Characterisation of the nonlinear viscoelastic material behaviour of ETFE and ECTFE foils for membrane structures

作为一个总括术语，膜结构包括由本申请中涉及的技术纺织品和技术箔制成的结构。两者的自重都非常低，这意味着它们用于屋顶区域和立面中的宽跨度、无支撑的表面。到目前为止，只有一种薄膜被用于薄膜结构：由乙烯-四氟乙烯(简称ETFE)制成的薄膜。ETFE铝箔结构是一种相对年轻的建筑类型。另一种可能的箔材料是乙烯-三氟氯乙烯，简称ECTFE，它还没有用于膜结构。ETFE薄膜在单轴和双向拉伸试验中都表现出非线性粘弹性材料行为。然而，目前，对于已经使用的箔材料ETFE和新的箔材料ECTFE，在不同边界条件下的力学行为的基本知识都是缺乏的。这项建议的中心研究目标是弥合关于机械材料行为的表征和模型的知识的基本空白。因此，必须从根本上研究和记录ETFE和ECTFE箔在单轴和双向短时拉伸试验中的应力-应变行为，包括所有材料参数(弹性模数、泊松比、屈服应力和应变、拉伸强度、断裂伸长率等)。此外，这两种类型的铝箔的单轴和双轴蠕变和松弛行为将在长期拉伸试验中进行研究。由于膜结构总是多轴张拉，双轴材料的性能对设计起着决定性的作用。考虑到材料厚度、试件几何形状、温度、应变速率、双轴应力状态下的应力比和加载历史等参数的影响，为了实现研究目标，本文对材料的拉伸加载行为(单轴/双轴)和蠕变和松弛行为(单轴/双轴)进行了详细的研究。包括来自不同制造商的材料。根据试验结果，确定了同向碎屑、反碎屑和平面结构作为质量特征的特征值和设计相关的强度值。建立了描述单轴和双轴状态下的非线性粘弹性应力-应变以及蠕变和松弛行为的解析模型。所开发的方法和模型还将在现有的大型构件试验台上进行大规模构件试验，并通过数值有限元参数研究进行推广。

As an umbrella term, membrane structures comprises structures made both of technical textiles and technical foils dealt with in the present application. Both have a very low dead weight, which means that they are used for wide, unsupported spans of surfaces in the roof area and in the façade. So far, only one type of film has been used for membrane structures made of foils: foils made of ethylene-tetrafluoroethylene, ETFE for short. ETFE foil stuctures are a comparatively young type of construction. Another possible foil material is ethylene-chlorotrifluoroethylene, or ECTFE for short, which has not yet been used in membrane structures. ETFE foils exhibit nonlinear viscoelastic material behavior in both mono- and biaxial tensile tests. At present, however, basic knowledge of the mechanical behaviour under varying boundary conditions such as temperatures or load speeds is lacking both for the already used foil material ETFE and for the new foil material ECTFE.The central research objective of this proposal is to close fundamental gaps in knowledge regarding the characterisation and modelling of mechanical material behaviour. Therefore, the stress-strain behaviour of ETFE and ECTFE foils in mono- and biaxial short-time tensile tests has to be fundamentally studied and recorded, including all material parameters (moduli of elasticity, poisson’s ratios, yield stresses and strains, tensile strengths, elongation at break, etc.). In addition, the mono- and biaxial creep and relaxation behaviour of both types of foil are to be investigated in long-term tension tests. Since membrane structures are always multi-axially tensioned, the biaxial material behaviour is decisive for the design. Since biaxial material characterization is associated with considerable effort, one research goal is also to develop a correlation between the mono- and biaxial material behaviour and to describe it analytically.In order to achieve the research objectives, both the tensile load behaviour (mono-/biaxial) and the creep and relaxation behaviour (mono-/biaxial) are examined in detail, taking into account the influencing parameters of material thickness, specimen geometry, temperature, strain rate, stress ratio in the biaxial stress state and load history. Materials from different manufacturers are included. Characteristic values as quality features and strength values as design relevant values for synclastic, anticlastic and plane structures are determined on the basis of the experimental results. Analytical models to describe the nonlinear, viscoelastic stress-strain as well as the creep and relaxation behaviour in the mono- and biaxial state are developed. The developed methods and models will also be validated in large scale component tests on an existing large component test stand and extended by numerical FE parameter studies.