The wet-assembly hybrid bonded/bolted (WHBB) joint is increasingly employed in aircraft fuel tank structures owing to its advantageous mechanical strength and sealing performance. However, the integral tank is susceptible to leakage during service, particularly at the joint, which seriously endangers the flight safety of the aircraft. In this paper, a leakage prediction method of WHBB joint based on porous media theory is proposed, in which the shape and characteristic length of the sealant layer are taken into consideration. The model parameters are determined by the analysis and treatment of the defect state of the WHBB joint section. The prediction results agree well with the experimental data, which were acquired by self-designed sealing leakage rate measurement system, and the deviation between the predicted results and the average value of the experimental data is less than 20%. Furthermore, in order to verify the environmental adaptability, the prediction results based on 2D cutting sections of the joints and experimental results under three different loading conditions are compared. The comparison results not only prove the accuracy of the prediction model, but also reveal the important influence of tensile fatigue load on the sealing performance of the structure. The tensile fatigue loads lead to two orders of magnitude increase in leakage rate, and the reason is that the repeated stretching and compression process lead to an increase in interfacial cracks between the adhesive layer and the hole wall, thereby accentuating the defects within the adhesive layer.

In this paper, the influence of forced installation caused by a hole-location error on the 3D stress distribution and damage of a composite bolted joint is investigated. An analytical model of stress distributed on composite holes is promoted, in view of non-uniform extrusion caused by forced installation. At first, non-uniform extrusion of the hole edge caused by forced installation is analyzed. According to the contact state, expression of hole deformation is given. Then, based on Hertz theory, the maximum extrusion load is obtained with help of deformation expression. By constructing an elastic foundation beam model, 3D stress distributed on a hole could be analyzed according to the extrusion load. Then, stress distribution predicted by the above analytical method is compared with that provided by FE considering composite damage. Finally, a forced installation experiment is carried out to analyze the damage distribution of the joint. Results show that a central-symmetrically distributed stress is introduced by the hole-location error. With an increment of the error, strength of composite decreases due to extrusion damage. Therefore, stress presents a concave distribution on the hole. As the hole-location error exceeding 3%, stress decreases gradually due to failure of composite. Damage of holes does not exhibit a centrosymmetric distribution. Serious damage is mainly distributed on the entrance of the hole at the lower sheet.

The countersunk bolt is widely used in aircraft bolt connection because it can satisfy the requirements of aircraft aerodynamic shape and reduce the radar reflection area. However, the countersunk bolt is affected by the geometric error of countersink and assembly error in the assembly process, which directly affects the joint strength of composite structure. The tensile strength of the composite structure of the countersunk bolt is affected by the countersink depth, angle error and nail pressing method. In this paper, the effects of sink depth, countersink angle error and nail pressing method on the tensile strength of single-bolt single-lap sink interference joint were studied by experiments and numerical methods. Nine experimental specimens are fabricated according to the ASTM-D5961 standard. According to the comparative analysis of experimental and simulation data, the results indicate that the oblique pressure connection should be better than the direct pressure connection; The ultimate strength of the joint increases with the increase of the countersink angle deviation; the increase of countersink depth error leads to the decrease of tensile ultimate load of joint; The ultimate tensile strength of the joint decreases with the increase of the angle of the deflection direction of the angle error, and the angle error of the countersink should be controlled within 1°. The empirical formula of the influence of countersink geometric error on the ultimate tensile strength of the interference connection structure of composite countersunk head bolts is established, which provides a reference for improving the connection strength of composite head bolts in engineering applications.