High Strain Rate Tensile Deformation of Low Carbon Steels (OLP-0160)

Abstract

An increase in demand for many advanced materials like high strength steels, polymers, reinforced composites etc. have been observed in automobile industries to meet the challenges of light weight, safe design and improved crashworthiness. These materials are often put into application considering their quasi-static properties. But successful design of components using these materials also demands consideration of dynamic properties where there is a possibility of high speed deformations like ‘crash’ in cars, ‘ballistic impact’ in missile nose cones, ‘foreign object damage’ in aero components etc. Amongst many advanced materials, high strength steels like transformation induced plasticity (TRIP), twinning induced plasticity (TWIP), interstitial free (IF) steels have gained importance in automobile industries due to their excellent combination of strength and ductility. Increase in application of these materials also demands determination of their dynamic properties to assess their crashworthiness. The current investigation aims at successful determination of dynamic tensile properties of low carbon steels at 101 and 102 s-1 order of strain rates using a special servo-hydraulic test system and comparing their properties at dynamic range with that of in quasi-static range. Sheets of an interstitial free and a plain carbon steel have been selected for the current investigation. The experiments consisted of characterization of their microstructure like examination of the existing phases and determination of average grain sizes and determination of the tensile properties at wide range of strain rates, ranging from 1x10-4 to 3x102 s-1. Tensile tests of the selected materials at quasi-static strain rates, ranging from 1x10-4 to 1x10-1 s-1, were conducted in a conventional servo-hydraulic test machine. Special servo-hydraulic high speed test machine was used to conduct tensile test in high strain rate regime, i.e. from 5x101 to 3x102 s-1. The external noise present in the load-time plots obtained at high strain rates is eliminated by using Fourier filtering. Strength values of both the steels were found to increase as the strain rate increases from 1x10-4 to 3x102 s-1, but percentage elongation decreased in the quasi-static range of 1x10-3 to 1x10-1 s-1, whereas it increased in the high strain rate regime from 1x102 to 3x102 s-1. Very low strain hardening exponent values and decrease in strain rate sensitivity with increase in strain levels at high strain rates have been attributed to thermal softening behavior which takes place due to adiabatic heating during deformation. Ductile-shear is the primary deformation mode for both the steels at quasi-static and high strain rates.