Studies of Auxetic Foam and Bridge Deformation Using Digital Speckle Photography

Abstract

The mechanical properties of auxetic (negative Poisson's ratio) foam are studied and reviewed with different experimental methods. Developments in design, manufacture, test and analysis of auxetic foam and related composites are presented. Auxetic foam can be made by open-cell conventional PVC foam, through volumetric compressing and heating process successively. When cooling down to room temperature, the cellular micro structure of PVC foam has been turned out to be re-entranted, which means the ribs of each cell of the foam are protrude inward permanently due to the external mechanical and thermal effects. The new re-entranted structure opens out many interesting and meaningful values. In this thesis, the most updated research in auxetic foam is studied experimentally. Stress-strain relationship and energy dissipation properties are presented through uniaxial loading and cyclic loading tests; force-deflection relationship and shear failure resistance behaviors are carried out through three points bending test of sandwich beam with auxetic foam core. Meanwhile, compared with conventional foam, advantages of auxetic foam are presented in several aspects as well. Additionally, a static deflection analysis of a wood bridge is analyzed by DSP and some correlation results are presented in order to do research of aging infrastructures. A truss bridge is structured with commercially available wood planks made of pine. One facet surface of the bridge is painted with a thin coat of retro-reflective paint and illuminated by two 200w flood light. The painted surface of the bridge was photographed before and after the application of static load. Each digital image of the bridge is subdivided into sub-images of 128 128 pixel arrays and then"compared" in CASI. The result of a displacement vector map of the entire bridge is verified by Finite Element Method. At last, the strain distribution of main beam is shown and compared with the values of strain gages. The correlation results show that the DSP technique and CASI methods are suitable for the study of these two projects mentioned above. Some future's works also listed. An optical technique called Digital Speckle Photography (DSP) is used as the analyzing method to measure 2-D or 3-D displacement and strain fields in micro or macro-scale, through which an entire structure deformation pattern and strain distribution from a small foam surface to a big bridge structure facade can be mapped simultaneously in a single step. DSP is an easy setup and the data can be obtained in semi-real time. The displacement of the random features and structures that are often present on many engineering surfaces when viewed in a microscope or CCD camera are measured with the DSP system, followed by an image correlation method. Computer Aided Speckle Interferometry (CASI). In this thesis, the background and the application of the DSP and the algorithms of CASI are introduced.