Study On Toughening Mechanism Of Nylon Net Between Metal-Metal Adhesive Composites

Metal-metal bonding area exists in the restricted district of entry helicopter honeycomb panel,such as empennage,doors and windows and other parts must be provided with edge sealing.The load distribution in the connecting part is very complex,making the bonding area produce cracks under low peel strength,and the phenomenon of constant expansion of cracks even occurs,which greatly reduces the functional performance and carrying capacity of bonding components.Therefore,in case of not changing the base materials and not increasing many processes,designing appropriate interlayer structure to restrain composite materials from delaminate damage to a large extent and improve its fracture toughness becomes a research focus in composite material design and analysis.Experimental results showed that implanting nylon net toughening layer inside the film adhesive layer could significantly improve the fracture toughness of composite material on the premise of assuring good process performance.However,at present,there are few researches focusing on the mechanism of film adhesive nylon net interlayer toughening,as a result,few experimental data and literatures are available for reference.Hence,studying the toughening mechanism of film adhesive nylon net interlayer by conducting tests and establishing numerical analysis model of interlayer layered damage of nylon net interlayer toughening composite material is of significant application value and theoretical significance.In this paper,through double cantilever beam（DCB）test and end notch flex（ENF）test,the facture toughness of aluminum-aluminum bonding parts containing toughening film adhesive and not containing toughening film adhesive was tested,and an analysis was offered for the influence of nylon net toughening film adhesive on model I and model II interlayer facture toughness of bonding parts.According to the test result,it showed that after implanting nylon net toughening layer in the film adhesive,the peak load,extension resistance andG_（40）c of the composite material at the time of model I layering was significantly improved.When the initial cracking length was 50mm,G_（40）c increased from 0.211N·mm^-1 to 0.386N·mm^-1.In comparison tointerlayer toughening layer without nylon net,it realized an improvement of 45%in respect of energy release rate.From the cross section analysis,we found that the cracks deflected to enter the nylon net toughening area after the introduction of toughening layer,so that stratification between nylon net and film adhesive interface and cohesive failure of film adhesive in the nylon grid area happened,resulting in an improved interlayer mechanical property of bonding composite material.The energy release rate of model II cracks increased from 0.667N·mm^-1 to 1.168N·mm^-1,indicating that the energy release rate increasing by 42%in comparison with that of interlayer toughening layer without nylon net.It showed that the capability of composite material in interlayer laying resisting was significantly improved after implanting nylon net toughening layer in the film adhesive layer.The existing researches indicated was associated with compression after impact of laminated plate,namely,ifG_{（40）（40）c} is high,so will the compression after impact.Using the MMB mixed-model loading scheme,four different ratios of I/II mixed-model tests were carried out,and the component valuesG_（40）m andG_{（40）（40）m} at each ratio were obtained.Combined with the results of pure model I test and pure model II test,the failure criteria of mixed model delamination was established,which provides a basis for the structural design of metal-metal bonded composite materials.The multi-scale analysis of the toughened film between nylon net layers was carried out,and the correctness of the multi-scale analysis model method adopted in this paper was verified by a numerical example.Predicting the macroscopic mechanical properties,obtaining the overall macroscopic performance parameters,and then establishing the finite element analysis model of the nylon net toughened plastic film double cantilever beam and verifying the test results.At the same time,the J integral of the crack tip of the untoughened and toughened finite element models was calculated.Compared with the untoughened finite element model,the J integral of the crack tip at the crack propagation in the toughened zone decreases significantly when interlaminar delamination occurs,while at the interface between the toughened film and the aluminum plate,the difference was not significant.Meanwhile,the Y-direction normal stresss_y and interlaminar shear stresst_yz at the crack tip were significantly reducedafter adding the nylon net toughening layer,and the stress concentration at the crack tip was effectively improved.