Three-dimensional elastic-plastic dynamic fracture analysis for stationary cracks using enriched elements

In this study, three-dimensional dynamically loaded stationary crack problems are evaluated by taking into consideration elastoplastic material properties. These types of problems, in which elastoplastic cracked structures are loaded dynamically, has been a great challenge for many scientists, due to the fact that the solutions are much more complicated and computationally time consuming than corresponding static problems. Analysis of dynamically loaded problems where elastoplastic materials exist within the structure has become a very important research subject recently, because of the inadequacy of the static solutions in many critical applications. The basic difference between a static and a dynamic problem is the fact that stresses or stress intensity factors in a dynamic analysis can be much higher than the corresponding static values. In the case of a sudden dynamic loading, failure due to fracture can happen unexpectedly, e.g. fracture during impact loading. This analysis may be very important in various applications in the semiconductor industry, automotive vehicles, high speed machinery and military applications.;There are some basic differences between static and dynamic fracture problems in which elastoplastic materials are present. These differences can be summarized with the inclusion of some effects which are not present in a typical static problem; inertia effects which leads to the propagation of stress waves, strain rate and temperature dependency of the material properties that affect the yield stress of the material, and the necessity for a time integration to calculate the fracture parameters w.r.t time, such as the stress intensity factors, dynamic J-integral and the crack tip opening displacement (CTOD). In this dissertation, the main task of interest can be subdivided into three different parts, in order to demonstrate the analysis methodology developed in the course of this study.;The first part will show the basic principles of linear elastic dynamic fracture modeling, including the finite element formulation of the dynamic analysis and the calculation of stress intensity factors for dynamic linear elastic fracture problems. These problems may be in the form of a homogenous cracked structure, or an interface crack which lies between two dissimilar materials. The analysis of the fracture problem is demonstrated with the aid of the Enriched Finite Element Method, which embeds special elements around the crack tip, called enriched elements. The dynamic analysis involves explicit and implicit time integration methods, and these methods will also be explained in this part. The engineering meaning of dynamic linear elastic fracture mechanics (LEFM) is demonstrated and compared with known results from the literature.;Analyzing cracked structures with elastoplastic material properties needs a step-by-step procedure and this will be described in the second part of this study. A dynamic time integration algorithm for nonlinear elastoplastic analysis is explained and comparisons are made with known results from the literature. Possible effects of strain rate and temperature on the yield stress of a plastically deforming material are also investigated. For a problem in which the crack tip zone is assumed to be elastic (small scale yielding conditions under LEFM rules), the stress intensity factors may still be considered as an appropriate parameter for fracture characterization. However, if the crack tip zone is not small and LEFM rules can not be justified, then different parameters should be utilized to characterize the fracture problem. At this point, it is expected that J-integral and CTOD calculations can be very useful to be able to quantify the crack problem. For the calculation of the J-integral in a cracked body, a special technique is definitely required, and the domain integral algorithm will be developed and applied for this purpose.;The analysis that is presented in this dissertation mostly involves dynamically loaded problems. In a dynamic problem, there are numerous time steps that may lead to a very computationally time consuming process, especially if the number of DOFs is high. In addition, if the problem requires an elastoplastic analysis, then these time considerations for the solution of a specific dynamic problem can become an overriding issue. Finite Element (FE) problems sometimes can become far too large, in terms of the total DOFs, for efficient in core memory storage. In order to deal with both of these computational matters, the development of parallelized versions of the current research FE code was considered to be necessary. Results regarding improvements associated with parallel computing developments are presented in the last chapter.;Most of the numerical examples presented in this study were evaluated using a specialized FE software that was developed at Lehigh University ME&M department. In a few instances, ANSYS commercial software, was used to compare results, with software developed by the author.