Strain hardening and its relation to Bauschinger effects in oriented polymers

Abstract

ABSTRACT:The nature of strain hardening in glassy polymers is investigated by studying the mechanical response of oriented polycarbonate in uniaxial extension and compression. The yield stress in extension is observed to increase strongly with predeformation, whereas it slightly decreases in compression (the socalled Bauschinger effect). Moreover, oriented specimens tend to display increased strain hardening in extension, whereas this nearly vanishes in compression. It is shown that these observations can be captured by the introduction of a viscous contribution to strain hardening in terms of a deformation dependence of the flow stress. This can originate either from a deformation-induced change in activation volume, as observed for isotactic polypropylene, or from a deformation-induced change of the rate constant, as observed for polycarbonate, which causes the room temperature yield kinetics of this material to shift from the into the ( + ) regime.

Investigation of Bauschinger effect in thermo-plastic polymers for biodegradable stents

“…Comparable to the results for polycarbonates of Senden et al [3] a phenomenon similar to Bauschinger effect can also be demonstrated in biodegradable thermoplastic polymers for stent application. The reduction of yield strength due to plastic pre-strain in the opposite loading direction most likely influences the mechanical properties of biodegradable polymeric stents, e.g.…”

“…[3] already showed a phenomenon similar to Bauschinger effect, which was basically described for metals, in polycarbonates. The aim of the present study is to investigate if there is such a phenomenon also for thermoplastic polymers used for stent application.…”

Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening

“…Recent experiments and modeling [47,53,57,64] have provided strong support to the notion that entanglements play only a secondary role in glassy-polymeric strain hardening, at least for the majority of synthetic polymers and in the weak hardening regime. Refs.…”

“…It must be noted that increasing l K also increases ρ e [52]. However, considerable evidence (e. g. [9,19,23,24,47,53,55]) suggests that chain orientation and local, secondary interactions which act over scales ∼ (l 0 ∼ a ∼ l K ) are the true controlling factors for σ, at least during the initial stages of hardening. Finally, Haward postulated that G R arises from the contraints imposed by the mesh of uncrossable chains [11]; our argument that hardening scales with ρ cr are consistent with this hypothesis.…”

Effect of deformation mode and grain size on Bauschinger behavior of annealed copper

“…The increase of flow stress occurs due to the formation of more barriers against dislocation movement because of more dislocation-dislocation interaction apart from the interactions between dislocations with different barriers. Accordingly, it is expected that increasing the stress rate in the forward direction would lead to the development of higher back stress and consequently the Bauschinger parameters would be influenced when the change in stress rate is sufficient to significantly alter the work hardening behavior of that material [5,[15][16][17][18][19][20].…”