The aim of this study is to investigate the behavior of fatigue cracks under mixed mode-I/III loading conditions, which combine opening and anti-plane shear modes. To achieve this goal, an experimental and numerical approach is adopted, using a novel spherical porous cylindrical specimen that allows for varying the mode mixity by changing the loading angle. The experimental setup consists of two digital cameras that capture the crack growth path and the fractured surfaces on the specimen during the cyclic loading. The images are then processed by Solidworks software to generate 3D CAD data of the crack geometry. The numerical approach involves modeling, meshing and solving the problem using ANSYS software, which provides the stress and displacement fields around the crack tip. The stress intensity factors (SIF) and equivalent SIF along the crack front are then computed using FCPAS software, which implements a modified version of the virtual crack closure technique (VCCT). The experimental and numerical results are compared in terms of the crack propagation path, the crack front profiles, the SIF distributions and the equivalent SIF values. The comparison shows a good agreement between the two methods, demonstrating the validity and accuracy of the proposed approach.
The spherical porous cylindrical specimen is made of aluminum alloy 2024-T3 and has a diameter of 50 mm and a height of 100 mm. The specimen has a central hole with a diameter of 10 mm and a spherical cavity with a radius of 25 mm. The cavity is filled with epoxy resin to create a porous region that simulates the presence of a crack. The specimen is subjected to cyclic loading by applying a constant axial force and a variable torsional moment. The loading angle, defined as the angle between the axial force and the crack plane, is varied from 0Â (pure mode-I) to 90Â (pure mode-III) in steps of 15Â. The applied stress ratio is kept constant at R = 0.1.
The crack growth path and the fractured surfaces on the specimen are observed by two digital cameras mounted on a tripod. The cameras are synchronized with the loading machine and take pictures at regular intervals during the fatigue test. The images are then imported into Solidworks software, which uses a photogrammetry technique to reconstruct the 3D CAD data of the crack geometry. The crack propagation path is obtained by tracing the contour of the crack opening on the specimen surface. The crack front profiles are obtained by slicing the CAD data along different planes perpendicular to the crack plane.
The numerical simulation of the problem is performed using ANSYS software, which employs the finite element method (FEM) to solve the linear elastic problem. The specimen is modeled as a solid body with isotropic material properties and geometric nonlinearity. The boundary conditions are applied according to the experimental loading conditions. A quarter model of the specimen is used to reduce the computational cost, taking advantage of the symmetry of the problem. The mesh is refined near the crack tip to capture the stress singularity. The SIF and equivalent SIF along the crack front are calculated using FCPAS software, which implements a modified version of the VCCT. The VCCT is based on the concept of energy release rate and uses nodal forces and displacements to estimate the SIF. The equivalent SIF is defined as the SIF that would produce the same crack growth rate as the actual mixed mode loading condition. 061ffe29dd