Papers by Sia Nemat-nasser
Volume 9: Mechanics of Solids, Structures and Fluids, 2013
ABSTRACT Polymers with thermally-reversible Diels-Alder cross-links have been previously shown to... more ABSTRACT Polymers with thermally-reversible Diels-Alder cross-links have been previously shown to heal cracks and regain structural integrity. Complete recovery of fracture toughness has been experimentally observed in neat samples under ideal conditions. In the present work, new healable polymer samples containing glass or carbon reinforcing fibers in [90,0]s cross-ply orientations are fabricated and characterized using dynamic mechanical analysis (DMA). The DMA results are compared with one-dimensional composite and beam analyses. Transverse cracks observed in microscopy images and attributed to residual thermal stresses are considered using a shear lag method. Crack healing is assumed to be occurring as a function of the sample temperature, where the limits of healing are established by other experiments. By considering the composite constituent properties, sample geometries, and the presence of cracks that heal during the test, the DMA measurements are accurately modeled.
Volume 8: Mechanics of Solids, Structures and Fluids; Vibration, Acoustics and Wave Propagation, 2011
The increasing demand for in-service structural health monitoring has stimulated efforts to integ... more The increasing demand for in-service structural health monitoring has stimulated efforts to integrate self and environmental sensing capabilities into materials and structures. To sense damage within composite materials, we are developing a compact network microsensor array to be integrated into the material. These structurally-integrated embedded microsensors render the composite information-based, so that it can monitor and report on the local
Volume 9: Mechanics of Solids, Structures and Fluids, 2013
ABSTRACT Polyurea is an elastomer that has been intensively researched due to its excellent therm... more ABSTRACT Polyurea is an elastomer that has been intensively researched due to its excellent thermal and mechanical properties. Polyurea based composite material has recently become a research interest to further explore what this polymer has to offer. In order to better understand the overall static or dynamic mechanical properties of the polyurea based composites, how to tailor and characterize the polyurea-filler interface has become a crucial problem. This study focuses on one of the filler materials, glass. Three types of polyurea-glass interfaces are studied by using silane reagents that have similar molecular structures but with different end functional groups to modify the glass surfaces. Accordingly, bonds with different strengths are formed between the glass and the polyurea through the different chemical character of the reagent molecules. The polyurea-glass interfacial properties are tested by the single-fiber fragmentation, which is a widely used method to test the shear properties of the interface between the fiber and the polymer. Single-fiber fragmentation samples are fabricated by casting a single glass fiber along the axial direction of the dogbone-shaped polyurea tension test sample. Tension tests are conducted and the continuous photoelastic videos are taken to observe the single fiber fragmentation process until the fragmentation reaches its saturation state. Meanwhile, stress-strain data are recorded. By analyzing the single-fiber fragmentation data, the polyurea-glass interfacial shear strengths are calculated. The observation of the debonding zones at the interface is used to find the approximate models for the interfacial shear adhesion of polyurea-glass interfaces for different reagents, hence proving the potential for tailoring of the interfacial strength using surface treatment.
Conference Proceedings of the Society for Experimental Mechanics Series, 2013
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
Behavior and Mechanics of Multifunctional Materials and Composites 2013, 2013
ABSTRACT Due to its excellent thermo-mechanical properties, polyurea is attracting more and more ... more ABSTRACT Due to its excellent thermo-mechanical properties, polyurea is attracting more and more attention in blast-mitigating applications. In order to enhance its capability of blast-induced stress-wave management, we seek to develop polyurea-based composites in this work. Fly ash which consists of hollow particles with porous shell and low apparent density was chosen as filler and a series of fly ash/polyurea composites with various fly ash volume fractions were fabricated. The dynamic mechanical behavior of the composites was determined by a personal computer (PC) based ultrasonic system in the 0.5-2MHz frequency range between -60°C to 30°C temperatures. Velocity and attenuation of both longitudinal and shear ultrasonic waves were measured. The complex longitudinal and shear moduli were then computed from these measurements. Combining these results provided an estimate of the complex bulk and Young's moduli of the fly ash/polyurea composites at high frequencies. These results will be presented and compared with those of pure polyurea elastomer.
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
Philosophical Magazine A, 1992
In polycrystalline materials, the microscopic strain field is always highly heterogeneous during ... more In polycrystalline materials, the microscopic strain field is always highly heterogeneous during inelastic deformation, even when the macroscopic strain field is uniform. This is particularly true near potential damage and failure nucleation sites, such as voids and inclusions. Also, in the context of high-strain-rate plastic flow, the only currently practical way to induce large strains and high strain rates simultaneously under experimental conditions is by means of heterogenous deformations. For these and other reasons, experiments involving highly heterogeneous deformations are of considerable interest. A joint experimental/computational study of heterogeneous deformations in copper single crystals will be presented. Carefully oriented, monocrystalline copper specimens were subjected to large-strain indentation at both high and low strain rates. The high-rate experiments were performed using a split Hopkinson pressure bar high-strain-rate apparatus. Scanning and transmission electron microscopy performed on the deformed specimens revealed strikingly different deformation morphologies and dislocation substructures between the two strain rates. In general, the substructure of the high-rate specimens showed relatively little evidence of dynamic recovery processes as compared to that of the low-rate specimens. This finding tends to support the notion of strain-rate sensitivity of structure evolution in face-centred cubic metals. A series of finite element simulations of the experiment was performed using the explicit finite element code PRONTO2D, into which a rate-dependent crystal plasticity constitutive theory has been implemented. In the constitutive model, the effects of strain-rate-dependent substructure evolution were simulated through the inclusion of a strain-rate-history-dependent hardening parameter. Comparison of the experimental and computational results reveals that strain-rate history has a profound influence on deformation morphology at medium to high strain rates. Also, the comparison serves to highlight the importance of proper characterization of both instantaneous rate-sensitivity and strain-rate-history effects.
Conference Proceedings of the Society for Experimental Mechanics Series, 2011
ABSTRACT Polyurea has been the material of choice in many applications due to its thermomechanica... more ABSTRACT Polyurea has been the material of choice in many applications due to its thermomechanical properties. These applications span a wide spectrum from abrasion-resistant coating to reinforcement against blast damage in structures, ships, and vehicles. The improved observed performance is linked to its microstructure, a lightly crosslinked (elastomeric) block copolymer. The constitutive modeling of such materials is generally based on a wide variety of experimental measurements, including dynamic mechanical analysis. Furthermore, the response under stress-wave propagation may be measured through ultrasonic tests. In this work we present our research towards improvement and modification of the dynamic mechanical properties of polyurea through inclusion of various size and distributions of fly ash hollow spherical particles. The extensive experimental results are reported and a micromechanical homogenization model is presented. The extent of application of such model will be established and alternative modeling techniques which include the possible inertia effects at high rates of deformation will be surveyed.
Conference Proceedings of the Society for Experimental Mechanics Series, 2013
ABSTRACT
Conference Proceedings of the Society for Experimental Mechanics Series, 2013
Behavior and Mechanics of Multifunctional Materials and Composites 2013, 2013
ABSTRACT Acoustic impedance is a material property that depends on mass density and acoustic wave... more ABSTRACT Acoustic impedance is a material property that depends on mass density and acoustic wave speed. An impedance mismatch between two media leads to the partial reflection of an acoustic wave sent from one medium to another. Active sonar is one example of a useful application of this phenomenon, where reflected and scattered acoustic waves enable the detection of objects. If the impedance of an object is matched to that of the surrounding medium, however, the object may be hidden from observation (at least directly) by sonar. In this study, polyurea composites are developed to facilitate such impedance matching. Polyurea is used due to its excellent blast-mitigating properties, easy casting, corrosion protection, abrasion resistance, and various uses in current military technology. Since pure polyurea has impedance higher than that of water (the current medium of interest), low mass density phenolic microballoon particles are added to create composite materials with reduced effective impedances. The volume fraction of particles is varied to study the effect of filler quantity on the acoustic impedance of the resulting composite. The composites are experimentally characterized via ultrasonic measurements. Computational models based on the method of dilute-randomly-distributed inclusions are developed and compared with the experimental results. These experiments and models will facilitate the design of new elastomeric composites with desirable acoustic impedances.
... always gave a first priority for my career interests and designed projects such a way that th... more ... always gave a first priority for my career interests and designed projects such a way that they can help me in achieving my career goals. Lastly, I would like to thank my wife, Deepthi Priyanka Poluri, for her constant moral boost during my busy schedules towards the end of my ...
The integration of sensors, actuators, and such devices into a structure is vital in smart applic... more The integration of sensors, actuators, and such devices into a structure is vital in smart applications. Essential to the application of smart composites is the issue of the mechanical coupling of the sensor to the host material.
... 0416, USA * [email protected] ABSTRACT The elastic moduli and wave speeds in low-impe... more ... 0416, USA * [email protected] ABSTRACT The elastic moduli and wave speeds in low-impedance materials including polyurea and other elastomers and elastomer-based composites strongly depend on the confining hydrostatic pressure and temperature. ... Steel PU ...
Uploads
Papers by Sia Nemat-nasser