The strain rate sensitivity of two commercial aluminium alloys AA6082 and A
A7108 in peak temper T6 and overaged T79 condition is analysed. These alloy
s are precipitation hardenable and as such, are expected to have a relative
ly low strain rate sensitivity. Their response to rapid loading has been te
sted over a wide range of strain rates, from 0.1 to 3000 s(-1) at room temp
erature, 375 and 515 degrees C for AA6082 and room temperature, 280 and 340
degrees C for AA7108. Specimens were cut from planar extruded sections and
tested in uniaxial compression with the deformation axis parallel to the e
xtrusion direction. Intermediate strain rate testing was carried out using
a computer controlled servo-hydraulic testing machine and high strain rate
testing above 500 s(-1) on a Split Hopkinson Pressure Bar (SHPB). Owing to
inaccuracies of the conventional Kolsky solution of the SHPB, a numerical s
imulation was performed. The flow stress at a plastic strain of 5% was plot
ted as a function of strain rate and temperature. Up to 2000 s(-1), there i
s very little change of the activation volume with increasing temperature f
or both alloys, indicating that barriers to thermal activation are large an
d approximately the same. However, at higher strain rates, the rate control
ling mechanism appears to change and at lower temperatures, a trend towards
negative strain rate sensitivity was observed. Metallographic examination
of AA7108 revealed strain localisation, seen as parallel deformation bands.
The strain localisation could be the cause of the negative strain rate sen
sitivity. (C) 2000 Elsevier Science S.A. All rights reserved.