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The spall strength of cast iron with varying microstructures has been investigated using plate impact at moderate speed. Stress history measurements were made with manganin stress gauges embedded between the back face of the specimen and a low impedance polycarbonate backing. Five separate cast irons were tested. Four of these consisted of gray cast iron with graphite in flake form, with three classified as Type VII A2 and the fourth containing a bimodal distribution of Types VII A4 and VII D8. The fifth casting consisted of ductile cast iron with graphite in nodular form, classified as Type I, size class 5. The spall strength for the Type VII A2 gray cast irons varied between 40 and 370 MPa, and that of the additional gray cast iron, between 410 and 490 MPa. The spall strength of the ductile cast iron fell within the range of 0.94–1.2 GPa. It is shown that the spall strength is linked to the damage level at the spall plane, where an increased level of tensile stress is required to generate higher levels of damage. Post mortem analysis was performed on the recovered samples, revealing the graphite phase to be the primary factor governing the spall fracture of cast irons, where crack nucleation is directly correlated to the debonding of graphite from the metal matrix. The average length of graphite found within a casting is linked to the material's strength, where strength increases as a function of decreasing length. The morphology and mean free path of graphite precipitates further govern the subsequent coalescence of initiated cracks to form a complete fracture plane. In cases where graphite spacing is large, increased energy level is required to complete the fracture process. A secondary factor governing the spall fracture of cast irons has also been linked to the microstructure of the metal matrix, with pearlite yielding higher spall strengths than free ferrite.

The spall strength of cast iron has been investigated by means of planar plate impact experiments conducted in a vacuum. A single stage gas gun was utilized to drive projectiles to velocities between 100 and 300 m/sec, resulting in low to moderate shock loading of the cast iron specimens. Measurement of the stress histories were made with the use of commercial manganin stress gauges that were imbedded between the back face of the cast iron specimen and a low impedance backing of polycarbonate. Spall strength values were calculated utilizing the measured peak stress and minimum stress pullback signals captured in the stress history. Spall Strength values were found to vary between 0.98 and 1.45 GPa for the cast iron tested. Post-Mortem analysis of recovered specimen has provided insight into the evolution of spall failure in cast iron and shed light on the varying nature of the spall strength values calculated. It was determined that the lower bound of strength values was associated with small scale micro-failure, while the upper bound values corresponded to complete spall fracture.

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