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Keywords: curved cracks, crack path prediction, computational fracture mechanics, finite elements, meshing algorithm, equivalent stress intensity factors.

Summary

The theory required to predict the two-dimensional propagation path of cracks under general bi-axial loading is well known, however its implementation in an efficient and reliable computational code is a far from trivial task [1].

The purpose of this work is to describe how the difficulties involved in translating such theoretical tools into practical numerical techniques have been solved, and how these techniques were used in a successful special-purpose academic program called Quebra2D. A brief description of the three most popular methods available to numerically compute stress intensity factors is presented, including details regarding the required steps to translate them into a workable finite element code.

The numerical computation of the crack increment direction in two-dimensional is reviewed, including a discussion on the three most used criteria used to predict the variable crack propagation direction in the linear-elastic regime. The computed values at each calculation step are used to obtain the crack incremental growth direction - and thus the fatigue crack path - in the linear-elastic regime.

The equivalent stress intensity factors used to treat the two-dimensional fatigue crack growth problem under general bi-axial loading are also presented.

Experimental results are used to verify the accuracy of the numerical predictions. The fatigue crack growth experiments are performed on C(T) specimens of cold-rolled AISI 1020 steel, measured according to the ASTM E 8M-99 standard. Three modified C(T) specimens are designed and tested, each one with a 7mm-diameter hole positioned at a slightly different position from the notch root. Two very different crack growth behaviours had been predicted by the finite element modelling of the C(T) specimens, depending on the hole position. The predictions indicated that the fatigue crack would always be attracted by the hole, but it could either curve its path and grow toward the hole ("sink in the hole" behaviour) or just be deflected by the hole and continue to propagate after missing it ("miss the hole" behaviour).

Using the Quebra2D program, the transition point between the "sink in the hole" and the "miss the hole" crack growth behaviour is identified. The three modified C(T) specimens are designed so that two of them had the hole just half a mm above the transition point, and the third specimen had the hole half a mm below it. The results show that the implemented computational code is able to predict the curved crack path in all experiments, either under constant or variable amplitude loading.

References

1: T.N. Bittencourt, A. Barry, A.R. Ingraffea, "Comparison of mixed-mode stress-intensity factors obtained through displacement correlation, J-integral formulation, and modified crack-closure integral", Fatigue and Fracture Mechanics: 22nd Volume, ASTM - Standard Technical Publication (STP), 1131: 69-82, 1992.

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Front Page Browse CCP CSETS CTR IJRT Other Authors Search Purchase Guide FAQ Contact us	Civil-Comp Proceedings ISSN 1759-3433 CCP: 88 PROCEEDINGS OF THE NINTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY Edited by: B.H.V. Topping and M. Papadrakakis Paper 243 Practical Aspects Concerning the Numerical Implementation of the Fatigue Growth of Curved Cracks M.A. Meggiolaro¹, A.C.O. Miranda², L.F. Martha² and J.T.P. Castro¹ ¹Mechanical Engineering Department, ²Civil Engineering Department, Pontifical Catholic University of Rio de Janeiro, Brazil doi:10.4203/ccp.88.243 purchase the full-text of this paper Full Bibliographic Reference for this paper M.A. Meggiolaro, A.C.O. Miranda, L.F. Martha, J.T.P. Castro, "Practical Aspects Concerning the Numerical Implementation of the Fatigue Growth of Curved Cracks", in B.H.V. Topping, M. Papadrakakis, (Editors), "Proceedings of the Ninth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 243, 2008. doi:10.4203/ccp.88.243 Keywords: curved cracks, crack path prediction, computational fracture mechanics, finite elements, meshing algorithm, equivalent stress intensity factors. Summary The theory required to predict the two-dimensional propagation path of cracks under general bi-axial loading is well known, however its implementation in an efficient and reliable computational code is a far from trivial task [1]. The purpose of this work is to describe how the difficulties involved in translating such theoretical tools into practical numerical techniques have been solved, and how these techniques were used in a successful special-purpose academic program called Quebra2D. A brief description of the three most popular methods available to numerically compute stress intensity factors is presented, including details regarding the required steps to translate them into a workable finite element code. The numerical computation of the crack increment direction in two-dimensional is reviewed, including a discussion on the three most used criteria used to predict the variable crack propagation direction in the linear-elastic regime. The computed values at each calculation step are used to obtain the crack incremental growth direction - and thus the fatigue crack path - in the linear-elastic regime. The equivalent stress intensity factors used to treat the two-dimensional fatigue crack growth problem under general bi-axial loading are also presented. Experimental results are used to verify the accuracy of the numerical predictions. The fatigue crack growth experiments are performed on C(T) specimens of cold-rolled AISI 1020 steel, measured according to the ASTM E 8M-99 standard. Three modified C(T) specimens are designed and tested, each one with a 7mm-diameter hole positioned at a slightly different position from the notch root. Two very different crack growth behaviours had been predicted by the finite element modelling of the C(T) specimens, depending on the hole position. The predictions indicated that the fatigue crack would always be attracted by the hole, but it could either curve its path and grow toward the hole ("sink in the hole" behaviour) or just be deflected by the hole and continue to propagate after missing it ("miss the hole" behaviour). Using the Quebra2D program, the transition point between the "sink in the hole" and the "miss the hole" crack growth behaviour is identified. The three modified C(T) specimens are designed so that two of them had the hole just half a mm above the transition point, and the third specimen had the hole half a mm below it. The results show that the implemented computational code is able to predict the curved crack path in all experiments, either under constant or variable amplitude loading. References 1 T.N. Bittencourt, A. Barry, A.R. Ingraffea, "Comparison of mixed-mode stress-intensity factors obtained through displacement correlation, J-integral formulation, and modified crack-closure integral", Fatigue and Fracture Mechanics: 22nd Volume, ASTM - Standard Technical Publication (STP), 1131: 69-82, 1992. purchase the full-text of this paper (price £20) go to the previous paper go to the next paper return to the table of contents return to the book description purchase this book (price £145 +P&P)
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