Composite scarf repair has been widely applied as a high efficiency choice for force transferring in aircraft structure repair. The damage tolerance and resistance to low velocity impact are significant for structure design especially for primary load-bearing structures. Some literature studied impact damage and resistance for scarf topology repairs or joints. However, it is not clear that which location in bonded area is the most sensitive to the impact load. The impact location effect on impact damage mechanisms and capacity of tension after impact (TAI) are studied in this paper. Central point in bonded zone has the strongest influence on impact damage. The scarf adhesive film fractured from the feather tip of the back side and oriented to the weakest material. The load capacities of tension after impact at different locations coincide with the adhesive damage area ratio. The finite element model based on CZM (Cohesive Zone Model) is built. And the simulation results validate that central location has the most sensitive characteristic to impact damage and TAI capacity.

Stepped and scarf topology are still the preferred choices for repairs and joints of composite primary load-bearing structures. The stepped bonding scheme is easy to be implemented in real engineering process. The precise stress distribution, load capacity, damage mode and failure mechanism are not adequately clear. This paper adopts static stress analysis of FEM (Finite Element Method), experimental studies and CDM (Continuum Damage Mechanics) to reveal the tension failure mechanism for composite stepped bonding structures. In the static stress analysis, the mesoscale FEM model considered the adhesive thickness was created. It investigated the detail stress distribution and concentration of the step topology adhesive. Then seven specimens with tensile load were studied experimentally through relation of load-deformation and the appearance of damage. For more details, the adhesive damage was simulated by using CZM (Cohesive Zone Model) which includes the mix-mode fracture of normal force and sliding shear force. The process of damage propagation evolution can be clearly observed in the simulation results. The damage initiates at the vertical location of steps adhesive and propagates to the horizontal overlaps adhesive. Simulation for predicting the ultimate load capacity coincides with experiments. This paper’s work will be useful for designing and analyzing the full composite materials airframe and wing repaired by stepped bonding technique.

Due to the high connecting strength and excellent load-transferring efficiency, the adhesively stepped-lap composite bonding has been widely used as joints and repairs in advanced aircraft structures. In the service life of composite structures, they frequently subject to bending loads which are rarely reported by the available reference. Hence, in this paper, three-point and four-point bending experiments are performed on composite stepped-lap structures. Typical damage mode of adhesive on the stepped surface has been found in experiments. And the failure of composite fiber, matrix, interlamination and adhesive materials appeared as competitive behavior. Furthermore, numerical models of stepped-lap composite joints, which used cohesive zone model and 3D Hashin criteria to simulate the inter- and intralaminar damages, were utilized to predict the bending strength and progressive damage. The FEM model, based on progressive damage method, captured the detailed failure as the loading displacement was increasing. The simulation results of load displacement and damage have good agreement with the experiments.

The ingested particles or debris will cause high-speed & low-energy impact to the hot end components of aeroengine. Few literature focus on the projectile impact of SiC/SiC with Environmental Barrier Coating(EBC). The impact of 2 mm projectile was specially studied for Si/Yb₂Si₂O_7/Yb₂SiO₅-SiC/SiC. Gas gun system was used to conduct impact testing at the speed of 3 m/s∼353m/s. The optical microscope, Scanning Electronic Microscope(SEM), micro CT were used to observe the impact and tension after impact(TAI) damage. For TAI, Digital Image Correlation (DIC) was used to conduct the full-field strain detection around the impact point. Meanwhile, the Smoothed Particle Hydrodynamics (SPH) methodology was programmed in C++ and developed to simulate SiC/SiC impact problem by introducing modified Hashin-Rotem criteria, extending interface method to 3D and adding GPU accelerated algorithm. The results indicate that threshold velocities of EBC huge peeling and SiC/SiC penetration are different, TAI residual strength also has extinct threshold at the velocity of 246 m/s with 37.1% declining extent, and the impact pit depth of SPH simulation is in good agreement with the experiment.

Composite bonded joints are widely used in the aircraft components, which are frequently subjected to impact loading. The present paper intends to evaluate the mode II fracture behavior of composite bonded joints under high loading rate conditions. Dynamic loading was applied on the end-notched flexure (ENF) specimen at 10 m/s and 15 m/s via an electromagnetic split Hopkinson pressure bar system. Then the dynamic fracture toughness was evaluated using an experimental-numerical hybrid method according to the virtual crack-closure technique and compared to the fracture toughness under quasi-static loading rate (2 × 10⁻⁵ m/s and 2 × 10⁻⁴ m/s). Results indicate that the mode II fracture toughness of adhesively bonded composite joints shows a positive rate-dependent trend. The mode II fracture toughness can increase about 31.5% when the loading rate increases to 15 m/s. At last, fracture surface inspections using scanning electron microscopy (SEM) were conducted to reveal the physical mark of such an effect.