Study On The Relationship Between Low Energy Impact Damage And Failure Of Composite Laminates

Structures subject to low energy impact can cause no significant damage to the structure,resulting in a performance degradation and a serious threat to the safety of the components.According to the injury site can be divided into internal damage and external injury,external damage is the most direct impact of the composite layer of impact.The observation and description of external damage are more direct and objective,and the performance of impact damage to the material is also the most persuasive,so the study of the external damage to the impact area is more research significance and value.The glass fiber reinforced resin matrix composites and carbon fiber reinforced resin matrix composites used in this paper are the most representative continuous fiber reinforced laminated composite materials.The unidirectional laminates with quasi isotropic fiber layer and multi ply laminated plates with the grid cloth as the layer are studied.The low impact impact test was carried out on the laminates with different impact energy and the impact energy of the unidirectional plate is 1 to 5 times of the thickness,and the impact energy of the multi-direction plate is 2 to 6 times of the thickness.The ultra-deep electron microscope was used to observe and describe the damage characteristics of the impact zone.Then,the leakage detection is carried out after observing the impact area.Under the microscope,it can be seen that there are different degrees of cracks around the impact zone of the laminates with the different impact energy.When the impact energy is small,there are one or two fine and short cracks around the impact position.As the impact energy increases,the number of cracks increases,the crack becomes thicker and more obvious,and the length increases.Analysis of the relationship between the impact energy and the pit depth of the composite laminate,found that laminates have an inflection point when the impact energy is 3 times the thickness of unidirectional laminates and the impact energy is 4times the thickness of multidirectional laminates.After the inflection point of the glass fiber epoxy laminated specimen impact area on the back of the "whitening" phenomenon and a slight uplift phenomenon.The variation curves of the pit depth of the carbon fiber multi-ply laminated plate appear at the inflection point when the impact energy is 3 times the thickness of the plate.These inflection points show that the impact of the impact of the test specimen to form a significant impact damage,composite laminates on the impact resistance of the sudden change.After the leak experiment showed that glass fiber unidirectional laminates when the impact energy is 2~3J/mm,composite laminates in the impact energy of 3~4J/mm leakage,while the carbon fiber composite laminates in the impact energy of 2~3J/mm leakage.The experimental results show that the glass fiber composite laminates have better impactresistance than the unidirectional laminates,and the glass fiber composite laminates have better impact resistance than the carbon fiber composite laminates under the same conditions.In this paper,the impact damage factors,such as the depth of impact pit,the impact damage area per unit thickness,and the apparent loss are quantified.The failure damage criterion and the practical engineering experience can be used to evaluate the damage degree of the composite laminates.It is difficult to give an accurate evaluation of the failure of composite laminates due to the complex interweaving of various damage factors.Therefore,the fuzzy theory is the most suitable method to deal with the problem of comprehensive evaluation.According to the damage of composite laminates after impact based on the theory of fuzzy mathematics to establish the comprehensive evaluation index system,and establish the calculation model and program of fuzzy comprehensive evaluation to determine the extent of damage in composite laminates.