Forecast of Adiabatic Shear Band Formation in Two Commercial Ultra-high-Strength Armor Steels by Split Hopkinson Pressur
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ltra-high-strength armored steels or structures have been developed to achieve an excellent protective capability against a ballistic impact, along with transportability and maneuverability. When the armor steels are ballistically impacted under extremely high strain rates, they are heavily deformed within a narrow localized area, particularly at a ballistically penetrated area, by a thermal-mechanical instability before the insufficient emission of high thermal energy, thereby forming adiabatic shear bands (ASBs).[1–10] ASBs deteriorate seriously the load-carrying capability of the armor steels[11–18] because cracks easily initiate and propagate along the ASBs. In order to improve the
MIN CHEOL JO, SELIM KIM, HYUNG KEUN PARK, HYOUNG SEOP KIM, and SUNGHAK LEE are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 37673, Korea. Contact e-mail: [email protected] SUNG SUK HONG and HONG KYU KIM are with the Materials Directorate, Agency for Defense Development, Daejeon 305-600, Korea. Manuscript submitted November 19, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
ballistic performance as well as basic dynamic properties by preventing or minimizing ASBs, their formation behavior should be carefully analyzed and predicted, but very few studies have been conducted regarding examination of ASBs and correlation with the ballistic behavior due to very hard work to interpret high strain rate behavior. In the present study, the formation behavior of ASBs or cracks in commercial ultra-high-strength armor steels was investigated by a laboratory-scale split Hopkinson pressure bar (SHPB). A ballistic impact experiment was also carried out to examine deformation and fracture behaviors and ASB microstructures and to correlate them with those of the SHPB. Formation possibility and extent of ASBs or cracks were evaluated by defining a critical strain for ASB formation obtained from interrupted dynamic compressive tests and by conducting quantitative analyses on ASBs or cracks in relation with basic microstructures, intrinsic properties, dynamic compressive properties, and finite element method (FEM) simulations. Two commercial ultra-high-strength steels whose brand names are ‘‘Mars300’’ and ‘‘Armox Advance’’ fabricated in steel companies of Industeel, France and SSAB, Sweden, respectively, were used, and their chemical compositions are shown in Table I. The ‘‘Mars300’’[19] and ‘‘Armox Advance’’[20] steel plates (thickness; 12 mm), which are referred to as ‘‘MA’’ and ‘‘AR’’, respectively, for convenience, basically, had a martensite microstructure and were compared mainly based on strength levels because their compositions and fabrication procedures were different. These plates were polished and etched in a 1-pct-nital solution, and the etched microstructures of longitudinalshort-transverse (L-S) plane were observed by optical and scanning electron microscopes (SEM). Constituent phases were examined by X-ray diffraction (XRD, Cu Ka) and electron backscatter diffraction (EBSD, step size;
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