ACS PRF | ACS
All e-Annual Reports
43848-AC8
Nano-to Macro-Scale Modeling of Rock Mechanical Behavior Under Extreme Loading Conditions
The main accomplishments that were achieved during the first year of the project include: 1) Development of a Matlab-based Atomistic Finite Element Method (AFEM) program, and 2) Validation of the AFEM program by simulation of the cyclic behavior of the zigzag CNT (Carbon Nanotube). AFEM is a very versatile and innovative technique for MD simulation recently developed by Liu et al. (2004). In AFEM, bonds between particles are treated as elements, and the FE formulation is obtained by minimization of the global energy potential. The main advantage of AFEM is that it requires only order-N calculations in comparison to the widely used MD simulation using the conjugate gradient method which requires order-NxN calculations. The substantial reduction in calculation times in AFEM is achieved at the same level of accuracy as the conjugate gradient method. Another advantage is that AFEM can be seamlessly coupled with FEM permitting multiscale simulations combining nanoscale MD elements and continuum finite elements.
AFEM simulations of the zigzag CNT showed that AFEM is capable of accurately predicting the mechanical behavior of nanoscale materials. AFEM results were found to be identical to those obtained from traditional MD simulation, with much shorter calculation times. In addition, cyclic simulation of CNT showed interesting results not hitherto reported in previous studies. It was shown that the cyclic strength and strain behavior of zigzag tubes is largely influenced by the loading manner within their elastic limits. It is concluded that the residual defect-free morphological deformation as a result of structural instability is the primary mechanism responsible for the fatigue failure of CNT. CNT under axial compression continuously buckled into a series of morphological patterns, with each pattern corresponding to an abrupt energy release and singularity in the stress-strain or load-displacement curve.
Work is currently being carried out to apply the AFEM program to clay and rock minerals. This work requires the formulation of the molecular structure and the establishment of parameters for the energy potential of the most comment types of minerals, including montmorillonite, kaolinite, quartz and calcium carbonate. The AFEM program will then be extended to account for the interaction between the mineral and fluids such as water, and the effects of temperature so that extreme loading and environmental conditions can be simulated. The nanoscale simulation will then be homogenized using nanoscale equivalent continuum models such as the Virtual Internal Bond (VIB). The equivalent continuum models in conjunction with coupled AFEM-FEM formulation will then be implemented in a Finite Element Code and used for multi-scale simulations of rock mechanical behavior under extreme loading such as borehole stability of HPHT reservoirs.
Work on this project was done with Dr. Jianfeng Wang, a post-doctoral research scholar who finished his Ph.D. in 2006. One journal paper is currently being reviewed and another is about to be submitted. In addition, one conference paper was published, and the PI (Dr. Marte S. Gutierrez) was invited to give two presentations on nano-geomechanics, which is an emerging field in the application of nanomechanics to geomaterials. We hope to write two to three more papers based on the results of the project.
