School of Mechanical Engineering Seminar
Wednesday, October 17, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Models and Simulations of the Surfactant-Driven

Fracture of Particle Rafts

John E. Dolbow

Professor of Civil and Environmental Engineering,

Mechanical Engineering and Materials Science,

and Mathematics

Duke University

When a densely packed monolayer of hydrophobic particles is placed on a uid surface

the particles interact through capillary bridges, leading to the formation of a particle raft or

\praft" for short. Densely packed monolayers exhibit a two-dimensional elastic response, and

they are capable of supporting both tension and compression. The introduction of a controlled

amount of surfactant generates a surface tension gradient, producing Marangoni forces and

causing the surfactant to spread, fracturing the monolayer. These systems are of interest to

materials scientists and engineers because they provide an idealized setting for investigating the

interplay between uid ow and fracture. Previous studies of the surfactant-induced fracture

of prafts have examined the role of viscosity and the initial packing fraction on the temporal

and spatial evolution of the fractures. The potentially important role of di_erences in surface

tension between the surfactant and the underlying uid has not been explored.

This seminar will describe a new continuum-based model and simulations that account for

the interplay between the pressure exerted by a spreading surfactant and the elastic response

of the praft, including the fracture resistance. This is e_ected through the use of a surfactant

damage _eld that serves as both an indicator function for the surfactant concentration, as well

as the damage to the monolayer. Stochastic aspects of the particle packing are incorporated into

the model through a continuum mapping approach. The model gives rise to a coupled system

of nonlinear partial di_erential equations, with an irreversibility constraint. We recast the

model in variational form and discretize the system with an adaptive _nite element method. A

comparison between model-based simulations and existing experimental observations indicates

a qualitative match in both the fracture patterns and temporal scaling of the fracture process.

Based on the model, we determine a dimensionless parameter that characterizes the ratio

between this driving force and the fracture resistance of the praft. Interestingly, while our

results indicate that the stochastic aspects of the packing are important to the fracture process,

we _nd that regimes of fracture are largely governed by di_erences in surface tension. Finally,

we support our _ndings with newly designed experiments that validate the model and con_rm

the trends inferred from the simulations.

School of Mechanical Engineering Seminar
Monday, May 28, 2018 at 14:00
Wolfson Building of Mechanical Engineering, Room 206

Ceramic Matrix Composites (CMCs): Manufacturing and Microstructural Effects on the Mechanical Properties using the Parametric HFGMC

Omri Yannay

 M.Sc. student of Prof. Rami Haj-Ali

Advanced Carbon-based Ceramic Matrix Composites (CMCs) are important in today's aviation industry because of their unique properties - can withstand high temperature and severe erosion conditions, while maintaining the composites strength at relatively lower weight.

However, these unique properties depend on the microstructure of the formed material through the CMC's production process. The use of refined micromechanical methods, such as the parametric High Fidelity Method of Cells (HFGMC) is crucial in order predict the overall thermos-mechanical properties and how they are related to the optimal ratio of the phases, towards improving the desired and objective properties. Furthermore, applying this new micro-scale analysis can save time and money by replacing the experiments on such expensive material system.  It can even generate added values that one cannot extrapolate in standard experimental approach such as predicting the overall anisotropic mechanical properties, and the stress states at the micro scales. It should be noted that all inputs for the proposed micromechanical simulations can be easily obtained by using basic physical measurements combined with data in the open literature, such as material's microstructure and phase's properties.

This research presents a new framework for prediction the overall thermo-mechanical properties of CMCs using the parametric HFGMC starting from the manufacturing process of CMCs by Liquid Silicon Infiltration (LSI) method. For each production stage, a Repeated Unit Cell (RUC) model is applied in order to achieve more reliable results at each production level.  The proposed micromodels are nested in a multi scale analysis in order to generate the overall effective properties of the CMC.  Finally, the effects of material microstructural features on the overall elastic properties are investigated, reported and discussed.

Micro-mechanics based Progressive Fatigue Damage models of laminated composites

Royi Yuval Safin

MSc Student of Prof. Rami Haj-Ali

The increased demand on new advanced composites in the aerospace, automotive, civil and military applications necessitates predictive fatigue failure models of composite materials and structures at different loadings. While the mechanical behavior under fatigue loading of metallic materials is well established, this is not the case for composites. Fatigue in composite materials is associated with several interacting damage mode systems, often leading to a sudden brittle failure.

Predictive fatigue damage models in composite materials are also challenging due to the complex failure mechanisms under static and fatigue loading and because of the anisotropic elastic and strength properties.  However, a good model have the potential to reduce the large experimental effort needed to test for different composite material systems and their constituents, such as fibers, matrices, lamination stacking sequences etc. In addition, fatigue experiments are expensive as a single coupon may need to be tested for up to several weeks.

In this study, two micromechanical methods are proposed for the fatigue failure prediction of unidirectional and laminated composites  under general loading with minimal dependence on empirical parameters. Both of the methods are based on using the generalized method of cells (GMC) micromechanical model. The first approach is based on fatigue micro-failure criteria, which are applied separately to the fiber and matrix regions, while the second approach is based on a damage law evolution for isotropic materials that applied only to the matrix.

The use of micromechanics allows the study of damage at both the micro and macroscales that explicitly recognize the fiber and matrix constituents. The proposed GMC-Fatigue constitutive equations enable a

multi-scale fatigue analysis of

laminated composite structures.  To that end, a new multi-scale fatigue module is implemented in the Abaqus FE code for the fatigue analysis of plates with an open hole.  Life predictions are demonstrated along with the possibility for residual strength analysis, see Fig.1.  Good prediction ability is demonstrated and compared to published test data in the literature.

Figure 1:  Schematic illustration a micromechanical repeating unit-cell GMC model (top row) coupled with fatigue analysis of laminated composite materials and structures