In this paper, the mechanical behavior and damage evolution of CFRP laminates under three-point bending loads were investigated. Based on the three-dimensional Hashin failure criterion and cohesive zone method, a progressive damage analysis model was constructed to predict seven different failure modes, and an improved stiffness matrix degradation law was proposed. Numerical simulations were performed using ABAQUS finite element software and UMAT user subroutines and verified by three-point bending tests. The experimental results show that the relative errors of the flexural strength and flexural modulus of the current model simulation results and the experimental data were −3.10% and −2.70%, respectively, and the damage evolution process after the ultimate load was accurately predicted. Meanwhile, the predicted final failure and damage modes were consistent with the micrographs of the failed specimens.

The purpose of this investigation is to study the performance degradation mechanism of CRRP composite materials used in the structure of a solar UAV under acid rain environment and to provide references for the structural design of the solar UAV. An aging test was designed according to the actual working conditions of the solar unmanned aerial vehicle and was carried out by continuously immersing the composite material in both deionized water and an acidic solution with a pH of 2.0 at 60°C. The mechanical behavior of the composites was studied through a three-point bending test. The results showed that after 120 days of aging test, the composite material exhibits different characteristics in the two conditions. The composite material under the hygrothermal conditions finally reached the equilibrium moisture absorption content of 1.07%, and the flexural strength decreased by 8.77%. The composite material under acidic conditions deviated from the Fick behavior in the final stage of the test, and the final moisture content was 2.88%, and the bending strength decreased by 26.43%. Several typical empirical models were analyzed, and a hyperbolic tangent function empirical model with moisture content as the main variable was used to predict the CFRP flexural strength degradation process, and good results were obtained. The effect of aging conditions on the microstructure of composite materials was observed by SEM, and the aging mechanism of composite materials was analyzed. The corrosion of the polymer matrix and the debonding of the fiber/matrix interface are the root causes of the performance degradation of composite materials.

In this paper, an attempt is made to describe the decline in fatigue life of an aluminum alloy structure containing bolted joints under the conjoint influence of an aggressive aqueous environment and loading that is essentially cyclic in nature. In an attempt to project the real situation experienced by an aluminum alloy structure containing bolted joints while in service, an alternate cycle test method was used to essentially include the damage caused by the environment and cyclic loading. A chloride-containing solution was used to simulate the damage caused to the bolted structure upon exposure of the aluminum alloy structure containing a joint to the environment. The nature of fatigue loading was essentially cyclic tension. It was found that following an observable amount of environment-induced damage caused by exposure to a wet–dry environment followed by cyclic loading, the fracture position revealed a noticeable shift from the centerline of the bolt hole to the loading end. This can be attributed to the occurrence of fretting-related damage at the fine microscopic level. Based on an assumption of an average fracture line, a life prediction model was established. Compared with experimental results, the prediction method considering the fracture position to shift from the centerline of the bolt hole does provide results with a reasonably high degree of accuracy. The method proposed in this study does provide a convincing approach for reliable estimation of life, or endurance, of an aluminum alloy structure containing joints.

We carried out a comparative study on both the stress corrosion response and corrosion damage characteristics of aluminum alloy 2219, both the base material and a friction stir welding (FSW) counterpart upon exposure to exfoliation corrosion (EXCO) solution. The results reveal that the test specimen containing an FSW joint reveals better electrochemical corrosion resistance than that taken from the base metal. When test specimens upon exposure to EXCO solution are concurrently subjected to a tensile stress, since the mechanical properties of the FSW joint are lower than the base metal, a test specimen containing an FSW joint is more easily prone to the early initiation of fine microscopic cracks. This makes the test specimen containing the FSW joint to be less resistant to stress corrosion damage than that taken from the base metal for the various levels of applied stress and exposure time to EXCO solution. The average corrosion depth of the test specimen containing the FSW joint is less than that of the base metal, while the maximum corrosion depth of it is greater than that of the base metal. This reveals that test specimen containing the FSW joint is more susceptible to damage and degradation than test specimen taken from the base metal.

In this paper, a new optimization design strategy is explored to achieve a rapid design of lightweight structures for unmanned aerial vehicles (UAVs) using honeycomb sandwich panels. Based on the existing performance equivalent method of honeycomb core material, an effective mechanical property homogenization model of honeycomb material is proposed and applied to finite element analysis. A finite element model is established to analyze the mechanical response of honeycomb sandwich panels under different bending loads, and the rationality of the model is verified by mechanical tests. The error between the simulation results and the experimental results is about 10%. Based on the variable density method and rational approximation of material properties interpolation method, a topology optimization model with the minimum structural flexibility as the objective is constructed. By comparing with the experiment-based design scheme, it is found that the optimized design scheme has a more reasonable structure and better performance. The optimized design can effectively reduce the structural weight and improve the load transfer path while ensuring structural stiffness. In this way, the influence of stress concentration and large deformation on the structure is avoided, which is of great significance for the design of UAV structures.

In this paper, a coating–substrate interfacial corrosion test method was developed to simulate and study the failure processes occurring at the coating interface as a direct consequence of environment-induced degradation or corrosion. It was found that the corrosion-induced failure rate of the coating–substrate interface upon exposure to an aggressive corrosive medium was high. Microscopic pits tend to appear at the interface of the coating and the substrate. The permeation channel at the coating interface did cause the corrosive medium, primarily the chloride ions, to gradually diffuse from the sides of the sample to the inner surface of the interface thereby enabling the initiation and continued progression of “local” corrosion. The process for failure due essentially to corrosion of the coating was established, while ensuring to include the infiltration phase, the presence of “local” corrosion phases, expansion, if any, due to corrosion, and eventually culminating in failure. Based on the experimental results, a finite element simulation of the “local” corrosion occurring at the coating interface was executed. The results revealed the microscopic pits at the interface to progressively increase the “local” stress concentration on the surface of the substrate but were found to have little influence on overall stress distribution in the coating. It was also found the shape of the etch pit had an effect on failure expansion under the influence of stress. The numerical method can be used to predict structural failure caused by corrosion pits at the interface of the coating–substrate system in an aggressive environment.