A finite element calculation of stress intensity factors by a modified crack closure integral
ABSTRACT An efficient technique for evaluating stress intensity factors is presented. The method, based on the crack closure integral, can be used with a constant strain finite element stress analysis and a coarse grid. The technique also permits evaluation of both Mode I and Mode II stress intensity factors from the results of a single analysis. Example computations are performed for a double cantilever beam test specimen, a finite width strip with a central crack, and a pin loaded circular hole with radial cracks. Close agreement between numerical results given by this approach and reference solutions were found in all cases.
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ABSTRACT: Coating spallation events have been observed along the slip-side joggle region of the Space Shuttle Orbiter wing-leading-edge panels. One potential contributor to the spallation event is a pressure build up within subsurface voids or defects due to volatiles or water vapor entrapped during fabrication, refurbishment, or normal operational use. The influence of entrapped pressure on the thermo-mechanical fracture-mechanics response of reinforced carbon-carbon with subsurface defects is studied. Plane-strain simulations with embedded subsurface defects are performed to characterize the fracture mechanics response for a given defect length when subjected to combined elevated-temperature and subsurface-defect pressure loadings to simulate the unvented defect condition. Various subsurface defect locations of a fixed-length substrate defect are examined for elevated temperature conditions. Fracture mechanics results suggest that entrapped pressure combined with local elevated temperatures have the potential to cause subsurface defect growth and possibly contribute to further material separation or even spallation. For this anomaly to occur, several unusual circumstances would be required making such an outcome unlikely but plausible.
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ABSTRACT: . The virtual crack closure technique (VCCT) is a well-established method for computing the energy release rate (ERR) when analysing fracture problems via the finite element method. For mixed-mode fracture problems, the VCCT is also commonly used to partition the fracture modes, i.e. to determine the ERR contributions related to fracture modes I, II, and III. A perhaps little known fact, however, is that in some circumstances the standard VCCT predicts physically inconsistent, negative values for the modal contributions to the ERR. Focusing on I/II mixed-mode problems, this paper presents a revised VCCT which furnishes a physically consistent partitioning of fracture modes by associating the mode I and II ERR contributions to the works done in a suitably defined two-step process of closure of the virtually extended crack. 1 INTRODUCTION The virtual crack closure technique (VCCT) is a well-established method for calculating the energy release rate (ERR) when analysing fracture problems via the finite element method (FEM). The technique is based on the numerical implementation of Irwin's crack closure integral , as first proposed for two-dimensional problems by Rybicki and Kanninen , and later extended to three-dimensional problems by Shivakumar et al. . In recent years, the VCCT has gained great popularity for the study of mixed-mode fracture problems, such as the delamination of composite materials and interfacial fracture between dissimilar materials. In these cases, the VCCT is used to compute not only the total ERR, but also the contributions of the three fracture modes (I or opening, II or sliding, and III or tearing) . A perhaps little known fact, however, is that in some circumstances (for instance, bodies with asymmetric cracks subjected to certain load conditions), the standard VCCT predicts physically inconsistent, negative values for the modal contributions to the ERR. Although this potential shortcoming of the technique has already been mentioned in the literature , it does not seem to have received the attention it deserves. Focusing on I/II mixed-mode fracture problems, we develop a revised VCCT that associates the mode I and II ERR contributions to the works done in a suitably defined two-step process of closure of the virtually extended crack. Furthermore, we suggest an implementation procedure based on computation of flexibility coefficients. The effectiveness of the proposed method is then tested by considering the problem of a delaminated cantilever beam subjected to bending couples. The overall thickness of the beam is kept constant, while several positions of the delamination are considered to highlight the effects of crack asymmetry. The mode I and II contributions to the ERR are computed using both the standard and revised VCCT. For the sake of comparison, the same quantities are also computed using the analytical solution by Suo and Hutchinson . Thus, the revised VCCT demonstrates its ability to furnish physically consistent predictions also in cases where the standard technique fails.
Conference Paper: Multidomain BEM for crack analysis in stiffened anisotropic plates[Show abstract] [Hide abstract]
ABSTRACT: The present paper is concerned with the application of a boundary element model for the analysis of cracks in stiffened composite panels. The panel stiffeners are reduced to equivalent strips and the multidomain technique is used to model panel zones presenting different properties (skin and stiffeners equivalent strip). Also the crack is modeled exploiting the multidomain formulation. Evaluation of stress intensity factors is performed for representative problems.International Conference on Boundary Element and Meshless Techniques XV, Florence, Italy; 07/2014