Membranes are stiff under tension but switch over to wrinkling when compression occurs. Roddeman proposed a kinematic model to handle this phenomenon under finite deformation conditions. The wrinkling conditions of Roddeman are transformed into the reference configuration. This results in a more simple nonlinear formulation. For application in a finite element code a consistent linearization was carried out. Numerical examples for linear and nonlinear orthotropic constitutive equations are discussed.
IntroductionMembrane structures are widely used. Airships, sails and airbags are examples of such structures. Therefore a growing interest of numerical simulation of membranes exists.Usually numerical analysis is performed by means of the Finite Element Method. We make use of a FE-formulation described in [1]. Thereby a spatial membrane is related to a plane reference (dressmaker's pattern). A nonlinear FE algorithm requires the determination of the state of stress in the integration points of each element. How to determine the state of stress in an integration point in case of wrinkling is the topic in this paper.Membranes are very suitable for carrying tensile loads, but they fail partially or completely if compressive in-plane loads occur. Since membranes do not possess any flexural stiffness they can not carry compressive in-plane loads. In this case membranes wrinkle. The wrinkling phenomenon was treated by authors in the past. In this field pioneer work was done by Reissner [2] in 1938, followed by many authors as for instance Mansfield [3], Wu [4], Miller, Hedgepeth [5] and Pipkin [6]. This paper is based on the work of Roddeman [7], who propose a kinematic approach. Taenzer gave a formulation of this kinematic model, suitable for spatial membrane elements with a plane reference (see [1] and [10]). However, Roddeman and Taenzer elaborated the theory in terms of the Cauchy stress tensor. As these formulations relate to the deformed configuration, their application in a FE-code can be quite complicated. Seokwoo and Seyoung [8], Lu et al. [11] and Schoop et al. [12] formulated the wrinkling conditions by means of the 2nd Piola-Kirchhoff stress tensor.In this paper the wrinkling conditions of Roddeman are transformed into the reference configuration. This results in simpler nonlinear equations, valid for linear and nonlinear anisotropic elastic materials. For application in a FE algorithm consistent linearization was performed.
Different states of a membraneThere are three possibilities: A membrane or regions of a membrane can be taut, regularly wrinkled or slack. In a FE-calculation we decide this question in all integration points. Some authors used stress based criterions to determine in which state the membrane is, other used strain based criterions. We in this paper use a mixed stressstrain-criterion proposed in [11] for linear material behaviour.A membrane is taut when both the principal stresses are positive. We use the usual membrane theory. If a membrane is not taut, it is regularly wrinkled if one princi...