Plane biaxial tensile test

Biaxial tensile test is a tensile testing in which the sample is stretched in two distinct directions. This technique is used to obtain the mechanical characteristics of anisotropic materials, such as composite materials, textiles, and soft biological tissues. There are three main types of biaxial tensile testing:

The scope of the test represents one of the most crucial aspects in developing the relative testing protocol. For foils and coated fabrics there are several areas of interest: the initial behaviour, the service behavior, the behavior at break, the long term behaviour (creep) and the dynamic behavior. The material response can be significantly different according to the loading condition considered.

A protocol on the initial behavior aims to investigate the material behaviour of the fabric at the early stages of the structure’s life span during the erection phase. The results represent a fundamental support for the compensation of the panels of fabric in order to refine the erection and pre-tensioning processes.

The service behaviour is another aspect of great interest because it represents the overall material response to load conditions occurring during the entire life span of the structure. The result of this type of test represents a fundamental input data for the software used for structural analysis and the determination of the stress distribution in the structure. The biaxial behaviour at breaking load is an important open issue that has not been investigated thoroughly until now. Previous researches demonstrate the difficulties concerning the rupture of a biaxial sample, it has been noted that generally the maximum biaxial tensile strength is lower than the corresponding ultimate tensile strength obtained by means of a monoaxial test. However, the test can be focused on the resistance of a joint, evaluating the strength of welded (high frequency welding, hot element welding), glued and sewn seams or the resistance of clamping plates and Keder rail joints or looped and laced joints.

Another field of research is the appearance and propagation of tears in the fabric, this issue has been partially investigated and represents a fundamental data in the determination of the safety factor which should be used for a specific structure. Because tear propagation generally occurs at 25% UTS, the safety factor is generally higher than four. The test is carried out on a sample taken from a dismounted structure or by using conditioned samples of fabric which aim to reproduce the in-situ conditions. The load profile should reproduce the expected overload by pulling the sample until breakage, which should start far from the sample edge and the clamping system. The repetition of the test and different temperatures (generally -20 °C and +70 °C) offers important information about the joint behaviour at extreme conditions, such as heavy snow fall with temperatures below zero or a blast of wind during a hot summer. This type of test is generally required by designers and manufacturers and those in charge of the evaluation of the structure behaviour and the final test of the structure. Finally, in order to prevent collapses due to ponding and fluttering, the creep behaviour should be carefully considered in the design, choosing anchorages which enable periodical re-tensioning according to the predicted fall in the level of pre-stress. It is based on a monoaxial test but an accurate investigation requires the use of virgin cruciform samples of fabric and the force is applied by means of counter weights. The creep is defined as the “increase in strain with time when a constant force is applied” and aims to describe the material behaviour when a constant force is applied over a long period of time. This has a considerable effect on the design and realisation of the membrane structure because a progressive increase in the material strain leads to a considerable reduction in the level of pre-stress initially induced in the structure. The sample should be maintained at a constant temperature and for specific applications it may require a proper climatic chamber for investigations at high and low temperatures. There are no complete studies about the dynamic behaviour of coated fabric and foils for structural applications. A dynamic test on coated fabrics can evaluate the response under fast loading and unloading cycles due to blasts of wind or other sources of stress. It should consider a conditioned fabric which reproduces the in-situ conditions and the testing apparatus should be able to apply a high speed load profile which is generally not possible with a common testing rig equipped with electric servomotors. The results of this type of test, despite the absence of research which can support these expectations, can highlight anomalies in the fabric strength and in the tear propagation with consequent adjustments in the safety factor applied.

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