Zouheir Fawaz

Numerical simulation of normal and oblique ballistic impact on ceramic composite armours

This paper presents three-dimensional finite element models that investigate the performance of ceramic–composite armours when subjected to normal and oblique impacts by 7.62 AP rounds. The finite element results are compared with experimental data from different sources both for normal and oblique impact, respectively. Simulation of the penetration processes as well as the evaluation of energy and stresses distributions within the impact zones highlight the difference between normal and oblique ballistic impact phenomena. The findings show that the distributions of global kinetic, internal and total energy versus time are similar for normal and oblique impact. However, the interlaminar stresses at the ceramic–composite interface and the forces at the projectile– ceramic interface for oblique impact are found to be smaller than those for normal impact. Finally, it is observed that the projectile erosion in oblique impact is slightly greater than that in normal impact. 2003 Published by Elsevier Ltd.

Applicability and viability of a GA based finite element analysis architecture for structural design optimization

Abstract A genetic algorithm (GA) based finite element analysis (FEA) procedure was developed for size and shape optimization of planar and space trusses. The purposed procedure interfaces a binary GA within a FEA software package in order to initially test the applicability and viability of such integration. In addition, special features of the GA were included to dynamically alter the population size, and the crossover and mutation rate in order to facilitate faster convergence and hence reduce the computational effort required. In other words, the GA adapted itself as search and optimization process progressed. The paper also brings a focus on the applicability of integrating a GA as an optimization tool within a FEA software. It was shown by way of many examples––solved by numerous mathematical, as well as other heuristic approaches in the literature––that the purposed methodology is quite efficient and capable of finding lighter and reasonable structural designs than that reported in the literature. Moreover, it is shown that the purposed method removes the immense effort required in coding ones own finite element codes by utilizing already existing finite element software. Nonetheless, it was found that even with a GA, optimization for very large problems was computationally extensive.

An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates

In this paper we investigated the fatigue damage of a unidirectional flax-reinforced epoxy composite using infrared (IR) thermography. Two configurations of flax/epoxy composites layup were studied namely, [0]16 unidirectional ply orientation and [±45]16. The high cycle fatigue strength was determined using a thermographic criterion developed in a previous study. The fatigue limit obtained by the thermographic criterion was confirmed by the results obtained through conventional experimental methods (i.e., Stress level versus Number of cycles to failure). Furthermore, a model for predicting the fatigue life using the IR thermography was evaluated. The model was found to have a good predictive value for the fatigue life. In order to investigate the mechanism of damage initiation in flax/epoxy composites and the damage evolution, during each fatigue test we monitored the crack propagation for a stress level and at different damage stages, a direct correlation between the percentage of cracks and the mean strain was observed.

Prediction of Bearing Strength in Fiber Metal Laminates

Finite element (FE) analyses are carried out on bolt bearing testing scenarios based on data found in the literature. Both layer-by-layer and smeared property FE models are created to calculate the compressive characteristic dimension (CCD) for three GLARE variants. A novel re-definition of conventional CCD is proposed which is governed by the yield strength of aluminum. The new definition also incorporates the two-phase nature of GLARE, as well as the delamination/ buckling phenomenon for pin/bolt bearing, in a bearing failure mode. A previously unconsidered, orthotropic plate buckling analysis is also conducted in a conservative, worst case scenario sense on the laterally unsupported prepreg layers. Results of the buckling analysis suggest that the prepreg contribution to bearing strength, in a bearing failure mode, is at best negligible and joint collapse is governed by the yielding and delamination of the aluminum layers. Calculation of a CCD, based on the new yield strength definition, produced consistent values amongst all GLARE variants considered in the layer-by-layer analysis suggesting that the CCD is a property of the material alone.

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 the mechanical properties and failure modes of hybrid natural fiber composites for potential bone fracture fixation plates

The purpose of this study is to investigate the mechanical feasibility of a hybrid Glass/Flax/Epoxy composite material for bone fracture fixation such as fracture plates. These hybrid composite plates have a sandwich structure in which the outer layers are made of Glass/Epoxy and the core from Flax/Epoxy. This configuration resulted in a unique structure compared to prior composites proposed for similar clinical applications. In order to evaluate the mechanical properties of this hybrid composite, uniaxial tension, compression, three-point bending and Rockwell Hardness tests were conducted. In addition, water absorption tests were performed to investigate the rate of water absorption for the specimens. This study confirms that the proposed hybrid composite plates are significantly more flexible axially compared to conventional metallic plates. Furthermore, they have considerably higher ultimate strength in tension, compression and flexion. Such high strength will ensure good stability of bone-implant construct at the fracture site, immobilize adjacent bone fragments and carry clinical-type forces experienced during daily normal activities. Moreover, this sandwich structure with stronger and stiffer face sheets and more flexible core can result in a higher stiffness and strength in bending compared to tension and compression. These qualities make the proposed hybrid composite an ideal candidate for the design of an optimized fracture fixation system with much closer mechanical properties to human cortical bone.

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.

Stepwise-equilibrium and Adaptive Molecular Dynamics Simulation for Fracture Toughness of Single Crystals

A stepwise-equilibrium and adaptive molecular dynamics (MD) simulation scheme for investigating the fracture toughness of single crystals is proposed in this study. The critical fracture toughness is found by conducting MD simulations along with the gradually increasing external load. At each load step, an equilibrium state is obtained by relaxing the system from the initial state generated. This is done by adjusting the atomic position using an additional displacement of linear elastic solution corresponding to the current load increment. The load increment is adjusted at each step in an adaptive way in order to achieve high computational efficiency and accuracy. A nickel crystal having 14256 atoms is investigated using this technique. The critical stress intensity factor in the (1[UNKNOWN]0) plane is found to be 0.7436 MPa √m, while the fracture stress is 4.7776 GPa. The effects of vacancies on the critical stress intensity factors are also investigated.

Biomechanical properties of a structurally optimized carbon-fibre/epoxy intramedullary nail for femoral shaft fracture fixation

Intramedullary nails are the golden treatment option for diaphyseal fractures. However, their high stiffness can shield the surrounding bone from the natural physiologic load resulting in subsequent bone loss. Their stiff structure can also delay union by reducing compressive loads at the fracture site, thereby inhibiting secondary bone healing. Composite intramedullary nails have recently been introduced to address these drawbacks. The purpose of this study is to evaluate the mechanical properties of a previously developed composite IM nail made of carbon-fibre/epoxy whose structure was optimized based on fracture healing requirements using the selective stress shielding approach. Following manufacturing, the cross-section of the composite nail was examined under an optical microscope to find the porosity of the structure. Mechanical properties of the proposed composite intramedullary nail were determined using standard tension, compression, bending, and torsion tests. The failed specimens were then examined to obtain the modes of failure. The material showed high strength in tension (403.9±7.8MPa), compression (316.9±10.9MPa), bending (405.3±8.1MPa), and torsion (328.5±7.3MPa). Comparing the flexural modulus (41.1±0.9GPa) with the compressive modulus (10.0±0.2GPa) yielded that the material was significantly more flexible in compression than in bending. This customized flexibility along with the high torsional stiffness of the nail (70.7±2.0Nm(2)) has made it ideal as a fracture fixation device since this unique structure can stabilize the fracture while allowing for compression of fracture ends. Negligible moisture absorption (~0.5%) and low porosity of the laminate structure (< 3%) are other advantages of the proposed structure. The findings suggested that the carbon-fibre/epoxy intramedullary nail is flexible axially while being relatively rigid in bending and torsion and is strong enough in all types of physiologic loading, making it a potential candidate for use as an alternative to the conventional titanium-alloy intramedullary nails.

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.