Elastic modulus

About: Elastic modulus is a research topic. Over the lifetime, 33153 publications have been published within this topic receiving 810247 citations.

Mechanical degradation and viscous dissipation in B2O3.

Abstract: The Brillouin light scattering technique was used to determine the complex mechanical modulus, which describes the dynamic response of molecular structures, for ${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$ and binary alkali borate. The effects of temperature, alkali concentration, oxygen and water vapor partial pressures on the structural developments and on the thermal activation of dissipative processes were examined. The glass transition in these systems is characterized by a discontinuity in the temperature dependence of the elastic component of this modulus. Above ${\mathit{T}}_{\mathit{g}}$, the elastic modulus decreases with a faster rate, the higher the alkali concentration. The complete structural evolution from a room temperature glass to a liquid near the boiling point was found to involve several distinct mechanisms, which become gradually activated with increasing temperature. By using a mechanical relaxation formalism, the activation energies and preexponential time constants describing the mechanical degradation, as well as the molecular rearrangements associated with each mechanism were determined. For a given system, the initial network degradation is characterized by the smallest activation energy. The motion involved in this process is that of boron atoms oscillating between triangular and tetrahedral coordination, upon exchanging one of their oxygen neighbors. During this phase boroxol rings open, without the formation of nonbridging oxygens. At intermediate temperatures the motion of alkali cations between network segments is activated, and at very high temperatures complete network disintegration takes place, leaving ionic species whose motion occurs by complete dissociation from their immediate neighbors.

Plasticity size effects in free-standing submicron polycrystalline FCC films subjected to pure tension

Abstract: The membrane deflection experiment developed by Espinosa and co-workers was used to examine size effects on mechanical properties of free-standing polycrystalline FCC thin films. We present stress–strain curves obtained on films 0.2, 0.3, 0.5 and 1.0 μm thick including specimen widths of 2.5, 5.0, 10.0 and 20.0 μm for each thickness. Elastic modulus was consistently measured in the range of 53– 55 GPa for Au, 125– 129 GPa for Cu and 65– 70 GPa for Al. Several size effects were observed including yield stress variations with membrane width and film thickness in pure tension. The yield stress of the membranes was found to increase as membrane width and thickness decreased. It was also observed that thickness plays a major role in deformation behavior and fracture of polycrystalline FCC metals. A strengthening size scale of one over film thickness was identified. In the case of Au free-standing films, a major transition in the material inelastic response occurs when thickness is changed from 1 to 0.5 μm . In this transition, the yield stress more than doubled when film thickness was decreased, with the 0.5 μm thick specimen exhibiting a more brittle-like failure and the 1 μm thick specimen exhibiting a strain softening behavior. Similar plasticity size effects were observed in Cu and Al. Scanning electron microscopy performed on Au films revealed that the number of grains through the thickness essentially halved, from approximately 5 to 2, as thickness decreased. It is postulated that this feature affects the number of dislocations sources, active slip systems, and dislocation motion paths leading to the observed strengthening. This statistical effect is corroborated by the stress–strain data in the sense that data scatter increases with increase in thickness, i.e., plasticity activity. The size effects here reported are the first of their kind in the sense that the measurements were performed on free-standing polycrystalline FCC thin films subjected to macroscopic homogeneous axial deformation, i.e., in the absence of deformation gradients, in contrast to nanoindentation, beam deflection, and torsion, where deformation gradients occur. To the best of our understanding, continuum plasticity models in their current form cannot capture the observed size scale effects.

Solvent-free, supersoft and superelastic bottlebrush melts and networks

Abstract: Solvent-free, supersoft and superelastic polymer melts and networks made from bottlebrush macromolecules can display low modulus, high strain at break, and extraordinary elasticity. Polymer gels are the only viable class of synthetic materials with a Young’s modulus below 100 kPa conforming to biological applications1,2,3, yet those gel properties require a solvent fraction4,5,6,7. The presence of a solvent can lead to phase separation, evaporation and leakage on deformation, diminishing gel elasticity and eliciting inflammatory responses in any surrounding tissues. Here, we report solvent-free, supersoft and superelastic polymer melts and networks prepared from bottlebrush macromolecules. The brush-like architecture expands the diameter of the polymer chains, diluting their entanglements without markedly increasing stiffness. This adjustable interplay between chain diameter and stiffness makes it possible to tailor the network’s elastic modulus and extensibility without the complications associated with a swollen gel. The bottlebrush melts and elastomers exhibit an unprecedented combination of low modulus (∼100 Pa), high strain at break (∼1,000%), and extraordinary elasticity, properties that are on par with those of designer gels8,9.

The response of solids to elastic/plastic indentation. I. Stresses and residual stresses

Abstract: A new approach for analyzing indentation plasticity and for determining indentation stress fields is presented. The analysis permits relations to be established between material properties (notably hardness, yield strength, and elastic modulus) and the dimensions of the indentation and plastic zone. The predictions are demonstrated to correlate with observations performed on a wide range of materials. The indentation stress fields are computed along trajectories pertinent to three dominant indentation crack systems: radial, median, and lateral cracks. The peak load and residual tensile stresses are shown to be consistent with observed trends in indentation fracture.