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As mentioned above, one of the more practical ways to defeat a given threat is by adding a relatively lightweight structure in front of the protected object, as shown in Fig.
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It is clear that the ballistic efficiency should be higher than 1.0 and the task of the armor designer is to increase these efficiencies to higher values. Frank (1981) suggested several measures for the ballistic efficiency of improved structures, through their mass (Em) and space (Es) efficiency, according to:$$ $$where the indices (r) and (s) denote the reference target and the improved structure, respectively, and P is the minimal thickness of the target which is needed to defeat the threat.
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The ballistic efficiency of a given structure is defined by its areal density, as compared with that of a reference target, which is needed to defeat a given threat. The relevant measure of the armor weight is its areal density (AD), given in units of kg/m2, which is simply the density of the protective structure multiplied by its thickness. Obviously, the aim of the armor designer is to increase the value of Vbl without increasing the weight of the structure. One of them is the ballistic limit velocity (Vbl) of the specific armor/threat combination. The protective capability of a structure against a given threat can be evaluated by several measures. The performance of these UHH steels will be compared to the current HHA MIL-DTL-46100E specification. This paper assesses the performance of these steels against two projectiles that will be used to generate a new military specification for Ultra High Hard Steel armor. ARMOX 600T is a nominal 600 BHN steel while ARMOX ADVANCE has a nominal hardness of Rockwell C58-63 (greater than 650 BHN). However, SSAB has two UHH armor steels designated ARMOX 600T and ARMOX ADVANCE that meet this hardness criteria. Swedish Steel Oxelösund AB (SSAB) produces a number of grades of steel which have previously been assessed against AP ammunition. This class of steels increases AP bullet defeat, reduces armor weight, and eliminates the manufacturing difficulties inherent in DHA. While these materials still serve their intended applications, monolithic Ultra-High Hardness (UHH) steels with a hardness of 600 BHN or greater have been developed. steel alloys for armor piercing (AP) bullet protection are manufactured to MIL-DTL-46100E, High Hardness Armor (HHA) with a hardness range of 477-534 Brinell Hardness Number (BHN) and to MIL-A-46099C Dual Hardness Armor (DHA) that is produced by roll bonding a 601-712 BHN front plate to a 461-534 BHN back plate.