Papers by Keyword: Mixed-Mode

Abstract: This article deals with the development of functional graded materials which is pursued by the collaborative research centre Transregio 30. The functional gradation is produced by a thermo-mechanical production process and is characterized by different fracture mechanical specific values. Because cracks are by all means responsible for failure of structural elements crack growth has to be considered during the development of these materials. In this contribution simulations of crack growth which were carried out with the system ADAPCRACK3D are presented to illustrate the influence of a material gradation on crack growth.

Abstract: Components in jet engines are subject to time-dependent multiaxial loading. This creates a time-varying mixed-mode stress state at the crack tip. Mixed-mode loading leads to out-of-plane crack propagation and has been treated in previous articles [1,2]. This paper concentrates on coping with the time-dependent character. Key issues are the crack propagation rate and the crack propagation direction in three-dimensional space. In order to determine the prevalent crack propagation direction the dominant loading case is determined based on its crack propagation rate. Then, a mixed-mode equivalent K-factor is calculated for all other loading cases based on the closeness of their associated crack propagation direction with the dominant one. Subsequently, a cycle extraction is performed on the crack propagation rate for all loading cases. The extracted cycles are processed based on their mimimum and maximum equivalent K-factor and the maximum temperature. The mission crack propagation rate consists of the sum of the rate of all extracted cycles.

Abstract: The original mixed-mode model is reformulated by considering the soft impingement effect and applying a general polynomial method of dealing with the concentration gradient in front of the interface. Comparison with the numerical solution shows that the reformulated mixed-mode model is more precise than the original model. The effect of soft impingement on the kinetics of partitioning phase transformation depends on both the growth mode and the degree of super-saturation.

Abstract: In this paper, mixed mode stress intensity factor (SIF) solutions are computed for inclined surface cracks in finite-thickness plates. The S-version finite element method (S-FEM), which is an adaptive method and multi-scale computing method, are employed in the analyses. When S-FEM is applied to solve the fracture problem, local mesh model including cracks can be built independently from the global mesh model for modeling overall structure. The local model is superposed on the g- lobal one. Therefore, it is easy to introduce cracks in an existing mesh model. The influences of inc- lination angles, crack shape and plate thickness on the mixed mode fracture solution were investiga- ted. The solutions presented can be used to assess fail/safe conditions and fatigue crack growth for the three-dimensional cracks studied.

Abstract: The present research examines analytically and experimentally the mode-I and mode-II and mixed mode-Interlaminar fracture toughness of PAN based carbon/epoxy composite. A modified Arcan fixture, well-suited for the study of the behavior of used composite assemblies, was developed in order to focus on the analysis of the fracture behavior of the material. The edge effects are minimized by using an appropriate design of the substrates so that experimental results give reliable data. Also the mode-I and mode-II stress intensity factors were computed for different crack lengths and load orientation angles using finite element analysis. The numerical results show that the modified Arcan specimen is able to provide pure mode-I, pure mode-II and any mixed mode loading conditions. It is shown that the results obtained from the fracture tests are consistent very well with mixed mode fracture theories. Obtained results indicated that fracture toughness and stress intensity factor for sliding mode enhanced up when the loading angle increased. Mechanism of fracture and toughening were examined by using scanning electron microscopy.

Abstract: The mixed-Mode interlaminar fracture toughness, GI/IIC, of z-pinned hybrid laminated composites is studied to investigate the effect of 3D-composites on the crack propagation resistance of delaminated composite structures. In this regard, the mixed-Mode interlaminar fracture toughness, GI/IIC, was measured using asymmetric double cantilever beam (ADCB) test method. The hybrid ADCB and z-pinned hybrid composite beams were laid-up from [G0/C0]4, [G0/C90]4, [G90/C0]4 and [G90/C90]4 to study the effect of z-pinning on the interlaminar fracture toughness. From the obtained results from test it was found that the resistance of z-pin fibres against the crack propagation in z-pinned hybrid composites can significantly increase the mixed-mode interlaminar fracture toughness.

Abstract: For most of structural failure in engineering structures , fracture often takes place due to a phenomenon called fatigue. Therefore, many studies about the effect of the various mode-mixities on fatigue characteristics have been performed. However, most of the former studies are about metal/metal interface or delamination of composite. In this study, the fatigue characteristics of a composite/metal interface are investigated. The fatigue tests were performed using single-leg bending (SLB) specimens bonded with composite and steel using co-cure bonding method. This paper focuses on the fatigue characteristics depending on different mode ratios (GⅡ/GT). The overall results obtained in this study show that the crack propagation rate increases with the mode Ⅱ loading component.

Abstract: Traditionally, applications of fracture mechanics have been mainly focused on cracks growing under the opening or mode I mechanism (three point bend specimen, middle tension specimen, crack tension specimen, etc.). When investigating instability under mixed mode loading conditions, three different alternatives can be envisaged consisting of: a) machining a hole in standard specimens, b) creating a crack oriented under a given angle, and c) using non-standard Arcan-Richard specimens. In this work, finite element calculations are performed to analyze the initial values of the fracture parameters in Arcan-Richard specimens. First, the influence of the normal stress mode to the shear stress mode ratio is analyzed, then the effect of the constraint level is discussed, and finally, the initial propagation angle of the daughter crack is derived.

Abstract: This document describes a mixed-element approach to perform mixed-mode crack propagation calculations in an arbitrary structure in a fully automatic way. In a preprocessing step a cylindrical volume about the crack front is meshed with hexahedral elements while the remaining parts of the structure at interest are filled with tetrahedrons. Subsequently, a finite element calculation is performed yielding the stress field at the crack tip. The postprocessor determines the stress intensity factors and the 3-dimensional crack growth, leading to a new crack front. This procedure is repeated until a user-defined criterion is reached. The method is illustrated by an aircraft casing component.

Abstract: Fatigue crack growth under mixed mode loading conditions is simulated using S-FEM. By using S-FEM technique, only local mesh should be re-meshed and it becomes easy to simulate crack growth. By combining with auto-meshing technique, local mesh is re-meshed automatically, and curved crack path is modeled easily. Plural fatigue crack problem is solved by this technique. For two parallel crack problem, criteria of crack coalescence are proposed. By simulating this problem by S-FEM, it is verified these criteria are conservative ones.