Micromechanics of a Bonded Non-Woven Glass Fiber Material
Bonded non-woven fabrics are porous engineering materials in which disordered fibers are consolidated by a polymeric binder. The bonded non-woven material under investigation is used for thermal and acoustical insulation purposes. The microstructure of such materials is tailored to minimize thermal conductivity. As external mechanical loadings disturb the microstructure, the mechanics of thermal insulation materials are of relevance to ensure the long-term effectiveness of insulation. The deformation of non-woven materials is known to be non-affine. Classic continuum mechanics theory is often not applicable. The difficulty in predicting the mechanical behavior of such materials arises from the heterogeneous microstructure and convoluted deformation mechanisms. It results in a lack of fundamental understanding in the mechanics of such a material. There are also industrial needs of quality control measures. This thesis focuses on 3D modeling and characterization with automated material phase separation, deformation and damage micromechanism in compression and notch effect in tensile failure of the bonded non-woven material. Major contributions have been achieved through experimentation and modeling. Firstly, 3D microstructure models with the binder phase effectively separated from the finder and porosity were established. With the realistic microstructural representation, characteristics of the microstructure are accurately measured. The mechanical response in monotonic and cyclic compression was investigated. Computations based on realistic 3D models reveal formations of fiber chains and fiber-fiber contact to accommodate compression. Therefore, both the compressive modulus and the degradation of the modulus due to cyclic loading were found to be dependent on the modulus of the binder phase. Acoustic emission analysis further enables to quantitatively relate fiber fracture events in the microstructure to the specimen damage state. In uniaxial tension, failure of notched and unnotched specimens of the bonded non-woven exhibits a notch effect, in which the net section strength increases with the depth depth. It results from the long-range interaction of fiber chains throughout the whole specimen domain. From a practical standpoint, this thesis successfully demonstrates the application of synchrotron CT system to multi-phase material systems and the acoustic emission technique to monitor the internal damage in the material system. In addition, two key mechanical characteristics are proposed to characterize mechanical performance. Specifically, the role of the binder phase in this bonded non-woven material is highlighted. Investigations on the failure of notched specimens provide insights in the context of material design with non-affinity.
Siegmund, Purdue University.
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