Bending Deformation Measurement And Damage Acoustic Emission Monitoring Of Fiber Composite Materials After Thermal Aging

Fiber-reinforced polymer matrix composites are widely used in aerospace,automotive,and energy fields because of their high specific strength,and excellent fatigue properties.Under the high temperature service environment,the degradation of matrix and interface properties easily leads to the significant reduction of mechanical properties of composite structures.Therefore,it is meaningful to study the bending deformation and damage evolution mechanisms of glass fiber reinforced resin matrix composites under different thermal aging conditions and the effect of multilayer graphene on the aging resistance of the composites.In this paper,combining the deformation characteristics,acoustic emission signal response and machine learning algorithm,and infrared thermal response of composite specimens,the effect mechanisms of different aging temperature and aging time on bending deformation and damage of composites are investigated by using the digital image correlation method,acoustic emission technology and infrared thermal imaging technology.The results show that:(1)When the thermal aging temperature was increased from 25℃ to 90,120 and 150℃,the bending strength of the composite specimens showed a slight decrease followed by an increase after 8 days of aging.Under the thermal aging condition of 90℃,the post-curing reaction of the resin matrix is not significant enough,and the load-bearing capacity of the composite specimen shows a certain decrease due to the high-temperature thermal decomposition,and the acoustic emission signal corresponding to the fiber/matrix interface damage increases obviously.The cross-linking reaction of the resin matrix increased after thermal aging at 120℃.The damaged areas of the composite specimens with fiber fracture prefabricated defects exhibit obvious strain concentration and correspond to more acoustic emission signals during the three-point bending process after thermal aging.(2)The flexural modulus decreased,and the flexural strain increased of the composite specimens after thermal aging at 120℃ for 0,4,8,and 16 d.Combined with the acoustic emission signal clustering analysis,it was found that the cross-linking reaction of the composites after thermal aging improved the interfacial properties,and the acoustic emission signals corresponding to the interfacial damage were reduced and mainly distributed after the failure load.For the specimens with prefabricated fiber breakage,after 4 d of thermal aging,the cross-linking phenomenon causes a decrease in the number and amplitude of fiber breakage signals during loading of the composite,which effectively improves the loadbearing capacity of the composite.After 8 d of thermal aging,the bending mechanical properties of the composite specimens decreased and their damage corresponding to high amplitude signals increased.The combination of acoustic emission clustering analysis and digital image correlation method can better describe the damage evolution of composites under different thermal aging conditions,which can help to further reveal the influence of defects and thermal aging time on the internal damage of composites.(3)Compared with pure resin-based composites,multilayer graphene-filled composites have higher heat transfer and heat dissipation efficiency,and their bending strength increases and maximum strain decreases after thermal aging,indicating that multilayer graphene filling improves the aging resistance of the composites.Combining supervised and unsupervised algorithms for acoustic emission signals of bending damage of composite specimens,a prediction model for bending damage modes of composites was constructed,which can identify the damage modes of composite specimens more accurately.Due to thermal aging,composite specimens undergo matrix damage earlier during bending loading and the cumulative energy of fiber/matrix damage increases.Based on the bending damage characteristics of composites after thermal aging,the acoustic emission response behavior and damage mechanism of bending damage of composite specimens can be well verified.