Şebnem Özüpek

A methodology for the structural assessment of Nickel-Titanium (Ni-Ti) endodontic files and a novel approach to predict their fatigue behavior using finite element method (FEM) were proposed. ProTaper-Universal F1 and F2 endodontic files were selected due to availability of extensive test data needed for the validation of the methodology. Bending and torsional loadings were analyzed since these provide essential data for the structural integrity assessment for the endodontic files. High-definition FEM models and their computationally efficient idealized versions were developed. The results for the bending and torsional stiffness of the F1 endodontic file agreed with the literature data validating the proposed methodology. Hysteresis energy density was shown to give promising results as a predictor of low cycle fatigue failure. The predictions with the idealized models matched those of the high-definition models, justifying the proposed idealizations. The validated models demonstrated that F2 has 60% higher bending and torsion resistance and 7% higher hysteresis energy density per cycle with respect to F1, leading to the conclusion that F1 has a lower structural stiffness but a longer fatigue life as compared to F2. In summary, the developed methodology allows for the structural and durability evaluation of various design parameters for Ni-Ti endodontic files.

The use of explicit finite element methods is practical for only a limited range of bulk modulus to shear modulus ratios. As a consequence, these efficient and robust methods have not found a large range of application in the analysis of rubber-like materials. On the other hand, the implicit finite element methods, which do not have a material compressibility limitation, fail to provide a solution to many challenging nonlinear rubber problems. A systematic investigation of the bulk modulus effect on the solution of a typical class of rubber problems is carried out with the aim of providing quantitative guidance for the use of explicit methods in the quasi-static analysis of nearly incompressible media with strong nonlinearities.

A three-dimensional stress analysis of a rocket motor under cyclic temperature loading was performed. The novelty of the analysis lies in the use of a propellant constitutive model that accounts for the effects of viscoelasticity, large deformation, temperature, pressure, and softening in monotonic and cyclic loadings. The material model was validated for the cyclic effects in terms of stress as well as dilatation responses. The role of various damage model parameters in the cyclic response was investigated. The distributions of these parameters were found to be consistent with the stress–strain field predictions of the rocket motor analysis, indicating efficiency and stability of the computational algorithm. It was concluded that the formulation and its implementation enables realistic stress analysis of solid rocket motors under general loading and environmental conditions, providing valuable information for their life assessment.

The study is concerned with the simulation of cracks in finitely deforming viscoelastic media. The main goal is to establish a computational methodology for crack propagation analyses in bulk material and interface debonding using an extended finite element method and cohesive zone modeling. First, a linear elastic plate with a single-edge crack and a linear viscoelastic plate with a double-edge crack are selected as benchmark problems. Upon verification of the extended finite element method and cohesive zone modeling results against those from the conventional crack propagation method and those from the literature, a nonlinear viscoelastic solid rocket motor subjected to various thermal loadings is analyzed for failure due to bore cracking or interface debonding. For a cooldown the stress distribution along the bond line is determined for the bore cracking mode and results are compared with those from the literature. For the cyclic temperature, the propagation of bore cracks is analyzed using an extended finite element method, and the propagation of initial debonding is studied using cohesive zone modeling. For both failure modes, crack propagation occurs only during the first cooldown. The relation between the crack growth and the initial crack size show parabolic behavior. Overall, it is concluded that the extended finite element method and cohesive zone modeling are suitable methods for crack propagation analysis in nonlinear viscoelastic media.