Mauricio Ponga de la Torre
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Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
A brush-like structure emerges from the stretching of long polymer chains, densely grafted on to the surface of an impermeable substrate. This structure is due to a competition between the conformational entropic elasticity of grafted polymer chains, and the intra and interchain excluded volume repulsions. Polymer brushes occur in biology: neurofilaments, articulate cartilage, extra cellular biopolymers etc. Recently, engineered soft active materials are developed to produce large controllable and reversible bending and stretching deformations. These materials are the focus of this work.New theoretical models, molecular simulations to assess them, and experimental studies are presented in this work. Mechanical stress within a brush and its dependence on the molecular parameters of the brush and external stimulus (temperature) is studied for the first time. A continuum beam model accounting for the Young-Laplace and the Steigman-Ogden curvature elasticity corrections is developed first to understand the large deformation of a flexible substrate due to a brush grafted on it. This model yields a generalized surface stress-curvature relation that enables one to determine stress from curvature measurements.Strong stretching theory (SST) from polymer physics is combined with continuum mechanics to obtain stress variation in a neutral brush with Gaussian chains. This theory predicts that the normal stress, parallel to the substrate, is a quartic function of the distance from the grafting surface with a maximum at the grafting surface. Idealizing the brush as a continuum elastic surface with residual stress, closed-form expressions for surface stress and surface elasticity as a function of molecular weight and graft density are derived. At a higher graft density, a more refined (semi) analytical SST with Langevin chain elasticity is advanced. Theoretical predictions are assessed by molecular dynamics simulation of a brush using bead-spring model.Experiments on a thermoresponsive brush grafted onto a soft beam showed the surface stress is ∼ −10 N/m and its magnitude decreases gradually, and reversibly, on increasing solvent temperature. Molecular scale parameters of the brush are estimated experimentally to enable qualitative comparison with SST theories.
Master's Student Supervision (2010 - 2020)
The performance of Niobium (Nb) superconducting radio frequency (SRF) cavities is extremely sensitive to defects near the surface. These imperfections include dislocations, interstitial solutes, or grain boundaries which have been shown to limit the material’s ability to expel magnetic flux during the transitions to its superconducting state. This gives rise to detrimental thermal effects that limit the quality factor, and hence performance, of an SRF cavity. However, beside its importance in the performance of SRF cavities, little is known about the effect of annealing on the distribution of dislocations, specially near the surface. In this work, a mixed multiscale computational and experimental approach is developed to study the evolution dislocation density in pure Nb samples during annealing.The model is based on a Discrete Dislocation Dynamics (D.D.D.) technique that couples glide and climb motion of edge dislocations to simulate the evolution of dislocation density within the bulk of a Nb substrate as well as near the surface. The D.D.D. model integrates information from multiple scales. Accelerated molecular dynamics simulations were used to calculate the required activation energies for vacancy migration and hence fed our dislocation climb constitutive laws. Finite element modelling and experimental techniques were used as inputs to the D.D.D. implementation which accounted for the presence of residual stresses for both kink-pair nucleation dominated glide motion and vacancy migration driven climb.The model showed good agreement with other empirical methods. Furthermore, experimental results showed that the resistance of Nb samples at temperatures slightly above the critical temperature (T = 10K) can be coupled to the dislocation density. Hence, together the D.D.D. framework provided herein, can serve as a useful tool to determine an optimal annealing recipe for a given initial state of material condition.
The dynamic behaviour and failure mechanisms of nano-sized voids in single crystals is studied for three hexagonal close-packed materials by means of molecular dynamics simulations. Our study reveals that in Magnesium the response is highly anisotropic leading to a brittle to ductile transition in the failure modes under different load orientations. This transition is accompanied by different mechanisms of deformation and is associated with the anisotropic HCP lattice structure of Mg and the associated barrier for dislocation motion. Remarkably, brittle failure is observed when external loads produce a high stress triaxiality while the response is more ductile when the stress triaxiality decreases. On the other hand, the failure in other two hexagonal close-packed materials studied in this work, i.e, Titanium and Zirconium, is more ductile, in high contrast with the brittle failure observed in Magnesium. We find that this difference is due to the fact that nano-sized voids in Titanium and Zirconium emit substantially more dislocations than Magnesium, allowing for large displacements of the atoms and plastic work, including non-basal planes. Based on our findings, we postulate that this brittle failure in Magnesium is due to a competition between dislocations emission in the basal plane and crack propagation in non-basal planes. Thus, we propose to use the ratio between unstable stacking fault and surface energy in these materials to assess the tendency of hexagonal close-packed materials and alloys to fail under brittle or ductile modes. Using this ratio, we critically identify the low surface energy of Mg as responsible for this brittle behaviour and recommend that Mg-based alloys with large surface energies can lead to better performance for dynamic applications. The fundamental mechanisms observed, therefore, explain the low spall strength of Mg and suggest the possibility of manipulating some mechanisms to increase ductility and spall strength of new lightweight Mg alloys.