Flow stress peaks at high strain rates in PMMA

Abstract
The results of an investigation into the stress-strain behaviour of PMMA in the strain rate region of 10² to 10⁴ s⁻¹ over a range of temperatures from room temperature up to 90°C are reported. The investigation was carried out in an effort to detect the presence, or absence, of peaks in the flow stress – strain rate response. All tests were carried out in compression on a standard Hopkinson Bar system. Although not as clearly defined as the sharp peak in flow stress reported by previous workers in PEEK, there is good evidence of a peak in the flow stress in the strain rate range 10² to 10³ which both narrows and moves to a higher strain rate as the temperature approaches the glass transition. Previously reported peaks in flow stress had only been observed in semi-crystalline polymers. The current observations in PMMA suggest that this may be a universal phenomenon. Preliminary work at similar strain rates and temperature ranges on PCTFE, show less conclusive indications of peaks in the flow stress at strain rates around 3×10³ s⁻¹.