Characterization of driving band material and simulation of driving band engraving process

Sammanfattning

Driving bands are an important part of projectiles used in mid and large caliber canons, it serves many important purposes. It gets exposed to high temperature combustion gases while engaged in the barrel´s rifling, exposed to high strain rates, which leads to extreme wear conditions. Because the driving band is what engages in the rifling, a driving band failure leads to high barrel wear and an unstable projectile while in air. Because of this, investigation of the material behavior and wear mechanisms of driving bands are of high importance. This thesis focuses on characterizing the driving band material and simulating the engraving process. Metallurgical analysis was conducted on both fired and unfired 57 mm driving bands using stereo microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) to assess microstructural changes due to firing, specifically focusing on evidence of dynamic recrystallization and deformation mechanisms. Mechanical properties of the driving band material were evaluated through quasi-static compression tests and high strain rate testing using a Split Hopkinson Pressure Bar (SHPB), enabling calibration of the Johnson-Cook material model parameters. Furthermore, finite element simulations were developed in LS-DYNA and Ansys to model the engraving process, accounting for variations in barrel wear, friction, and material behavior. The metallurgic investigation showed a large quantity of decoppering agents on the driving band surface, with high concentration in areas experiencing microcracks in the surface. Regarding the dynamic recrystallization however, no clear evidence was found of this phenomenon, only indications and arguments are provided to support dynamic recrystallization. Therefore, more testing is needed to ensure whether dynamic recrystallization has occurred or not. The mechanical testing provided, together with graphical alignment of test results at dynamic strain rates, two sets of material model parameters, one suited for lower strains and one suited for higher strains. To suggest one set of parameters for further simulation, the high strain set of parameters are suggested. The reason being that the strains experienced by the driving band in simulation are in the higher strain regime. The simulations showed quite big variations of the resistance force between the barrel and driving band depending on which friction coefficient that was used and which material parameters in the Johnson-Cook material model that was used. When comparing the resistance force between a new and a worn barrel, the new barrel had a much higher resistance force than the worn barrel.