report #3
TRANSCRIPT
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Date: 7/09/2011
Report Number: 3
Title: Characterization and Peridynamic Modeling of Shape Memory Alloy based Self‐Healing Composite Aerospace
Keywords: Peridynamic, Horizon, Local/Nonlocal model, Length scale, Bonds, Bond Elongation, Pairwise Force
Additional notes about Peridynamic equation of motion In Peridynamic theory, the forces are exactly nonlocal because material inside and outside of a region exert force together, but in classical elasticity the force interactions are restricted only on the surface of a meshed material (Figure 1), in other words, when force occurs over a volume and not a surface, we say the force is nonlocal (Figure 2), it is known as difference of local/nonlocal among Peridynamic and classical elasticity. [4]
Figure 1. Local model
Figure 2. Nonlocal model
Peridynamic from orthographic point of view is composed of two parts of peri which means all around, near, surrounding and dyna which means force or power. [1] Peridynamic is a nonlocal reformulation of classical continuum mechanics and depends on the length scale it can behave between MD1 and Classical Elasticity, the benefit of this method are in decrease of computational costs of nonlocal model and increasing the accuracy of classical methods which is using mesh to discrete the model, in other words, MD is hard to use in practical applications and there is limited validity for classical elasticity. [3]In equations that have not length scales (eq.1), when we change the
1 . Molecular Dynamic
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scale of variable the dynamic of system will remain unchanged, but in equations like (eq.2) by changing scale of one parameter like x some terms in equation will be dominant, but in Peridynamic authors have added the ability of changing length scales in a wide range of scales in the formulation of materials behaviors, in simple words, it is a single multiscale material model [4] and the scale of EOM2 will be changed by defining different horizons.
(1)
(2)
In a Peridynamic integral equation which is nonlocal (eq.3) if we extend it by Taylor series we will have (eq.4) that shows the effect of selection of horizon by vanishing HOT3 and if we tend the horizon to zero, so the result will be a local scale invariant equation (eq.5).
, | | , , (3)
, (4)
, lim (5)
Classical elasticity have implement stress‐strain relationship but in Peridynamic it is replaced by an integral that shows the summation of internal forces between particles in a limit distance and this integral is not a function of deformation gradient so it can be used in different types of deformations instead of classical elasticity that is align with kinematic assumption of molecular dynamics, so we can say Peridynamic is a generalized continuum mechanics. [4]
In the first report we discussed about benefits of using Peridynamic equation instead of implement of PDE4 in continuum mechanics for dynamical motions, moreover I find that in Peridynamic equation of motion for simulation of growth of crack, we don't need to use extra laws and special techniques of fracture mechanic which is used in classical equations in continuum mechanics and it is based on laws stress intensity factor. [1] The Peridynamic model involves displacement field only instead of its gradient so it formally appears to be a continuum version of MD. [5]
Figure 3. Pairwise force function
2 . Equation of Motion 3 . Higher Order Terms 4 . PDE
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The interaction between two point x and x' is called bond, and the vector valued function f (Figure 3) is the force density that its dimension is force per volume squared and Pairwise force is a function of current relative position of points and also their relative displacement (u) in order to deformation (eq.6) and contains all the constitutive (material dependent) properties and its vector is in the direction of connected line among deformed position of x and deformed position of x' which is coming from using equilibrium of angular momentum. (eq.7)
, , , , (6)
, , 0 (7)
A Pairwise force function can be specified by bond elongation (eq.8), and we can draw bond force based on bond elongation, as it can be seen from (Figure 4), by increasing relative distance among two points after their deformation the bond force will rise, but when this distance passes the horizon limitation, bond breakage will be happen.
| | | | (8)
Figure 4. Bond force vs. Bond elongation and Bond breakage horizon
Crack will be happened when the bonds break irreversibly, because of exceeding elongation to some prescribed value [2]. In PD5, cracks are part of solution not part of the problem. [6]
5 . Peri‐Dynamic
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Reference(s) [1]. "http://en.wikipedia.org/wiki/Peridynamics", 2011‐08‐24.
[2]."Peridynamic Analysis of Damage and Failure in Composites", Ebrahim Askari, Jifeng Xu, Stewart Silling, 44th AIAA Aerospace Science Meeting and Exhibit, Jan 2006, Reno, Nevada
[3]." http://www.math.cmu.edu/cna/macro2011/seleson.html", 2011‐08‐31
[4]."Peridynamic for multiscale materials modeling", E Askari, F Babaru, R B Lehoucq, M L Parks, S A Silling, O Wekner, Journal of Physics, Conference series 125 012078, 2008
[5]. "Poro‐mechanics I", Hoe I. Ling, Andrew Smyth and Raimondo Betti, Proceeding of fourth Biot conference on poromechanics, 2009, Google Books website.
[6]."Computational Peridynamics", Michael Parks, Computational Research Center Sandia National Laboratories Albuquerque New Mexico, 16‐22 Jan 2011.