Page 432 - Mechanics of Asphalt Microstructure and Micromechanics

424 C hapter T h ir te en

to perform these computational simulations for both material design and structural be-
havior prediction.
The critical point here is the failure modes and mechanism dominating the initia-
tion and propagation of localized deformation and cracking at different length scales.
While all the failures (cracking) or large plastic or permanent deformations are attrib-
uted ultimately to either the breakage of atomic bonds or dislocations of atoms, a con-
tinuum approach may be valid in many cases where failure mechanisms are well un-
derstood empirically, and the deformation gradient or strain rates, etc. are not so large.
For example, microstructure defects far away from the crack tip may not assist in the
arrest or speed up of cracking; nano-size defects or inhomogeneity may not affect a mil-
limeter-size crack at high-speed fracturing. Therefore, determining where a smaller
scale model is needed also requires multiscale modeling.
With high performance and even nano-composite materials, the failure mechanism
has not been well understood, therefore modeling and simulation at the atomistic scale
becomes critical. Mechanical properties of the thin films, etc., are so difficult to test.
Macroscopic properties such as strength and strain relationships, modulus, strength,
and toughness may not be able to be conveniently characterized. In addition, lack of
understanding of the deformation and failure mechanisms of the structural materials
under extreme loadings such as explosion, high-speed impact, and burning all require
understanding the fundamental mechanisms across the different scales. Even friction
between tires and pavement materials, and ice may require understanding the cohesion
and adhesion of these materials at nanoscale.
Steel corrosion and alkaline silicon reaction both involve complex chemo-mechani-
cal interactions. Even the hydration process has not been well understood. Recently,
chemo-mechanics of bituminous materials has caught the attention of many research-
ers. Phenomena such as self-healing mechanisms of polymers require the use of quan-
tum mechanics and nanoscale devices or techniques to understand. Furthermore, the
use of nanosensors involves the interactions at nanoscale between the hosting materials
and the sensor materials. Understanding of chemo-mechanical and electromagnetic
coupling is also important for the design and interpretation of the sensing data.
Developments in nanoscale science and technology as well as high performance
computation make it possible to perform such multiscale characterization, modeling,
and simulation. Multiscale modeling techniques and their numerical implementation
have reached a point where multiscale modeling techniques may help select materials,
develop mix design, and characterize mixture properties more rationally.

13.2.2 A Brief Overview of Multiscale Modeling Methods

13.2.2.1 Continuum Modeling
Continuum modeling based on governing constitutive equations has dominated the
realm of numerical modeling since its advent. The constitutive equations are actually a
set of phenomenological relationships between cause and effect such as stress and
strain. Continuum methods have been successful in characterizing large-scale struc-
tures and components, for which the detailed response of materials is not so critical. In
rigorous mechanics sense, constitutive equations in continuum mechanics are built on
the statistical description of atomic and molecular scale processes (average stress, strain,
or velocity over time and space). In other words, constitutive equations represent the
mechanical behavior of materials over long-time and large-length scales.