Steel fibers or fibers in general are utilized in concrete to control the tensile cracking and to increase its toughness. In literature, the effects of fiber geometry, mechanical properties, and volume on the properties of fiber reinforced concrete have often been experimentally investigated by numerous studies. Those experiments have shown that useful improvements in the mechanical behavior of brittle concrete are achieved by incorporating steel fibers. This study proposes a simulation platform to determine the influence of fibers on crack propagation and fracture behavior of fiber reinforced concrete. For this purpose, a finite element (FE) simulation tool is developed for the fracture process of fiber reinforced concrete beam specimens subjected to flexural bending test. Within this context, the objective of this study is twofold. The first one is to investigate the effects of finite element mesh size and element type on stress intensity factor (SIF) calculation through finite element analysis. The second objective is to develop a simulation of the fracture process of fiber reinforced concrete beam specimens. The properties of the materials, obtained from literature, and the numerical simulation procedure, will be explained. The effect of fibers on SIF is included by unidirectional elements with nonlinear generalized force-deflection capability. Distributions and orientation of fibers and possibility of anchorage failure are also added to simulation. As a result of this study it was observed that with the adopted simulation tool, the load-deflection relation obtained by experimental studies is predicted reasonably.

In today&#039 / s technology severe working conditions increase demands on structural materials. A class of materials which are developed to meet these increased demands is Functionally Graded Materials (FGMs). These are inhomogeneous structural materials which are able to withstand large temperature gradients and corrosive environment. Application areas of FGMs are in aerospace industry, nuclear reactors, chemical plants and turbine systems. FGMs have gradual compositional variation from metal to ceramic which give them mechanical strength, toughness and heat resistance. However under high temperature gradients, cracking problems may arise due to thermal stresses. In layered structures the final stage of failure may be delamination due to crack extension. The objective of this study is to model a particular type of crack problem in a layered structure consisting of a substrate, a bond coat and an orthotropic FGM coating. There is an internal crack in the orthotropic layer and it is perpendicular to material gradation of coating. The position of the crack inside the coating is kept as a variable. The steady-state temperature distribution between the substrate and the coating causes a buckled shape along crack face. The critical temperature change, temperature distribution, mixed mode stress intensity values and energy release rates are calculated by using Displacement Correlation Technique. Results of this study present the effects of geometric parameters such as crack length, crack position, etc as well as the effects of the type of gradation on buckling behavior and mixed mode stress intensity factors.

In this study finite element modeling of three dimensional elliptic and semielliptic cracks in a hollow cylinder is considered. Three dimensional crack and cylinder are modeled by using finite element analysis program ANSYS. The main objectives of this study are as follows. First, Ansys Parametric Design Language (APDL) codes are developed to facilitate modeling of different types of cracks in cylinders. Second, by using these codes the effect of some parameters of the problem like crack location, cylinder&rsquo / s radius to thickness ratio (R/t), the crack geometry ratio (a/c) and crack minor axis to cylinder thickness ratio (a/t) on stress intensity factors for surface and internal cracks are examined. Mechanical and thermal loading cases are considered. Displacement Correlation Technique (DCT) is used to obtain Stress Intensity Factors.

A phenomenological cohesive term is proposed and added to an existing cohesive constitutive law (by Roy and Dodds) to model the crack tip high inertia region proposed by Gao. The new term is attributed to fracture mechanisms that result in high energy dissipation around the crack tip and is assumed to be a function of external energy per volume input into the system. Finite element analysis is performed on PMMA with constant velocity boundary conditions and mesh discretization based on the work of Xu and Needleman. The cohesive model with the proposed dissipative term is only applied in the high inertia zone i.e., to cohesive elements very close to the crack tip and the traditional Roy and Dodds model is applied on cohesive elements in the rest of the domain. It was observed that crack propagated in three phases with a speed of 0.35cR before branching, which are in good agreement with experimental observations. Thus, modeling of high inertia zone is one of the key aspects to understanding brittle fracture.

Fracture of polymer-metal interfaces is one of the main failure modes occurring in micro-electronic components. This phenomenon is particularly true when considering the delamination of several layers of an interconnect structure. In order to predict the failure nucleation and the crack propagation into the composite material, the finite element analysis is one of the key procedures. Even though simple linear models have been considered for years, we are now facing the necessity of using more complex models including non-linearity which can occur, in this case, with the presence of high local stresses near the crack front. However, the computational time can sometimes be incredibly long. Moreover, the fact that the considered materials are quasi-brittle brings some numerical difficulties such as sharp snap-back and snap-through. The actual challenge resides in obtaining a reliable result in a reasonable time of calculation. The present work considers the implementation of a new non-linear finite element solver, developed for the MSc. Marc/Mentat package software. It is based on a general arc-length constraint which considers the energy released during the propagation of the crack. This offers the advantage of being directly linked to the failure process, and no previous knowledge of the failure behavior is required. The models considered in this work represent the simulation of crack propagations in multilayer electronic systems, such as SIP devices, and are based on a cohesive zone approach. In order to clearly understand the issues of this problem, this work includes a brief description of the fracture mechanics and reviews the existing nonlinear finite element solvers. After explaining the principle of the energy release solver and the different issues due to its implementation, its efficiency is compared to pre-implemented solvers, such as the Crisfield method. The results show a significant improved robustness of the new energy released method compared to the previous arc-length methods.

This work shows that toughness (resistance) to fracture propagation is an inherent characteristic of cohesionless particulate materials, which is significant for understanding hydraulic fracturing in geotechnical, geological, and petroleum applications. We have developed experimental techniques to quantify the initiation and propagation of fluid-driven fractures in saturated particulate materials. The fracturing liquid is injected into particulate materials, where the fluid flow is localized in thin crack-like conduits. By analogy, we call them 'cracks' or 'hydraulic fractures'. Based on the laboratory observations and scale analysis, this work offers physical concepts to explain the observed phenomena. When a fracture propagates in a solid, new surfaces are created by breaking material bonds. Consequently, the material is in tension at the fracture tip. In contrast, all parts of the cohesionless particulate material (including the tip zone of hydraulic fracture) are likely to be in compression. In solid materials, the fluid front lags behind the front of the propagating fracture, while the lag zone is absent for fluid-driven fractures in cohesionless materials. The compressive stress state and the absence of the fluid lag are important characteristics of hydraulic fracturing in particulate materials with low, or no, cohesion. Our experimental results show that the primary factor affecting peak (initiation) pressure is the magnitude of the remote stresses. The morphology of fracture and fluid leak-off zone, however, changes significantly not only with stresses, but also with other parameters such as flow rate, fluid rheology, and permeability. Typical features of the observed fractures are multiple off-shots and the bluntness of the fracture tip. This suggests the importance of inelastic deformation in the process of fracture propagation in cohesionless materials. Similar to solid materials, fractures propagated perpendicular to the least compressive stress. However, peak injection pressures are significantly greater than the maximum principle stresses in the experiments. Further, by incorporating the dominate experimental parameters into dimensionless form; a reasonable power-law fit is achieved between a dimensionless peak injection pressure and dimensionless stress. Scaling indicates that there is a high pressure gradient in the leak-off zone in the direction normal to the fracture. Fluid pressure does not decrease considerably along the fracture, however, due to the relatively wide fracture aperture. This suggests that hydraulic fractures in unconsolidated materials propagate within the toughness-dominated regime. Furthermore, the theoretical model of toughness-dominated hydraulic fracturing can be matched to the experimental pressure-time dependences with only one fitting parameter. Scale analysis shows that large apertures at the fracture tip correspond to relatively large 'effective' fracture (surface) energy, which can be orders of magnitude greater than typical for hard rocks.

The problems with the crack length determination by the unloading compliance method are well known for Charpy size specimens. The final crack lengths calculated for bent specimens do not fulfil ASTM 1820 accuracy requirements. Therefore some investigations have been performed to resolve this problem. In those studies it was considered that measured compliance should be corrected for various factors, but satisfying results were not attained. In the presented work the problem was attacked from the other side, the measured specimen compliance was taken as a correct value and what had to be adjusted was the calculation procedure. On the basis of experimentally obtained compliances of bent specimens and optically measured crack lengths the investigation was carried out. Finally, a calculation procedure enabling accurate crack length calculation up to 5mm of plastic deflection was developed. Applying the new procedure, out of investigated 238 measured crack lengths, more than 80% of the values fulfilled the ASTM 1820 accuracy requirements, while presently used procedure provided only about 30% of valid results. The newly proposed procedure can be also prospectively used in modified form for the specimens of different than Charpy size.

Thesis (Ph. D.)--University of Missouri-Columbia, 2000. / Typescript. Vita. Includes bibliographical references (leaves 182-188). Also available on the Internet.

Thesis--Universität Paderborn. / Includes bibliographical references.

Thesis (M.S.)--Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.