Non-linear and time-dependent viscoplastic material properties of ETFE films as membrane elements with multiaxial stress states in civil engineering

在膜结构中，由乙烯基聚乙烯（ETFE）制成的箔主要用于着名建筑物的屋顶或立面系统，因此它们以其高透明度和低自重给人留下深刻印象，从而实现大的无柱跨度。在ETFE箔结构设计中，由于施加载荷而在ETFE箔中产生的多轴应力被简化为双轴对齐的正交主应力。虽然多轴承载性能是决定性的，但设计是基于单轴确定的材料特性。这就导致了材料性能和结构承载性能方面的方法不足。为了应对这种情况，需要研究ETFE箔在多轴应力条件下的材料和承载行为。目前，只有双轴材料行为是通过十字形试样确定的，因此，由于开槽十字形试样的几何形状总是一个试样，没有材料失效发生，实际的整体双轴材料行为无法确定。在STR 482/6-1中，申请人能够证明ETFE箔的多轴、非线性粘弹到粘塑性材料行为不能通过双轴试验台确定，但必须在多轴试验台中进行研究。在初步的多轴试验中，可以表明，在多轴应力状态下，与单轴应力状态相比，发生不可忽略的材料强化和硬化效应。本计画的目的是分析ETFE薄膜的黏塑性材料特性，以及在多轴简单与循环荷载下的强化与刚化特性。为了扩大ETFE薄膜在建筑业膜结构中的应用领域，并能够更安全地同时更经济地设计这些薄膜结构，需要确定材料储备。在这种情况下，目前的结构形式-反曲和同向-以及不同的应变率，不同的测试温度，不同的箔厚度和箔产品的ETFE箔的多轴承载性能的影响进行了调查。在STR 482/6-1中建立的描述单轴和双轴材料行为的分析模型、相关性和方法将在将要进行的多轴试验和数值参数研究的基础上进一步发展，以便能够对多轴粘塑性材料行为进行分析建模，可以考虑由于循环载荷引起的材料强化和饱和效应。

In membrane structures, foils made of ethylene tetrafluoroethylene (ETFE) are mostly used on prestigious buildings as roof or façade systems, whereby they impress with their high translucency and low dead weight, enabling large, column-free spans. In the design of ETFE foil structures, the multi-axial stresses in the ETFE foils due to the applied loads are simplified into biaxially aligned orthogonal main stresses. Although the multiaxial load bearing behaviour is decisive, the design is based on monoaxially determined material properties. This leads to deficient approaches with respect to the material properties and the load bearing behaviour of the structure. To counteract this situation, the material and load bearing behaviour of ETFE foils in multiaxial stress conditions needs to be investigated. Currently, only the biaxial material behaviour is determined by means of cross-shaped specimens, whereby due to the slotted cross-shaped specimen geometry always a specimen and no material failure occurs and the actual overall biaxial material behaviour cannot be determined. In STR 482/6-1, the applicant was able to show that the multiaxial, non-linear viscoelastic to viscoplastic material behaviour of ETFE foils cannot be determined by biaxial test rigs, but must be investigated in multiaxial test rigs. In preliminary multiaxial tests it could be shown that in the multiaxial stress state, compared to the monoaxial stress state, non-negligible material-strengthening and -stiffening effects occur. The aim of this project is to analyse and characterise the viscoplastic material behaviour of ETFE foils as well as the strengthening and stiffening behaviour in multiaxial simple and cyclic loading. Material reserves are to be identified in order to expand the fields of application of ETFE foils in membrane structures of the building industry and to be able to design these foil structures more safely and at the same time more economically. In this context, the current structural forms - anticlastic and synclastic - as well as influences from varying strain rates, varying test temperatures, different foil thicknesses and foil products on the multiaxial load bearing behaviour of ETFE foils are to be investigated. The analytical models, correlations and methods for the description of the mono- and biaxial material behaviour established in STR 482/6-1 are to be further developed on the basis of multiaxial tests to be carried out and numerical parameter studies, so that the multiaxial, visco-plastic material behaviour can be modelled analytically and material hardening, material strengthening and saturation effects due to cyclic loads can be taken into account.