Mechanical Engineering MS Thesis Defense By Ayşe Su Giz

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

MECHANICAL ENGINEERING

MS THESIS DEFENSE BY AYŞE SU GİZ

Title: Characterization Of Tensile And Relaxatıon Properties Of Sodium Alginate Films

Speaker: Ayşe Su Giz

Time: August 10, 2018, 14.00

Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Assoc. Prof.  B. Erdem Alaca (Advisor, Koç University)

Assoc. Prof. E. Murat Sözer (Koç University)

Assoc. Prof. Halil Bayraktar (Istanbul Technical University)

Abstract:

Mechanical behavior of alginate films plays significant roles in many applications, hence good understanding and control of its mechanical properties are necessary for optimal fulfillment of these functions. In this work stress-strain and relaxation behavior of alginate films were investigated in tensile tests.

In the first part tensile tests were performed on alginate films with different glycerol and calcium chloride concentrations. For tensile testing, strain was applied to the samples with constant speed until failure. Elastic modulus, ultimate strain and tensile strength values were obtained for each film composition. It was found that, calcium chloride and glycerol significantly affected elastic modulus, tensile strength and ultimate strain. The highest strength and strain values obtained when both calcium chloride and glycerol concentrations were at moderate concentrations and consequently a saturation point was seen for both parameters

In the second part, the viscoelastic response to increasing glycerol was examined in relaxation tests. For relaxation tests, the samples were subjected to a step displacement, and the resulting stress was monitored for 9 h during relaxation process. The experiments were performed in two different relative humidity’s, 38∓1% and 51∓1%. It was found that increasing glycerol and humidity decreased the final normalized stress of the films. Moreover, humidity had a synergic effect on the glycerol concentration. The relaxation behavior was modeled with three different models,1- three Maxwell elements, 2- two Maxwell elements plus an isolated spring, 3- a stretched exponential Kohlrausch-Williams-Watts model plus an isolated spring.

The correlation of strain and cross-head displacement was acquired through finite element modeling and image processing. For this purpose, a model of 9319 tetrahedral elements was constructed with 61970 degrees of freedom. The crosshead displacement was imposed as a boundary condition, and the corresponding strain was computed. The strains computed with finite element modeling were verified through image processing, where the paths of surface irregularities were tracked from consecutive images obtained during the experiments.