Xi Chen's blog

Although it is more realistic to study the mechanical properties of nanostructures such as the carbon nanotubes (CNTs) at room temperature, atomistic simulations at finite temperature (such as molecular dynamics, MD) may cause the following problems: (1) Due to the limitation of the time scale achievable in MD (typically at the nanosecond scale), the loading rate in MD simulation at any finite temperature is not realistic. Very often, the loading rate used in MD simulations may well exceed 10m/s at 300K and thus many orders of magnitude higher than the real loading rate used in experiments. (2) A great advantage of simulation is to be able to turn on and turn off certain features and explore their effects, which is otherwise impossible in experiments. For example, the buckling behavior of CNTs is very sensitive to geometrical perturbations, which is prominent at room temperature and such perturbations causes severe uncertainties and makes it difficult to explore the intrinsic buckling behaviors. Therefore, by removing the temperature effect, we could better evaluate other key factors affecting the intrinsic buckling behavior, such as tube chirality, radius, and length, which could be otherwise covered by the thermal fluctuation effect. (3) Due to both time and length scale limitations, the MD simulations of large system are not yet possible, and thus the effective continuum models must be developed which need to be calibrated by atomistic simulations. At present, the temperature factor is still absent in most continuum models. Therefore, atomistic simulations at 0K or near 0K may provide a useful benchmark for the development of parallel continuum models, focusing on the most intrinsic and basic mechanical properties of nanostructures. Based on the above analysis, atomistic simulations at 0K by using the molecular mechanics (MM) method are still very useful, especially to us as mechanicians.

The conventional indentation analysis uses finite element simulations on a wide range of materials and studies their indentation responses, which is known as the forward analysis; then, from the reverse analysis it may be possible to extract material properties from the indentation responses on a particular specimen. In doing so, it is important to selecte a wide yet appropriate range of materials during the forward analysis. Often times when I read or review papers, I found some authors "randomly" select a large range of materials without really knowing what does that mean and whether it is practical; in many cases the materials employed in their forward/reverse analyses do not exist in reality or are actually not suitable for conventional indentation analysis.

In indentation analysis the constitutive elastoplastic properties of the specimen is often expressed by the power-law form. It is important to note that most brittle ceramic or glass materials crack upon indentation, and polymers creep during indentation experiment, moreover the tension and compression behaviors of polymers are often very different; thus, they typically cannot be well-described by the power-law form and their mechanical properties cannot be obtained from the conventional indentation analysis. Thus, ceramics and polymers should be excluded from the present analysis, as well as the highly anisotropic woods. In addition, composite materials, nanocomposites and other nano-structured materials, as well as thin films also need to be excluded from the continuum analysis because the underlying micro/nanostructures play a key role in their mechanical responses. Therefore, only the more ductile and "plastic" polycrystalline bulk metals and alloys are suitable for conventional indentation analysis at room temperature since large strain will occur beneath the indenter during indentation, and also because the conventional plasticity theory is developed for metals which is the foundation of the elastoplastic finite element analysis. The indentation depth also has to be sufficient large on the bulk specimen so as to overcome the strain gradient effect.

The material selection chart taken from page 425 of the famous handbook"Materials selection in mechanical design" by Mike Ashby can be used as a guide. In general, for most engineering metals and alloys suitable for conventional indentation study, the Young's modulus is from about 10 to 600GPa, and the yield strength is from roughly 10MPa to 2GPa, and the inverse of yield strain is in the range roughly from 100 to about 5000 (some pure metals may have even higher inverse yield strain, but should not far exceed such bound). Note that since the specimen must undergo relatively large strain during indentation without cracking, thus the material must be sufficiently ductile (i.e. plastic or soft).

In forward analysis, however, the material range chosen in finite element simulation needs to be moderately larger than the aforementioned bound, so as to avoid possible numerical ill conditions at the boundaries. The reverse analysis, however, should focus on the more practical materials, i.e. the range of metals and alloys listed above.

(originally written by Yuye Tang) A key procedure of the molecular-dynamics decorated finite element method (MDeFEM) is to determine the effective properties of components of a macromolecule. Here I illustrate how could one use the NMA computed from MD to estimate the elastic properties of loops in mechanosensitive channels, which is related with my research.