A Study On The Durability Of Glass Fiber Reinforced Thermoplastic Composite

In recent fifty years,glass fiber reinforced thermoplastic composite materials were widely used in the aerospace,industrial machinery,civil engineering,automobile industry,electro-chemical,and live entertainment and other fields instead of many traditional materials such as metal,wood because of light weight,high strength,excellent resistance to chemical corrosion,well designability.In the process of application in real environment,composite materials will be affected by various physical or chemical factors in the environment,which will deteriorate their properties.Therefore,the research on long-term durability of the composite materials plays a significant role in practical application.In this research,the long-term durability of glass fiber reinforced thermoplastic composites is discussed and analyzed from two aspects of hygrothermal aging and the dependence on time and temperature of flexural property of composites.For this reason,accelerated hygrothermal aging test,static bending test at different temperatures and speeds,and dynamic mechanical analysis test at multiple temperatures and frequencies were carried out,and the long-term life of the composite was predicted by using the time-temperature equivalence principle,and the following conclusions were obtained.Firstly,GF/PA and GF/PET composites with 20%and 40%glass fiber content were immersed in 80?deionized water for 0h,10h,25h,120h,436h,900h,respectively.Experiments showed that the weight gain rate increased linearly for both GF/PA and GF/PET composites before aging 120h,and then achieved balance.As the aging time increase,GF/PA composite weight gain rate fell slightly.Otherwise,the increase of the glass fiber reduced the diffusion of water molecules in the composite material,and the diffusion velocity in the GF/PA is greater than that in the GF/PET composites,moisture absorption process has a good agreement with the Fick's law.Secondly,during the whole hygrothermal aging process,the tensile strength of GF/PA-20%and GF/PA-40%composites decreased by 29.58%and 23.71%at the start,then increased by 3.95%and 0.48%,respectively,and then decreased by 65.82%and51.88%,respectively.The tensile strength of GF/PET-20%and GF/PA-40%composites increased by 2.02%and 2.11%,respectively,then decreased by 44.2%and 49.88%.After hygrothermal aging for 900h,for GF/PA composites,the strength retention rate increased with the increase of glass fiber in GF/PA,but for GF/PET it was the opposite.In the process of hygrothermal aging,the modulus variation trend of GF/PA and GF/PET composites was similar,both increasing at the beginning and then decreasing,but the modulus of GF/PA composites decreased by 7 times as much as that of GF/PET composites.Firstly,with the increase of the aging time,the glass transition temperature of GF/PA composite material gradually decreases,resulting in the deterioration of heat resistance of the composite material.However,for GF/PET composites,the glass transition temperature has little change with the increase of aging time,and the storage modulus?E'?of the composite decreases.Thermal decomposition temperatures of GF/PET and GF/PA composites did not change after the hygrothermal aging for 900h.Therefore,aging for 900h did not have effect on the thermal decomposition property of GF/PA and GF/PET composites.Secondly,the effects of temperature and time on bending properties were investigated by static and dynamic bending tests.In the static bending test,the bending strength and modulus of GF/PET composites decreased with the increase of temperature and the decrease of loading rate.Moreover,the increase of temperature and decrease of loading rate have equivalent effects on flexural properties of GF/PET composites.Viscoelastic mechanics data of composite was obtained by dynamic thermal mechanical analysis?DMA?,using time-temperature superposition principle,a smooth master curve of storage modulus vs logt could be obtained,and shift factors of master curve have good agreement with Arrhenius equation,based on this two conditions,it is concluded that time-temperature superposition principle is suitable for the GF/PET composite and the static bending modulus,in addition,the long-term durability of the GF/PET composites can be predicted using this principle,from the results of the test,it can be predicted that flexural modulus would decrease by 80%under the reference temperature 30?in 10¹⁰ minutes later.Through the analysis of the SEM photo,the effect of temperature on flexural fracture mode was discussed,when the temperature varied from 30?to 60?,brittle fracture of GF/PET composites occurred,40?and 60?,the fracture appears in the sheet rock in the shape of a compressive zone and tear-tensile zone,with the rise of temperature,the area of tear-tensile zone decreased,but compressive zone increases.However,when the temperature increased to 80?,the PET resin changed from glass state into high elastic state,plastic deformation appeared,there is no obvious crack of the specimen.With the increase of temperature from 30?to 100?,the fracture of the composite experienced four processes—glass fiber and PET resin break almost at the same time,glass fiber breaks precedes PET resin,glass fiber breaks while PET resin does not break,and neither glass fiber nor PET resin breaks...