5th International Conference on Composites Testing and Model Identification
February 14th to 16th, 2011, EPFL, Lausanne, Switzerland
ENVIRONMENTAL EFFECTS ON FATIGUE LIFE of IMPACTed COMPOSITE PIPES
Karakuzu R., Deniz M.E. and Icten B.M.
Department of Mechanical Engineering, University of Dokuz Eylul, Bornova, 35100 Izmir, Turkey , ,
1. INTRODUCTION
The fatigue life of fibre reinforced composite pipes is an important design parameter in order that they are subjected to various loading during service life. Their response to these loading ought to be understood. A crucial loading condition is foreign object-induced impact load, which if not designed against properly, may lead to catastrophic failure of the composite. There were fewer investigations on fatigue life and impact behaviour of fibre reinforced composite pipes. So this study focuses on transverse impact and fatigue life after-impact of composite pipes according to environmental condition. Samanci et al. [1] studied on fatigue damage behaviour of [±75°]3 filament wound composite pipes with different surface cracks under alternating internal pressure. Naik [2] studied many different effects of environmental conditions such as sea water immersion, dry heat, salt spray, humidity and impact on the burst pressure performance of the glass fibre reinforced thermoset pipes which consist of two type glass-epoxy and glass vinylester. Gning et al. [3-4] and Tarfaoui et al. [5] have carried out the identification and modelling of damage initiation and development in pipes subjected to quasi-static and drop weight impact for underwater applications. Some indications of how this damage affects the capacity of cylinders to resist external pressure loading were also presented. They noticed that the impact damage has been shown to reduce the residual implosion strength of pipes significantly. Kaneko [6] studied on the impact analyses of pressurized vessels by simulation. It is concluded that the relationship between burst mode and inner pressure can be clarified. It is seen that few studies have been identified the effects of the environmental conditions and impact events on fatigue life of composite pipes until now.
The objective of this study is to identify the effects of the sea water on fatigue life of filament wound composite pipes under alternating internal pressure before and after low velocity impacts. Firstly some of the specimens were subjected to fatigue tests as non-impacted and impacted (5.0, 7.5 and 10.0 J) for dry condition. Others specimens according to environmental condition were subjected to fatigue tests as non-impacted and impacted.
2. EXPERIMENTAL DETAILS
2.1. Production and preparation of the filament wound composite pipes specimens
Composite pipes were manufactured with [±55]3 winding orientation angle and band width of 14 mm by using a CNC filament winding machine in Izoreel Firm, Izmir-Turkey (Figure 1). E-Glass fibre with 600 Tex and 17 µm diameter was selected as reinforcement. The matrix material was selected Bakalite EPR 828 EL epoxy. Also, EPH 875 was selected as hardener. After production the pipes were cured at 130 oC for 3 h on the mandrel in a slow motion rotary oven. The fibre volume fraction (Vf) of the pipes studied in this investigation was about 65%. The inner diameter, length and thickness of the specimens were 100 mm, 400 mm and approximately 1.75 mm, respectively.
Figure 1: CNC filament winding machine and composite pipe
The composite pipes were immersed in artificial seawater having salinity about 3.5% at laboratory conditions for 3, 6 and 9 months.
2.2. Impact test
The impact tests were performed by using an instrumented drop-weight testing system, CEAST-Fractovis Plus, which is suitable for a wide variety of applications requiring low to high impact energies. It consisted of a drop-weight tower, an impactor, a velocity sensor and a data acquisition system. A hemispherical nose of 12.7 mm in diameter was attached to the lower end of a 22.4 kN force transducer (Figure 2). The total impact mass including striker and the crosshead was 5.02 kg. Data acquisition system records the time versus force data. The software used by the impact testing machine calculates time dependent velocity, deflection and absorbed energy values of the specimen using Newton’s second law and kinematic equations.
Figure 2: Impact test fixture and specimen
2.3 Fatigue test
Fatigue tests were applied to the filament-wound composite pipes in close-ended condition by using PLC controlled servo-hydraulic testing machine as shown in Figure 3. Tests performed according to ASTM D-2992. The pressurized liquid is oil.
Figure 3: PLC controlled servo-hydraulic testing machine with specimens
3. EXPERIMENTAL RESULTS AND DISCUSSION
In this study, the transverse impact test was carried out for three various impact energies; 5.0 J, 7.5 J and 10.0 J on the pipes at room temperature. Every test was performed by four times and average values were calculated. The contact force-deflection curves give significant information about the impact response of composite materials tested. In generally, there are three types of contact force-deflection curves: rebounding, penetration and perforation. In this study, rebounding type curve was observed for all tests. The load–deflection curve turned toward the origin of the diagram after reaching a maximum force. As long as a closed curve is observed it is possible to say that the impact loading does not result in a serious damage to the specimen. In Fig. 4, load-deflection curves of the specimens for four environmental conditions at three different impact energy levels (5.0 J, 7.5 J and 10.0 J) are shown. As can be seen from the figure, peak force and maximum deflection increase with increasing impact energy. Sea water immersed time does not have important effect on stiffness in loading except for 3-month wet condition while sea water affect significantly on maximum deflection and unloading behaviour of composite pipe.
(a) / (b)(c)
Figure 4: Contact force-deflection curves of composite pipes (a) 5.0 J, (b) 7.5 J, and (c) 10.0 J impact energy
Figure 5 shows the pictures of damaged specimens corresponding to three difference energy levels, 5.0 J, 7.5 J and 10.0 J. Delaminations occur with debonding of lower interfaces between [±55] orientations for 5.0 J impact energies. These delaminations propagate by the increasing of intralaminar cracks for 7.5 J and 10.0 J impact energies.
Figure 5: Illustration of impacted surface for three difference energy levels of the composite pipes
Fatigue test was carried out for the non-impacted and impacted composite pipes. Applied maximum and minimum stresses were calculated from the values of 70% and 20% of burst hoop stress of the weakest sample. In these tests the maximum and minimum stresses were found as σmax=120 MPa and σmin=34 MPa respectively. In the non-impacted and impacted samples, same R=σmin/σmax ratio is used. The low cycle tests were applied with 0.5 Hz frequency and cycle type was selected as punch type. Every test was performed four times and average cycle values were calculated.
In fatigue tests three damage modes named as perspiration, leakage, and eruption occurred. These modes were observed according to the increasing of the fatigue cycle. The whitening stage also can be observed in the filament winding direction because of the matrix cracks and debonding of the fibre and the matrix. Results show that the sea water and the transverse impact have significant effect on the fatigue life of the composite pipes. Fatigue life of the pipes decreases by transverse impact comparing with non-impacted pipes for eruption failure mode (Fig. 6).
Figure 6: Fatigue cycle of eruption mode–immersed time diagram of composite pipes for non-impacted and impacted cases
4. CONCLUSIONS
In this experimental investigation, the fatigue life after low velocity impact was obtained. From the results obtained the following conclusions can be drawn:
● Peak force and maximum deflection increase with increasing impact energy. Sea water immersed time does not have important effect on stiffness in loading except for 3-month wet condition while sea water affect significantly on maximum deflection and unloading behaviour of composite pipe.
● Fatigue life of the pipes decreases by transverse impact with comparing non-impacted pipes. So in serves, impact event in the pressured composite pipe should be taken into consideration.
● Seawater and transverse impact have significant effect on fatigue life of the composite pipes.
Acknowledgements
Financial support for this study was provided by TUBITAK-The Scientific and Technological Research Council of Turkey, Project Number: 108M471.
5. REFERENCES
[1] A. Samanci, A. Avci, N. Tarakcioglu and O.S. Sahin, Fatigue crack growth of filament wound GRP pipes with a surface crack under cyclic internal pressure, Journal of Material Science, 43, 2008, pp. 5569–5573.
[2] M.K. Naik, The effect of environmental conditions on the hydrostatic burst pressure and impact performance of glass fiber reinforced thermoset pipes, MSc Thesis, King Fahd University of Petroleum & Minerals, Dhahran, 2005.
[3] P.B. Gning, M. Tarfaoui, F. Collombet, L. Riou and P. Davies, Damage development in thick composite tubes under internal loading and influence on implosion pressure: experimental observations, Composites: Part B: Engineering, 36(4), 2005, pp. 306-318.
[4] P.B. Gning, M. Tarfaoui, F. Collombet and P. Davies, Prediction of damage in composite cylinders after impact, Journal of Composite Materials, 39(10), 2005, pp. 917–928.
[5] M. Tarfaoui, P.B. Gning and F. Collombet, Residual strength of damaged glass/epoxy tubular structures, Journal of Composite Materials, 41(18), 2007, pp.2165-2182.
[6] T. Kaneko, S. Ujihashi, H. Yomoda, and S. Inagi, Finite element method failure analysis of a pressurized FRP cylinder under transverse impact loading, Thin-Walled Structures, 46, 2008, pp. 898–904.