Ahmed Al-Ostaz

Crack initiation and propagation in materials with randomly distributed holes

Abstract We investigate fracture of both elastic-brittle (epoxy) and ductile (aluminum) sheets, each containing circular holes, and subjected to a uniaxial tension. The holes are distributed at random but with a restriction of no overlap and a minimum separation distance. In this study, which includes both numerical and experimental results, we consider a single configuration of holes but several samples and find that crack paths in these perforated sheets are not unique. This illustrates the need for a stochastic fracture mechanics analysis for this class of problems.

Fracture of random matrix-inclusion composites: scale effects and statistics

Abstract We study crack patterns and effective stress-strain curves in unidirectional fiber-matrix composites subjected to a uniform out-of-plane shear. The fibers are aligned in the longitudinal direction and arranged randomly, with no overlap, in the transverse plane. Both fibers and matrix are isotropic and elastic-brittle. We conduct this analysis numerically using a very fine two-dimensional spring network and simulate the crack initiation and propagation by sequentially removing bonds which exceed a local fracture criterion. In particular, we focus on effects of scale and geometric randomness in these composites. We consider several “windows of observation” (scales) and study crack patterns, types of constitutive responses, and statistics of the corresponding scale dependent effective elastic stiffness and strength of such composites. In the parametric study we cover a wide range of material combinations defined by the stiffness ratio and the strain-to-failure ratio and we employ a damage plane in terms of these two parameters to illustrate the results.

The influence of interface and arrangement of inclusions on local stresses in composite materials

Abstract We studied a model composite material consisting of a thin epoxy plate (matrix) reinforced with stiff circular disks (inclusions) and subjected to a uniaxial tension. At each inclusion—matrix interface there is an interfacial layer, an interphase, which has uniform properties. The inclusions are arranged in the matrix at random but with no overlap. For comparison we also considered square and triangular periodic arrangements. We have studied elastic fields of such composites both experimentally, using a photoelasticity method, and numerically via a finite element method. We have found that random inclusion arrangements give higher stress concentrations than the periodic ones owing to stress localizations. The highest stress increase is in a compliant interface case, which also exhibits the highest scatter in stress magnitudes for different random arrangements at the same volume fraction.

Computation of Elastic Properties of Portland Cement Using Molecular Dynamics

There is a growing interest in relating nanostructures to the macro properties of engineering materials such as composites and cement materials. Better understanding of structure and elastic properties of nanoparticles in concrete by modeling and experiment could lead to nanoengineered concrete with much better performance and energy efficiency. In this study, the molecular dynamics (MD) atomistic simulation technique was applied to study the elastic properties of major portland cement compounds (i.e., alite, belite, and aluminate). Applicability of three commonly used force fields: COMPASS, Universal force field (UFF), and Dreiding were evaluated in the MD simulation. The combination of different simulation cell sizes and force fields was investigated. MD simulation results of cement were comparable to the experimental data. The results could be used as nanoparticle properties for multiscale modeling of concrete, cementitious composites, and aggregate.

Reactive Molecular Simulation of the Damage Mitigation Efficacy of POSS-, Graphene-, and Carbon Nanotube-Loaded Polyimide Coatings Exposed to Atomic Oxygen Bombardment

Reactive molecular dynamics simulation was employed to compare the damage mitigation efficacy of pristine and polyimide (PI)-grafted polyoctahedral silsesquioxane (POSS), graphene (Gr), and carbon nanotubes (CNTs) in a PI matrix exposed to atomic oxygen (AO) bombardment. The concentration of POSS and the orientation of Gr and CNT nanoparticles were further investigated. Overall, the mass loss, erosion yield, surface damage, AO penetration depth, and temperature evolution are lower for the PI systems with randomly oriented CNTs and Gr or PI-grafted POSS compared to those of the pristine POSS or aligned CNT and Gr systems at the same nanoparticle concentration. On the basis of experimental early degradation data (before the onset of nanoparticle damage), the amount of exposed PI, which has the highest erosion yield of all material components, on the material surface is the most important parameter affecting the erosion yield of the hybrid material. Our data indicate that the PI systems with randomly oriented Gr and CNT nanoparticles have the lowest amount of exposed PI on the material surface; therefore, a lower erosion yield is obtained for these systems compared to that of the PI systems with aligned Gr and CNT nanoparticles. However, the PI/grafted-POSS system has a significantly lower erosion yield than that of the PI systems with aligned Gr and CNT nanoparticles, again due to a lower amount of exposed PI on the surface. When comparing the PI systems loaded with PI-grafted POSS versus pristine POSS at low and high nanoparticle concentrations, our data indicate that grafting the POSS and increasing the POSS concentration lower the erosion yield by a factor of about 4 and 1.5, respectively. The former is attributed to a better dispersion of PI-grafted POSS versus that of the pristine POSS in the PI matrix, as determined by the radial distribution function.

Measurement of Mechanical Properties of Hydrated Cement Paste Using Resonant Ultrasound Spectroscopy

The resonant ultrasound spectroscopy (RUS) experimental technique had been used successfully to measure the modulus of elasticity and Poisson’s ratio of hydrated cement paste. RUS is a modern nondestructive acoustic technique, which can be applied to determine the elastic properties of solids with high precision and great efficiency. By measuring the natural resonance frequencies for a single small parallelepiped cement paste sample with water cement ratio of 0.4, the elastic constants and, subsequently, the modulus (E) and Poisson’s ratio (v) were obtained. The results from RUS measurement, E=21.55 GPa and v=0.225, agree with the numerical modeling values.

Erosion Study of New Orleans Levee Materials Subjected to Plunging Water

During Hurricane Katrina, overtopping water caused erosion and subsequent failure of several sections of I-type flood walls in New Orleans. Erosion stemmed from the kinetic energy of water falling from the top of the flood wall, unlike the typical surface erosion caused by shear flow. This study evaluated the effects of important parameters of levee soils—fines content, degree of compaction (DOC), clay mineralogy, and water content in relation to the erosion behavior of New Orleans levees subjected to the plunging water. In general, test results showed that a higher fines content contributed to greater erosion resistance. The trend became unclear when fines content exceeded 20–25%. A higher degree of compaction did not necessarily contribute to greater erosion resistance. Underwater soaked soils showed much less erosion resistance than nonsoaked soils. Soils containing expansive clay minerals showed less erosion resistance than soils containing nonexpansive clay minerals.

Evaluation of a T-Wall Section in New Orleans Considering 3D Soil-Structure Interaction

T-walls in New Orleans survived Hurricane Katrina whereas I-walls obviously failed in several sections. However, it is still unclear whether these T-walls truly survived the hurricane with a fair amount of safety margins or barely survived it with undetected damages. The initial design of T-walls was based on simplified loading conditions with limited consideration of soil-structure interaction. In this study, three-dimensional (3D) numerical analyses were conducted, incorporating realistic loading conditions and soil-structure interactions but with time-saving techniques to evaluate the detailed behavior of T-walls. This paper addressed the procedure of innovative 3D numerical analyses and important findings by using special structural elements in FLAC3D. From this study, T-walls were found to have adequate stress levels in H-piles and concrete walls. However, it showed that the major factor that may cause the instability of the T-wall was the slope instability-type unbalanced force. This unbalanced forc...

Experimental Evaluation and Numerical Simulations of Nanocoatings in Infrastructure Fire Applications

AbstractAlthough coating masonry walls with polymeric elastomers can mitigate the effects of a projectile strike in a blast or explosion, it may also increase the fire hazard to the structure and the occupants. Although a rigorous assessment of this problem would require full-scale fire tests, considerable insights can be gained by performing bench-scale tests in conjunction with computer simulations. A joint experimental/numerical methodology is presented in this paper to evaluate the extent to which blast-resistant coatings, applied on masonry walls, may contribute to the spread of existing fire. The experimental data are obtained by performing cone calorimeter heat release rate (HRR) measurements. Flammability characterization and the heat flux generated for structural systems and components that are coated with blast-resistant and fire-retardant polymeric coatings are performed using the NIST fire dynamic simulator (FDS). In these simulations, structural concrete columns and masonry walls are exposed ...

Modeling the elastic modulus of exfoliated graphite platelets filled vinyl ester: analytical predictions with consideration of filler percolation

Mechanical response of nano-based composites is generally influenced by interaction of filler and matrix at interface. Increasing filler-loading within the composite may cause spatial limitation toward best dispersion of filler, and since synthesizing a totally agglomerated-free nanocomposite is difficult, filler and matrix interaction needs to be perfectly modeled. A micromechanical model is developed in this study based on the common Halpin–Tsai theory to predict the elastic stiffness of vinyl ester/exfoliated graphite platelet nanocomposites. The model considers near-rational ideal (uniformly dispersed) mixed with clustered filler-network to simulate filler-distribution conditions. A filler-dispersion level based on the filler concentration has been proposed mathematically in this study. Predictions of the proposed model considering filler morphology were compared with the predictions of the Halpin–Tsai model and the experimentally obtained results as well. The proposed model shows better accuracy in terms of stiffness over predictions of the Halpin–Tsai model and appears in a very good agreement with the experimental results obtained for vinyl ester nanocomposites.