EXPOSURE TEST AND LONG-TERM CHARACTERISTICS FOR CFRP STRUCTURAL MEMBERS

Transcription

1 Fourth Asia-Pacific Conference on FRP in Structures (APFIS 13) December 13, Melbourne, Australia 13 International Institute for FRP in Construction EXPOSURE TEST AND LONG-TERM CHARACTERISTICS FOR CFRP STRUCTURAL MEMBERS Y. Matsumoto 1 and K. Yonemaru 2 1 Toyohashi University of Technology, Japan. y-matsum@ace.tut.ac.jp 2 Shimizu Corp., Japan ABSTRACT FRP material has good characteristics such as light-weight, high-strength and high-corrosion resistance. Light-weight structure possesses some advantages over the seismic load and rational constructing procedure. In Japan, the innovative light-weight spatial structures having double layered truss roof consisting of CFRP were constructed in The structural weight is approximately 15N/m 2, and its member weight is only N/m. It means that the CFRP structural system may be an alternative revolutionary class in comparison to others using conventional structural materials such as steel and concrete. Thus, it would be very important to make clear the long-term characteristics of the CFRP structural members. In this study, the long-term exposure test of the prestressed CFRP members have been carried out in the past twelve years. And now, the mechanical characteristics of CFRP members are discussed on the results of the static loading-unloading test. KEYWORDS CFRP; double layered truss roof structure; long-term experiment; exposure test INTRODUCTION Light-weight structure possesses some advantages over the seismic load and rational constructing procedure that is why the FRP material is an appropriate material to adopt. FRP material has good characteristics such as light-weight, high-strength and high-corrosion resistance. A light-weight spatial structures which have double layered truss roof consisting of carbon fibre reinforced polymer (CFRP) have been constructed in 1998, Japan as shown in Figure 1. The structural weight is approximately 15N/m 2, and its member weight is only N/m. However, CFRP material is a revolutionary class of material in comparison to other structural material such as steel and concrete and can be applied widely on various fields. On the course of application, this research predicts a newly way of clarifying mechanical characteristics of CFRP spatial structures and it is very important the long-term characteristics of the CFRP structural members. Based on these, the long-term exposure test of the pre-stressed CFRP members have been carried out in the past twelve years. And now, the mechanical characteristics and aged changes of CFRP members are investigated through the static loading-unloading test. Figure 1. CFRP double layer truss roofs

2 DETAIL OF CFRP MEMBER AND EXPOSURE TEST Method of exposure test Figure 2 shows a detail of the connection of the present CFRP double layered truss structures consisting of CFRP circular tube, stainless steel and aluminium alloy. The outside diameter, thickness and length of the adopted CFRP members are 94mm, 2mm and mm, respectively. The adopted CFRP members using exposure test are same as the actual CFRP truss structure as shown in Figure 1. Figure 3 and 4 shows the setup of exposure test; the adopted experimental parameters are axial load, coating and place of exposure. In the axial loaded exposure specimens, the CFRP members are assembled as a regular octahedron and the axial tension has been applying to central tension-rod as shown in Figure 3. The value of axial load corresponding to allowable axial compressive stress for sustained loading is 45.5MPa. Table 1 shows the mechanical characteristics of the CFRP truss member. Connection Joint CFRP 1: Truss member (CFRP pipe) 2: Nosecone (stainless steel) 3: Bearing bolt (chrome molybdenum steel) 4: Collar (aluminium) 5: Hub (aluminium) 6: Blind Rivet (stainless steel) T C Mechanism of force transfer Figure 2. Detail of connection of the present CFRP truss structure T C T: Tensile force C: Compressive force Tension M1 M4 M2 M3 M8 M5 M7 M6 Tension rod M9 M12 M M11 Figure 3. Experimental setup 2

3 Table 1. Mechanical characteristics of the CFRP and CFRP truss member Tensile strength 1532 MPa Compressive strength 555 MPa Bearing strength 444MPa Tension 112 GPa Axial elastic modulus CFRP material Compression 4 GPa Circumferential elastic modulus 41.4 GPa Shear modulus 5.95 GPa Axial Poisson s ratio.86 Circumferential Poisson s modulus.33 Length mm Sectional area 578mm 2 CFRP Truss Bearing area for rivet 614mm 2 member Tension (rivet) 49.2 kn Allowable stress for sustained loading Compression (linear buckling) 26.3 kn Environmental Condition The exposure test was carried out in Ehime Prefecture, Japan and located nearby the existed actual CFRP truss structure as shown in Figure 1. The corrosion content of steel obtained by JIS Z 2383 (ISO 9226) is.24mm/year and the amount of ultraviolet light (UV-A; wavelength equal to nm) 272MJ/m 2. year (estimate value) on the area of exposure test. Results of observation Figure 4 shows the condition of the exposure specimens in October 26,. The remarkable changes are not observed in CFRP members. However, the polymer on the surface of the CFRP member had been decreasing and the carbon fibres were brought to the surface. In the case of coating CFRP members, a chalking was observed as shown in Figure 4. Meanwhile, in the dining hall as shown in Figure 1, the CFRP members with coating are having gloss; it is considered that the degradation of the indoors coating surface did not occur in the past twelve years. Figure 4. Overview of exposure test 3

4 UNLOADING/LOADING TEST Experimental Method Figure 5 shows the axial load history of tension rod for loading protocol and Figure 3 shows an experimental setup. The details of the loading protocol are as follows; Step 1: Step 2: Step 3: Step 4: Unloading until the axial load becomes zero and the monitor strains of CFRP few minutes. Pre-loading until the axial load becomes 5% of allowable axial load for sustained loading and the monitor strains of CFRP few minutes. Unloading until the axial load becomes zero and the monitor strains of CFRP few minutes. Loading until the axial load becomes allowable axial load and the monitor strains of CFRP long time. The axial load of CFRP member is controlled by the strain value thorough strain gauges attached with the members. Axial force N L : Allowable axial force for sustained loading N L Remaining Load.5N L Time Step 1 Step 2 Step 3 Step 4 Figure 5. Axial loading protocol Results of Unloading/Loading Test Figure 6 shows the axial load history of tension rod obtained by unloading/loading test. The remaining axial load of tension rod was evaluated to 74.1 kn and axial load after testing was 75.5 kn. Therefore, it is suggested that the stress relaxation had not been occurring in the past twelve years. Also, the axial force of CFRP member at pre-loading was 35. kn (11:45a.m.), and at beginning of the Step 3 was 33.4 kn (1: p.m.). Then the stress relaxation was not occurred because of the change of the force in a time between pre-loading and preunloading which is about 2%. Actually, the relaxation problems had been noticing for a structural member consisting of a polymer such as FRP but it is suggested that the present CFRP members will not cause the relaxation or creep deformations. Table 2 shows the axial strains of CFRP members obtained by the unloading/loading tests in February 1998 and October. In compressive members, the strains in were decrease to 5~17% compared to Also in tensile members, decrease to 14~19%. However, these strain values come to the analytical values because of the CFRP pipe seems to be fitting in the connections. The mechanical property of the present structural connection is shown in Figure 2; in the tensile members, the internal force transfers through only bearing of ribbed joints. On the other hand, in the compressive members, the internal force transfers through bearing and face of nosecone. Therefore, the effects of fitting of tensile members are affected more severely than the compressive members. It is proposed that the internal forces had been kept to approximately uniform and its values come to the analytical values. So, it is thought that the present CFRP truss structural system has been kept healthy. Figure 7 shows the axial stress axial strain and the axial stress circumferential strain curves of CFRP members obtained by the unloading/loading tests in October. The linear behaviour is obtained in all stress strain curves. Also, the axial elastic modulus is kept to approximately equal of each member. Table 3 shows an axial elastic modulus and Poisson s ratio of CFRP members calculated by stress strain curves as shown in Figure 7. The axial elastic moduli have been estimated 11~15% lower than results of material coupon tests in However, the strain values did not have considerable change in 1998 and ; the source of these differences is considered that the connections are not perfect pinned joints. Then it is suggested that the cause of differences does not correspond to aging degradation. 4

5 Axial force of tension rod [kn] 9: 9:45 : :15 : :45 11: 11:15 11: 11:45 12: 12:15 12: 12:45 13: 13:15 13: 13:45 14: 14:15 14: 14:45 15: 15: (Step1) (Step4) Unloading Pre-loading Figure 6. Axial load history of tension rod obtained by unloading/loading test Loading Table 2. Axial strain of CFRP members obtained by the unloading/loading tests Compressive member Tensile member Compressive member Member M1 M2 M3 M4 M5 M6 M7 M8 M9 M M11 M12 Strain Strain Average Strain Strain Average Ratio of to Strain Strain Average Ratio of to Analytical result Time 5 [N/mm 2 ] 5 [N/mm 2 ] [N/mm 2 ] 5 4 Axial strain Circumferential strain 4 Circumferential strain Axial strain 4 Axial strain Circumferential strain :M1 :M2 :M3 :M [μstrain] :M5 :M6 :M7 :M [μstrain] (a) Results for M1~4 (b) Results for M5~8 (c) Results for M9~12 Figure 7. Stress strain relationships of CFRP members obtained by the unloading/loading tests Table 3. Elastic modulus and Poisson s ratio of CFRP members obtained by the unloading/loading tests Compressive member Tensile member Compressive member Member M1 M2 M3 M4 M5 M6 M7 M8 M9 M M11 M12 Axial elastic modulus [GPa] Average Official value Axial Poisson s ratio Average Official value.86 :M9 :M :M11 :M12 [μstrain] 5

6 CONCLUSION In this study, the long-term exposure test for axial loaded CFRP pipes as a structural member for special structures has been carried out. Also, the aging degradations have been investigated. The conclusions are as follows. 1) Relaxations had not been occurring under allowable axial load for sustained loading in the past twelve years. 2) Mechanical characteristics of the present CFRP member does not change even if the polymer on the surface of the CFRP member had been decreasing and the carbon fibres were brought to the surface. 3) The strains in were decrease to 5~17% compared to 1998 in compressive members. On the other hand, the strains decrease to 14~19% in tensile members. This is because the mechanical property of the present structural connection as shown in Figure 2. In the newly developed material, it is necessary that the long-term characteristics are clarified. Then, this study will become valuable results for the FRP structures. So, it can be expected that the present CFRP spatial structures can achieve high durability and light-weight structures. Also, we intend to ongoingly carry out the observation and strain measurement of the CFRP members. ACKNOWLEDGEMENTS The authors greatly acknowledge to Prof. Seishi Yamada who coached us politely. Also, this study was supported by Toray Industry Co.,Ltd. This study is supported by the Maeda engineering foundation in. REFERENCES YONEMARU K., FUJISAKI T., NAKATSUJI T., SUGISAKI K. and KIMOTO Y., Development of Space Truss Structure with CFRP, Proceedings of the Mouchel Centenary Conference, Civil-Comp Press, 1997, pp LAWRENCE C. BANK, Composites for Construction: Structural Design with FRP Materials, Wiley, 6. XIAO-LING ZHAO, FRP-Strengthened Metallic Structures, CRC Press, 13. 6