Damage mechanism in high hardness armor (HHA) steel subjected to V50 ballistic impact A.G. Odeshi 1 , M.N. Bassim 2 and M. Bolduc 3 1 Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Dr., Saskatoon, Saskatchewan, S7N 5A9, Canada 2 Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, EITC, Manitoba R3T 2N2, Canada 3 Defence Research and Development Canada, Valcartier, Quebec, Canada Abstract. A high hardness armor (HHA) steel plate was subjected to a standard V50 ballistic im- pact test. Microstructural evolution leading to deformation and perforation of the plates during projectile penetration is investigated. Penetration of the steel plate by the projectile is promoted by shear strain localization and occurrence of adiabatic shear bands which were observed around the perforations. Cracking of the shear bands triggered the failure and perforation of the steel plate during the ballistic impact. The primary transformed bands enveloping the projectile has an average thickness of about 55 mm, while narrower secondary shear bands of about 12 mm wide branched out of the primary band leading to secondary cracking which propagated from the wall of perforation into the armor plate. 1. INTRODUCTION Material deformation at high strain rates is dominated by the formation of narrow bands of intense shear strain localization known as adiabatic shear bands (ASB), along which shear failure occurs at high strain rates [1–4]. The mechanisms of occurrence of adiabatic shear bands are complex and have continued to attract increasing attention among researchers who are engaged in a sustained effort to understand the controlling variables and thereby improve materials performance under high strain-rate loading conditions such as ballistic impact, explosion fragmentation, and mechanical forming operations. Adiabatic shear bands occur when the plastic strength of a material drop drastically as a result of thermal softening effect of adiabatic heating in some regions at high strain rates [1]. Temperatures of up to 1100xC were measured inside adiabatic shear bands during formation in a titanium alloy undergoing high strain rate deformation in torsion [5]. Batra and Love [2, 4] suggested that adiabatic shear band are initiated at a point where the maximum shear stress drops to 80% of its peak value in homogeneous materials, although they reported that this criteria does not hold for particulate composites. The possibility of a simultaneous initiation of multiple shear bands at the onset thermal-softening- induced strain localization that eventually grows into one full-blown adiabatic shear band has been suggested [1, 5]. Whereas Marchand and Duffy [1] proposed the coalescence of the initial multiple shear bands, Ranc et al. [5] proposed another mechanism which suggests deactivation of other starting shear bands by stress relaxation created by one of them. Experimental investigations and finite element calculations by Bassim et al. [3, 6] suggest that the initiation of adiabatic shear bands is promoted by the presence of inhomogeneities such as secondary precipitates in high strength metallic alloys. The threshold strain-rate and critical strain for the occurrence of adiabatic shear band varies depending on the material and on its microstructure [7]. Intense shear strain localization in adiabatic shear bands induces finer microstructure and increased martensite content inside the shear bands in dual-phase steels [8] and DYMAT 2009 (2009) 563–567 Ó EDP Sciences, 2009 DOI: 10.1051/dymat/2009080