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In order to study the mechanical properties of ZT7G/LT-03A carbon fiber reinforced composite bolted joints, the single-bolt double-lap joints tests were carried out with 3 kinds of ply and 3 kinds of width, and the effects of the different ply ratios, width to diameter ratios on the failure load and failure mode were analyzed. Then, the failure process of the specimens was simulated by using the progressive damage and failure analysis method. The failure mode and failure load of the model are consistent with that of the specimen, which verified the accuracy of the model. On this basis, the stress and damage status of each ply during initial load shedding were studied, and the influence of the ply ratios and width to diameter ratios on the performance of laminates was further studied. The results show that when the width to diameter ratio of laminates increases, the failure mode changes from tensile failure to bearing failure, and to change the ply ratios does not affect the failure mode in a certain range.

Composite structures have been widely used in the field of aerospace.In order to study the mechanical properties of composites in hygrothermal environments,the pull-through tests of a type of polyimide composite laminates are carried out under three test environments:cold temperature dry(CTD),room temperature dry(RTD)and elevated temperature wet(ETW).The numerical simulation analysis of the RTD test process is carried out.The pull-through strength and failure mode of two typical laminates are obtained by experiment,and the failure load is predicted by simulation analysis.The results show that the type of polyimide composites have excellent thermal stability.The pull-through strength of the composites under CTD condition is 8.1%~9.0% higher than that under RTD condition,and the pull-through strength of the composites under ETW condition is equivalent to that under RTD condition.The pull-through strength of(30/60/10)ply is slightly higher than(40/50/10)ply.

A new longitudinal failure criterion for unidirectional fibre reinforced (UD) composites is proposed, incorporating the contribution of local fibre misalignment to fibre failure. The failure function was established based on the quadratic polynomial expansion of stress components in the local coordinate system parallel to the misaligned fibres, with coefficients rationally determined based on the basic strength parameters of UD composites. The local stress components were used to determine the failure status associated with fibre compression, while the tensile load along fibres was assumed to have negligible influence on the development of the fibre misalignment. The predicted failure envelopes agreed well with the experimental data for different stress combinations, particularly in the regimes where the fibre failure was influenced by transverse stresses. This validates the predictive ability of the proposed criteria and highlights the importance of the contribution of matrix-failure-related stresses to the fibre-dominated failure behaviour of fibre reinforced composites. [ABSTRACT FROM AUTHOR]

The joint part is usually a weak link of the composite structural strength, and the multi-bolt joint of the composite greatly reduces the joint strength duo to the uneven bolt load distribution. Therefore, it is of important significance for increasing efficient load transfer to determine the distribution of the bolt load and its influencing factors. Taking the composite-to-titanium hybrid structure as the research object, the load distribution characteristics and influence of different assembly forms of co-cured, bonded and separated on bolt load under the joint of two kinds of fasteners, such as the protruding and the countersunk, are analyzed. The stress distribution of the bolt hole in the joint area is measured by strain electric measurement. The numerical simulation model is established by ABAQUS software to analyze the bolt load distribution of joints in composite laminate with metal. The results show that the load distribution of the co-cured composite laminate with countersunk bolt joint is stepped from the loading end to the opposite end of the branch, while the load distribution of the other joints is high on both sides and low in the middle. The only difference is the proportions of bolt load in each row. The bolt load differences of the separated composite laminate are the largest, the bonded are the second, and the co-cured is the smallest.

Different phenolic resins have a direct impact on the properties of aramid paper honeycombs. It is important to study the relationship between phenolic resin and honeycomb properties. Two different phenolic resins are used to prepare aramid paper honeycomb core materials, and the compression properties, shear properties, flame retardancy and dielectric properties of the prepared honeycomb core material are tested. Combining the observation of resin end face resin distribution and contact angle measurement, the reasons for the difference in performance of honeycomb core materials prepared by two kinds of resins are analyzed. The results show that the temperature resistance of the resin itself directly affects the temperature resistance of the honeycomb and the volatilization rate of solvent water in the resin and the surface tension between the resin and the aramid paper affect the resin distribution of the resin on the honeycomb wall. When the content of the honeycomb core resin is the same, the resin rubber column at the end of the joint is beneficial to increase the compressive strength of the honeycomb core material, but the accumulation of the resin at the end of the node reduces the wall thickness of the honeycomb, resulting in a decrease in the shear strength and modulus of the honeycomb.

High-performance thermoplastic composites, due to their high toughness, excellent impact damage resistance and better fatigue resistance, are especially suitable for constructing special helicopter structures such as the landing gear, tail section, rotor hub and transmission drive shaft, which requires high damage-resistant, ballistic-tolerant, anti-fatigue properties. The application of high-performance thermoplastic composites in foreign helicopters is expanding from the application of key laminate structures to honeycomb sandwich structures, and a large number of low-cost and auto-mated manufacturing techniques such as in-situ automated fiber placement (AFP) and low-cost verification techniques such as virtual testing is being used. Limited by the low level maturity of domestic thermoplastic composites technology, high cost and long cycle of the traditional building block verification methods for the new composite system, the application of thermoplastic composites in domestic helicopter is still in initial stage, the performance stability and consistency of thermoplastic composites, supplementary low cost automatic manufacturing process, low density and high temperature core material and high-confidence efficient virtual qualification technology are needed to focus on the future.

The fastener pull-through resistance is a key performance index of composite laminates used for engineering application, and increasing research attention is being paid to developing methods for its calculation or estimation. The currently available research methods mainly focus on the standard test and the finite element analysis for determining the pull-through resistance of composite laminates suffering transverse load by the fasteners. Based on the results of the fastener pull-through resistance experiment performed on X850 composite laminates, a model for estimating the maximum affordable load of composite laminates for the fastener pull-through resistance is proposed, using generalized regression neural network technology. The inputs of this model are simplified to six parameters: the proportion of the ±45° layer of the laminates, the number of the layers, the thickness of the laminates, the bolt head shape, whether the bolt has a washer or not, and the nominal diameter of the bolt; the Gauss function is used as the hidden layer function. The model uses a large portion of the experimental data to train for finding the optimal smoothness factor, which is used to reconstruct the model, and simulation is performed with the remainder of the experimental data. The comparison between the estimated results using the model and the experimental results shows that the generalization ability of the proposed model can meet the estimation requirements. Moreover, the pull-through resistance of composite laminates under transverse load from a fastener can be estimated with high accuracy after some standard fastener pull-through resistance tests of the composite laminates.