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

作为一个总括性术语，膜结构包括由本申请中处理的技术纺织品和技术箔制成的结构。两者都有一个非常低的自重，这意味着它们用于在屋顶区域和立面的宽，无支撑的表面跨度。到目前为止，只有一种薄膜被用于由铝箔制成的膜结构：乙烯-四氟乙烯制成的薄膜，简称ETFE。ETFE箔结构是一种相对年轻的结构类型。另一种可能的箔材料是乙烯-三氟氯乙烯，简称ecfe，它尚未用于膜结构。ETFE箔在单轴和双轴拉伸试验中均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ecfe，缺乏在不同边界条件下（如温度或加载速度）的机械行为的基本知识。本提案的中心研究目标是缩小关于机械材料行为的表征和建模的知识的基本差距。因此，ETFE和ECTFE箔在单轴和双轴短时间拉伸试验中的应力-应变行为必须进行基本的研究和记录，包括所有材料参数（弹性模量、泊松比、屈服应力和应变、抗拉强度、断裂伸长率等）。此外，两种类型的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拉伸，双轴材料的行为是决定性的设计。由于双轴材料的表征需要相当大的努力，一个研究目标也是发展单轴和双轴材料行为之间的相关性，并对其进行分析描述。为了实现研究目标，在考虑材料厚度、试样几何形状、温度、应变率、应力比在双轴应力状态和载荷历史等影响参数的情况下，对拉伸载荷行为（单/双轴）和蠕变松弛行为（单/双轴）进行了详细的研究。包括来自不同制造商的材料。根据试验结果确定共裂、抗裂和平面结构的质量特征值和强度设计相关值。建立了描述非线性粘弹性应力-应变以及单轴和双轴状态下蠕变和松弛行为的分析模型。所开发的方法和模型还将在现有大型部件试验台的大型部件试验中进行验证，并通过数值有限元参数研究进行扩展。

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.