A fresh look at why spinel outperforms sapphire during ballistic tests despite its inferior properties- A mechanisms perspective
Ghatu Subhash (University of Florida)
Eringen Medal Symposium in honor of G. Ravichandran
Mon 10:45 - 12:15
Salomon 001
Compared to magnesium aluminate spinel, Sapphire has many superior mechanical properties: density 3.97 g/cm3 vs 3.58 g/cm3; Young’s modulus 386GPa vs. 277 GPa; Biaxial flexural strength 760 MPa vs. 172 MPa; Vickers hardness 17.4 GPa vs. 14 GPa; and HEL 23GPa vs. 11.3 GPa. However, the ballistic performance of sapphire has been reported to be inferior compared to spinel. For comparable aerial density, V50 for spinel has been reported to be almost 300 ft/sec higher than sapphire. To unlock this mystery, we have conducted a range of studies including static indentation, dynamic indentation and ball impact studies and investigated the fundamental mechanisms of fracture in these two materials. While hardness is an important factor that contributes to superior performance of any armor material, it is not the only factor that determines the performance. A range of mechanisms including decreased tendency for brittle fracture, mixed mode fracture, grain boundary shielding etc ., act in favor of spinel, whereas interacting cracks and propensity for fracture along preferred crystallographic planes act against sapphire and contribute to lower performance. The details of analysis from various experiments will be presented in detail.