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The Effect of Sandblasting on the Initiation of Stress Corrosion Cracking in Unidirectional E-Glass/Polymer Composites Used

in High Voltage Composite (Non-Ceramic) Insulators

L. Kumosa, D. Armentrout and M. Kumosa

Center for Advanced Materials and Structures Department of Engineering

University of Denver 2450 S. Gaylord Street,

Denver, CO 80208 Key words: E-glass/polymer composites, stress corrosion cracking, composite (non-ceramic) insulators, brittle fracture, sandblasting. ABSTRACT The purpose of this research is to examine the effect of sandblasting on the initiation of stress corrosion cracking (SCC) in unidirectional E-glass/polymer composite materials with modified polyester, epoxy and vinyl ester resins. The composites with different amounts of surface damage due to sandblasting have been subjected to mechanical bending loads in the presence of nitric acid (pH=1.2). It has been shown in this research that the resistance to SCC in nitric acid of the investigated E-glass/polymer composites, commonly used in high voltage composite (non-ceramic) insulators, is not reduced by the applied low and medium sandblasting conditions. On the contrary, the sandblasting slightly increase the resistance of the composites to the initiation and propagation of stress corrosion cracks. 1. INTRODUCTION Composite (non-ceramic) suspension high voltage insulators can fail in service by brittle fracture [1-17]. The failure process is catastrophic and its most likely cause is the stress corrosion cracking of the insulator composite rod materials due to the formation of nitric acid solutions in-service [9]. SCC of unidirectional glass/polymer composite materials depends on many factors. The dominant factors are: types of glass fibers and polymer resins, acid type and concentration, magnitudes of mechanical loads, surface conditions, etc. Some insulator manufacturers apply sandblasting to E-glass/polymer composite rods during the insulator manufacturing process to increase the adhesion of either silicone or EPDM rubbers to the composite rods. The surface damage caused by sandblasting could have a very negative effect on the resistance of the insulators to brittle fracture failures, however, the effect of sandblasting on the stress corrosion cracking in the composites and thus brittle fracture of composite high voltage insulators has not yet been investigated.

Published version can be found in Composites Science & Technology, Vol. 62, No. 15 (2002) pp. 1999-2015.

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The recent work of Megel et al. [15] investigated the effect of the combination of nitric acid and mechanical bending loads on the formation of stress corrosion cracking in pultruded unidirectional E-glass/polymer composite materials based on epoxy, vinyl ester and modified polyester polymers. These three different composite systems from Glasforms, Inc, in an as-supplied state, were exposed to nitric acid and bending loads in a four-point bend fixture. The stress corrosion process in the composites was monitored using acoustic emission (AE) techniques. A schematic of the experimental setup used by Megel et al. [15] can be seen in Figure 1. It was shown in Ref. 15 that the most acoustically active material was E-glass/modified polyester whereas the least acoustically active composite was the E-glass/vinyl ester system. E-glass/epoxy was in between. It was also observed that the acoustic emission vs. time curves obtained from the stress corrosion tests could be broken up into three distinct stages (see Figure 2a-c). Stage I involves the initiation of stress corrosion cracking as seen in the form of random transverse single fiber cracks on the tensile surface of the specimen. Stage II combines continued initiation through single fiber cracks, as well as sub-critical crack growth seen in the single fiber cracks spreading into neighboring fibers to form multiple fiber cracks. Stage III is the stable propagation of one of the multiple fiber cracks across the specimen inevitably ending in the failure of the specimen and a total load drop. Regarding the resistance to the initiation of stress corrosion cracking of the composites, it was found that the E-glass/vinyl ester composite is approximately 10 times better than the E-glass/epoxy composite whereas the E-glass/modified polyester exhibits approximately 5 times worse resistance to SCC initiation than the E-glass/epoxy system. As far as the sub-critical crack extension process is concerned, the E-glass/epoxy composite exhibits approximately 5 times better resistance than the E-glass/modified polyester system. The sub-critical crack growth (stage II) was not achieved in the E-glass/vinyl ester specimens after 72 hours of testing. In this study, the effect of surface conditions on the initiation and propagation of stress corrosion cracks in the same unidirectional E-glass/polymer composites was examined using the approach developed in Ref. 15. Particular attention was given to the determination of the effect of sandblasting on the stress corrosion fracture process in the composites and thus its effect on the resistance to brittle fracture of high voltage composite insulators. 2. EXPERIMENTAL PROCEDURE 2.1. Materials and testing conditions Three unidirectional composite systems (E-glass/vinyl ester, E-glass/epoxy and E-glass/modified polyester) were tested with as-supplied, low sandblasting and medium sandblasting surface conditions. The composites were supplied by Glasforms, Inc and the sandblasting of the composite specimens was performed by Crystal Mark, Inc.

Table 1 shows the specific details of the sandblasting procedure used in this project and compares them to the conditions used by one insulator manufacturer which applies

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sandblasting during insulator manufacturing. In Tables 2a and 2b the surface roughness of the sandblasted specimens with the low and medium sandblasting conditions are shown, respectively. Table 1: Details of sandblasting procedure used in these experiments and the sandblasting used by an insulator manufacturer.

Manufacturer Rod Pretreatment

Light Abrasion (Low Sandblasting)

Medium Abrasion (Medium Sandblasting)

Blasting Particles Al3O2 Grade 80 Al3O2 17.5µm Al3O2 17.5 µm Pressure 344 kPa 275 kPa 550 kPa Feed Rate Unknown 3.4 mm/s 1.7 mm/s Angle Incidence Grazing Perpendicular Perpendicular Table 2a. Surface roughness measured on the light sandblasted E-glass composite specimens (units of µm).

Specimen A Specimen B Specimen C Average E-glass Modified Polyester 0.44 0.44 0.38 0.42 E-glass Epoxy 1.49 1.44 1.30 1.40 E-glass Vinyl Ester 0.97 0.75 0.74 0.82

Table 2b. Surface roughness measured on the medium sandblasted E-glass composite specimens (units of µm).

Specimen A Specimen B Specimen C Average E-glass Modified Polyester 0.99 1.08 1.04 1.04 E-glass Epoxy 1.99 2.55 2.87 2.47 E-glass Vinyl Ester 0.89 0.81 0.99 0.90

Examples of the three surface conditions (as-supplied, low sandblasting, medium sandblasting) on each of the three E-glass/polymer composite materials can be seen in Fig. 3a-c. For comparison, a sandblasted composite surface from one of the insulator manufacturers is shown in Figure 4. It is clear that the surface damage to the composite generated by the manufacturer closely resembles the damage conditions on the surfaces of the specimens tested in this research. Furthermore, the surface damage generated by the manufacturer is intermediate between the applied low and medium sandblasting conditions. 3. STRESS CORROSION RESULTS 3.1. E-glass/polymer composites; as supplied and with low and medium sandblasting In the initial stage of this research, stress corrosion tests for each type of E-glass/polymer composite (three tests for each composite material) without sandblasting were performed using the experimental set-up shown in Figure 1. The tests were performed in nitric acid (pH 1.2). During testing, acoustic emission and displacements were monitored as a function of time. The acoustic emission vs. time and displacement vs. time curves from

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the tests were obtained and compared with the previously published data for the same composites tested under the same testing conditions [15]. The new and already published AE vs. time curves for the E-glass/modified polyester, E-glass/epoxy and E-glass vinyl ester composites are shown in Figures 5a-c, respectively, for comparison. The previously published data from Megel et al. [15] are indicated in these figures by the letter “M”. The average displacement vs. time curves for three E-glass/modified polyester, E-glass/epoxy and E-glass vinyl ester specimens are shown in Figure 6. The E-glass/epoxy system almost always exhibited the largest bending displacements followed by the E-glass/modified polyester and E-glass/vinyl ester systems. Acoustic emission vs. time curves for the E-glass/modified polyester, E-glass/epoxy and E-glass vinyl ester specimens with low sandblasting are presented in Figures 7a-c, respectively. For each composite system three stress corrosion tests were performed. However, one tests on the low sandblasted E-glass/epoxy specimens was unsuccessful due to a problem with the AE setup. Therefore, only two AE vs. time curves are shown in Figure 7b. The E-glass/polymer specimens with medium sandblasting were also subjected to the stress corrosion tests in nitric acid under four-point bending conditions. The AE data from these tests are shown in Figures 8a-c. Similar to the tests on the low sandblasted composites, three tests were performed on each composite system with medium sandblasting conditions. However, one test on the medium sandblasted E-glass/epoxy specimens was also unsuccessful due to a problem with the AE setup, therefore only two AE vs. time curves are shown in Figure 8b. There are also two curves presented in Figure 8c for the E-glass/vinyl ester system with medium sandblasting. One test did not generate any acoustic emission events. It was however considered to be successful. For comparison, the averaged stress corrosion AE data (up to 24 hours) for the as supplied E-glass/modified polyester, E-glass/epoxy and E-glass/vinyl ester composites are shown in Figure 9. It can be seen that the previous rating of the composites [15], as far as their stress corrosion properties in nitric acid are concerned, has not changed. The E-glass/vinyl ester system exhibits the highest resistance to SCC followed by the E-glass/epoxy and E-glass/modified polyester systems. The effects of surface conditions on the stress corrosion behavior of the composites are illustrated in Figures 10a-c. The average acoustic emission data from the tests up to 24 hours are shown in these figures. 3.2. SEM analysis Detailed SEM analyses of stress corrosion damage in the as supplied E-glass/polymer composites subjected to nitric acid under four point bending conditions have already been performed and reported in Ref. 15. Similar findings were discovered in this study and therefore no new major conclusions are made. Sandblasted composites after the SCC testing were not analyzed using SEM because of their substantial surface damage. Due to

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the large amount of cracking on the surface caused by the sandblasting, individual fiber cracks caused by SCC were very difficult to locate. 4. MICRO-HARDNESS TESTING It is assumed that the resistance to SCC of unidirectional glass/polymer composites is related to the fracture toughness properties of the polymer resins (among several other factors). Therefore, the micro-hardness of six materials was investigated to determine the hardness of the polymer resin in each material and thus to indirectly determine their fracture toughness, assuming that hard polymers should exhibit low fracture toughness properties. Three resins – epoxy, vinyl ester and modified polyester – were investigated in two different applications: neat resin and E-glass fiber/polymer composite. All hardness measurements were performed using a Wilson Tukon Tester fitted with a Vickers indenter. The experimental procedure was based on the ASTM standard E 384-89. All filar measurements were performed at 20x magnification. The conversion from filar to µm at 20X magnification is 1 filar = 0.485 µm. Vickers hardness Hv was then calculated based on the following equation obtained from the ASTM standard:

2

2sin2d

PAP

Hs

V

��

��

�

==

α

Equation 1

where, P is the load (kilogram-force), As is the surface area of indentation (mm2), d is the mean diagonal of indentation (mm) and α is the face angle of the indenter (136°). The preceding equation can be simplified to conventionally used units of grams and µm by the following equation:

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14.1854

d

PHV = Equation 2

where, P1 is the load (grams) on the indenter and d1 is the mean diagonal of indentation (µm). The length d1 was obtained by making five indentations in each of the materials tested and measuring both diagonals. This resulted in 10 measurements of the diagonal. These measurements in filar at 20x magnification were converted into µm and then the average values were entered into equation 2 to obtain the Vickers hardness ratings. Three E-glass composite materials were tested at 5 different loads to establish the load effect on the hardness measurements of the resin within a composite. In Figures 11a and 11b the length of the diagonal and the Vickers hardness number are shown, respectively for the as-supplied E-glass/polymer systems as a function of load. The Vickers hardness data of the resins in the composites at a load of 10 grams and the neat resins at a load of 100 grams are presented in Figures 12 and 13, respectively. The interpretation of the hardness test data is presented in the section below. 5. DISCUSSION

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To examine the effect of specimen surface conditions (as supplied vs. sandblasted) and the polymer resin (modified polyester, epoxy and vinyl ester), the average number of AE signals after 72 hours from all the successful testes performed by Megel et al. [15] and in this study are illustrated in Table 3. By examining the data presented in Table 3, the effects of resin and specimen surface conditions on the rates of SCC in the composites can be clearly seen assuming that the acoustic emission activity is proportional to the amount of stress corrosion damage (predominantly fiber fractures). Table 3. Comparison of the acoustic emission data (average number of events) from the tests performed on the as supplied, low sandblasted and medium sandblasted E-glass composites.

Average AE Events After 72 Hours Material As-Supplied Low Sandblasting Medium Sandblasting

E-glass/Modified Polyester 43698.7 54039 (1) 39380 E-glass/Epoxy 1455 (2) 395 (3) 792 (4)

E-glass/Vinyl Ester 34.2 (5) 18 7.5 1 One sample cracked before 72 hours, therefore the average should be higher. Average was performed from two sets of data. 2 One sample cracked before 72 hours, therefore the average should be higher. Average was performed from 5 sets of data. 3 One test failed from a system malfunction and one sample cracked before 72 hours. Average should be higher and was performed from only one data set. 4 One test failed from a system malfunction. Average was performed from two sets of data. 5 One test failed from a system malfunction. Average was performed from five sets of data. It has been demonstrated in this study that the initial rating of the as-supplied E-glass/polymer composites with the modified polyester, epoxy and vinyl ester resins regarding their resistance to SCC in nitric acid [15] has not changed. The additional tests performed on the composites clearly show that the E-glass/modified polyester system exhibits the lowest resistance to SCC (highest AE activity) followed by the E-glass/epoxy system. The as-supplied E-glass/vinyl ester material showed the highest resistance to SCC in nitric acid. The new stress corrosion data and conclusions are consistent with the results presented by Megel et al. [15]. It has been indicated in Refs. 15 and 16 that there are three dominant factors which control the initiation and propagation of stress corrosion cracks in unidirectional E-glass/polymer composites, namely the acid diffusion through the composites, the fracture toughness of the polymer resin and the amount of exposed glass fibers on the composite surfaces. In addition, the residual stress in the fibers from the extrusion process and the residual stresses in the fibers from pultrusion can affect the resistance to SCC of the composites [7, 16]. The amount of exposed glass fibers can vary significantly and appears to be strongly dependent on the polymer type. The E-glass/vinyl ester composites exhibit the lowest amount of exposed fibers (4.3 ± 1.82%) followed by the E-glass/modified polyester (11.7 ± 3.53%) and the E-glass/epoxy (35.6 ± 5.38%) [16]. The water absorption properties of the composites are also dependent on the polymer type [8].

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Since the toughness of a polymer resin should depend on its hardness, with soft resins exhibiting high fracture toughness, an attempt was made to evaluate indirectly the fracture toughness properties of the three polymer resins used in the composites considered. The data shown in Figures 11a and 11b strongly indicate that the hardness of a resin, as determined by the Vickers hardness test, depends not only on its type but also on the applied force. Certainly, for large applied forces, the hardness of the surface should be closer to the macroscopic hardness of the entire composite including the effect of glass fibers (see Figure 16a) and should be proportional to the amount of exposed fibers on the composite surface. For very low forces, the hardness of the surface should be very close to the hardness of the polymer resin, especially if this was measured in areas between the fibers (see Figure 16b), which is not possible if high forces are applied. For a force of 10 g, the hardness of the E-glass/vinyl ester composite was the lowest (15.3) followed by the hardness of the E-glass epoxy (21.8) and the E-glass/modified polyester (23.0) (see Figure 12). It can also be seen that the hardness of the E-glass/polymer composites measured with a force of 500g was strictly related to the amount of exposed fibers on the composite surface. The hardness of the E-glass/epoxy specimen was 48.6 (the largest amount of exposed glass fibers) followed by the hardness of the E-glass/modified polyester (38.0), followed by the hardness of the E-glass/vinyl ester system (34.6, the smallest amount of exposed fibers) for very similar volume fractions of glass fibers in the composites. Table 4: Comparison of the critical factors affecting SCC resistance.

Material

Diffusivity (10-6 mm2/s) / Maximum Moisture Content

(%)*

Vickers Hardness #

(@ 10 g Load)

Surface Fiber Exposure

(%)**

Corrosion Resistance (Ranking)

E-glass/Modified Polyester 7.04 / 0.225 23.0 11.7 ± 3.53 C

E-glass/Epoxy 1.489 / 0.210 21.8 35.6 ± 5.38 B

E-glass/Vinyl Ester 1.218 / 0.177 15.3 4.3 ± 1.82 A

*, ** From Ref. 8 and Ref. 16, respectively. In Ref. 16 the standard deviations are erroneous. The resin hardness data in conjunction with the water diffusion results and the amounts of exposed fibers on the composite surfaces (see Table 4) further support the conclusions made in Refs. 15 and 16 regarding the effects of these three factors on the SCC in the composites. Since the fracture toughness of the vinyl ester resin is the highest (as deduced from the lowest hardness) with the lowest amount of exposed fibers on the surface of the E-glass/vinyl ester composite and the lowest moisture absorption, the resistance of the E-glass/vinyl ester system to SCC in nitric acid is much higher in comparison with the other systems based on the epoxy and modified polyester resins. The fact that the vinyl ester resin is significantly less brittle than the epoxy and modified polyester resins can also be clearly seen in the sandblasting data presented in Tables 2a

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and 2b. The average surface roughness of the medium sandblasted composite specimens with the modified polyester resin increased from 0.42 to 1.04 µm. The increase in the surface roughness of the epoxy based composite specimens was approximately twice (from 1.40 to 2.47µm). The roughness of the medium sandblasted composite surfaces with the vinyl ester resin did not change. The same roughness was measured for the low (0.82) and medium (0.90µm) sandblasted specimens. With regard to the effect of sandblasting on SCC, the data presented in Figures 10a-c are slightly confusing. It could be said that in some cases sandblasting worsens the resistance to SCC (see Figure 10a) whereas in several other cases the resistance to SCC actually was improved due to sandblasting (see Figures 10b and 10c). Considering the amount of scatter in the SCC data presented in Figures 10a-c, it can be stated that the overall effect of sandblasting on the corrosion properties of the composites is either neutral or slightly positive. However, the effect of sandblasting can be much better observed if the individual stages of SCC in the composites are examined. As was done with the results from Megel et al. [15], stage analyses of each of the E-glass/polymer composites was performed with the inclusion of both sandblasting conditions. These analyses are shown in Figures 14 and 15 for both the crack initiation stage (Stage I) and the sub-critical crack extension stage (Stage II), respectively. It can be seen in the data presented in Figures 14 and 15 that for the as supplied surface condition, the same ranking as shown by Megel et al. [15] is present. The E-glass/modified polyester composite exhibits a faster rate of growth in both stages than E-glass/epoxy, which has a faster rate of growth in both stages than E-glass/vinyl ester, which actually does not move beyond the first crack initiation stage. These two figures (14 and 15) show that not only is the ranking previously observed by Megel et al. still valid, but also that the low and medium sandblasting surface conditions noticeably lower the rates of growth in both the crack initiation stage (Stage I, see Figure 14a-c) and in the sub-critical crack extension stage (Stage II, see Figure 15a-c). Despite the fact that the stage analysis can provide useful interpretations of the overall data presented in Figures 10a-c, caution must be observed when this type of analysis is applied. In some cases, the separation of the stages can be quite difficult. Moreover, in some instances the crack initiation stage (Stage I) can be actually very short, quickly followed by the sub-critical stage (Stage II) and almost immediately followed by Stage III where a large macroscopic crack propagates across the entire specimen. In such cases specimens fail catastrophically during testing. This was observed for specimens #3 (E-glass/modified polyester with low sandblasting), #3 (E-glass/epoxy as supplied) and #2 (E-glass/epoxy with low sandblasting). Stress corrosion cracking of E-glass fiber/polymer composites and bare E-glass fibers can occur in the presence of mechanical tensile stresses and free hydrogen ions. The stresses can be either externally applied or residual from fabrication. During pultrusion of unidirectional E-glass/polymer composites, residual compressive stresses are generated in E-glass fibers inside the composites. However, on the composite surface, partially exposed glass fibers will develop tensile residual stresses which can initiate SCC in

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unidirectional E-glass/polymer composites in the absence of external mechanical loads [16]. There is also another type of residual stress in E-glass fibers. During fiber fabrication significant residual thermal stresses are generated in the fibers, the magnitudes of which are controlled by the cooling rate and the drawing stress and speed [18,19]. High cooling rates cause a severely non-uniform distribution of temperature across a fiber, with the lowest temperature at the fiber surface and the highest in the fiber center. Under the influence of such a temperature profile, together with the applied drawing stress, a considerable thermal stress gradient is established. The residual stresses in the bare fibers are dominated by axial stress but also have a small portion of radial stress. These stresses are concentrated in a thin surface layer, the thickness of which is highly dependent on the drawing stress as well as the fiber diameter. It has been shown that the residual stresses can affect the fracture morphology in bare E-glass fibers [7]. During sandblasting of the unidirectional E-glass/polymer composites, numerous surface cracks in the fibers on the composite surfaces are formed (see Figures 3 and 4). Such surface cracks must significantly relieve the residual axial stresses in the fibers both from fiber and composite fabrications. The relaxation of the residual tensile stresses in the fibers on the low and medium sandblasted surfaces of the E-glass/polymer composites was responsible for the noticeable improvement in the resistance to SCC of the composites in nitric acid. However, more severe sandblasting conditions might have a negative effect on the resistance to SCC of the composites. Moreover, the applied stress on the tensile side of the composite specimens subjected to four point bending was approximately 130 MPa for a load of 90 N [15]. Such a load and stress correspond to an average load applied to a 345 kV composite insulator in-service. For much higher bending loads, the magnitudes of the applied stresses would be higher which would decrease the ratio between the residual stresses in the fibers and the axial stresses generated by the bending forces. This could reduce the stress relaxation effect caused by sandblasting. 6. CONCLUSIONS The results obtained from the stress corrosion tests performed on the as supplied E-glass/polymer unidirectional composites subjected to four-point bending in the presence of nitric acid clearly indicate that the previously determined ranking of the composites regarding their resistance to stress corrosion cracking in nitric acid as presented by Megel et al. in Ref. 15 has not changed. The E-glass/modified polyester exhibits the lowest resistance to SCC and thus brittle fracture. The highest resistance to SCC has been found in the case of the E-glass/vinyl ester composite system with the stress corrosion resistance of the E-glass/epoxy system in-between the other two systems. It has also been shown that the resistance to SCC is in direct correlation with the following material characteristics: acid/water absorption, fracture toughness/softness of the polymer resin and the amount of exposed glass fibers on the composite surfaces. E-glass/vinyl ester has the slowest diffusivity of water and the lowest moisture content, the least hard resin (highest fracture toughness) and the least amount of exposed fibers on its surface, and in

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this research it has been shown that E-glass/vinyl ester has the highest resistance to SCC. It has also been shown in this study that the effect of sandblasting on the initiation and propagation of stress corrosion cracks in low and medium sandblasted composite specimens is slightly positive. The sandblasting applied to the composites relieved the residual thermal stresses in the fibers improving the resistance of the composites to SCC. REFFERENCES 1. M. Kumosa et al., Micro-Fracture Mechanisms in Glass/Polymer Insulator Materials

under Combined Effects of Electrical, Mechanical and Environmental Stresses, Final Report to the Bonneville Power Administration, Electric Power Research Institute and the Western Area Power Administration, Oregon Graduate Institute, Portland, Oregon, July 1994.

2. M. Kumosa, Q. Qiu, E. Bennett, C. Ek T. S. McQuarrie and J. M. Braun, Brittle

Fracture of Non-Ceramic Insulators, in the Proceed. Fracture Mechanics for Hydroelectric Power Systems Symposium'94, Canadian Committee for Research on the Strength and Fracture of Materials (CSFM), BC Hydro, Sept. 1, 1994, pp. 235-254.

3. Q. Qiu, Brittle Fracture Mechanisms of Glass Fiber Reinforced Polymer Insulators,

Ph.D. Thesis, Oregon Graduate Institute of Science & Technology, Portland, Oregon, October 1995.

4. M. Kumosa and Q. Qiu, Failure Analysis of Composite Insulators (Failure

Investigation of 500 kV Non-ceramic Insulators for Pacific Gas & Electric Company), Final Report to the Pacific Gas and Electric Company, May 1996, Department of Engineering, University of Denver.

5. M. Kumosa, H. Shankara Narayan, Q. Qiu and A. Bansal, Brittle Fracture of Non-

Ceramic Suspension Insulators with Epoxy Cone End-Fittings, Composites Science and Technology, Vol. 57, (1997) pp. 739-751.

6. Interview with Maciej Kumosa, Research of Brittle Fractures in Composite

Insulators, Insulator News & Market Report, July/August (1997) pp. 46-51. 7. Q. Qiu and M. Kumosa “Corrosion of E-Glass Fibers in Acidic Environments”,

Composites Science and Technology, Vol. 57 (1997) pp. 497-507. 8. Y. Zhao, An Electrical Study of the Brittle Fracture Failure of Composite Insulators,

MS Thesis, Department of Engineering, University of Denver, November 1997. 9. A. R. Chughtai, D. M. Smith and M. Kumosa, Chemical Analysis of a Field-Failed

Composite Suspension Insulator, Composite Science and Technology, Vol. 58, (1998) pp. 1641-1647.

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10. M. Kumosa et al., Fracture Analysis of Composite Insulators, Final Report to the Electric Power Research Institute, Department of Engineering, University of Denver, Denver, Colorado, March 1998.

11. M. Kumosa et al., Micro-Fracture Mechanisms in Glass/Polymer Insulator Materials

under the Combined Effect of Mechanical, electrical and Environmental Stresses, Final report to BPA, APA, PG&E, WAPA and NRECA, University of Denver, Denver, Colorado, December 1998.

12. S. H. Carpenter and M. Kumosa, An Investigation of Brittle Fracture of Composite

Insulator Rods in an Acidic Environment with Static or Cyclic Loading, J. Materials Science, Vol. 35, Issue 17 (2000) pp. 4465-4476.

13. T. Ely and M. Kumosa, The Stress Corrosion Experiments on an E-glass/Epoxy

Unidirectional Composite, J. Composite Materials, Vol. 34 (2000) pp. 841-878. 14. T. Ely, D. Armentrout and M. Kumosa, Evaluation of Stress Corrosion Properties of

Pultruded Glass Fiber/Polymer Composite Materials, J. Composite Materials, Vol. 35 (2001) pp. 751-773.

15. M. Megel, L. Kumosa, T. Ely, D. Armentrout and M. Kumosa, Initiation of Stress

Corrosion Cracking in Unidirectional Glass/Polymer Composite Materials, Composites Science and Technology, Vol. 61 (2001) pp. 231-246.

16. L. Kumosa, D. Armentrout and M. Kumosa, An Evaluation of the Critical Conditions

for the Initiation of Stress Corrosion Cracking in Unidirectional E-glass/Polymer Composites, Composite Science and Technology, Vol. 61 (2001) pp. 615-623.

17. D. Armentrout, M. Kumosa and T. S. McQuarrie, Boron Free Fibers for Prevention of

Acid Induced Brittle Fracture of Composite Insulator GRP Rods, IEEE Transactions on Power Delivery, submitted.

18. A. Schmiemann, M. Gehde and G.W. Ehrenstein, Effect of Internal Stresses on

Corrosion Behavior of Glass Fibers, Proc. 44th Annual Conf. On Reinforced Plastics/Composites, Session 2-C, The Society of the Plastics Industry, Inc., New York, 1989.

19. H. Stockhorst and R. Bruckner, Structure Sensitive Measurements on E-glass Fibers,

J. Non-Cryst. Solids, Vol. 49 (1982) pp. 471-. ACKNOWLEDGMENTS This research has been supported by the Western Area Power Administration and Electric Power Research Institute under Contracts #DE-AP65-00WA14437 and #EP-P2971/C1399, respectively.

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Figure 1: Schematic of the four-point bend fixture and experimental setup.

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Figure 2a

Figure 2b

Figure 2c

Figure 2: Schematic stages of SCC and SEM examples from specimens exposed to nitric acid for 72 hours

under four-point bending loads.

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Figure 3a: SEM micrographs of the as-supplied surfaces of (from top to bottom) E-glass/modified polyester, E-glass/epoxy and E-glass/vinyl ester.

E-glass/Modified PolyesterAs-Supplied (No Sandblasting)

E-glass/EpoxyAs-Supplied (No Sandblasting)

E-glass/Vinyl EsterAs-Supplied (No Sandblasting)

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Figure 3b: SEM micrographs of the low sandblasted surfaces of (from top to bottom) E-glass/modified polyester, E-glass/epoxy and E-glass/vinyl ester.

E-glass/Vinyl EsterLow Sandblasting

E-glass/EpoxyLow Sandblasting

E-glass/Modified PolyesterLow Sandblasting

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Figure 3c: SEM micrographs of the medium sandblasted surfaces of (from top to bottom) E-glass/modified polyester, E-glass/epoxy and E-glass/vinyl ester.

E-glass/Modified PolyesterMedium Sandblasting

E-glass/EpoxyMedium Sandblasting

E-glass/Vinyl EsterMedium Sandblasting

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Figure 4: GRP rod with sandblasting performed by an insulator manufacturer.

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Figure 5b: AE vs. time plots for the as-supplied E-glass/epoxy system (new and previously published data from Ref. 15).

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Eve

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(#)

E-glass/Modified Polyesterwith Low Sandblasting

0

2000

4000

6000

8000

10000

0 6 12 18 24 30 36 42 48 54 60 66 72

Time (h)

AE

Eve

nts

(#)

E-glass/Epoxy with Low Sandblasting

Figure 7a: AE vs. time plots for the E-glass/modified polyester system with low sandblasting.

Figure 7b: AE vs. time plots for the E-glass/epoxy system with low sandblasting (one set of data was damaged and therefore only two

plots are shown).

0

5

10

15

20

25

30

0 6 12 18 24 30 36 42 48 54 60 66 72

Time (h)

AE

Eve

nts

(#)

E-glass/Vinyl Ester with Low Sandblasting

Figure 7c: AE vs. time plots for the E-glass/vinyl ester system with low sandblasting.

21

0

10000

20000

30000

40000

50000

60000

70000

0 6 12 18 24 30 36 42 48 54 60 66 72

Time (h)

AE

Eve

nts

(#)

E-glass/Modified Polyester with Medium Sandblasting

0

200

400

600

800

1000

1200

1400

0 6 12 18 24 30 36 42 48 54 60 66 72

Time (h)

AE

Eve

nts

(#)

E-glass/Epoxy with Medium Sandblasting

Figure 8a: AE vs. time plots for the E-glass/modified polyester system with medium sandblasting.

Figure 8b: AE vs. time plots for the E-glass/epoxy system with medium sandblasting (one test set of test data was damaged and

therefore only two plots are shown).

0

1

2

3

4

5

6

7

8

9

10

0 6 12 18 24 30 36 42 48 54 60 66 72

Time (h)

AE

Eve

nts

(#)

E-glass/Vinyl Ester with Medium Sandblasting

Figure 8c: AE vs. time plots for the E-glass/vinyl ester system with medium sandblasting.

22

0

200

400

600

800

1000

1200

1400

1600

0 3 6 9 12 15 18 21 24Time (h)

Ave

rage

d A

E E

vent

s (#

)

E-glass/Polymer As-supplied


E-glass/Epoxy

E-glass/Vinyl Ester

Figure 9: Averaged AE vs. time curves for the As-supplied E-glass/polymer materials for the first 24 hours of testing (from the new and previously published data from Ref. 15).

23

0

2000

4000

6000

8000

10000

12000

0 3 6 9 12 15 18 21 24

Time (h)

Ave

rage

d A

E E

vent

s (#

) Low Sandblasting

Medium Sandblasting

As-Supplied


Figure 10a: Averaged AE vs. time plots comparing the three different surface conditions in the E-glass/modified polyester system.

0

20

40

60

80

100

120

140

160

180

0 3 6 9 12 15 18 21 24

Time (h)

Ave

rage

d A

E E

vent

s (#

)

E-glass/Epoxy

As-Supplied

Low Sandblasting

Medium Sandblasting

Figure 10b: Averaged AE vs. time plots comparing the three different surface conditions in the E-glass/epoxy system.

0

2

4

6

8

10

12

14

16

0 3 6 9 12 15 18 21 24

Time (h)

Ave

rage

d A

E E

vent

s (#

) As-Supplied

Low Sandblasting

Medium Sandblasting

E-glass/Vinyl Ester

Figure 10c: Averaged AE vs. time plots comparing the three different surface conditions in the E-glass/vinyl ester system.

24

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 100 200 300 400 500Load (grams)

Ave

rage

Dia

gona

l Len

gth

(mm

) .

E-glass/Epoxy

E-glass/Vinyl Ester


Figure 11a: Average diagonal length measurements from Vickers hardness indenter vs. load plots for the three E-glass/polymer materials.

10

15

20

25

30

35

40

45

50

0 100 200 300 400 500

Load (grams)

Vic

kers

Har

dnes

s N

umbe

r .

E-glass/Epoxy

E-glass/Vinyl Ester


Figure 11b: Vickers hardness numbers vs. load plots for the three E-glass/polymer materials.

25

23.0

15.3

21.8

0

5

10

15

20

25

30

E-glass Epoxy E-glass Vinyl Ester E-glass Modified Polyester

Vic

kers

Har

dnes

s N

umbe

r (@

10

g)

.

Figure 12: Vickers hardness numbers at 10 grams for the three E-glass/polymer materials.

13.9

21.622.4

0

5

10

15

20

25

Epoxy Resin Vinyl Ester Resin Modified Polyester Resin

Vic

kers

Har

dnes

s N

umbe

r (@

100

g)

.

Figure 13: Vickers hardness numbers at 100 grams for the three polymer resins.

26

0

20

40

60

80

100

120

140

160

180

200

#1 #2 #3 #1 #2 #3 #1 #2 #3

Cra

ck In

itiat

ion

Rat

es(A

E E

vent

s/H

our)

No Data

E-glass/Modified PolyesterAs-Supplied

E-glass/EpoxyAs-Supplied

E-glass/Vinyl EsterAs-Supplied

Average = 148.3

Average = 14.6

Average = 0.7

Crack Initiation (Stage I)

0

5

10

15

20

25

30

35

40

#1 #2 #3 #1 #2 #3 #1 #2 #3

Cra

ck In

itiat

ion

Rat

es(A

E E

vent

s/H

our)

No Data




Average = 19.8

Average = 16.4

Average = 0.2


Figure 14a: Crack initiation (Stage I) rates for as-supplied E-glass/polymer composites.

Figure 14b: Crack initiation (Stage I) rates for E-glass/polymer composites with low sandblasting.

0

5

10

15

20

25

30

35

#1 #2 #3 #1 #2 #3 #1 #2 #3

Cra

ck In

itiat

ion

Rat

es(A

E E

vent

s/H

our)




No Data

Average = 20.4

Average = 1.5Average 0.1


Figure 14c: Crack initiation (Stage I) rates for E-glass/polymer composites with medium sandblasting.

27

0

200

400

600

800

1000

1200

1400

#1 #2 #3 #1 #2 #3 #1 #2 #3

Sub

-Crit

ical

Cra

ck E

xten

sion

Rat

es(A

E E

vent

s/H

our)

E-glass/Modified PolyesterAs-Supplied

E-glass/EpoxyAs-Supplied

Average = 782.9

Average = 55.1

Sub-Critical Crack Extension (Stage II)

E-glass/Vinyl EsterAs-Supplied

Average = 0

0

50

100

150

200

250

#1 #2 #3 #1 #2 #3 #1 #2 #3

Sub

-Crit

ical

Cra

ck E

xten

sion

Rat

es(A

E E

vent

s/H

our)



No Data

Average = 106.06Average = 48.13



Average = 0

Figure 15a: Sub-critical crack extension (Stage II) rates for as-supplied E-glass/polymer composites.

Figure 15b: Sub-critical crack extension (Stage II) rates for E-glass/polymer composites with low sandblasting.

0

20

40

60

80

100

120

#1 #2 #3 #1 #2 #3 #1 #2 #3

Sub

-Crit

ical

Cra

ck E

xten

sion

Rat

es(A

E E

vent

s/H

our)



Average = 79.2

No DataAverage = 6.5


Average = 0


Figure 15c: Sub-critical crack extension (Stage II) rates for E- glass/polymer composites with medium sandblasting.

28

Figure 16a: Indentation area consisting of polymer resin and glass fibers.

Figure 16b: Indentation area in the polymer matrix between the glass fibers.

the effect of sandblasting on the initiation of stress ... · pdf filethe effect of...

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