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Crack Growth Analysis Using Ansys Software VERIFIED

For the static crack growth simulation, ANSYS Mechanical provides two common fracture criteria, which are the J-integral and stress intensity factor. In this study, the stress intensity factor criterion was used, which states that the crack grows when the equivalent stress intensity factor exceeds the fracture toughness of the material. This computation is performed along the distributed crack front, where the distribution of the stress intensity factor controls the adapting crack front shape. The equivalent stress intensity factor is represented by [33], as follows:

crack growth analysis using ansys software

Figure 3 shows the predicted crack growth path for two different thicknesses, Figure 3a for 10 mm thickness and Figure 3b for 20 mm thickness. As seen in Figure 3, the crack begins to grow in a straight line because there is no influence of holes. As it approaches the hole, it grows in the direction of the first upper hole, which was not close enough to cause the crack to sink into the hole; therefore, the crack has changed its direction and continues to propagate in a straight line until it arrives near the second upper hole, which is close enough to the crack trajectory for the crack to sink into it.

This work compares the fatigue crack growth under constant amplitude load between two softwares, ANSYS Mechanical R19.2 and FRANC2D/L, as an alternative tool for modeling fatigue crack growth problems in mixed-mode loads. Four different configurations of the modified compact tension were simulated in both softwares. The crack growth trajectory, SIFs, stresses distribution, and fatigue life cycles were predicted using both softwares, and the outcome from other researchers validated the results.

The Cornell Fracture Mechanics Group at Cornell University developed the free two-dimensional fracture analysis software FRANC2D/L, which was funded by the US National Science Foundation, NASA, the US Navy, and other agencies [19]. The FRANC2D/L analysis is carried out in two stages, with CASCA which is used as a mesh generator with different types of mesh and associated with FRANC2D/L. In the second part, the boundary constraints, problem characteristics, stress computation, input crack singularity, crack growth, and problem outcomes were determined [48, 49]. SIFs were computed using three approaches in FRANC2D/L: method of displacement correlation, method of modified crack closure integral, and method of J-integral. The crack orientation was estimated via maximum circumferential stress criterion, while fatigue crack growth rate was calculated based on Paris law equation with the same procedure used in ANSYS as represented in equations (1) and (2), respectively. The step-by-step procedure of the FRANCD2D/L software is shown in Figure 2.

Abstract:This paper presents computational modeling of a crack growth path under mixed-mode loadings in linear elastic materials and investigates the influence of a hole on both fatigue crack propagation and fatigue life when subjected to constant amplitude loading conditions. Though the crack propagation is inevitable, the simulation specified the crack propagation path such that the critical structure domain was not exceeded. ANSYS Mechanical APDL 19.2 was introduced with the aid of a new feature in ANSYS: Smart Crack growth technology. It predicts the propagation direction and subsequent fatigue life for structural components using the extended finite element method (XFEM). The Paris law model was used to evaluate the mixed-mode fatigue life for both a modified four-point bending beam and a cracked plate with three holes under the linear elastic fracture mechanics (LEFM) assumption. Precise estimates of the stress intensity factors (SIFs), the trajectory of crack growth, and the fatigue life by an incremental crack propagation analysis were recorded. The findings of this analysis are confirmed in published works in terms of crack propagation trajectories under mixed-mode loading conditions.Keywords: XFEM; ANSYS mechanical; smart crack growth; stress intensity factors; LEFM; fatigue life prediction

I must select three named geometric regions to define the crack. These are the crack edge, the top surface of the crack and the bottom surface of the crack. Each of these regions is then associated with a node set for use in the analysis.

The Pre-Meshed crack object is selected, the option to carry out a Static Fatigue analysis is chosen and the Critical Fracture Toughness is defined. The Stress Intensity Factor method is selected in this case. The crack will propagate when the calculated Stress Intensity Factor, K exceeds the Fracture Toughness Kc . This calculation is done along the distributed crack front, and the distribution of Stress Intensity Fac-tor will control the adapting crack front shape. The Failure Criteria can also be set to the J-Integral method.

I have also included a probe for the in-plane shearing Mode 2, which calculates KII as SIFS (K2). These values are secondary for this configuration but may become important for a crack that changes direction significantly with a resulting shear stress environment. I have also included a crack extension probe to monitor the crack growth. Each of these probes will produce an XY plot.

The Stress Ratio is used to define the ratio of minimum stress to maximum stress. This gives the stress range and hence the Stress Intensity factor range. In my case, the stress is cycling from zero to maximum, so the ratio is 0.0. If the stress was fully reversed, the ratio would be -1.0.The load is dropped to a much lower level of 27.0 KN, appropriate for a continuous cyclic operational loading. A data probe is added to report the number of cycles for each crack increment.A solution for an approximate 1D solution can be found using this expression:

Grain boundaries typically dominate fracture toughness, strength and slow crack growth in ceramics. To improve these properties through mechanistically informed grain boundary engineering, precise measurement of the mechanical properties of individual boundaries is essential, although it is rarely achieved due to the complexity of the task. Here we present an approach to characterize fracture energy at the lengthscale of individual grain boundaries and demonstrate this capability with measurement of the surface energy of silicon carbide single crystals. We perform experiments using an in situ scanning electron microscopy-based double cantilever beam test, thus enabling viewing and measurement of stable crack growth directly. These experiments correlate well with our density functional theory calculations of the surface energy of the same silicon carbide plane. Subsequently, we measure the fracture energy for a bi-crystal of silicon carbide, diffusion bonded with a thin glassy layer.

In light of these previous geometries8, it would be ideal to have test samples with geometrical features enabling stable crack growth beyond any damaged region, in order to measure fracture toughness as the crack evolves and to overcome limitations imposed by FIB-induced damage. It would be also useful to have freedom in the positioning of the notch combined with a relatively simple sample geometry, thus facilitating sample fabrication and fracture or surface energy analysis. Furthermore, a minimization of the effect of frame compliance and friction between the indenter and the sample would make evaluation of the measured energy easier.

The coordinates of the crack tip position were selected by hand, augmenting contrast within the script to make this easier. Each cantilever beam width was measured using frames toward the end of the test, when the crack was longer. As the nanoindenter sits on the SEM stage with its indentation axis tilted 30 with respect to the horizontal plane, all the measurements along the vertical direction of the image were corrected for foreshortening.

For the purpose of fracture energy measurements, only the time interval during which the crack growth was observed was taken into account (see crack growth interval indicated in Fig. 3b). In order to reduce noise we performed a linear fit of the data for the energy measurement calculation.

These slight asymmetries require the energy stored in the cantilevers to be measured individually to avoid the fracture energy being significantly underestimated or overestimated. While this subtlety may not be obvious, in practice it is straightforward to implement asymmetric analysis with this in situ geometry. A comparison of the differences between crack growth measurements using asymmetric or symmetric analysis is presented in Fig. 4a.

FIB milling normal to the sample surface is known to produce tapered final geometries; however, the analysis presented in this work considers the cantilevers made of constant cross-section along its length. It is therefore important to design the milling steps to minimize the taper in the final geometry or alternatively the analysis must be performed using a more complicated elastic model, as afforded for instance with cohesive zone-based finite element models.

The geometry of the DCB was optimized using an elastic finite element analysis in Ansys in order to magnify the stress at the bottom of the notch in comparison to the load that would cause fracture toward the top of each of the bending beams (see Supplementary Fig. 4).

".. engineers are using nCode DesignLife, for instance, to analyze hood slams. After gathering data from three slam velocities, the engineers can run the stress time history through nCode DesignLife to determine the number of cycles required to induce cracking. With this process, nCode DesignLife saves both cost and time in the testing."


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