The first proper paper from the ‘Structure and forces in orb webs’ project is out in the Journal of Experimental Biology. In this paper we measure web geometry and silk material properties, and use these, together with published data on pre-stress, to build a simple finite element model in ABAQUS. The model confirms that the zigzag shape of the Nephila non-sticky spiral is caused by its high pre-stresses and predicts these to be 1-2 times higher than the pre-stresses in the radii.
Abstract
Detailed information on web geometry and the material properties of the various silks used enables the function of the web’s different structures to be elucidated. In this study we investigated the non-sticky spiral in Nephila edulis webs, which in this species is not removed during web building. This permanent non-sticky spiral shows several modifications compared with others, e.g. temporary non-sticky spirals – it is zigzag shaped and wrapped around the radial thread at the elongated junctions. The material properties of the silk used in the non-sticky spiral and other scaffolding structures (i.e. radii, frame and anchor threads) were comparable. However, the fibre diameters differed, with the non-sticky spiral threads being significantly smaller. We used the measured data in a finite element (FE) model of the non-sticky spiral in a segment of the web. The FE analysis suggested that the observed zigzag index resulted from the application of very high pre-stresses to the outer turns of the non-sticky spiral. However, final pre-stress levels in the non-sticky spiral after reorganisation were down to 300MPa or 1.5–2 times the stress in the radii, which is probably closer to the stress applied by the spider during web building.
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