Mechanical response of polyether polyurethane foams under multiaxial stress and the initial yielding of ultrathin films.

摘要

In the first part of this thesis, we study the mechanical response of elastic polyether polyurethane (EPP) foams by means of experiments, theory, and modeling. The experiments include five loading cases: uniaxial compression along the rise direction; uniaxial compression along two mutually perpendicular transverse directions; uniaxial tension along the rise direction; shear combined with compression along the rise direction; and hydrostatic pressure combined with compression along the rise direction.;We use our experimental results to calibrate a mean-field model of EPP foams. In this model, a unit cell composed of several bars is cut off from an idealized, perfectly periodic foam microstrusture. The tips of the bars of the cell are subjected to a set of displacements affine with the applied mean deformation gradient, and left to rotate freely. The unit cell is characterized using a few physically meaningful material and geometric parameters whose values may be readily estimated for any given foam.;We verify that under uniaxial loading the model predicts configurational phase transitions, stress plateaus, and two-phase fields for low-density foams; a critical point for foams of a critical density; and monotonically hardening stress-strain curves for foams of density higher than the critical density. The critical exponents associated with the critical point are the same as in other mean-field models such as the Van der Walls model of a fluid.;We conclude that the mechanical response of EPP foams is dominated at large strains by either one of two mechanisms at the level of a foam cell: snap-through buckling, which leads to nonconvex strain energy functions, stress plateaus, and two-phase strain fields; or bending, which leads to convex strain energy functions, monotonically increasing stresses, and homogeneous strain fields.;This conclusion allows us to interpret an extensive series of experiments in which EPP foam specimens are penetrated with a wedge-shaped punch. For low-density foams, we find experimentally that the mechanical response is linear up to a penetration of the punch of about 40% of the height of the specimen. We surmise that the strain field in the specimen consists of a high-strain phase in a region close to the tip, where a phase transition has taken place, and a low-strain phase in a region far from the tip, where the phase transition is yet to take place. The two regions are separated by a sharp interface, where the strain is discontinuous. We use DIC to trace the sharp interface as it grows and sweeps through the specimen during a test.;In the second part of this thesis, we study the initial yielding of ultrathin metallic films (thickness of a fraction of a mum). Recent experiments indicate that in free-standing metallic films of constant grain size the initial yield stress increases as the film becomes thinner, it peaks for a thickness on the order of 100 nm, and then starts to decrease. This reversing (first hardening, then softening) size effect poses two challenges: (1) It cannot be explained using currently available models and (2) it appears to contradict the little-known but remarkable experimental results of J. W. Beams [1959], in which the size effect in bulge tests did not reverse even for a thickness of 20 nm.;We show that the reversing size effect can be explained and the contradiction dispelled by taking into account the effect of the surface stress on the initial yielding. We also predict that the mode of failure of a film changes from ductile to brittle for a thickness on the order of 100 nm, in accord with experiments. Our successful application of methods of continuum mechanics to films as thin as 100 times a typical lattice parameter adds to a growing realization of the robustness of these methods at ultrasmall length scales. (Abstract shortened by UMI.)