Abstract
Under ballistic impact or blast loading, the high strain rate and high temperature behaviour of armour steels is key to their response to a given threat. This experimental and numerical investigation examines the tensile response of a class 4a improved rolled homogenous armour steel (IRHA) and a high hardness armour steel (HHA). Cylindrical tensile specimens were tested at a range of strain rates from 0.001 s-1 to 2700 s-1. Quasi-static, elevated temperature tests were performed from room temperature up to 300° C. While the HHA is strain rate insensitive, the IRHA displays a significant increase in strength across the range of loading rates reducing the ultimate strength difference between the materials from 19% at 0.001s-1 to 4.6% at 2700s-1. An inverse numerical modelling approach for constitutive model calibration is presented, which accurately captured the dynamic material behaviour. The modified Johnson-Cook strength and Cockcroft-Latham (C-L) fracture models were capable of predicting the ballistic limit of each material to within 5% of the experimental result and to within 10% for deformation under blast loading. The blast rupture threshold of both materials was significantly over-estimated by the C-L model suggesting stress state or strain rate effects may be reducing the ductility of armour steel under localised blast loading.
Highlights
In protective structures armour steels are utilised to defeat various blast and ballistic threats
A comparison of four grades of armour steel under localised blast loading was presented by McDonald [7] and showed that the normalised deformation resistance of an improved rolled homogenous armour (IRHA) grade exceeded that of two higher yield strength RHA grades and approached that provided by High hardness armour (HHA), regardless of the 20% higher tensile yield strength
This paper evaluates the mechanical response of an IRHA and a HHA grade of steel under a range of tensile loading conditions, which include high strain rates and temperatures
Summary
In protective structures armour steels are utilised to defeat various blast and ballistic threats. Numerous grades of armour steel are defined in defence standards [1–3] with mechanical performance and metallurgy optimised for protection against specific threats whilst maintaining adequate engineering properties. High hardness armour (HHA) steel [2] is optimised for ballistic protection with a Brinell hardness between 477 and 534 and lower Charpy fracture toughness requirements than the IRHA. While no specific explanation was given for the similar performance, Showalter commented that with negligible differences in ballistic protection, higher toughness and improved weldability, the IRHA grade would be better choice of armour material than the traditional HHA in a structural. This paper evaluates the mechanical response of an IRHA and a HHA grade of steel under a range of tensile loading conditions, which include high strain rates and temperatures. Elongation was measured by a non-contact extensometer about a 45 mm gauge length defined at the beginning of the test
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