Lower-Frequency Damping Properties of Fiber Reinforced Composite*

LI Jianzhi，SUN Baochen

(Key Laboratory of Structural Health Monitoring and Control，Shijiazhuang Tiedao University，Shijiazhuang 050043，China)

In recent years，more and more composites have been used in various areas，such as commercial aerospace，auto，marine，apace，civilengineering and sporting goods and expanded military applications.They were always regarded as the most suitable materials for building long-span bridges due to the outstanding mechanical properties compared with conventional steel material.Thus，Fiber Reinforced Plastics(FRP)composites can exhibit essential advantages that address the shortcomings of conventional steel cable such as large sag effectand durability problems.Carbon FRP(CFRP)cables were initially most investigated to replace steel cables［1－3］.To take the advantage of the composites to solve the problems of the traditional steel cables，composite bridge has been the fourth generation of stayed-cable bridge.However，the cable vibration has been the most main influence factors of bridge［4－6］.The active damper is usually not effective for long length of cable due to the limitation on its ratio of installing location to the cable length［7］.Semiactive dampers may integrate the advantages of passive and active dampers，but much research and development still needs to be conducted［8］.The material dampers had not require optimization of position or proper tuning，which can effectively suppress different kinds of vibration due to fundamental improvement of the damping.Therefore，it is very important to investigate the damping properties of various composites according to the working frequency of cable.

Many vibration control methods were used to change the form or appearance of cable，such as installing a damper and dynamical absorber having some many limitations［4－5］.The most efficient technique is to increase the damping value of cable material.However，the damping properties of composite material are the most susceptible to the ambient temperature and vibration frequency.The damping characteristics of hybrid composites in the condition of stayed cable working are investigated in this paper.

1 Materials and methods

1.1 Raw materials

Carbon fiber and glass fiber are used as reinforced material.Epoxy resin and vinyl ester resin are used matrix，which are CYD－128 epoxy produced by Yueyang petrochemical corporation and R－802 vinyl ester resin，respectively.Various parameters of material are shown in Table 1.

Table 1 Property of Various Fiber and Matrix

1.2 Specimen

The used specimen is unidirectional orientated fiber reinforced composites.The damping sample was cut into long strands，then was cleaned and placed in drier.The dimension is 45 mm×5 mm×2 mm(L×W×H).

1.2.1 Experimental method

The pultrusion technology was used in this paper(Fig.1).The pultrusion is one of the most costeffective methods for the production of composite materials.It is a continuous process that produces little waste material.Numerous process variables such as pull speed，die temperature，quality of fiber/resin wet-out，and the volume of fiber can affect the quality of pultruded composites.In this paper，the traction velocity，the cured pressure and temperature are 0.1 m/min，0.4 MPa～0.5 MPa and 170 ℃～180 ℃ respectively.The dimension of this mould is 2mm in thickness，20 mm in width.The fiber and matrix ratio is 75%and 25%.Four kinds of composites，carbon fiber reinforced composites produced with the epoxy resin and vinyl esterresin and glass fiber reinforced composite produced by the epoxy resin and vinyl ester resin were produced and tested.

Fig.1 Schematic diagram of the Pultrusion

The damping test based on ASTM D2236－69.The three-point bending modes were adopted and the specimen was scanned in frequency and temperature by the VA400 dynamic testing analyzer.

Based on the vibration characteristic of stayed cable，low-frequency damping behavior of various composites from 0.1 Hz to 2 Hz was investigated in this paper.

2 Results and discuss

2.1 The effects of vibration frequency on damping properties

In this section，the effect of the frequency on the damping ofvarious composites was done.Data pertaining to the frequency as well as resulting impact damage is analyzed by means of experimental results.

The relation between the loss factor of various composites and frequency is shown in Fig.2.From the figure，the loss factor of four kinds of composites is in-creasing with the frequency.Curves subsequently rise with a slope.The correlative literature showed that the relation between the loss factor of various composites and frequency is complicated and the damping property of composites is from interfaces damping between the fiber and the matrix［9］.Therefore，interfaces damping fully displayed in the external and fast excitation.，so the loss factor is increasing with increasing frequency.For example，the loss factor of glass fiber/vinyl ester resin composites changes from 0.1 to 0.26 at the frequency range from 0.1 Hz to 2 Hz.The loss factor of the metallic building material is 10－3～10－4.Therefore，the unidirectional composite is better than the metallic material for stayed cable.Carbon fiber/epoxy resin composite material.The follow-up research should focus on how to increase the low frequency damping.

Fig.2 Relation between Damping and Frequency

The damping property of various unidirectional composites is shown in Fig.2.For the same reinforced fiber composites，the damping property of Vinyl composites is better than that of the epoxy Composites.The reason is that the damping performance of composites is influenced by the interface of composites besides fiber and matrix.Therefore，some interfaces results from relative slip when shear stress is larger than interface bond strength，the slide friction damping is a key factor for the damping performance of composites.The interface would slid，involve micro crack，and absorb energy due to the weak bond interface.The interface is not destroyed by shearstress in the low frequency condition.Only the micro crack involved absorbed energy.Thus，the he damping property ofVinyl composites is better than that of the epoxy Composites.

From the Fig.3 and Fig.4，the interface bonding of carbon fiber composites is better than that of glass fiber composites.The composite displays the whole fracture features;carbon fiber has much resin matrix in the surface of fiber.The glass fiber has less resin matrix in the surface of fiber.Based on the above analysis，the interfacial shear strength of carbon fiber is higher than that of glass fiber.According to the interface sliding mechanism，the good interface will lower make the damping properties.Thus，the damping property of glass fiber composites should be better than that of the glass fiber Composites.However，from the Fig.1，for the same matrix composites，the damping property of carbon fiber composites is better than that of the glass fiber composites，which the experimental result is contradictory to the theoretical analysis.The author thinks the theoreticalanalysis is based on the same fiber diameter.In this paper，the diameter of carbon fiber is 5 μm and that of glass fiber is 12 μm.It is remarkable from Fig.3(B)and Fig.4(B).Therefore，the surface area of carbon fiber is larger than that of glass fiber.The larger is the surface area;the better is the damping property.In addition，elastic modulus of carbon fiber is higher than that of glass fiber.The larger shear stress is involve in the carbon fiber composites that results in micro-deformation and absorbs the vibration energy.Thus，the damping property is intensified.

Fig.3 SEM Image of GFRP

Fig.4 SEM Image of CFRP

From the above，in addition to the mismatch between elastic modulus of the carbon fiber and matrix，the interface damping has contributed a lot to the material damping.This research result is proved in the literatures［10－14］.

2.2 The effects of temperature on damping properties

The relation between the loss factor of various composites and temperature is shown in Fig.5.From the figure，the loss factor of four kinds of composites has a notable decline as temperature rose from －20℃to 0℃and the loss factor of four kinds of composites has a notable decline as temperature rose from －20℃to 0℃ and has a slight decline from 0℃ to 60℃.At last，the developmental level trends to harmony.The reason forthis phenomenon is thatfiber/matrix interface has larger shear stress since material is perfectly rigid in lower temperature.Therefore，the more energy sliding needs，the larger damping factor is.Curves subsequently decrease with a slope that lowers increasing the temperature.This in fact was caused by the lower interface shear stress in composites.

Fig.5 Relation between Damping and Temperature

However，it was found that the damping property of vinyl composites is better than that of the epoxy composites for the composites with the same fiber.This is because that the interface bonding of vinyl composites is weaker than that of the epoxy composites For the same matrixcomposites，the damping propertyof carbon fiber composites is better than that of the glass fiber composites.The reason is that the surface area of carbon fiber/matrix bonding is larger than that of glass fiber/matrix bonding because of the smaller diameter of carbon fiber.In addition，the noticeable mismatch between elastic modulus of the carbon fiber and matrix，the interface damping has contributed a lot to the material damping.This result indicates that the dominant mechanism fordampingin thecompositesystem appears to be frictional sliding at the fiber polymer interfaces.

3 Conclusions

The loss factors of all four kinds of composites are increasing with frequency.For the same fiber reinforced composites，the damping factor of vinyl composites is better than that of the epoxy Composites.For the same matrix composites，the damping factor of CFRP is better than that of GFRP.Interface damping mechanism contributed a lot to the material damping.So the fiber diameter is smaller，the damping is higher.These results will provide a guideline for stayed cable vibration.

［1］Wu Z S，Wang X，Iwashita K.State-of-the-Art of Advanced FRP Applications in Civil Infrastructure in Japan［J］.Proceedings of Composites and Polycon，2007:1－13.

［2］Johannes F N.Carbon Fibre Composites as Stay Cables for Bridges［J］.Applied Composite Materials，2000，7(2－3):139－150.

［3］Cheng L J，Karbhari V M.New Bridge Systems Using FRP Composites and Concrete:A State-of-the-Art Review［J］.Progress in Structural Engineering and Materials，2006，8(4):143－154.

［4］Wang X，Wu Z S.Modal Damping Evaluation of Hybrid FRP Cable with Smart Dampers for Long-Span Cable-Stayed Bridges［J］.Compos Struct，2011，93(4):1231－1238.

［5］Wang X，Wu Z S.Integrated High-Performance Thousand-Metre Scale Cable-Stayed Bridge with Hybrid FRP Cables［J］.Composites Part B，2010，41(2):166－175.

［6］Guan Y J，Wei Y T.A New Effective 3－D FE Formulation of FRP Structural Modal Damping for Thick Laminate［J］.Compos Struct，2009，87(3):225－231.

［7］Caetano Elsa de Sa.Cable Vibrations in Cable-Stayed Bridge［M］.Zurich:IASBSE-AIPC-IVBH，2007:78－79.

［8］Johnson E A，Baker G A，Spencer B F J，et al.Semiactive Damping of Stay Cables［J］.Eng Mech，2007，133(1):1－11.

［9］Babkina N V，Lipatov Y S，Alekseeva T T.Damping Properties of Composites Based on Interpenetrating Polymer Networks Formed in the Presence of Compatibilizing Additives［J］.Mech Compos Mater，2006，42(4):385－392.

［10］Kireitseu M，Hui D，Tomlinson G.Advanced Shock-Resistant and Vibration Damping of Nanoparticle-Reinforced Composite Material［J］.Composites Part B，2008，39(1):128－38.

［11］Suhr J H，Koratkar N A.Energy Dissipation in Carbon Nanotube Composites:A Review［J］.J Mater Sci，2008，43(13):4370 －4382.

［12］DeValve C，Pitchumani R.A Numerical Analysis of Carbon Nanotube-Based Damping in Rotating Composite Structures［C］//Composite Structures，2013，(103):18－26.

［13］DeValve C，Pitchumani R.Experimental Investigation of the Damping Enhancement in Fiber-Reinforced Composites with Carbon Nanotubes［J］.Carbon，2013，(63):71－83.

［14］Shafi Ullah Khan，Chi Yin Li，Naveed A Siddiqui，et al.Vibration Damping Characteristics of Carbon Fiber-Reinforced Composites Containing Multi-Walled Carbon Nanotubes［J］.Composites Science and Technology，2011，71(12):1486－1494.