In aircraft engineering, an increase of internal pressure in a hydraulic pipe increases the probability of pipe damage, leading to crack propagation becoming a serious issue. In this study, the extended finite element method (XFEM) is applied to simulate initial crack propagation in hydraulic pipes and to investigate the influence factors. Stress intensity factors are extracted to verify the mesh independence of XFEM, which is based on the level set method and unit decomposition method. A total of 30 finite element models of hydraulic pipes with cracks are established. The distribution of von Mises stress under different initial crack lengths and internal pressures is obtained to analyze the change of load-carrying capacity in different conditions. Then, a total of 300 finite element models of hydraulic pipes with different initial crack sizes and locations are simulated under different working conditions. The relationship between the maximum opening displacement and crack length is analyzed by extracting the opening displacement under different initial crack lengths. The length and depth of the initial crack are changed to analyze the factors affecting crack propagation. The opening size and crack propagation length are obtained in different directions. The results show that radial propagation is more destructive than longitudinal propagation for hydraulic pipes in the initial stage of crack propagation.

Featured Application: This work has a potential application to anti-lightning-strike design of aircraft carbon fiber/epoxy composite panels. Abstract:In order to investigate the lightning damage behavior of an aircraft carbon fiber/epoxy composite-reinforced panel and its protection structures, four types of panels were selected to carry out a lightning experiment. Panels were without protection, with a full-sprayed aluminum coating, a local-sprayed aluminum coating, and a full-embedded copper mesh filling, respectively. Their surface and internal damage was detected via ultrasonic C-scanning. Results showed delamination damage for the protected and unprotected specimens due to substantial lightning Joule heat, thermal shock, and internal explosion. The aluminum coatings and the copper mesh had good shielding performance against anti-lightning strike damage. The protection method with a full-sprayed aluminum coating is more effective compared with the other two methods. This study is valuable to investigate the protection effectiveness of metal covers when aircraft composite structures are struck by lightning.

For type-II superconducting bulks used as trapped-field magnets, the thermomagnetic instability, manifested as flux jumps and temperature spikes, frequently takes place, resulting in a large amount of energy dissipation in a short time and further the crack problem due to electromagnetic and thermal stresses. In this paper, based on the magnetic and heat diffusion equations and fracture theory, we develop a thermal-magnetic-mechanical coupling model to analyze the flux-jump and fracture behaviors in bulk samples of BiSrCaCuO under various magnetization processes. This model has an important advantage that the simulation domain can be restricted to the sample itself, without having to consider the air region around it, and its reliability is verified by the existing experimental and numerical results. The effects of the sample size, the ambient temperature, and the sweep rate, direction, and uniformity of the external magnetic field on the flux jumps, and Mode I and Mode II stress intensity factors are fully analyzed under different cooling conditions. It is found that as ambient temperature or field inclined angle increases or field sweep rate decreases, the first flux-jump field presents a trend of monotonically increasing for zero-field-cooling magnetization but it has an opposite trend for field-cooling magnetization. The flux jump can lead to the jump of temperature, electromagnetic force, and stress intensity factor. In addition, the sensitivity of flux-jump and fracture behaviors to different parameters and the influence of flux jump on the demagnetization behavior under crossed magnetic fields are discussed. We also find the levitation force jumping phenomenon when the bulk sample is magnetized in a nonuniform magnetic field. From the results obtained, we provide some general guidelines on how the system parameters of superconducting bulk magnets could be chosen to improve the thermal-magnetic-mechanical stability.

3D orthogonal woven composites are receiving increasing attention with the ever-growing market of composites industries. New challenge what we face to is how the damage tolerance improve in such composites with orthogonal and layer-to-layer structure under both mechanical and extreme environment. In this paper, a novel impact damage suppression strategy is proposed by combining structural and electromagnetic properties to realize advanced functionalities. An integrated experimental platform is designed with a power system, a drop-testing machine and data acquisition devices to investigate the synergistic effects of pulse current and impact force on composites. Experimental results exhibit that pulse current can effectively reduce delamination damage and residual deformation. A multi-field coupled damage model is developed to analyze the evolutions of temperature, current and damage. The microcrack formation and extrusion deformation in yarns causes the local current redistribution in carbon fibers, and its interaction with the self-field produces an obvious anti-impact effect. The obtained results reveal the mechanism of damage suppression and provide a potential orientation for improving damage tolerance of these composites.