Samit Roy

Bio: Samit Roy is an academic researcher from University of Alabama. The author has contributed to research in topics: Fracture toughness & Finite element method. The author has an hindex of 20, co-authored 108 publications receiving 1764 citations. Previous affiliations of Samit Roy include Missouri University of Science and Technology & Southwest Research Institute.

Anomalous Moisture Diffusion in Viscoelastic Polymers: Modeling and Testing

Abstract: It is now well known that Fick's Law is frequently inadequate for describing moisture diffusion in polymers or polymer composites Non-Fickian or anomalous diffusion typically occurs when the rates of diffusion and viscoelastic relaxation in a polymer are comparable, and the ambient temperature is below the glass transition temperature (T g ) of the polymer, As a result, it is necessary to take into account the time-dependent response of a polymer, analogous to viscoelastic relaxation of mechanical properties, in constructing such a model In this paper, a simple yet robust methodology is proposed that would allow characterization of non-Fickian diffusion coefficients from moisture weight gain data for a polymer below its T g Subsequently, these diffusion coefficients are used for predicting moisture concentration profiles through the thickness of a polymer Moisture weight gain data at different temperatures for an epoxy adhesive is employed to calibrate the model Specimen thickness independence of the modeling parameters is established through comparison with test data A finite element procedure that extends this methodology to more complex shapes and boundary conditions is also validated

The Interfacial Shear Strength of Carbon Nanotube Sheet Modified Carbon Fiber Composites

Abstract: Carbon fiber reinforced polymer composites have low density and high tensile strengths. However, their compressive strengths are much lower than their corresponding tensile strengths due to fiber micro-buckling and interface failure between fiber and matrix. To address this issue, we report a method for fabricating carbon nanotube (CNT) sheet scrolled carbon fibers or fiber tows to improve the interfacial shear strengths. A CNT sheet is drawn from a drawable carbon nanotube forest grown on a silicon substrate, it is used to wrap around individual carbon fibers. The CNT wrapped carbon fiber is subsequently impregnated into a polymer to form a composite. Scanning electron micrograph shows that the wettability of CNT wrapped carbon fiber composite increases drastically in comparison with the composite without CNT, indicating significantly increased bonding between carbon fiber and polymer due to the addition of aligned CNT at the interphase. Fiber push-out and push-in nanoindentation characterization indicates increased interfacial shear strengths, consistently at over 80% with the use of wrapped aligned CNT sheet. The results from scrolling CNT sheet around individual carbon fibers to enhance compressive strengths indicate the potential performance enhancement of composites when this approach is scaled up.

Non-linear analysis of adhesively bonded joints

Abstract: An updated Lagrangian formulation is used to develop a 2-D finite element for the analysis of adhesively bonded joints. The finite element accounts for the geometric non-linearity. The present finite element results are in good agreement with those reported in the literature. The effect of boundary conditions and mesh on the stress distributions in lap joints is also investigated.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

Abstract: The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.