The 2nd Joint International Conference on Multibody System Dynamics May 29–June 1, 2012, Stuttgart, Germany An energy-momentum scheme for frictional contact in flexible multibody dynamics. Marlon Franke, Christian Hesch, Peter Betsch Chair of Computational Mechanics University of Siegen Paul-Bonatz-Str. 9-11, 57068 Siegen, Germany [marlon.franke, christian.hesch, peter.betsch]@uni-siegen.de ABSTRACT The present contribution deals with the frictional contact of flexible bodies. In particular we investigate the time integration of dynamic contact problems with large deformations. The overall aim is to compare standard with recently developed integrators. To this end we propose an energy and momentum consistent integrator which exhibits superior numerical stability. 1 INTRODUCTION The aim of our work is to develop stable integrators for transient large deformation contact problems (see e.g. Reference [13]). Using the widely-used node-to-surface method, where we enforce the constraints by Lagrange multipliers, we obtain a unified system of index-3 differential-algebraic equations (DAE). Within this common framework we are able to incorporate structural elements such as shells, beams, continua as well as rigid bodies for which we use a rotationless formulation (see References [12, 11]). For the underly- ing DAE’s even second order implicit integrators like e.g. the midpoint rule or the Newmark method fail to deal in a consistent way with various physical properties of the system, resulting in an unstable numerical behavior of the system. To avoid this disadvantage we apply an energy-momentum scheme, which has al- ready been investigated in the context of nonlinear elastodynamics (see References [2, 4]). In the context of contact mechanics similar consistent integrators have been applied previously (see References [1, 10]), vio- lating the non-penetration condition. The present work builds on the previous works (see References [3, 6]). 2 Continuum and contact mechanics In order to simplify the notation for frictional multibody contact problems we restrict our consideration on a two body contact problem (see Figure 1). Therefore the bodies are depicted in the reference configuration (i) R3 (i) R3 B0 ⊂ ,i ∈ [1, 2] and in the current configuration Bt ⊂ ,i ∈ [1, 2], respectively. The material boundaries of the bodies can be decomposed into the Neumann boundary Γn, the Dirichlet boundary Γd and in the contact surface Γc. The spatial counterparts of the boundaries are denoted with γn, γd and γc. In this connection we presume that the boundaries satisfy: (i) (i) (i) (i) (i) (i) (i) (i) (i) (i) ∂B0 =Γn ∪ Γc ∪ Γd and Γn ∩ Γc =Γn ∩ Γd =Γc ∩ Γd = ∅ (1) Any material configuration of the bodies can be addressed via X(i) and we obtain the spatial counterpart using the mapping ϕ(i) X(i) . We assume that the bodies come in contact within the considered time interval t ∈ I := [0,T ]. The balance of linear momentum with respect to the reference configuration reads (i) ¨(i) (i) (i) ρ0 ϕ = DIV(P )+ B (2) (i) (i) ¨(i) where B denotes the body force per reference volume, ρ0 ϕ denotes the inertia force term corre- (i) (i) sponding to the i-th body B0 using the reference density ρ0 and P denotes the first Piola-Kirchhoff stress tensor, which depends on the considered constitutive law described via the strain energy function W (i). For the initial/boundary value problem (see Equation (2)) we have to take the boundaryand the initial conditions (1) Γn (1) ϕ(1) (1) (1) B0 Bt γn (1) Γd (1) γd (1) (1) (1) ϕ (X ,t) Γc (1) (1) γc X ϕ(2)(X(2),t) (2) ei,i =1, 2, 3 (2) (2) X (2) γc B0 Γc (2) Bt Γ(2) d ϕ(2) (2) (2) γd Γn (2) γn B(i) B(i) Figure 1. Configurations of two body contact ( 0 : bodies in the reference configuration, t : bodies in the current configuration) into account. Hence the boundary conditions are given as (i) ¯ (i) (i) ϕ = X in Γd ∀ t ∈ I (3) (i) (i) (i) ¯(i) (i) T = P N = T in Γn ∀ t ∈ I (4) (i) (i) (i) ¯ (i) where T denotes the Piola traction vector and N denotes the unit outward normal of Γn . X and T¯(i) denote the prescribed displacement and traction respectively. At last we provide the initial conditions (i) (i) (i) ϕ (t =0)= X0 in B0 (5) ˙ (i) (i) (i) ϕ (t =0)= V0 in B0 (6) In order to deduce the contribution of each body to the weak form of the boundary value problem we define the solution space (i) (i) 1 (i) (i) ¯(i) (i) Vs = {ϕ ∈ H (B ): ϕ = ϕ on Γd } (7) and the space of test functions (i) (i) 1 (i) (i) 0 (i) Vt = {δϕ ∈ H (B ): δϕ = on Γd } (8) where the Sobolev space H1(B(i)) consists of the spaces of square integrable functions and their first derivatives. The weak form for the problem at hand reads =:G(i),dyn =:G(i),int (i) ¨(i) (i) (i) (i) ρ0 ϕ · δϕ dV + P : Grad(δϕ ) dV B(i) B(i) 0 0 (i) ¯(i) (i) ¯(i) (i) (i) (i) (i) (i) = ρ0 B · δϕ dV + T · δϕ dA + t · δϕ dA ∀ δϕ ∈Vt (9) B(i) Γ(i) Γ(i) 0 n c =:G(i),ext =:G(i),c As indicated in Equation (9) we obtain the virtual work for the considered bodies: 2 G (ϕ,δϕ)= G(i),dyn + G(i),int − G(i),ext − G(i),c =0 (10) i=1 Additionally we postulate that the local momentum balance on the contact boundary −t(2) dA = t(1) dA (11) holds. Thus, the contact contribution to the virtual work reads 2 Gc = G(i),c = t(1) · δϕ(1) − δϕ(2) dA (12) (1) i Γc The fundamental balance principles, namely the balance laws for the linear momentum L, the angular momentum J as well as the total energy E of the system should hold during the considered time interval I as depicted in Table 1, where F˜(i) denotes the resultant forces, M˜ (i) the resultant momenta and P (i),ext the resultant external power. In case of a conservative problem (e.g. frictionless contact and pure stick contact), the fundamental balance principles are maintained. Balance principles ˙ 2 ˜(i) Linear momentum: L = i=1 F Angular momentum: J˙ = 2 M˜ (i) i=1 Energy: E˙ 2 T˙ (i) V˙ (i) 2 P (i),ext = i=1 + = i=1 B(i) Table 1. Balance principles for bodies 0 For detailed derivation of the balance principles like the linear and angular momentum as well as the total energy of the system we refer to Reference [7]. 3 Contact description (1) Bt (1) (1) X (1) B0 ϕ B(2) 0 ¯ (2) ξ2 X ei ϕ¯(2) a2 n a1 (2) Bt ξ1 Figure 2. Parametrization of the contact surfaces For the contact description we use the so-called direct approach (see Reference [9]) but also other descrip- tions like e.g. the covariant approach (see Reference [8]) are suitable. For further considerations we use the (1) (1) well-known slave-master concept (see Reference [5]). To this end we determine X ∈ Γc and its spatial (1) (1) (1) counterpart ϕ (X ,t) ∈ γc as the slave node, which should be prohibited to penetrate the opposing (2) (2)∗ master surface Γc rather than its spatial counterpart γc . Using the closest point projection min ϕ(1)(X(1)) − ϕ(2)(X(2)) (13) we obtain ϕ(2)(X¯ (2)(X(1))) with closest distance to the slave node ϕ(1)(X(1)). In the following we ne- glect the arguments of the slave point ϕ(1) := ϕ(1)(X(1)) and its corresponding master point ϕ¯(2) := ϕ(2)(X¯ (2)(X(1))). The master surface corresponds to a 2-D manifold which we parametrize using convec- tive coordinates ξ1, ξ2 (see Figure 2). Therefore the aforementioned master node is characterized by ϕ¯(2) := ϕ(2)(ξ¯1, ξ¯2), X(2) := X(2)(ξ¯1, ξ¯2) (14) where ξ¯1, ξ¯2 denote the projection point corresponding to X¯ (2). Thus, we are able to define the spatial bases for the projection point moving with the slave point ϕ(1) (2) ¯1 ¯2 aj := ϕ,ξj (ξ , ξ ) (15) In this connection we define the gap function g = n · ϕ(1) − ϕ¯(2) (16) (2) (2) where n denotes the unit outward normal to γc at ϕ¯ . With the bases for the projection point at hand we introduce the outward unit normal a × a n = 1 2 (17) a1 × a2 The contact traction can be subdivided into a normal and a tangential part due to their different physical behaviour in the different directions: (1) t := t = tN n + tT (18) The normal contact conditions, well-known as the Karush-Kuhn-Tucker conditions, are given by g ≥ 0 (19) tN = t · n ≤ 0 (20) tN g =0 (21) For frictional contact we further have to incorporate tangential contact conditions in terms of a frictional constitutive law. Here we make use of Coulomb’s friction law, for which the tangential contact conditions can be stated as (see Reference [9]): φ = tT − µtN ≤ 0 (22) tT vT = −ζ (23) tT ζ ≥ 0 (24) ζ φ =0 (25) where vT denotes the tangential velocity and ζ denotes the consistency parameter. Finally we obtain the virtual contact work decomposed into the normal and the tangential part via c j (1) ¯(2) G (ϕ,δϕ)= tN n + tTj a · δϕ − δϕ dA (26) (1) Γc ¯j = tN δg + tTj δξ dA (27) (1) Γc ∗In the continuous setting the choice whether X(1) or X(2) is defined as slave node is arbitrary.