Experimental investigation and constitutive modeling of randomly oriented electrospun nanofibrous membranes / Wong Dannee
The recent advancement of nanotechnology has enabled the fabrication of nanofibers through a number of processing techniques. Among these, electrospinning offers a unique ability to produce nanofibrous membranes for different materials and of different assemblies or textures that make them suitab...
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Format: | Thesis |
Published: |
2018
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Online Access: | http://studentsrepo.um.edu.my/9362/1/Wong_Danee.pdf http://studentsrepo.um.edu.my/9362/6/wong_dannee.pdf http://studentsrepo.um.edu.my/9362/ |
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Summary: | The recent advancement of nanotechnology has enabled the fabrication of nanofibers through
a number of processing techniques. Among these, electrospinning offers a unique
ability to produce nanofibrous membranes for different materials and of different assemblies
or textures that make them suitable for various applications including filtration, tissue
engineering, nanocomposites and textiles. In these applications, electrospun nanofibrous
membranes are frequently subjected to complex stresses and strains which could lead to
fiber failures. Therefore, the understanding of their mechanical properties becomes crucial
in order to facilitate the design and performance evaluation of the materials. In view of
probing the mechanical response of nanofibrous membranes, relevant experimental characterizations
are conducted such as atomic force microscopy (AFM), nanoindentation, nanotensile
tests or conventional tensile tests. These experimental techniques are often daunting,
costly and time-consuming. If a robust and cost-effective alternative method in evaluating
the mechanical properties of electrospun nanofibrous materials through numerical simulation
can be established, the strong dependence on experimental works can therefore be
significantly reduced. The present thesis focuses on the development of a simple constitutive
model with reduced number of material parameters for the mechanical response of
randomly oriented electrospun PVDF nanofibrous membranes. To this end, the thesis is
divided into two parts. The first part focuses on the experimental aspects that include the
fabrication of electrospun nanofibrous membranes using different sets of electrospinning
parameters and the characterization of their surface morphology. Subsequently, samples
obtained using the optimum set of parameters are chosen for further characterizations, i.e.
physical evaluation of undeformed and deformed membranes, mechanical testing and fiber orientation analysis. Three types of uniaxial mechanical tests are conducted: monotonic
tensile tests, cyclic loading tests with increasing maximum strain and cyclic-relaxation tests.
Results show that the material exhibits complex mechanical responses, which include
finite strain, irreversible deformation, hysteresis and time-dependent response. Furthermore,
fiber orientation analysis suggests that the material is initially isotropic in the plane
(transversely isotropic) and the deformation-induced fiber re-orientation takes place. The
second part of the thesis deals with the development of a constitutive model capturing the
observed responses. Motivated by the experimental observation, the model development
starts from the description of material response at fiber-scale in order to describe individual
fiber response and irreversible inter-fiber interactions using hyperelastic and large strain
elasto-plastic frameworks respectively. The macroscopic response of the membranes is subsequently
obtained by integrating the fiber responses in all possible fiber orientations. The
efficiency of the proposed model is assessed using experimental data. It is found that the
model is qualitatively in good agreement with uniaxial monotonic and cyclic tensile loading
tests. Two other deformation modes, i.e. equibiaxial extension and pure shear (planar
extension) are simulated to further evaluate the model responses. |
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