Alireza Sayyidmousavi

Thermomechanical viscoelastic response of a unidirectional graphite/polyimide composite at elevated temperatures using a micromechanical approach

In this work, the thermomechanical viscoelastic response of a high temperature polymer matrix composite system made up of T650-35 graphite fibers embedded in PMR-15 resin is studied through a micromechanical model based on the assumptions of simplified unit cell method within a temperature range of 250–300℃ corresponding to aerospace engine applications. The advantage of this particular micromechanical model lies in its ability to give closed-form expressions for the effective viscoelastic response of unidirectional composites as well as each of their constituents. Using the experimental data of the creep behavior of thermostable PMR-15 polyimide, the micromechanical model is first calibrated to account for the effect of temperature. The resulting elastic and viscoelastic responses are found to be in good agreement with the existing experimental data. The validated model is then used to predict the behavior of the composite material under different combinations of thermal and mechanical loadings. The results clearly demonstrate the importance of accounting for the viscoelastic effect of the matrix material as the temperature increases. Current works on modeling temperature-dependent viscoelastic behavior of polymer matrix composites are mainly based on the assumption of thermorheologically simple material. However, through the present approach where the matrix is modeled as a thermorheologically complex material, the effect of temperature on the elastic and viscoelastic response of the composite system can be individually investigated.

Investigation of stress shielding around the Stryker Omnifit and Exeter periprosthetic hip implants using an irreversible thermodynamic-based model.

This study investigates stress shielding by predicting bone density around two different implants following total hip arthroplasty using a new thermodynamic-based model for bone remodeling. This model is based on chemical kinetics and irreversible thermodynamics in which bone is treated as a self-organizing system capable of exchanging matter, energy, and entropy with its surroundings. Unlike the previous works in which mechanical loading is regarded as the only stimulus for bone remodeling, this model establishes a coupling between mechanical loading and the chemical reactions involved in the process of bone remodeling. This model is incorporated into the finite element software ANSYS by means of a macro to investigate stress shielding around two different implants: Stryker Omnifit and Exeter periprosthetic hip stems. The results of the simulation showing bone density reductions of 17% in Gruen zone 1 and 27% in Gruen zones 7 around the Omnifit hip stem agree well with dual-energy X-ray absorptiometry (DEXA) measurements reported in the literature. On the other hand, the Exeter implant is found to result in more severe resorption in the proximal femur. This is consistent with clinical studies, which report a higher survivorship rate for HA-coated Omnifit hip stems.

A 3D micromechanical energy-based creep failure criterion for high temperature polymer matrix composites

In the present study, a generalized three-dimensional (3D) energy-based criterion for the creep failure of viscoelastic materials is developed. Unlike the existing approaches which are restricted to uniaxial loading, the proposed criterion can predict failure under any combination of loads. This criterion is then incorporated into a simplified unit cell micromechanical model to predict the time-delayed failure of unidirectional polymer–matrix composites at elevated temperatures. The composite material used in this study is T300/934 which is suitable for service at high temperatures in aerospace applications. The use of micromechanics can give a more accurate insight into the failure mechanisms of the composite materials, in particular at high temperatures, where the general behavior of the polymer–matrix composite is governed by matrix viscoelasticity and matrix time-dependent failure due to creep is a localized phenomenon. The micromechanical model is also used to estimate the ultimate strength of the constituents from the knowledge of the allowable strengths of the unidirectional composite in the principal material directions. The obtained creep failure stresses are found to be in reasonable agreement with the experimental data particularly for the 90° unidirectional laminate, where failure is totally matrix dominated.

A multiscale approach for fatigue life prediction of polymer matrix composite laminates

The present study introduces a progressive fatigue damage model within a multiscale framework by incorporating a Simplified Unit Cell Micromechanical model into a Finite Element program. The use of micromechanics will allow the study of damage at the micro-scale which can therefore identify modes of failure in each of the composite’s constituents, separately. The use of finite element method at the macro-scale enables the model to capture the geometric complexities including regions of stress concentration, which expedites the failure of the material. Damage progression is modeled through the degradation of the material property corresponding to the failure mode detected by the micromechanical model. The results of the model are in good agreement with the experimental data for both unidirectional and multidirectional laminates. The present approach is capable of predicting the fatigue life of composite laminates of any arbitrary geometry and lay-up configuration with minimum dependence on empirical parameters.

Finite element buckling analysis of laminated composite sandwich panels with transversely flexible core containing a face/core debond

In this article, a new three-dimensional finite element modeling approach with less computing time and space is introduced to study the buckling behavior of sandwich panels, containing a face–core debond. The finite element model presented in this study relates the motion of the face sheets to the core through constraint equations utilizing the concept of slave and master nodes, thus representing a more realistic model of the sandwich panel. The composite face sheets are modeled with shell elements, and the core is modeled using the 3D structural solid elements capable of taking transverse flexibility into consideration. In order to model the debond, the constraints between the nodes of the face sheet and the core are removed and replaced with contact elements in the debonded region to avoid interpenetration. The model is validated through comparison with experimental results reported in the literature. The validated model is then used to study the effects of the size, shape, aspect ratio of the debond, as well as fiber orientation of the face sheets and the influence of core stiffness on the buckling load of the panel subject to different boundary conditions on the top and bottom face sheets of the panel.

An efficient hybrid method for stochastic reaction-diffusion biochemical systems with delay

Many chemical reactions, such as gene transcription and translation in living cells, need a certain time to finish once they are initiated. Simulating stochastic models of reaction-diffusion systems with delay can be computationally expensive. In the present paper, a novel hybrid algorithm is proposed to accelerate the stochastic simulation of delayed reaction-diffusion systems. The delayed reactions may be of consuming or non-consuming delay type. The algorithm is designed for moderately stiff systems in which the events can be partitioned into slow and fast subsets according to their propensities. The proposed algorithm is applied to three benchmark problems and the results are compared with those of the delayed Inhomogeneous Stochastic Simulation Algorithm. The numerical results show that the new hybrid algorithm achieves considerable speed-up in the run time and very good accuracy.

A micromechanical approach for the fatigue failure prediction of unidirectional polymer matrix composites in off-axis loading including the effect of viscoelasticity

A novel micromechanical approach is used to study the fatigue failure of unidirectional polymer matrix composites subject to off-axis loading. The main advantage of the present micromechanical model lies in its ability to give closed form solutions for the effective nonlinear response of unidirectional composites and to predict the material response to any combination of shear and normal loading. The fatigue failure criterion is expressed in terms of the fatigue failure functions of the constituent materials. The micromechanical model is also used to calculate these fatigue failure functions from the knowledge of the S–N diagrams of the composite material in longitudinal, transverse, and shear loadings; thus, eliminating the need for any further experimentation. Unlike previous works, the present study accounts for the viscoelasticity of the matrix material rendering it the capability of modeling creep damage accumulation in high-temperature composite materials. The results are found to be in good agreement with the literature. In particular, for higher off-axis angles, the results are seen to be in better concurrence with the experimental data compared to when the effect of viscoelasticity is overlooked. The present approach is also capable of accounting for the strain evolution due to viscoelasticity of the matrix material.

Finite element buckling analysis of laminated composite sandwich panels with transversely flexible core carrying attached elastic strip

In this article, a three-dimensional finite element model is proposed to study the effect of distributed attached mass with thickness and stiffness on the buckling instability of sandwich panels with transversely flexible cores. Unlike the previous works in the literature which have made use of unified displacement theories, the present model uses different types of finite elements to model the core and the face sheets. It utilizes shell elements for the face sheets and three-dimensional solid elements for the core which enables the model to account for the transverse flexibility of the structure. The motions of the face sheets and the core as well as the attached mass are related through defining constraint equations between the nodes of their respective finite elements based on the concept of master and slave nodes which is incorporated into the finite element analysis program ANSYS through a user-defined subroutine. The validated finite element model is then used to study the effects of size, thickness, material property, aspect ratio, and the position of the attached mass on the buckling load of a sandwich panel under different combinations of boundary conditions. The results presented in this study have hitherto not been reported in the literature.