Habilitation à diriger des recherches : Génie mécanique : Villeurbanne, INSA : 1999. / Document rédigé également en version française. Titre provenant de l'écran-titre. Bibliogr. p. 135-155.

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The objective of this work is to study crack path deflection under proportional and non-proportional mixed-mode fatigue and predict crack branching direction based on linear elastic fracture mechanics (LEFM) driving forces. Under proportional in-phase mixed Mode I / Mode II loading conditions, crack growth direction has previously been observed in some materials to shift from tensile-dominated Mode I to shear-dominated Mode II or mixed-mode crack growth at higher proportions of initial Mode II loading, but non-proportional loads are not well-characterized. An LEFM approach is desired in order to implement the model in crack growth software such as the boundary element-based fracture analysis package FRANC3D. A novel specimen configuration has been designed and analyzed for generation of wide ranges of mixed-mode loading conditions in a single test. This specimen and a more conventional thin-walled tubular specimen have been used to test polycrystalline nickel-base superalloy Inconel 718 under proportional in-phase and 3 kinds of non-proportional fatigue loading. Stress intensity factors for the various configurations have been analyzed with FRANC3D. Modal transition from Mode I (tensile) to Mode II (shear) crack branching has been observed in several load cases. Qualitative microscopy of fracture surfaces was used to characterize the difference between crack branch modes. An LEFM approach based on an effective stress intensity factor range, which incorporates the maximum value and range of each appropriate stress intensity (Mode I or Mode II), has been used to successfully predict the crack deflection angles, and in some cases to quantify modal transition, within each load case considered. Variability between load cases and specimen configurations points to the limitations of LEFM in providing a general predictor of crack path behavior across all types of non-proportional mixed mode loading.

Driven by the need for increase in system¡¯s functionality and decrease in the feature size, International Technology Roadmap for Semi-conductors has predicted that integrated chip packages will have interconnections with I/O pitch of 90 nm by the year 2018. Lead-based solder materials that have been used for many decades will not be able to satisfy the thermal mechanical requirements of these fines pitch packages. Of all the known interconnect technologies, nanostructured copper interconnects are the most promising for meeting the high performance requirements of next generation devices. However, there is a need to understand their material properties, deformation mechanisms and microstructural stability. The goal of this research is to study the mechanical strength and fatigue behavior of nanocrystalline copper using atomistic simulations and to evaluate their performance as nanostructured interconnect materials. The results from the crack growth analysis indicate that nanocrystalline copper is a suitable candidate for ultra-fine pitch interconnects applications. This study has also predicts that crack growth is a relatively small portion of the total fatigue life of interconnects under LCF conditions. The simulations result conducted on the single crystal copper nano-rods show that its main deformation mechanism is the nucleation of dislocations. In the case of nanocrystalline copper, material properties such as elastic modulus and yield strength have been found to be dependent on the grain size. Furthermore, it has been shown that there is competition between the dislocation activity and grain boundary sliding as the main deformation mode This research has shown that stress induced grain coarsening is the main reason for loss of mechanical performance of nanocrystalline copper during cyclic loading. Further, the simulation results have also shown that grain growth during fatigue loading is assisted by the dislocation activity and grain boundary migration. A fatigue model for nanostructured interconnects has been developed in this research using the above observations Lastly, simulations results have shown that addition of the antimony into nanocrystalline copper will not only increase the microstructure stability, it will also increase its strength.

The goal of this academic project was to study the effects of different variables on the damage initiation and progression around four bolt holes of a joint in carbon fiber/graphite epoxy composite coupons. The tracked variables included the type of layup, R values, stress levels, and damage mechanisms observed in each specimen. In-situ x-ray of the individual coupons recorded the extent of damage, mostly longitudinal splitting and bearing failure, as a function of the cycle count. The following lay-ups were included: [45/90/-45/02/45/02/-45/0]s, [04/45/03/90/0]s, [±5/65/(±5)2/-65/±5]s, and [±5/65/(±5)2/-65/5/65]s, In particular, the objective was to determine the stress levels at which detectable damage starts developing by applying 50,000 cycles at incremental stress levels. Once damage was initially detected, we typically raised the stress level 2.5 ksi and cycled another 50,000 cycles until damage reached a point where the bolt holes had elongated 10% of the original diameter of 0.25 inches. This type of testing was be continued for several different R ratios and comparisons were be made between the performances of different lay-ups at varying load levels. A finite element model was created in ABAQUS to help understand the stress fields within the laminates.

Thesis (Ph. D.)--Mechanical Engineering, Georgia Institute of Technology, 2008. / Committee Chair: David. L. McDowell; Committee Member: Min Zhou; Committee Member: Naresh N. Thadhani; Committee Member: Rami M. Haj-Ali; Committee Member: Richard W. Neu.

Thesis (Ph.D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Rami Haj-Ali; Committee Member: Arash Yavari; Committee Member: Bruce R. Ellingwood; Committee Member: Kenneth M. Will; Committee Member: Richard W. Neu.