33 research outputs found
Strain Hardening in Polymer Glasses: Limitations of Network Models
Simulations are used to examine the microscopic origins of strain hardening
in polymer glasses. While traditional entropic network models can be fit to the
total stress, their underlying assumptions are inconsistent with simulation
results. There is a substantial energetic contribution to the stress that rises
rapidly as segments between entanglements are pulled taut. The thermal
component of stress is less sensitive to entanglements, mostly irreversible,
and directly related to the rate of local plastic arrangements. Entangled and
unentangled chains show the same strain hardening when plotted against the
microscopic chain orientation rather than the macroscopic strain.Comment: 4 pages, 3 figure
Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening
A simple theory for glassy polymeric mechanical response which accounts for
large scale chain relaxation is presented. It captures the crossover from
perfect-plastic response to strong strain hardening as the degree of
polymerization increases, without invoking entanglements. By relating
hardening to interactions on the scale of monomers and chain segments, we
correctly predict its magnitude. Strain activated relaxation arising from the
need to maintain constant chain contour length reduces the dependence of
the characteristic relaxation time by a factor during
active deformation at strain rate . This prediction is consistent
with results from recent experiments and simulations, and we suggest how it may
be further tested experimentally.Comment: The theoretical treatment of the mechanical response has been
significantly revised, and the arguments for coherent relaxation during
active deformation made more transparen
Strain Hardening of Polymer Glasses: Entanglements, Energetics, and Plasticity
Simulations are used to examine the microscopic origins of strain hardening
in polymer glasses. While stress-strain curves for a wide range of temperature
can be fit to the functional form predicted by entropic network models, many
other results are fundamentally inconsistent with the physical picture
underlying these models. Stresses are too large to be entropic and have the
wrong trend with temperature. The most dramatic hardening at large strains
reflects increases in energy as chains are pulled taut between entanglements
rather than a change in entropy. A weak entropic stress is only observed in
shape recovery of deformed samples when heated above the glass transition.
While short chains do not form an entangled network, they exhibit partial shape
recovery, orientation, and strain hardening. Stresses for all chain lengths
collapse when plotted against a microscopic measure of chain stretching rather
than the macroscopic stretch. The thermal contribution to the stress is
directly proportional to the rate of plasticity as measured by breaking and
reforming of interchain bonds. These observations suggest that the correct
microscopic theory of strain hardening should be based on glassy state physics
rather than rubber elasticity.Comment: 15 pages, 12 figures: significant revision
Giving shape to the past: Pre-columbia in nineteenth-century Mexican literary journals
Is FAD fishing an economic trap ? Effects of seasonal closures and other management measures on a purseâseine tuna fleet
International audienceThe management of fish aggregating devices (FAD) creates heated debates in tuna fishery management organizations striving to reduce the number of deployed floating objects. Through several econometric models and a machine learning approach, we evaluate the consequences of three management scenarios on the catch and profit of the French purseâseine fleet operating in the Indian Ocean: (1) a half reduction in the number of authorized buoys per vessel, (2) a 72âday closure of FAD fishing with and (3) without reâallocation of effort on free schools. The results show a significant decrease of fleet profits by 7%, 10% and 18% respectively. We hypothesize an âeconomic trapâ of FAD fishing caused by the far greater efficiency of this harvesting technique for larger vessels searching for economies of scale, and by the overfished status and catch limitation of yellowfin ( Thunnus albacares ) and bigeye ( Thunnus obesus ) tunas in the Indian Ocean. The results are compared with other studies looking at the impact of FAD management measures in other oceans