PSI - Issue 14

Kartikeya et al. / Procedia Structural Integrity 14 (2019) 514–520 Kartikeya/ Structural Integrity Procedia 00 (2018) 000 – 000

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composite materials, foams and concrete are used as ballistic materials owing to different level of protection and weight considerations. Composite panels and foams are used in Explosive Ordinance Disposal (EOD) suit of a solider whereas armor steels are mainly used for protecting vehicles, used by defense forces, from land-mines and other buried improvised explosive devices (IEDs). Plates of armor steel are lined on the body of a vehicle for blast protection so that it becomes armored vehicle. Mitigation of effects of the blast could be done by reducing peak overpressure, blast wave velocity and resulting high temperature due to blast, Kinney and Graham (2014). Ballistic steels absorb, reflect and deflect blast energy to prevent structures unlike mild steels which deform plastically and then fail due to ductile tearing according to Langdon et al (2015). Armor steels store more elastic energy during deformation than mild steels due to their higher strength. At greater charge mass, when both type of steel fail, mild steels exhibit ductile tensile rupture whereas armor steels exhibit comparatively more brittle failure involvement. Three failure modes are observed in impulsively loaded steel plates, Mode I large plastic deformation, Mode II partial tearing and Mode III capping at central region of plate according to Olson (1993). Jacob et al. (1993) showed as the loading impulse is increased, Mode I is responsible for maximum energy dissipation. Almohandes et al. (1996) tested monolithic plates, layered plates and composite plates with Fiberglass/polyester composites under ballistic impacts. It was observed that unbounded layered plates provided least protection. Børvik et al. (2003) determined that thick targets exhibit shear plugging and thin targets exhibit global phenomenon like bending and membrane stretching. This change of energy dissipation route makes laminated configuration more attractive than monolithic. Palta et al (2018) showed that laminated configuration with layers of Kevlar/epoxy composite with steel offer better protection against bullet penetration with reduced weight. Design by simulation can be a powerful method for fabrication and deployment of blast resistant structures. Appropriate material model and effective blast simulation are two critical elements for effectively simulating blast resistance of a material. There are three ways available in literature for simulating blast waves according to Børvik et al (2009). First is CONWEP empirical model provided in Abaqus FE package. It is most economical computationally. Second is Coupled-Eulerian-Lagrangian (CEL) modeling. It models air and explosive as Eulerian domain and solid structures as Langrangian. Since user has to validate both domains this method is quite expensive computationally, although most realistic simulations of all the methods can be performed. Third method is extracting blast pressure wave history from a Computational Fluid Dynamics (CFD) simulation and applying it on structure in an explicit analysis. 2. Materials and Methods Armor steels are well established as blast resistant materials. Commercially available steel known as Armox 440T, SSAB (2017), is used for this study due to availability of relevant data. These steels have Young’s Modulus (E = 207 GPa) and Poisson’s ratio (ν = 0.3) equivalent t o mild steels but they are much stiffer during loading due to higher strength (σ y = 1100 MPa and UTS = 1250-1550 MPa). During interaction of blast wave with structure, the material experiences plastic flow at high strain rates, increase in temperature and subsequent material fracture. Since Johnson-Cook material model incorporates effects from strain rate, hardening and temperature, it is most popular in current literature of numerical modeling. Model equivalent stress is given by Eq. 1. 2.1. Material Model

(1)

A, B, C, n and m are material constants which can be determined by performing certain tests as specified by Johnson and Cook (1985). First parenthesis of Equation 1 signifies contribution on equivalent stress from strain-rate sensitivity of material; second term signifies effect of strain-hardening and third determines the effect of temperature softening. Non-dimensional temperature ( is the controlling term for thermal softening as defined in Eq. 2.