intensity factor. To examine the physical relationships between material properties and particle size distributions from blast-loaded structures, two hypereutectoid steels FS-01 and HF-1 were heat- treated to a number of brittle conditions and evaluated in terms of static tensile properties, plane strain fracture toughness, crack branching behavior, and explosive fragmentation. The distribution parameters did not correlate with strength, toughness, or hardness, but the coarseness of the particles appeared to increase with increasing tensile elongation.