Soft tissue can be viewed as a multi-layered composite structure reinforced with different collagen fiber systems used to control the overall mechanical properties. The objective of this study was to manufacture, test and model the... more
Soft tissue can be viewed as a multi-layered composite structure reinforced with different collagen fiber systems used to control the overall mechanical properties. The objective of this study was to manufacture, test and model the mechanical behavior of a multidirectional collagen reinforced composite material system. To that end, novel bio-composite constructs were fabricated from long collagen fibers reinforced polyacrylamide–alginate (PAAm–Alg) matrix. The collagen fibers were aligned in multi-directions within thin circular matrix plates. The constructs were clamped and subjected to flexure using a rigid spherical indenter. Three dimensional finite element (FE) models were generated with continuum and beam elements representing the matrix and fibers, respectively. The hyperelastic behavior predicted by the calibrated FE models was validated based on the results of one tested configuration. The new bio-inspired composites can be tailored to generate similar mechanical behavior of native tissues. The FE model can be employed to design future complex constructs for soft tissue substitutes.
Research Interests:
A novel collagen-based bio-composite was constructed from micro-crimped long collagen fiber bundles extracted from a soft coral embedded in alginate hydrogel matrix. The mechanical features of this bio-composite were studied for different... more
A novel collagen-based bio-composite was constructed from micro-crimped long collagen fiber bundles extracted from a soft coral embedded in alginate hydrogel matrix. The mechanical features of this bio-composite were studied for different fiber fractions and in longitudinal and transverse loading modes. The tensile modulus of the alginate hydrogel was 0.6070.35 MPa and in longitudinal collagen-reinforced construct it increased up to 9.7172.80 for 50% fiber fraction. Ultimate tensile strength was elevated from 0.0870.04 MPa in matrix up to 1.2170.29 for fiber fraction of 30%. The bio-composite demonstrated hyperelastic behavior similar to human native tissues. Additionally, a dedicated constitutive material model was developed to enable the prediction of the longitudinal mechanical behavior of the bio-composite. These findings will allow tailor-designed mechanical properties with a quantitatively controlled amount of fibers and their designed spatial arrangement. This unique bio-composite has the potential to be used for a wide range of engineered soft tissues.
The objective of this study was to introduce a class of collagen-fiber reinforced bio-composite laminates as biomimetic of soft tissues. These novel all-natural bio-composite laminates include long collagen fibers from soft coral embedded... more
The objective of this study was to introduce a class of collagen-fiber reinforced bio-composite laminates as biomimetic of soft tissues. These novel all-natural bio-composite laminates include long collagen fibers from soft coral embedded in an alginate hydrogel matrix. Controlling the fiber orientation and volume fraction enabled the fabrication of laminates with wide range of mechanical behaviors. Four material systems were investigated in the current study having different fiber orientations: longitudinal (0), transverse (90), cross-plied (0/90) and angle-plied (±30). The range of Fiber volume fractions (FVFs) for the laminated membranes is between 0.21 and 0.31. The laminates were subjected to uniaxial loading, yielding hyperelastic stressestrain behavior. A hyperelastic finite element (FE) model was constructed for the heterogeneous laminate, based on the fiber and matrix hyperelastic material behavior and their FVF, in order to predict the overall bio-composite mechanical behavior. The predictions of the FE model were verified from the tested laminated systems. The FE model consisted of beam elements representing the collagen fibers embedded in the solid matrix (alginate). Good predictions were demonstrated by the proposed FE model compared with the tested bio-composites for all orientations up to 10% strain. The overall hyperelastic stressestrain behavior was in a similar range to known native soft tissues. In addition, the model allowed for examining the mechanical behavior of laminates with other FVFs. The new bio-composite material can be used for future soft tissue mimicry and repair.
Soft tissue can be viewed as a multi-layered composite structure reinforced with different collagen fiber systems used to control the overall mechanical properties. The objective of this study was to manufacture, test and model the... more
Soft tissue can be viewed as a multi-layered composite structure reinforced with different collagen fiber systems used to control the overall mechanical properties. The objective of this study was to manufacture, test and model the mechanical behavior of a multidirectional collagen reinforced composite material system. To that end, novel bio-composite constructs were fabricated from long collagen fibers reinforced polyacrylamide–alginate (PAAm–Alg) matrix. The collagen fibers were aligned in multi-directions within thin circular matrix plates. The constructs were clamped and subjected to flexure using a rigid spherical indenter. Three dimensional finite element (FE) models were generated with continuum and beam elements representing the matrix and fibers, respectively. The hyperelastic behavior predicted by the calibrated FE models was validated based on the results of one tested configuration. The new bio-inspired composites can be tailored to generate similar mechanical behavior of native tissues. The FE model can be employed to design future complex constructs for soft tissue substitutes.
The cusps of native aortic valve (AV) are composed of collagen bundles embedded in soft tissue, creating a heterogenic tissue with asymmetric alignment in each cusp. This study compares native collagen fiber networks (CFNs) with a goal to... more
The cusps of native aortic valve (AV) are composed of collagen bundles embedded in soft tissue, creating a heterogenic tissue with asymmetric alignment in each cusp. This study compares native collagen fiber networks (CFNs) with a goal to better understand their influence on stress distribution and valve kinematics. Images of CFNs from five porcine tricuspid AVs are analyzed and fluid-structure interaction models are generated based on them. Although the valves had similar overall kinematics, the CFNs had distinctive influence on local mechanics. The regions with dilute CFN are more prone to damage since they are subjected to higher stress magnitudes.
Research Interests:
Research Interests:
ABSTRACT
Research Interests:
Research Interests:
Research Interests:
Research in optimum structural design has shown that mathematical programming techniques can be employed efficiently only in conjunction with explicit approximate constraints. In the course of time a well-established approximation for... more
Research in optimum structural design has shown that mathematical programming techniques can be employed efficiently only in conjunction with explicit approximate constraints. In the course of time a well-established approximation for homogeneous functions (scalable structures) has emerged based on the linear Taylor expansion of the displacement functions in the compliance design space (Reciprocal approximation). It has been shown that the quality of this approximation is based on the property that homogeneity of the constraints is maintained and consequently the approximation is exact along the scaling line.
Research Interests:
Research Interests: Materials Engineering, Mechanical Engineering, Civil Engineering, Modeling, Finite element method, and 21 moreNeural Network, Structural Analysis, Mechanics of Materials, Finite Element Analysis (FEA), Nonlinear Analysis, Composite Material, Notch, Composite Structure, Fiber Reinforced Polymer, Glass Fiber, Constitutive model, Direct Shear Test, Plane Strain, Material Model, Experimental Data, Artificial Neural Network, Constitutive Equation, Fiber Reinforcement, Pultrusion, Volume Fraction, and Functional Response
ABSTRACT
Research Interests:
Research Interests:
... E-mail: rami.haj-ali ... mechanics in heterogeneous composite materials is to include these effects at the constituent level, rather than the homogenized material level (Lagoudas et al., 1991; Ju, 1991; Haj-Ali, 1996; Haj-Ali et al.,... more
... E-mail: rami.haj-ali ... mechanics in heterogeneous composite materials is to include these effects at the constituent level, rather than the homogenized material level (Lagoudas et al., 1991; Ju, 1991; Haj-Ali, 1996; Haj-Ali et al., 1993, 2001, 2006; Haj-Ali and El-Haiiar, 2003 ...
Research Interests:
Abstract: The development of artificial neural network (ANN) models to predict the long-term expansion response of concrete cylinders while exposed to a 2.1% Na2SO4 sulfate solution is described in this paper. The experimental data used... more
Abstract: The development of artificial neural network (ANN) models to predict the long-term expansion response of concrete cylinders while exposed to a 2.1% Na2SO4 sulfate solution is described in this paper. The experimental data used in this study was collected by the ...
Research Interests:
Piezoresistive composites are materials that exhibit spatial and effective electrical resistivity changes as a result of local mechanical deformations in their constituents. These materials have a wide array of applications from... more
Piezoresistive composites are materials that exhibit spatial and effective electrical resistivity changes as a result of local mechanical deformations in their constituents. These materials have a wide array of applications from non-destructive evaluation to sensor technology. We propose a new coupled nonlinear micromechanical-microelectrical modeling framework for periodic heterogeneous media. These proposed micro-models enable the prediction of the effective piezoresistive properties along with the corresponding spatial distributions of local mechanical–electrical fields, such as stress, strain, current densities, and electrical potentials. To this end, the high fidelity generalized method of cells (HFGMC), originally developed for micromechanical analysis of composites, is extended for the micro-electrical modeling in order to predict their spatial field distributions and effective electrical properties. In both cases, the local displacement vector and electrical potential are expanded using quadratic polynomials in each subvolume (subcell). The equilibrium and charge conservations are satisfied in an average volu-metric fashion. In addition, the continuity and periodicity of the displacements, tractions, electrical potential, and current are satisfied at the subcell interfaces on an average basis. Next, a one way coupling is established between the nonlinear mechanical and electrical effects, whereby the mechanical deformations affect the electrical conductivity in the fiber and/or matrix constituents. Incremental and total formulations are used to arrive at the proper nonlinear solution of the governing equations. The micro-electrical HFGMC is first verified by comparing the stand-alone electrical solution predictions with the finite element method for different doubly periodic composites. Next, the coupled HFGMC is calibrated and experimentally verified in order to examine the effective piezoresistivity of different composites. These include conductive polymeric matrices doped with carbon nano-tubes or particles. One advantage of the proposed nonlinear coupled micro-models is its ability to predict the local and effective electro-mechanical behaviors of multi-phase periodic composites with different conductive phases.
A modified Arcan fixture with butterfly specimen geometry is designed to measure the in-plane shear response of thick-section pultruded FRP composites. The objective of the proposed testing method is to determine both the material shear... more
A modified Arcan fixture with butterfly specimen geometry is designed to measure the in-plane shear response of thick-section pultruded FRP composites. The objective of the proposed testing method is to determine both the material shear stiffness and its non-linear stress – strain response up to ultimate stress. The tested pultruded specimens include two alternating layers in the form of a unidirectional glass roving and continuous filament mat layers. Finite element models for the butterfly specimen are generated to examine the effects of the notch radius and material orthotropy on the uniformity and distribution of stresses in the gage area. Butterfly geometry with a blunted notch and roving orientation parallel to the applied load is found to have a uniform shear stress in the gage section. The axial-shear response is measured under different biaxial stress states by varying the angle of the applied load. The tested non-linear shear stress – strain responses compare favorably to results previously obtained from off-axis compression tests used to calibrate a multi-axial constitutive model for this material. Results from strain gage and Infrared thermography measurements provide confirmation for the effectiveness of the fixture and the specimen geometry.
Research Interests: Mechanical Engineering, Civil Engineering, Thermodynamics, Engineering Mechanics, Microstructure, and 10 moreFinite element method, Finite Element, Neural Network, Numerical Method, Chaos Theory Evolution Equation, Nonlinear Analysis, Composite Material, Composite Structure, Material Model, and Artificial Neural Network
An experimental and analytical study is carried out to characterize the fracture behavior of fiber reinforced plastic (FRP) pultruded composites. The composite material system used in this study consists of roving and continuous filament... more
An experimental and analytical study is carried out to characterize the fracture behavior of fiber reinforced plastic (FRP) pultruded composites. The composite material system used in this study consists of roving and continuous filament mat (CFM) layers with E-glass fiber and polyester matrix materials. Eccentrically loaded single-edge-notch-tension ESE(T) fracture toughness specimen were cut with the roving transverse to the loading
Research Interests:
Thermography is a method used mainly for the detection of warmer arterial wall regions, as an indication for the presence of inflamed atherosclerotic plaques. A new method, utilizing injection of cold saline to the bloodstream and... more
Thermography is a method used mainly for the detection of warmer arterial wall regions, as an indication for the presence of inflamed atherosclerotic plaques. A new method, utilizing injection of cold saline to the bloodstream and measuring temperature gradients within the flow instead of the wall is numerically investigated. Results show an almost 12-fold increase in expected temperature gradients, emphasizing
Research Interests:
ABSTRACT The aortic valve (AV) is a complex structure that functions in a complex dynamic environment. During systole, the valve leaflets bend at the base to open and experience fluid shear stress on both ventricular and aortic sides of... more
ABSTRACT The aortic valve (AV) is a complex structure that functions in a complex dynamic environment. During systole, the valve leaflets bend at the base to open and experience fluid shear stress on both ventricular and aortic sides of the leaflet. During diastole, adverse pressure gradient closes the valve causing it to structurally support the systemic afterload pressure. Ex vivo experiments has shown that isolated mechanical forces such as pressure, membrane tension, and fluid shear stress affects the remodeling activities of the valve leaflets and also elicit pathological responses [1], potentially leading to calcific aortic valve disease in the long term. Clinically, patients with hypertension have increased risk of developing calcific aortic valve disease [2], which could be a result of the increased pressure or the increased stretch on the valve leaflets.
A study was conducted to investigate and validate the effect of free-edge delamination in the midplane of IM7/8551-7 eight-plied symmetric quasi-isotropic laminates and joints under both monotonic and cyclic loading. For this purpose, a... more
A study was conducted to investigate and validate the effect of free-edge delamination in the midplane of IM7/8551-7 eight-plied symmetric quasi-isotropic laminates and joints under both monotonic and cyclic loading. For this purpose, a novel approach of using Infrared ...
ABSTRACT
Research Interests:
Research Interests:
Research Interests:
Calcific aortic valve disease (CAVD) is a progressive pathology characterized by calcification mainly within the cusps of the aortic valve (AV). As CAVD advances, the blood flow and associated hemodynamics are severely altered, thus... more
Calcific aortic valve disease (CAVD) is a progressive pathology characterized by calcification mainly within the cusps of the aortic valve (AV). As CAVD advances, the blood flow and associated hemodynamics are severely altered, thus influencing the mechanical performance of the AV. This study proposes a new method, termed reverse calcification technique (RCT) capable of re-creating the different calcification growth stages. The RCT is based on three-dimensional (3D) spatial computed tomography (CT) distributions of the calcification density from patient-specific scans. By repeatedly subtracting the calcification voxels with the lowest Hounsfield unit (HU), only high calcification density volume is presented. RCT posits that this volume re-creation represents earlier calcification stages and may help identify CAVD initiation sites. The technique has been applied to scans from 12 patients (36 cusps) with severe aortic stenosis who underwent CT before transcatheter aortic valve implant...
Research Interests:
A bicuspid aortic valve (BAV) is a congenital cardiac disorder where the valve consists of only two cusps instead of three in a normal tricuspid valve (TAV). Although 97% of BAVs include asymmetric cusps, little or no prior studies... more
A bicuspid aortic valve (BAV) is a congenital cardiac disorder where the valve consists of only two cusps instead of three in a normal tricuspid valve (TAV). Although 97% of BAVs include asymmetric cusps, little or no prior studies investigated the blood flow through physiological three-dimensional BAV and root. This study presents four fully coupled fluid-structure interaction (FSI) models, including native TAV, asymmetric BAV with or without a raphe and an almost symmetric BAV. The FSI simulations are based on coupled structural and fluid dynamics solvers that allow accurate modeling of the pressure load on both the root and the cusps. The partitioned solver has non-conformal meshes and the flow is modeled employing an Eulerian approach. The cusps tissue in the structural model is composed of hyperelastic finite elements with collagen fiber network embedded in the elastin matrix. The tissues behavior of the aortic sinuses is also hyperelastic. The coaptation is modeled with master-slave contact algorithm. A full cardiac cycle is simulated by imposing the same physiological blood pressure at the upstream and downstream boundaries, for all the TAV and BAV models. The latter have significantly smaller opening area compared to the TAV. Larger stress values were also found in the cusps of the BAV models with fused cusps, both at the systolic and diastolic phases. The asymmetric geometry cause asymmetric vortices and much larger wall shear stress on the cusps, which is a potential cause for early valvular calcification in BAVs.
Research Interests:
A nonlinear viscoelastic and multi-scale modeling framework is used for the analysis of creep behavior and collapse of thick-section and layered composite structures. Previously developed nonlinear viscoelastic micromodels by the authors... more
A nonlinear viscoelastic and multi-scale modeling framework is used for the analysis of creep behavior and collapse of thick-section and layered composite structures. Previously developed nonlinear viscoelastic micromodels by the authors [Haj-Ali RM, Muliana AH. Micromechanical constitutive framework for the nonlinear viscoelastic behavior of pultruded composite materials. Int J Solids Struct 2003;40(5):1037–57; Haj-Ali RM, Muliana AH. A multi-scale constitutive formulation
Research Interests:
This study combines a simple damage modeling approach with micromechanical models for the progressive damage analysis of pultruded composite materials and structures. Two micromodels are used to generate the nonlinear effective response... more
This study combines a simple damage modeling approach with micromechanical models for the progressive damage analysis of pultruded composite materials and structures. Two micromodels are used to generate the nonlinear effective response of a pultruded composite system made up from two alternating layers reinforced with roving and continuous filaments mat (CFM). The layers have E-glass fiber and vinylester matrix constituents.
Research Interests:
Experimental and numerical analyses are performed to determine the translayer mode-I fracture toughness of a thick-section fiber reinforced polymeric composite using the eccentrically loaded, single-edge-notch tension, ESE(T) specimen.... more
Experimental and numerical analyses are performed to determine the translayer mode-I fracture toughness of a thick-section fiber reinforced polymeric composite using the eccentrically loaded, single-edge-notch tension, ESE(T) specimen. Finite element analyses using the virtual crack closure technique were performed to assess the effect of material orthotropy on the mode-I stress intensity factors in the ESE(T) specimen. The stress intensity factors