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Luca Reyes
Luca Reyes

3d Crack Analysis Using Ansys |LINK|

Hi, I am doing project on mixed mode crack growth of semi elliptical crack in gas turbine components under cyclic loading. Ansys has capability for finding J,K using CINT command. But, In mixed mode crack growth, i dont know how to deal with nodes and how to create random crack with new node coordinates obtained from previous steps. If you have any ansys code related to creating crack and finding fracture parameters like K, J (mode 1 or mixed mode), Can you please give me the code.

3d Crack Analysis Using Ansys

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I am a beginner in FEM and just started using ANSYS. I need to study the stresses and SIF at the crack front of an elliptical crack inside a material subjected to a stress perpendicular to the crack plane. Any details on how I should build my model?

I am obtainning the Stress Intensity factors along the front of cracks in a 3D FE models (ANSYS). The loading mode is mixed and Ki, Kii and Kii should be estimated. I have noticed that the SIF strongly depends on the poisson's ratio but i understand Ki does not depends on the elastic properties of material. I know ANSYS calculates the SIF from the displacements at the crack tip using E and mu, but i did not expect so huge differences in SIF values, In fact all analytical predictions for SIF does not depends on poisson's ratio....

i m dan doing ME Engineering Design.I m doing project in the area of probabilistic fracture mechanics.,So i need how to find SIF,J integral,Energy release rate,CDOT for all type of crack (2D,3D)..,I want some of the material related to this..,i m using Ansys..,so complete step by step procedure in Crack analysis using analysis i want 3d Crack analysis

This paper presents a methodology for conducting a 3-D static fracture analysis with applications to a gas turbine compressor blade. An open crack model is considered in the study and crack-tip driving parameters are estimated by using 3-D singular crack-tip elements in ANSYS\(\circledR \). The static fracture analysis is verified with a special purpose fracture code (FRANC3D). Once the crack front is perfectly defined and validated, a free vibration study is conducted by analyzing the natural frequencies and modeshapes for both a single blade and bladed disk system. Taking advantage of high performance computing resources, a high fidelity finite element model is considered in the parametric investigation. In the fracture simulation, the influence of the size of a single edged crack as well as the rotational velocity on fracture parameters (stress intensity factors and J-Integral) are evaluated. Results demonstrate that for the applied loading condition, a mixed mode crack propagation is expected. In the modal analysis study, increasing the depth of the crack leads to a decrease in the natural frequencies of both the single blade and bladed disk system, while increasing the rotational velocity increases the natural frequencies. The presence of a crack also leads to mode localization for all mode families, a phenomenon that cannot be captured by a single blade analysis.

Given the severity of fatigue induced cracks, several numerical and experimental studies have been conducted to study the effect of crack initiation and propagation as well as the effect of cracks on the dynamic response of the rotor and bladed disk. While significant attention has been devoted to failure analysis of cracked turbine blades in recent years, with advances in three-dimensional fracture analysis, efforts have been made to characterize the crack propagation in cracked blades. Witek [2] proposed a hybrid procedure of crack growth dynamic estimation in an aero-turbine compressor blade subjected to resonant vibrations. Poursaeidi and Bakhtiari [3] used the fracture analysis code FRANC3D [4] to simulate fatigue crack growth in a gas turbine compressor blade, while using the Paris and Forman-Newman-De Koning model to predict fatigue life. The two studies use the Raju-Newman analytical solution [5] to calculate stress intensity factors for a semi-elliptical flaw in a turbine blade assumed as a flat plate. Kirthan et al. [6] used the finite element code ANSYS to calculate stress intensity factors for a simplified approximation of a gas turbine blade while performing a fatigue crack growth simulation using the Paris-Erdogan crack growth model. While most of the above studies focus on a quasi-static fracture analysis, in reality, cracks in rotating bodies demonstrate a breathing effect due to the opening and closing of the crack. To that effect, Liu and Jiang [7] presented a dynamic crack simulation in rotating blades using a hexahedral finite element method.

The geometry of both the single blade and the bladed disk was created in SolidWorks and the three dimensional finite element mesh was generated using the ANSYS finite element package. The finite element mesh for the uncracked single blade is shown in Fig. 1a and for bladed disk is shown in Fig. 1b. The single blade is divided into separate regions to apply the required mesh controls in order to refine the mesh in regions of interest (such as high stress concentration and/or around the crack front). Since high mesh refinement is introduced around the region of the crack front, the size of the finite element model increases once the crack is introduced. For the fracture analysis study of single cracked blades, 20 noded solid brick elements with quadratic interpolation were used away from the crack and 15 noded quarter-point singular wedge elements around the crack front. The ANSYS finite element model with the associated boundary conditions for the uncracked single blade was imported into FRANC3D.

For the modal analysis study, 3-D 8 noded solid elements with linear interpolation were used. The uncracked single blade had a total of 56,177 elements and 1074,36 nodes, while the healthy bladed disk had 880,520 elements and 1,681,517 nodes. The number of elements ranged with an average of 370,000 elements and 540,000 nodes for the cracked single blade and an average of 1,116,000 elements and 2,137,500 nodes for the bladed disk with a single cracked blade. The variation in the number of elements and nodes is attributed to the varying size of the crack front used in the analysis.

An initial stress analysis was conducted to determine zones of high stress concentration. In addition to the boundary conditions specified above, a linearly increasing hydrostatic pressure was applied on the pressure side of the blade ranging from 16 kPa near the root of the blade to 30 kPa at the tip. An appropriate mesh density was considered after conducting a convergence study using h-refinement and regions of high stress concentration were observed near the root of the blade on both the leading and the trailing edge as shown in Fig. 2. In the absence of any foreign object damage, the crack is most likely to develop in such an area, and therefore a single edge crack is introduced close to the root of the blade. In other fracture simulations related to crack propagation, other investigators [15] have also inserted cracks in similar locations. In the current study, a crack is inserted in the geometry at 5 mm above the root of the blade and at various depths from the leading edge of the blade, the idea being that as the depth increases, so does the size of the crack.

Once the location of the crack and the size of the crack is identified, the region around the crack needs to be meshed appropriately to accurately calculate fracture parameters such as stress intensity factors. The following sections first outline the theory behind both programs employed in this paper to carry out the fracture analysis, describe the meshing strategy employed in both programs and a validation study is conducted to determine the effectiveness of both approaches.

The static fracture analysis study is concerned with estimating the crack-tip driving parameters such as the stress intensity factor and the J-Integral to determine the subsequent crack propagation process. In each case, a crack is inserted in the geometry at 5 mm above the root of the blade and at depths of 4, 8 and 16 mm from the leading edge of the blade. Figure 5a shows the finite element mesh generated around the open crack inserted in the ANSYS\(\circledR \) model. In addition to studying the effect of the crack size and location on the fracture parameters, three different rotational velocities (500, 1000 and 2000 rad/s) are used in the parametric study to determine the effect of the inertial load on the fracture parameters. In each study, the fracture parameters calculated in ANSYS\(\circledR \) are compared with those calculated in FRANC3D. FRANC3D requires a model defined without a crack and this is generated in ANSYS\(\circledR \) and imported into FRANC3D, which supports ANSYS\(\circledR \) input files. Figure 6b shows the finite element mesh that is automatically generated around the crack front in FRANC3D. Another attractive feature of employing FRANC3D for fracture parameter calculation is the ability to sub-divide the finite element model, select only the region around the crack front and carry out the analysis. This significantly reduces computation cost, especially when compared to using ANSYS\(\circledR \).

Figure 9 compares the Mode I stress intensity factor for each mode obtained using ANSYS\(\circledR \) for rotational speeds of 1000 and 2000 rad/s for a crack depth of 4mm. It can be seen that increasing the inertial loads on the structure dramatically increases the overall stress distribution and thereby the stress intensity factor.

To capture the effect of the crack on the dynamic behavior of the structure, a modal analysis simulation was performed on both a cracked single blade as well as a cracked bladed in a rotor-disk system. The Block-Lanczos Method [20] is utilized to extract the natural frequencies of the structure and a three-dimensional deformation regime corresponding to each natural frequency also provides the mode-shape of vibration. A case study is initially considered to determine the effectiveness of considering singular elements around the crack-tip for the modal analysis study. This is done to reduce computational costs for a modal analysis study of a bladed disk with a cracked blade. The percentage difference between the natural frequencies calculated using elements with quadratic interpolation and those calculated using linear interpolation for a single blade with a 4mm crack are shown in Table 1. Although the percentage difference increases as the mode number increases, it is still small. It is therefore concluded that for the modal analysis study with a cracked blade, 3-D 8 noded solid elements with linear interpolation can be used.


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