Brittle Fracture Resistance Test on Composite Core of ACCC/TW

Transcription

1 To: Dave Bryant Composite Technology Corporation 2026 McGraw Ave Irvine, CA USA Brittle Fracture Resistance Test on Composite Core of ACCC/TW University of Southern California, M.C. Gill Foundation: Composites Center Report No: USC-BrittleFractureTest January 4, 2006 Eric J. Bosze, O. Bertschger, Yun-I. Tsai and Steven R. Nutt Summary Brittle fracture tests were conducted on a pultruded hybrid composite rod (Drake size) consisting of carbon fibers and B-free E-glass fibers using a proprietary high glass transition temperature (T g ) epoxy resin system, in accordance with the procedure outlined in [1, 2]. The rod was subjected to a constant tensile load of 10,000 lbf (44.5 kn) while immersing a portion of the gauge section in a 1N and 5N nitric acid (HNO 3 ) solutions. The applied load and acid immersion were continued for a period of 96 hours or longer. No failures occurred, and macroscopic inspection revealed no damage or change in the composite after this exposure. Subsequent to acid immersion, the tensile strength of the samples at room temperature was found to be considerably greater than the rated tensile strength of the core (34,410 lbf (153 kn)). The results indicate that the polymer composite used in the ACCC conductor is immune to brittle fracture under the conditions specified in industry-standard test protocols and under more aggressive acidic environments. Report 1. Introduction to Brittle Fracture in the Electric Power Industry Tensile strength in unidirectional fiber composites is typically a fiber-dominated phenomenon. Transverse failures occur across fiber bundles, as shown in Figure 1. This type of failure is called brittle fracture and normally occurs only at the ultimate tensile strength of the composite. The phenomenon of brittle fracture has a special meaning in the electric power industry, particularly in the context of polymer composite or non-ceramic insulators (NCI s). In this context, the phenomenon of brittle fracture 1 USC-BrittleFractureTest

2 occurs at unusually low loads when the composite is exposed to adverse chemical environments, and is called acid attack brittle fracture. The phenomenon has been studied experimentally and in field applications over the last three decades. A good review of brittle fractures in the electrical power industry was recently published [3]. Typical NCI s consist of a fiberglass rod and an elastomeric outer housing [4]. Studies have shown conclusively that brittle fractures are produced by electrolytic corrosion of the fiberglass rod in combination with a tensile load [4-6]. Figure 1. Typical transverse fracture across carbon fibers in unidirectional composite. While field studies have focused on the conditions leading to brittle fractures, experimental studies highlight material-specific issues that contribute to the existence of brittle fractures. These issues include 1) glass fiber grade and fiber sizing, 2) resin matrix and hardener, 3) curing temperature and 4) matrix post-curing and filler [5, 6]. Published reports indicate that the type of glass fiber used in composites can make a significant difference in the likelihood of brittle fracture [7]. Specifically, boroncontaining E-glass fibers are particularly susceptible to brittle fractures when composites are subjected to mechanical loads in the presence of dilute acids. The extent of susceptibility depends not only on the type of fiber, but also on the applied stress, the concentration of the corrosive medium, and the duration of exposure. Fortunately, brittle fracture in NCI s can be prevented through the use of boron-free glass fibers. In fact, B-free glass fibers have been used in NCI s for the past 15 years, and no brittle fractures have been reported [2, 5]. All of the reported case studies and experimental findings agree that brittle fracture can be prevented if no acid or water comes into contact with the composite rod and there are no boron-containing fibers. The polymer composite of the ACCC conductor (developed by Composite Technology Corporation), features a core region of carbon (C) fibers, surrounded by a shell region of B-free E-glass fibers. The composite is then wrapped with multiple layers of Al wires to form the conductor. The glass-fiber shell region prevents the possibility of galvanic corrosion between the C fibers in the core region and the overlaid Al wires of the conductor. Furthermore, the (exclusive) use of B-free glass fibers is intended to ensure against the possibility of brittle fracture [8]. To assess the potential for acid-attack brittle fracture in the ACCC design, the composite rod was exposed to solutions of nitric acid (HNO 3 ) while simultaneously applying a load of ~25% of the rated tensile load (10,000 lbf (44.5 kn)), which is the 2 USC-BrittleFractureTest

3 expected load upon installation of the ACCC conductor. In normal (non-acid) environments, tensile failure typically occurs at loads of ~40,000 lbf (178kN). 2. Experimental Methods The acid attack brittle fracture test was conducted in accordance to the procedures described in IEEE and similar publications [1, 2]. A Drake size (0.375 in., 9.53 mm) composite rod comprised of B-free E-glass fibers (Owens Corning AdvanTex), carbon fibers (T700SC, Toray), and a proprietary high glass transition temperature (T g ) epoxy (labeled 3896) was used for acid-attack brittle fracture experiments. The boron-free E- glas is presently used for ACCC production specifically to eliminate potential brittle fracture caused by acid attack. The composite rod was laid up in custom-made gripping fixtures, as described in the Tensile Testing Procedure Report. A Teflon sleeve container was placed around the rod in the gauge section to hold the nitric acid solution. The sample was mounted in a 20:1 creep frame, as pictured in Figure 2. Weights were then loaded into the creep frame to apply a total load of 10,000 lbf (44.5 kn) on the sample, resulting in an applied stress of 91 ksi (624 MPa). This load was selected because it is similar to the initial load expected when the conductor is installed in service. Nitric acid (HNO 3 ) solutions of 1 and 5N (1 and 5 molarity), which give ph values of 0 and -0.7, were poured into the container to begin the experiment. The samples remained under load in the creep for up to 96 hours or longer with periodic checks of the acid solution level and the sample elongation. According to [1], if failure does not occur during the first 96 hours of exposure, the composite can be considered resistant to acid-attack brittle-fracture. Figure 2. Shows the composite rod in the creep frame and the nitric acid container. After the acid was removed from the container, the tensile strength of the sample was measured following the procedure described in Tensile Testing Procedure Report to determine the retained strength of the composite after exposure to the nitric acid 3 USC-BrittleFractureTest

4 solutions. The rate tensile strength (RTS) of the composite is stated to be 34,410 lbf (153 kn). Carbon fibers, boron-containing and boron-free E-glass fibers were also placed into the same nitric acid solution in separate beakers to determine any changes in the appearance of the fibers caused by similar time exposure to the nitric acid. The intent was to assess any evidence of acid attack on the fibers after exposure. The fibers were examined microscopically using a scanning electron microscope (SEM) before and after exposure to the nitric acid solution. 3. Results 3.1. Tensile Strength After Exposure Two rods, exposed to 1N and 5N solution of nitric acid were tested. The 1N solution is the recommended concentration by the industry standards and exhibits a ph of 0. The 5N solution was used to test under a more aggressive environment, and has a ph of Pure nitric acid is typically used in order to dissolve the polymer matrix, but it is not expected that a concentration higher than 1N would exist even in the high electrical environments of overhead conductors. Figure 3 shows rods that were exposed up to 5N solution for 12 days. As is seen, even at 5N solution, the only visible difference with the unexposed rod was the slightly fraying of the glass fibers on the surface. No other color changes or damage was observed. Figure 3. Shows rods exposed to different nitric acid concentrations for up to 12 days. When the rods were under tension and acid attack, if damage was occurring in the composite, a change in the gauge length should have been detected. For the rod tested with the 1N attack, the gauge length at the beginning of the test was measured to be 22 14/16 in. (581 mm) between the gripping fixtures. After 11 days, the gauge length 4 USC-BrittleFractureTest

5 did not change, and creep deformation was thus negligible. Furthermore, the sample did not undergo tensile failure or show any evidence of failure initiation. (In contrast, similar conditions produced failures of non-ceramic composite insulators, or NCI s, used in the electrical power industry, in as little as 2 hours [7]). The composite easily passed the requisite 96 hours of exposure without failure, and survived the entire 264 hours of exposure. The composite is thus classified as acid attack brittle-fracture resistant. The tensile strength of the sample was measured at room temperature to determine if this exposure affected the overall strength of the composite. Tensile failure occurred at an unusually high 125% of RTS. Thus, the exposure to the 1N HNO 3 solution had no adverse affect on the composite tensile strength. A rod exposed to the 5N solution also passed the 96 hour requisite. The original gauge length was 26 1/16 in. (662 mm) and after 96 hours of exposure was found to be the same. The rod was then tensioned until failure, which occurred at 108% of the RTS. The differences in the tensile strengths of the 1N and 5N exposure are mostly attributed to difference in the composite manufacturing between the two samples, and are not due to the exposure to different concentrations of nitric acid Individual Fibers Subjected to Dilute Acid Exposure To determine if nitric acid exposure caused microstructural damage to the carbon fibers or to the boron-free E-glass fibers, fibers were observed before and after exposure in the same 1N nitric acid solution. Figure 4 (a) shows the boron-free E-glass fibers prior to exposure while (b) shows the boron-free E-glass fibers after 12 days of exposure in the acid solution. No microstructural damage was apparent in these fibers. (a) (b) Figure 4. SEM s of boron-free E-glass (a) before exposure and (b) after 12 days of exposure to 1N HNO 3. In contrast though, Figures 5 (a) and (b) shows boron-containing E-glass before and after the acid exposure. The images show that after 12 days of exposure, there is considerable change to the microstructure and is consistent to the damage observed in [1]. There are spiral patterns that appear in the glass, decreasing the diameter of the fibers. If boron-containing E-glass was used in the composite, the decreased diameter of the E-glass fibers would reduce the strength of the composite. With the glass 5 USC-BrittleFractureTest

6 exhibiting lower strength, this would decrease the ultimate tensile strength of the composite, and could lead to brittle fracture at loads much lower than the ultimate tensile load expected for a composite without this type of damage to the glass fibers. (a) (b) Figure 5. SEM s of boron-containing E-glass (a) before and (b) after exposure to 1N HNO 3 for 12 days. Carbon fibers were also exposed to the same acid solution. Figure 6 (a) and (b) shows the carbon fibers before and after acid exposure. The carbon fibers experienced no microstructrual damage as a result of exposure to the acid solution. The diameter of the fibers also did not change. (a) (b) Figure 6. Carbon fibers (a) before exposure and (b) after 1N HNO 3 exposure for 12 days. Since the area fractions of the carbon to glass are approximately equal in the composite core material, even if the glass were to undergo some erosion as seen in Figure 5, the carbon fiber and matrix do not appear to be susceptible to the nitric acid solution. Therefore, acid attack brittle fracture is not expected to be a problem for the ACCC. 6 USC-BrittleFractureTest

7 4. Conclusions The composite core of the ACCC conductor (Composite Technology Corporation, Irvine, CA) was subjected to an industry-standard acid attack brittle fracture test to determine susceptibility to brittle fracture under aggressive service environments. After 264 hours, the composite showed no detectable damage or change, and did not fail when attacked with a 1N solution of nitric acid. Under 5N for 96 hours, there was also no degradation in the composite or in its tensile strength. The acid solution exposure produced no change in color of the samples, suggesting that the matrix is also resistant to these acid concentrations. The test methods used are widely accepted by the electric power industry to assess resistance to acid-attack brittle fracture. The results from this study consistently showed that the composite is resistant to acid attack brittle-fracture, and acid-attack brittle fracture should not be problematic for the ACCC conductor. 5. References 1. H. Dietz, H. Karner, K.H. Muller, H. Patrunky, G. Schenk, P. Verma and H.J. Voss, "Latest Developments and Experience with Composite Longrod Insulators" in International Conference on Large High Voltage Electrical Systems, Paris, (1986). 2. M. Kuhl. FRP Rods for Brittle Fracture Resistant Composite Insulators. IEEE Transactions on Dielectrics and Electrical Insulators 2001; 8 (2): M. Kumosa, L. Kumosa and D. Armentrout. Failure Analyses of Nonceramic Insulators Part 1: Brittle Fracture Characteristics. In: IEEE Electrical Insulation Magazine, vol. 21 (3), p J.T. Burnham, T. Baker, A. Bernstrof, C. de Tourreil, J-M. George, R. Gorur, R. Hartings, B. Hill, A. Jagtiani, T. McQuarrie, et al. IEEE Task Force Report: Brittle Fracture in Nonceramic Insulators. IEEE Transactions on Power Delivery 2002; 17 (3): F. Schmuck and C. de Tourreil. Brittle Fractures of Composite Insulators: An Investigation of their Occurrence and Failure Mechanisms and a Risk Assessment. CIGRE, WG C. de Tourreil and F. Schmuck. Brittle Fractures of Composite Insulators - Failure Mode Chemistry, Influence of Resin Variations and Search for a Simple Insulator Core Evaluation Method. In: Electra, vol. 215 August-September p. B D.L. Armentrout, M. Kumosa and T.S. McQuarrie. Boron-Free Fribers for Prevention of Acid Induced Brittle Fracture of Composite Insulator GRP Rods. IEEE Transactions on Power Delivery 2003; 18 (3): Netcomposites, "Ownes Corning Develop Boron Free Glass for High Voltage Applications," Netcomposites, 7 USC-BrittleFractureTest

8 Prepared by: Eric J. Bosze Research Associate University of Southern California, Materials Science Department Approved by: Professor Steven Nutt Principle Researcher University of Southern California, Materials Science Department The University of Southern California has prepared this report in accordance with, and subject to the terms and conditions of the contract between USC and Composites Technology Corporation, dated 4 January University of Southern California, USC-BrittleFractureTest