Long Term Thermal-Mechanical Durability Study Of Short Fiber Reinforced Composites With Glass Fiber And Natural Fiber

Topics such as composite joining durability and long-term thermodynamic and mechanical performance fatigue of fiber-reinforced composites in practical applications are increasingly attracting the attention of researchers.In particular,due to the global warming and environmental issues,the application of natural fibers in composite materials in addition to chemical fibers is also being actively explored as composite reinforcement materials.Therefore,in this thesis,the thermosetting of short fibers（glass fibers and kenaf fibers）and recyclable thermoplastic resin composites are used as research objects,and the deterioration of high and low temperatures and mechanical fatigue is investigated,and the changes of the strength of the open hole and the influence of weld lines are discussed,and the mechanical properties and durability are predicted by the principle of time-temperature equivalence.The research work consists of three parts.The first part includes the study of long-term durability and notch sensitivity analysis of glass fiber mat and kenaf fiber mat reinforced unsaturated polyester resin composites aged in high and low temperature environments.The second part is to analyze the durability and notch sensitivity of glass fiber-kenaf fiber-polypropylene（PP）hybrid composites aged in high and low temperature environments.The third part includes the study of the time-temperature dependence of glass fiber reinforced polyethylene terephthalate（PET）injection molding composites with welded lines,and the analysis of their long-term mechanical durability by the principle of time-temperature equivalence.In the second chapter,firstly,glass fiber mat unsaturated polyester resin composite （GM/UP）and kenaf fiber mat/UP composite（KM/UP）were prepared by hand lay-up and compression molding.The test sample was subjected to high and low temperature cycles（after 12 hours at-25℃,the temperature was raised to+58℃ and kept for 12 hours,100cycles）,and the impact of the notch and the results of environmental degradation were discussed.During the whole experiment,the weight of the experimental materials was closely monitored and no obvious changes were found.DMA（dynamic viscoelasticity）experiments showed that the degradation of high and low temperature cycles reduced the glass transition temperatures of both materials to a certain extent（about 8%）,that is,GM/UP is reduced from 157.6 degrees to 150.7 degrees（30 temperature aging）and 143.7degrees（100 temperature aging）,KM/UP is reduced from 161.9 degrees to 154.2 degrees（30 cycles）and 150.1 degrees（100 cycle）.It shows that temperature aging reduces the heat resistance of these two materials,and it is basically caused by the deterioration of unsaturated resin.The data shows that compared with GM/UP,KM/UP materials not only have lower mechanical properties,but also are less resistant to environmental degradation,because regardless of elastic modulus or tensile strength,non-degraded KM/UP is significantly lower than non-deteriorated GF/UP,and after 100 high and low temperature cycles,KM/UP decreased by 30%,while GF/UP was 13%.In addition to the insufficient strength of the kenaf fibers in KM/UP,SEM observations show that the kenaf fibers have different thicknesses and uneven dispersion,which exacerbates the gap in static mechanical properties between KM/UP and GM/UP composites.On the other hand,in the notched case,the high and low temperature cycle temperature degradation seems to have a similar effect on the two composites.Specifically,after perforation,the notch strength decreased by about 10%compared with the un-notch strength,and the notch strength of GM/UP decreased by another 17%after environmental thermal ageing.The same situation also occurred in KM/UP materials,that is,after KM/UP notching,the notch strength is reduced by about10%and then reduced by 17%after the environment ageing.However,the notched strength was almost close to the un-notched strength at higher thermal cycles of KM/UP.Since the reinforcing fiber and the matrix resin coexisting in the composite material are two different component materials,any external influence（including temperature change,mechanical fatigue,etc.）will cause and enhance thermal stress and strain（especially in its interface area）.Therefore,on the basis of environmental degradation,the study carried out the low-cycle fatigue LCF experiment（the low-cycle fatigue stress-strain experiment with 55%strength load was selected）,and the characteristic distance（d₀）measurement;the characteristic distance（d₀）of the material which was calculated by finite element method to evaluate the effects of high and low temperature cycles on the notch strength and notch sensitivity of the materials.Due to the low number of cycles adopted and the fatigue stress is not high,the experimental data show that the tensile modulus and tensile strength of GM/UP and KM/UP before and after high and low temperature cycle degradation are significantly affected by the tensile stress strain fatigue cycle test（30 times or 100 times）both have little impact（basically less than 5%）.Then,for the test samples after notching,the samples before and after the high and low temperature cycle degradation will not be able to return to the original position after the first tensile fatigue test,that is,the internal local damage will cause irreversible deformation,damage and elongation.The notch sensitivity is discussed by the ratio of the notch strength to the static tensile strength（notch strength ratio）.The higher the value,the closer to or greater than 1,the less sensitive the material is to the notch.After making open hole,the notch strength ratio data shows that after notching GM/UP is lower,while KM/UP is higher,indicating that KM/UP is less sensitive to notch.The characteristic distance（d₀）refers to the length from the hole edge to the point where the stress equals to un-notch strength while the notched specimen carries maximum load.The numerical value（d₀）calculated by the finite element shows that the GM/UP material can withstand 100 high and low temperature cycles.The characteristic distance in GM/UP was not changed,but the characteristic distance in KF/UP gradually increased,which indicated that the notch sensitivity of KM/UP was low.Although GM/UP has better mechanical properties,after high and low temperature cycle aging,KM/UP shows more stable fatigue resistance and better resistance to notch sensitivity.Secondly,the glass fiber-kenaf fiber hybrid polypropylene composite material test sample was compounded by direct fiber fed injection molding（DFFIM）,in which the twisted kenaf fiber yarn was directly fed into the molten polypropylene through the opening of the injection molding machine.In the polypropylene melt or glass fiber reinforced polypropylene melt,the kenaf yarn is cut off by the rotation of the screw,and the kenaf fiber is dispersed in the melt and injected into the standard test sample mold from the nozzle.Both un-notch and notch samples were subjected to thermal environment experiments（keeping at 25℃ for 12 hours,then heating up to+80℃ for 12 hours,then cooling down to 25℃,and cycle 100 times）to study the influence of environmental thermal fatigue effect on the mechanical properties of composite materials.Compared with kenaf fiber polypropylene injection molded parts,the presence of glass fiber significantly improves the static mechanical properties of hybrid composites.However,as the number of materials fed increases,the static tensile strength of the sample increases.During thermal ageing fatigue,the tensile properties also tends to increase,and the tensile modulus and strength of the samples mixed with kenaf fibers or glass-kenaf fibers hybrid show an overall increase trend during thermal fatigue,while the notch strength does not decrease.Furthermore,the ratio of the notch strength to the conventional strength also shows that the addition of kenaf fiber reduces the notch sensitivity of the composite material and better resists the damage caused by the notch.Potential changes in the microstructure of the composites due to the thermal environment were evaluated by XRD analysis,and it was found that the crystallinity of the polymer increased as the number of thermal environmental cycles increased.Third,glass fiber reinforced polyethylene terephthalate（PET）injection molding composites（GF/PET）of two types of test specimens（with/without weld line）were molded using single and double injection gate equipment.By their quasi-static tensile and dynamic viscoelastic DMA results were used to study the effect of weld lines on the mechanical properties of composites（GF/PET）.The experimental results found that the weld strength was about 50%-60%lower than the tensile strength at different temperatures and tensile speeds.As the fiber content increased from 15 wt%to 30 wt%,the weld strength continued to decrease by another 10%,indicating that the impact of weld-lines presence on the mechanical properties of composite materials is obvious,mainly because the fibers are perpendicular to the stretching direction on the fusion surface,not only has no reinforcing effect,but also because the interfacial bonding force between fibers and polymer is lower than the strength of polyethylene terephthalate resin,leading to a decrease in weld strength.On the other hand,tensile strength and weld strength are inversely proportional to temperature and linearly proportional to extending speed.Tensile modulus has a negative correlation with temperature and a non-linear relationship with stretching speed.The ratio of weld strength to that of a non-weld line shows that below T_g（approximately 80 degrees）,these ratios are essentially constant and do not change with stretching speed and temperature.Detailed cross-sectional observation revealed that fiber-resin bonding was better in the fractured surface at higher test speeds.The long-term durability of composite materials was analyzed by the principle of time-temperature superposition,and the Arrhenius type shift factor was obtained by fitting the tensile strength data,and the service life of composite materials with or without weld joints was predicted.It was found that the specimens without welded joints were better than those with weld line ones,and as the glass fiber increased from 15%to 30%,the life of the weld line specimens decreased further.