Finite-element model with contact: oblique impact of neo-Hookean cube

3.6 Numerical examples

3.6.3 Finite-element model with contact: oblique impact of neo-Hookean cube

Then, distances are measured with respect to the𝑊^2,∞-norm

∥𝑞∥_𝑊2,∞(0,𝑇)=∥𝑞(𝑡)∥𝐿^∞+∥𝑞(𝑡)∥˙ 𝐿^∞+∥𝑞(𝑡)∥¨ 𝐿^∞ (3.86)

and a relative𝑊^2,∞-error is defined by

𝑊^2,∞-error =

∥𝑞ℎ∥𝑊^2,∞(0,𝑇)− ∥𝑞∥𝑊^2,∞(0,𝑇)

∥𝑞∥_𝑊^2,∞_(0,𝑇) (3.87)

where ∥𝑞∥_𝑊2,∞(0,𝑇) is estimated from numerical results. The relative𝑊^2,∞-error of force-stepping with and without parity-invariance is shown on the right side of Figs. 3.15 and 3.16, respectively, as a function of the simplicial grid size ℎ, for 𝑇 = 0.1 nsec. The slope of the convergence plot is also directly related to the rate of convergence in grid size, that is 𝑊^2,∞-error = 𝑂(ℎ^𝑟). This gives an estimated rate of convergence in the 𝑊^2,∞-norm of 𝑟 ≃1. Furthermore, it is interesting to observe on the left side of Figs. 3.15 and 3.16 that the average and the maximum time steps selected by force-stepping are𝑂(ℎ). Additionally, the minimum time step selected by the method is 𝑂(ℎ^7/4) when an approximate potential𝑉_ℎwith parity-invariance is employed, and𝑂(ℎ²) when the approximate potential breaks the parity symmetry of the exact potential𝑉. It is also interesting to note that force-stepping with parity-invariance exhibits a region of asymptotic convergence larger than force-stepping without parity-invariance. Therefore, the sequence𝑞ℎ is indeed convergent and transversal, as expected from the analysis in Section 3.5, and force-stepping with parity-invariance exhibits better convergence properties.

3.6.3 Finite-element model with contact: oblique impact of neo-Hookean

10⁻⁶ 10⁻⁵ 10⁻⁴ 10⁻³ 10⁻² 10⁻¹ 10⁻¹³

10⁻¹⁰ 10⁻⁷ 10⁻⁴ 10⁻¹ 10²

Simplicial grid size [nm]

Time increment [fsec]

10⁻⁶ 10⁻⁵ 10⁻⁴ 10⁻³ 10⁻² 10⁻¹ 10⁻⁵

10⁻⁴ 10⁻³ 10⁻² 10⁻¹ 10⁰ 10¹

Simplicial grid size [nm]

W 2,∞ error

Figure 3.15: Convergence analysis of the frozen argon cluster using force-stepping with parity- invariance. Right: Convergence is observed in the𝑊^2,∞-norm with estimated convergence rate of 𝑟≃1. Left: The average (★) and the maximum (∙) time steps selected by force-stepping are𝑂(ℎ), whereas the minimum (■) time step is𝑂(ℎ^7/4).

10⁻⁶ 10⁻⁵ 10⁻⁴ 10⁻³ 10⁻² 10⁻¹ 10⁻¹³

10⁻¹⁰ 10⁻⁷ 10⁻⁴ 10⁻¹ 10²

Simplicial grid size [nm]

Time increment [fsec]

10⁻⁶ 10⁻⁵ 10⁻⁴ 10⁻³ 10⁻² 10⁻¹ 10⁻⁵

10⁻⁴ 10⁻³ 10⁻² 10⁻¹ 10⁰ 10¹

Simplicial grid size [nm]

W 2,∞ error

Figure 3.16: Convergence analysis of the frozen argon cluster using force-stepping without parity- invariance. Right: Convergence is observed in the𝑊^2,∞-norm with estimated convergence rate of 𝑟≃1. Left: The average (★) and the maximum (∙) time steps selected by force-stepping are𝑂(ℎ), whereas the minimum (■) time step is𝑂(ℎ²).

to illustrate the performance of force-stepping in that area of application is the oblique impact of a neo-Hookean cube in a rigid wall. To this end, we assume a strain-energy density of the form

𝑊(𝐹) = 𝜆0

2 (log𝐽)²−𝜇0log𝐽+𝜇0

2 tr( 𝐹^𝑇𝐹)

(3.88)

which describes a neo-Hookean solid extended to the compressible range. In this expression,𝜆0and 𝜇0 are Lam´e constants, and𝐽 is the Jacobian of the deformation gradient𝐹.

In problems involving contact we additionally consider the kinematic restrictions imposed by the impenetrability constraint. We recall that the admissible configuration set𝒞 of a deformable body is the set of deformation mappings which are globally one-to-one. In so-called barrier methods, the interpenetration constraint may be accounted for by adding the indicator function𝐼_𝒞(𝑞) of the admissible set 𝒞 to the energy of the solid. We recall that the indicator function of a set𝒞 is the extended-valued function

𝐼_𝒞(𝑞) =

⎧



⎨



⎩

0 if𝑞∈ 𝒞

∞ otherwise

(3.89)

Often in calculations, the indicator function 𝐼_𝒞 is replaced by a penalty approximation 𝐼_𝒞,𝜖 ≥ 0 parameterized by a small parameter𝜖 >0 and such that𝐼𝒞,𝜖= 0 over𝒞. In this approach, as𝜖→0, 𝐼_𝒞,𝜖→𝐼_𝒞 pointwise and interpenetration is increasingly penalized. A convenient choice of penalty energy function for contact with a rigid hyperplane is of the form

𝐼_𝒞,𝜖(𝑞) = 1 2𝜖

∑

𝛼∈𝐼

𝑔𝛼(𝑞) (3.90)

where the index set𝐼ranges over all boundary nodes and

𝑔𝛼(𝑞) =

⎧



⎨



⎩

0, if (𝑞−𝑂)⋅𝑛≥0,

∥(𝑞−𝑂)⋅𝑛∥², otherwise,

(3.91)

where 𝑂 and 𝑛 are the hyperplane reference point and outer-pointing normal. We note that the admissible set 𝐼𝒞,𝜖 is not invariant under the action of translations and rotations. It therefore

follows that the constrained Lagrangian does not retain its momentum preserving properties and the Lagrangian reduction described in Section 3.4 cannot be carried out.

The application of force-stepping to dynamic contact problems is straightforward. The case in which the constraints are represented by means of a penalty energy function𝐼_𝒞,𝜖falls right within the general framework and requires no special considerations, that is 𝑉(𝑞) = 𝑊(𝐹(𝑞)) +𝐼_𝒞,𝜖(𝑞).

In addition, collision is automatically captured by the intrinsic time adaption of force-stepping (cf., e. g., [10, 80], for a detailed discussion of time-step selection considerations in contact problems).

Our example concerns the oblique impact of an elastic aluminum cube of size equal to 0.1 m.

The mesh comprises 192 4-node tetrahedral isoparametric elements and 71 nodes. The cube is a compressible neo-Hookean solid characterized by a strain-energy density of the form (3.88). The values of the material constants in (3.88) are: 𝜆0 = 60.5 GPa,𝜇0 = 26 GPa, and𝜌= 2700 kg/m³. The cube is imparted an initial velocity 𝑣0 = (1,1,1) km/s. The rigid wall is described by a hyperplane with𝑂 = (0.101,0.101,0.101) m and 𝑛= (0,−1,−1). A sequence of snapshots of the force-stepping trajectory corresponding toℎ= 5⋅10⁻⁴ m are shown on Figs. 3.17 and 3.18.

0 5 10 15 20 25 30

35 40 45 50 55 60 65

70 75 80 85 90 95 100

Figure 3.17: Oblique impact of neo-Hookean cube. Sequence of snapshots of the force-stepping trajectory withℎ= 5⋅10⁻⁴ m displaying the 𝑦𝑧-midplane of the cube. Numbers indicate times in 𝜇s and oblique lines represent the rigid wall.

Figure 3.19 compares the time histories of total energy, potential energy and kinetic energy attendant to the force-stepping trajectory for ℎ= 5⋅10⁻⁴ m and Newmark’s trajectory for Δ𝑡= 0.2 nsec, the latter chosen to be in the order of the average time step resulting from the force-

t = 0 t = 20 t = 35 t = 40

t = 60 t = 80 t = 100

Figure 3.18: Oblique impact of neo-Hookean cube. Sequence of snapshots of the force-stepping trajectory withℎ= 5⋅10⁻⁴ m. Numbers indicate times in𝜇s and red lines provide a reference for the first points of impact, that is the (𝑥,0.101,0.101)-line.

stepping calculations. As expected, the kinetic energy of the system is partly converted to potential energy during the approach part of the collision sequence, and vice versa during the release part.

Also characteristically, force-stepping is observed to conserve energy exactly through the collision, whereas the Newmark energy remains within tight bounds due to the small time step and the appropriate penalty energy, i.e. (3.90) and (3.91) with 𝜖 = 10⁻¹⁴. Other notable features of the calculations are the ability of force-stepping to detect the time of collision and to automatically modulate the time-step so as to resolve the fine structure of the intricate interactions that occur through the collision.

Figure 3.20 shows the time history of the linear and angular momenta of the cube. Because the cube impacts with a rigid wall, the total linear and angular momenta of the system are not conserved through the collision but evolve in a characteristic way with time. We note from Figure 3.20 that force-stepping trajectory does indeed follow closely Newmark trajectory, even though force- stepping does not conserve linear and angular momenta. We also note that the transfer of linear momentum from the rigid wall to the cube is in the direction normal to the hyperplane and the motion in the orthogonal direction is unperturbed. Similarly, the oblique impact introduces angular momentum into the system. The good momentum behavior of force-stepping is noteworthy and

0 50 100 150 4044

4046 4048 4050 4052 4054

Time [μsec]

Total Energy [kJ]

0 50 100 150

0 1000 2000 3000 4000

Time [μsec]

V, T [kJ]

Figure 3.19: Oblique impact of neo-Hookean cube. Blue line: Force-stepping solution with ℎ = 5⋅10⁻⁴m. Black line: Newmark solution with Δ𝑡= 0.2 nsec.

0 50 100 150

−3

−2

−1 0 1 2 3

y , z x

Time [μsec]

Linear Momentum [kNs]

0 50 100 150

−0.6

−0.4

−0.2 0 0.2 0.4 0.6

x y

z

Time [μsec]

Angular Momentum [kJs]

100 120 140 2.696

2.698 2.7 2.702

100 120 140

−5 0 5 10x 10⁻³

Figure 3.20: Oblique impact of neo-Hookean cube. Blue line: Force-stepping solution with ℎ = 5⋅10⁻⁴ m. Black line: Newmark solution with Δ𝑡 = 0.2 nsec. The insets show in detail the 𝑥-component of momentum maps.

may be understood as a consequence of the near-momentum conservation properties of the method (Theorem 3.4.2).

Finally, we investigate the relationship between the average time step and the number of degrees of freedom of the system 𝑑= 3𝑁. To this end we consider three different meshes with increasing level of refinement, i.e. meshes I, II and II with 213, 375 and 1287 degrees of freedom, respectively.

Figure 3.21 shows the average time step selected by force-stepping is 𝑂(ℎ/𝑁²), where ℎ is the simplicial grid size. We note that these results are consistent with and extend the results obtained in Section 3.6.2, i.e. a dependence of the form𝑂(ℎ).

10⁻¹¹ 10⁻¹⁰ 10⁻⁹ 10⁻⁸ 10⁻⁷ 10⁻⁵

10⁻⁴ 10⁻³ 10⁻² 10⁻¹

Mesh I, h=1.2⋅10⁻⁴ Mesh II, h=1.2⋅10⁻⁴

Mesh III, h=1.2⋅10⁻⁴ Mesh I, h=5.0⋅10⁻⁴ Mesh II, h=5.0⋅10⁻⁴

h/N ²

Time increment [nsec]

I

II

III

Figure 3.21: Oblique impact of neo-Hookean cube. The average time step selected by force-stepping is 𝑂(ℎ/𝑁²), whereℎis the simplicial grid size and 3𝑁 is the number of degrees of freedom of the system. Three different meshes are considered and depicted on the right hand side.