Modeling and analysis of spider orb webs under impact loads

Abstract

Over the past several decades, there have been tremendous theoretical, experimental and computational works towards better understanding how spider orb webs work. Part of the underlying physics and biology knowledge are being gradually revealed. However, a thorough investigation on the multi-scale modeling and dynamic analysis covering the microstructure of silk fibers and the architecture of silk assembly is still lacking. The objective of this dissertation is to fill this gap. The contributions are as following: 1. A new constitutive model for spider dragline silk is proposed and compared to other models. Spider dragline silk is composed of a dominant soft amorphous phase (α-phase) and a reinforcing hard crystal phase, which is classified as rigid β-sheets and extremely soft β-spirals. β-sheets result in the Mullins effect whileβ-spirals produce plastic deformation. In addition to these effects, the damping effect due to finite viscoelasticity is accounted for by using a double network model. It is found that the proposed model accurately captures the complex mechanical behavior of spider dragline silk subjected to cyclic loading with merely 7 "physically-based" material constants. 2. To better understand the prey capture process, a multi-scale modeling approach is proposed to uncover the underlying energy dissipation mechanisms. Simulation results show that the microstructures of spider dragline silk play a significant role on energy absorption during prey capture. The alteration of the microstructures, material internal friction, and plastic deformation lead to energy dissipation, which is called material damping. In addition to the material damping in the micro-scale modeling, the energy dissipation due to drag force on the prey is also taken into consideration in the macro-scale modeling. The results indicate that aerodynamic drag plays a significant role when the prey size is larger than a critical size. 3. A finite element procedure for modeling and analyzing full orb webs is proposed. A full model of spider web is then developed and its dynamic response due to prey impact is investigated. The results show that dynamic response of the web is related to the amount of silk materials used in the web construction and may be little affected by the number of radial lines. The results also indicate that as the number of radial lines increases the displacement of the web with a spider at its center reduces exponentially. The effects of pre-stretch on the dynamic response of web showed that as the pre-stretch increases, the web could be subjected to the over shoot when impacted by a prey. Furthermore, the effects of damage (missing radial line) on the web on its dynamic response are investigated. The results show that while the prey-impacted radial line transmits the largest force to the spider for undamaged web, for damaged web, other fibers transmit larger force to the spider. The frequency of peak force that is inserted to the spider also shifts to the lower frequencies for damaged web compared to intact web. This may provide a signaling mechanism to spider that its web is damaged and should be repaired--Author's abstract