ABSTRACT of the Doctoral Thesis

Mechanical Simulation of Slit Sensors of Arachnids

by Bernhard Hößl (2007)

In the present thesis the intriguing mechanics of strain sensitive arachnid slit sensilla are studied. Individual slits occur singly, in loose groups, and in so called lyriform organs, where they are arranged in close neighborhood. The emphasis of this work is put on the complex deformation patterns of lyriform organs under mechanical far field loading, i.e., on the role of interaction effects, namely shielding and amplification, between the slits, and on the effects of details of the slits' morphology and the three dimensional shape of the cuticle at the site of the organs on the slits' deformation. Two modeling approaches are employed, an analytical method developed within the framework of fracture mechanics, Kachanov's method, and the Finite Element (FE) method. The results of these simulations are slit face displacements evaluated along the individual slits and at discrete points along the slits' faces as well as stress and strain fields in the vicinity of the slits.

The accuracy of Kachanov's analytical approximations for planar arrangements of slits is assessed by comparisons with results obtained by Finite Element analysis. The limits of the applicability of Kachanov's approximation to slit sensilla are found to be reached when the lateral spacing between interacting slits is less than half their length, i.e., the method is suitable for studying single slits and loose groups but not lyriform organs.

For studying more closely spaced slits the FE method is used. This method is not subject to intrinsic limitations in the closest neighboring distance between the slits, allows a wide variety of slit shapes to be studied, and can be easily extended to three dimensional configurations.

In a first step the influence of the geometrical parameters describing single isolated slits, such as aspect ratio, slit shape, geometry of the slits' centerlines, and slit orientation, on the slit deformation is analyzed. In arachnids slits can have a C- or S-shaped centerline but are usually straight. Insects show ellipse-like openings of low aspect ratio for their strain sensitive campaniform sensilla rather than slits with parallel faces and rounded ends. In a next step the directional response of generic planar arrangements of five slits similar to those studied with Kachanov's method, i.e. non-staggered, oblique bar, and triangular arrangements, is investigated for lateral distances as found in lyriform organs.

Lyriform organs are generally highly developed slit arrangements with variations in slit width, shape of the slits' centerline, orientation and length of the individual slits. The influence of these variations on the directional responses of the arrangements is investigated via planar slit formations based on actual lyriform organs of the spider Cupiennius salei and the bird spider Aphonopelma. For the case of the organ HS8 located on the tibia of C. salei the accuracy of the predictions of the slit deformation of the planar FE model is assessed by comparison with measurements obtained by white light interferometry.

Finally we extend the FE models to the third dimension and use structural models to study the influence of the cross section and the stiffness of the membranes as well as the material parameters of the slits' filling on the slit deformation. In addition three dimensional shells explore different three dimensional shapes of the regions of cuticle in which the slits are situated, especially global and local curvature effects. Series of numerical experiments use flat and cylindrical regions some of which contain local features in the form of dimples and ridges onto which single slits and generic arrangements of slits were placed.

A section of this thesis is devoted on how slits might be arranged in bio-inspired micro strain sensors.

revised 070417 (hjb)