Strain Rate Effects

Abstract

The mechanical properties of most materials vary with the rate of loading and this effect is particularly evident in polymers. It is usual when considering rate of loading effects in a material, rather than a structure, to quantify the loading rate in terms of a strain rate where the strain rate can conveniently be visualised as the number of times a sample will increase its own length in a second. Strain rates of interest vary from very low ~ 10⁻⁵ s⁻¹ which represent the lowest rates achievable in standard compression/tensile test rigs (though even lower rates are involved in creep tests) to ~ 10⁴ s⁻¹ which represents the highest rates achievable in standard high rate test equipments (see The Hopkinson bar). Even higher rates are attainable using exploding wire and plate impact techniques. Increasing strain rate has the effect of increasing both the modulus and the yield stress and flow stress of a polymer. Reported increases in yield stress (and the flow stress at a given strain) are in the region of 1 to 6 MPa per decade of strain rate for rates from 10⁻³ to 10³ s⁻¹ and the increase is almost independent of strain upto strains of ~ 0.3. Typical values for a selection of polymers are listed below in units of MPa per decade of strain rate increase.