Maen Alkhader

Synergistic Effects of Fatigue and Marine Environments on Carbon Fiber Vinyl-Ester Composites

Fiber-reinforced polymer (FRP) composites used in the construction of composite-based civil and military marine crafts are often exposed to aggressive elements that include ultraviolet radiation, moisture, and cyclic loadings. With time, these elements can individually and more so cooperatively degrade the mechanical properties and structural integrity of FRP composites. To assist in increasing the long-term reliability of composite marine crafts, this work experimentally investigates the cooperative damaging effects of ultraviolet (UV), moisture, and cyclic loading on the structural integrity of carbon fiber reinforced vinyl-ester marine composite. Results demonstrate that UV and moisture can synergistically interact with fatigue damage mechanisms and accelerate fatigue damage accumulation. For the considered composite, damage and S–N curve models with minimal fitting constants are proposed. The new models are derived by adapting well-known cumulative fatigue damage models to account for the ability of UV and moisture to accelerate fatigue damaging effects.

Large Strain Mechanical Behavior of HSLA-100 Steel Over a Wide Range of Strain Rates

High-strength low alloy steels (HSLA) have been designed to replace high-yield (HY) strength steels in naval applications involving impact loading as the latter, which contain more carbon, require complicated welding processes. The critical role of HSLA-100 steel requires achieving an accurate understanding of its behavior under dynamic loading. Accordingly, in this paper, we experimentally investigate its behavior, establish a model for its constitutive response at high-strain rates, and discuss its dynamic failure mode. The large strain and high-strain-rate mechanical constitutive behavior of high strength low alloy steel HSLA-100 is experimentally characterized over a wide range of strain rates, ranging from 10^(−3) s^(−1) to 10^4 s^(−1). The ability of HSLA-100 steel to store energy of cold work in adiabatic conditions is assessed through the direct measurement of the fraction of plastic energy converted into heat. The susceptibility of HSLA-100 steel to failure due to the formation and development of adiabatic shear bands (ASB) is investigated from two perspectives, the well-accepted failure strain criterion and the newly suggested plastic energy criterion [1]. Our experimental results show that HSLA-100 steel has apparent strain rate sensitivity at rates exceeding 3000 s^(−1) and has minimal ability to store energy of cold work at high deformation rate. In addition, both strain based and energy based failure criteria are effective in describing the propensity of HSLA-100 steel to dynamic failure (adiabatic shear band). Finally, we use the experimental results to determine constants for a Johnson-Cook model describing the constitutive response of HSLA-100. The implementation of this model in a commercial finite element code gives predictions capturing properly the observed experimental behavior. High-strain rate, thermomechanical processes, constitutive behavior, failure, finite elements, Kolsky bar, HSLA-100.

Influence of Cellular Topology on Dynamic Response of Solid Foams

Current processing techniques enable the manufacture of cellular cores to prescribed cell sizes and densities. Moreover, the rapid advance in additive manufacturing techniques promises that, in the near future, the fabrication of functional cellular structures will be achieved with desired cellular topologies tailored to specific application in mind. In this perspective, it is essential to develop a detailed understanding of the relationship between mechanical response and topology in cellular structures. The present work reports the initial results of a computational investigation in this direction. The fundamental issues addressed in the present study are (i) generation of stochastic cellular structures by using Voronoi tessellations, (ii) quantitative measure of cellular topology, (iii) uniqueness of mechanical response, (iv) specimen size effect, (v) boundary effect, and (vi) high-strain-rate effects.© 2006 ASME

Metabolic rate monitoring and weight reduction/management.

Engineering research may provide tools to the individual as well as to the public in general, to effectively monitor wellness and health patterns, such as metabolic rate and weight control. Ketone bodies and acetone gas emissions in exhaled breath and skin, in particular, may be used as biomarkers of fatty acid metabolism and may be used in diet control. Two types of technologies, resistive chemosensors and chemomechanical actuators are outlined here as examples of such tools currently under development and of great promise.

The influence of pressure on the large deformation shear response of a Polyurea

A new shear-compression experiment is developed to characterize the influence of hydrostatic pressure on the shear constitutive response of nearly incompressible viscoelastic materials undergoing large deformations. In this design, a uniform torsional shear stress is superposed on a uniform hydrostatic compressive state of stress generated by axially deforming samples confined by a stack of thin steel disks. The new design is effective in applying uniform multiaxial compressive strain while preventing buckling and barreling during inelastic deformation. In addition, it allows for the direct measurement of the stress and strain fields during the deformation history. The new shear-compression setup is developed to aid in characterizing the influence of pressure or negative dilatation on the shear constitutive response of viscoelastic materials in general and Polyurea in particular. Experimental results obtained with this technique illustrate the significant increase in the shear stiffness of polyurea under moderate to high hydrostatic pressures.

Band Gaps in Bravais Lattices Inspired Periodic Cellular Materials and the Effect of Relative Density and Strain Fields

Periodic cellular (lattice) materials, by virtue of their periodic structures and associated geometric impedance mismatch, exhibit wave dispersion, frequency dependent transmissibility, and directional characteristics that are inherently dependent on their constituent material and mesoscale microstructural features. These characteristics render lattice materials as potential candidates to perform as low frequency phononic crystals and metamaterials for radar, sonar, wave guiding, wave modulation and isolation applications. Accelerating the wide-spread implementation of lattice materials as phononic crystals hinges on establishing the ability to engineer them to exhibit application-tailored properties and tunable behavior (e.g. to activate/deactivate band gaps). Achieving tunability and application-oriented tailorablity requires, first, establishing an understating of phononic, acoustic, wave dispersion and directional properties of the lattices and how they are affected by lattices’ inherent features. Accordingly, using Bloch’s theorem in conjunction with finite element analysis, this work investigates the relationships between inherent microstructural features (such as lattice symmetry, relative density (i.e. volume fraction) and constituent material) and the acoustic properties (such as wave dispersion, band gaps, and acoustic anisotropy) of architectured lattice materials. The coupling between microstructural features and band gaps is investigated in a hexagonal lattice geometry which is inspired by the two dimensional Bravais family of lattices. Results illustrate that band structure and phononic properties are highly sensitive to relative density and can scale non-uniformly with it as eigenmodes are associated with relative density dependent deformation mechanisms. Moreover, results show that band gaps can potentially be activated and deactivated using macroscopic strain fields. The latter opens horizons for realizing cellular based phononic crystals with tunable properties.Copyright

Effect of Microstructure in Cellular Solids: Bending vs. Stretch Dominated Topologies

Rapid advance in additive manufacturing techniques promises that, in the near future, the fabrication of functional cellular structures will be achieved with desired cellular microstructures tailored to specific application in mind. In this perspective, it is essential to develop a detailed understanding of the relationship between mechanical response and cellular microstructure. The present study reports on the results of a series of computational experiments that explore the effect cellular topology and microstructural irregularity (or non-periodicity) on overall mechanical response of cellular solids. Compressive response of various 2D topologies such as honeycombs, stochastic Voronoi foams as well as tetragonal and triangular lattice structures have been investigated as functions of quantitative irregularity parameters.

Experimental investigation of the synergistic effects of moisture and freeze-thaw cycles on carbon fiber vinyl-ester composites

Carbon fiber-reinforced vinyl-ester polymer composites are increasingly used as structural members in applications (e.g., marine crafts and offshore structures) where they can be frequently exposed to the environmental elements of moisture and cold temperature fluctuations that cause freeze-thaw cycles. These harsh elements can individually and possibly synergistically damage carbon fiber-reinforced vinyl-ester composites. More importantly, their damage can accumulate over time and significantly degrade the structural properties, long-term integrity and durability of carbon fiber-reinforced vinyl-ester composites. This work experimentally investigates the individual and cooperative degrading effects of moisture and freeze-thaw cycles on the structural properties of carbon fiber-reinforced vinyl-ester composites, particularly on their flexural stiffness and strength. Results show that the combined damaging effects of moisture and freeze-thaw cycles are more significant than their individual effects, confirming the synergy between the damage mechanisms of the two elements.

Experimental Analysis of Process Parameters in Composite Manufacturing

With the growing world demand on composite products, composite manufacturers are increasingly moving towards adopting automated composite manufacturing. However, current automated machines are not optimized and have throughput limitations. Most of them only target specific composite processes. Because the shortage of understanding on the processing factors and knowledge on precise process control, the products get high probability to come with defect. This paper provides an experimental methodology to explore the relationship between the structural properties of composite parts and processing factors. The paper also provides composite manufacturers a guide for processing condition optimization, productivity improvement and cost reduction. The raw material involved in this paper is an aerospace grade carbon fiber prepreg and was used to fabricate composite samples in a hot press machine system in the experiment. To investigate the effects of processing factors, specimens were fabricated using a series of combinations of different factor values. Three point bending flexural test was performed to evaluate the quality of each sample. Collected data are analyzed by statistical methods and fitted to a multiple linear regression model, which provides insights for composite manufacturing.© 2016 ASME

Experimental Investigation of Failure in Viscoelastic Elastomers Under Combined Shear and Pressure

An experimental approach, based on Split Hopkinson Pressure Bar (SHPB) apparatus, is developed to elucidate failure of viscoelastic elastomers under combined shear and high pressures such as are encountered in explosive and/or armor-impact scenarios. In this experimental arrangement, thin cylindrical polyurea specimens with an aspect ratio (Diameter to thickness) greater or equal to 15 are tested, up to failure, using Split Hopkinson Pressure Bar (SHPB). Specimens with large aspect ratio are used to guarantee the close approximation of a triaxial state of stress in the specimen upon loading; hence the measured normal stress would be approximately equal to the hydrostatic pressure in the specimen. Friction at the loading interfaces forces the stress state to deviate from uniformity, restrict both the circumferential and radial displacements and lead to the development of shear stresses and strains. Hence, induced failure occurs under conditions combining high-strain-rate, high pressure and shear stresses. By using this setup, repeatable failure modes were detected and elucidated using finite element simulations.