Carbon fiber reinforced polymers (CFRP) are crucial for many industries due to their superior material properties. CFRPs have strength and toughness that are comparable to metals but with the advantage of possessing lighter weight and higher corrosion resistance. Typically, structural parts are joined by bolts and rivets resulting in difficulties keeping the integrity of these joints. In CFRP joints, screw holes are stress concentration sites that may develop cracks, splits and delamination. Alternatively, adhesive bonding can be used as a joining method for CFRP substrates to overcome the disadvantages bolts and rivets. Structural parts are usually subjected to cyclic loading. Therefore, fatigue is considered as a major design tool for these parts. Finite element analysis is a powerful tool for modeling damage in components. This paper aims to simulate fatigue crack growth in adhesively bonded carbon fiber reinforced polymer (CFRP) composite substrates using a double cantilever beam (DCB) specimen. ANSYS XFEM is utilized to simulate the crack path using the enrichment technique to assign elements to the crack path. The model calculates the stress intensity factor (SIF) based on the domain integral over the contour around the crack tip. Then, it is converted at each sub-step to the energy released rate (ERR) which includes a correction factor estimated from the cohesive zone model (CZM). The model is idealized as a 2D geometry with the nodes at the unloaded edge and the corner being constrained in the longitudinal and the tangential directions, respectively. Displacement was applied at the other end of the specimen separating the two beams in a mode I condition. Finally, the number of cycles is estimated from Paris law. To verify the proposed model, fatigue crack growth (FCG) tests were performed on an 8-layer unidirectional CFRP laminates (HexPly T700/M21) fabricated into DCB specimens. The substrates is joined by an aerospace grade adhesive (Araldite 420). The estimated energy release rate (ERR) using the developed finite element model is within 90% or more of that determined experimentally.

Magnesium alloys are attractive lightweight structural materials that have the potential to be used in aerospace, automotive and medical industries. However, fatigue is a major cause of failure in structural components. It is also known that notches, which are unavoidable, have a detrimental effect on fatigue resistance. Fatigue damage is associated with the formation of crack(s). Therefore, it is necessary to understand the fatigue crack growth behavior for better design analysis. In this research, a V-notched specimen machined from ZK60 magnesium extrusion is tested under cyclic axial loading. Fatigue experiment was performed under completely reserved force-controlled loading with an amplitude of 12 kN and under standard laboratory conditions. The fatigue test was interrupted 14 times in order to perform ex-situ x-ray tomographic analysis using the Nikon XTH 225 ST system. The sample failed after 184,005 cycles. A gray-level thresholding process was performed using MATLAB on the selected slices to convert each pixel to either white or black color. Then, these slices were stacked using imaging software to make 2D representations of the crack surface at different cycles. As a result, the areas of the crack surfaces were measured for all scans. The result suggests an exponential relationship between the crack surface area and the number of cycles.

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CFRP is gaining interest in several industries such as aerospace, sports, and oil field. When this material is assembled, the adhesive is considered a preference over screws and fasteners as screws holes can lead to matrix delamination. Prior applying an adhesive, surface pre-treatment is done to enhance bonding. Due to the complexity of the composite material namely in complex geometry, one can consider finite element analysis as an optimum method to model the material behavior. Failure of crack growth under cyclic loading is typically modeled using the CZM. However, finding the constitutive behavior parameters is considered challenging. In this work, the maximum stress, which is difficult to calculate experimentally, is estimated using the virtual closure technique (VCCT) as it is considered less complicated and costy than the conventional methods. The VCCT is a finite element method that is employed to simulate monotonic crack growth. From this model, the maximum stress is recorded and used as the maximum traction stress in the cohesive zone model (CZM) to simulate fatigue crack growth. The bilinear traction separation law was employed to simulate the cohesive process zone. To calibrate the model results, an experiment is conducted on two samples those were treated by two different methods. One sample has a sandblasting surface pre-treatment and the other is pre-treated by peelply. Each pre-treatment enhances different material toughness and hence validity of the results if supported. Both samples were tested under both static and cyclic loadings. The maximum energy release rate and the crack length were selected as comparison parameters between the models results and the experimental observations. Overall, it was noticed that the results are considered having reasonable fit.