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Abviewer 10 Crack
The SMART fatigue option requires a tetrahedral element mesh. The mesh size needs to be refined around the crack tip. I have used a sphere of influence method based around the geometric edge running through thickness.
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 Factor will control the adapting crack front shape. The Failure Criteria can also be set to the J-Integral method.
Fig. 4 shows output requests and includes general stress and displacement contours as well as fatigue-specific probes. The main fracture mechanism in this model is the crack tensile opening Mode 1, which calculates KI. The probe SIFS (K1) reports these values.
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.
I have constrained the bottom surface of the test piece and applied a vertical force of 1.5E5N. The initial crack length is 6.4 mm. A manual calculation gives a value for KI of 6.152E7 . This is below the Critical Kc of 1.5e8 input in Fig. 5. The component will not fracture under this loading. The main objective is to check the value and distribution of KI.
There are five curves shown in Fig. 6. These represent the five contours requested in the Pre-Mesh Crack object. They are used to check for convergence of the KI value. The first curve is from a contour close to the crack tip; the other curves represent contours at increasing distances. The curves converge quickly, showing the mesh is adequate.
I then increased the load to 7E5N, which gives a KI of 3E8 for the initial crack length of 6.4 mm. This is greater than Kc and results in a static fracture. Running the model with 40 Substeps results in a crack extension of 14.5 mm. The crack extends to just over 50% of the plate width. Fig. 7 shows the changing crack and mesh configuration. The maximum stress contours are adjusted at each step.
Fig. 8 shows the average KI as the crack progresses, together with the threshold Kc, shown as an orange dashed line. The location to sample along the crack front can be selected, and I have used the midpoint of the crack front.
Two ANSYS runs are carried out: a relatively coarse mesh and a finer mesh. The local element size is reduced from 0.8 mm to 0.65 mm. There is clear evidence of convergence with the life to create a 25 mm crack reducing from 3.74 E6 cycles to 3.62 E6 cycles. The 1D model provides a useful comparison with a life to 25 mm of 3.24 E6 cycles.
The crack is now attracted to the hole and joins it on the far side from the crack origin. The crack is then arrested and there is no further extension. The crack is arrested at around 8.631 E5 cycles and the structure is damage tolerant with respect to this particular initial crack and loading.
The sharp notch does attract and arrest the crack. It now takes 6.76 E5 cycles to reach this point. The notch is intercepted partway along its length. This is shown more clearly in the displacement plot, which is inset in the last frame of Fig. 13. Experimenting with the notch shows there is a critical length to attract the crack to an arrested state. Below that length the notch acts to accelerate the crack.
The Arbitrary Crack and Semi-Elliptical Crack look useful in creating cracks in more complex geometry. The other fracture solution methods, Interface Delamination and Contact Debonding, provide solutions for pre-defined crack paths, such as along bond lines.