Brittle Fracture in NCI
Brittle fractures in NCI refer to cracks running perpendicular to the long axis of the insulators inside the GRP (glass-fiber reinforced plastic) rods. The composite rods are known to be the principle load-bearing component of NCI insulators. Rod fracture failures that are identified as brittle fractures are lethal to insulators and must be spotted on time. The causes that lead to brittle fracture can be a combination of manufactures' mal treatment, mechanical stress, improper sealing and chemical reactions due to electrical discharge. Either way, this kind of failure must be spotted and attended on time because of the characteristics of the processes involved. Corona cameras are handy in these cases, because corona cameras can clearly show surface discharge that exists on insulators with housing imperfections.
Brittle fractures are usually initiated by water ingress into the insulator's core causing stress corrosion cracking of the GRP rod. The cracking is caused by the combined action of mechanical tensile stresses along the fibers and a corrosive acid that is created during an electrical discharge. The cracking will eventually allow water to penetrate to the NCI rod until a brittle fracture will occur. Electrical discharge such as corona and arcing can be detected by a corona camera and if attended on time and treated properly prevent the failure.
Though brittle fracture is not common it has drawn much attention because this kind of failure is catastrophic and because it is unpredictable. "If we consider the average crack tip velocity across a glass fiber to be about 200m/s then the rime for fracture of an average fiber with 14µm diameter s approx. 70ns. From the moment of the flaw initiation until the final failure of the fiber, the process is dynamic not static." (T. Ely and M. Kumosa, "The Stress Corrosion Experiments on an E-glass/Epoxy Unidirectional Composite," J. Composite Materials, Vol. 34, pp. 841-878, 2000)As to the location along the insulator of the brittle fractures, the most influencing factors are the position, type & size of the grading ring applied. Grading rings affect the electrical field spread along the insulator and reduce the electrical stress on the end fitting. Indeed it was shown that for higher voltages with larger corona rings, the location of the brittle fracture was at larger distances from the end fitting (For more information on how to avoid corona by choosing the correct grading ring please (click here)
A research on brittle fractures conducted by the Oregon Graduate Institute and University of Denver described in "Brittle Fracture Failure of composite (Non Ceramic) Insulators" by Maciej S. Kumosa, summarizes the typical characteristics of this type of damage and the ways to distinguish between mechanical fractures and brittle fractures. The distinction is important because the processes involved are different as are the remedies. The study suggests three tests as follows:
Macroscopic test: The fiber fracture under stress corrosion is caused by the ionization forming corona that produces an acid replacing the metal ions in the fibers while weakening and fracturing them under low tensile stresses. Though brittle fracture can occur either inside or outside of the insulator's energized end fitting, generally bare eyes inspection of the failure position and size will help with a macroscopic analysis. For example, brittle fractures will always transverse fracture surfaces on the GRP rod and run perpendicular to the rod axis.
Microscopic test: close look on the NCI with brittle fracture will show a formation of mirror, mist and hackle zones on the fracture surface of the broken fibers (as shown in the picture).
Mirror (*), mist (**) and hackle (***) zones on the fracture surface
When hydrogen ions replace metal ions weakening the fiber causing fracture under low tensile stress a fiber fracture will occur. The size of mirrors will depend on the mechanical stress applied on the material of the NCI. Depending on the size of the mirrors one can analyze the type of failure.
Chemical test: The most important information about the causes of brittle fracture is in the surface deposits. The causes for brittle fractures can be revealed by analyzing the chemical residues; Tracking metal ions such as iron and zinc ions will reveal the extent of water or acid ingress inside the insulator and the depletion of calcium and/or aluminum ions in the fiberglass core rod can explain the corrosive morphological damage and thus the relative decreases in calcium and aluminum (from the silicon) can be determined.
Post FailurePost failure is another important factor while analyzing brittle fracture damage. It happens after a fracture of a single fiber occurs and it is related to the chemical attack of a corrosive environment on the newly formed fiber fracture related to the corrosion process. The characteristics of the post failure are used to help determine the cause of the cracking on the composite insulator. The amount of post failure damage is related to the acid type, its concentration and the time of exposure on the GRP. The nature of deposits on the fracture surfaces are also important issues in the analysis process of the brittle fracture especially when the fracture is formed inside the fitting where the deposits are so thick that the fracture surface of the fibers cannot be seen.
Post failure damage on the fracture surface of E-glass fiber exposed to oxalic acid.
Prevention and conclusionsThe prevention of brittle fracture is possible and can be controlled by the manufacture and by the user. During the manufacturing it is important to use an excellent moister sealing and to assure that rods are free of large voids, cracks or any other type of macroscopic damage. Another important issue to address is the use of proper sheath thickness in order to protect the interface between the fitting and the rod.
On the other hand the user can install grading rings since they will minimize the corona effect in the rubber rod and thus prevent (or postponed) brittle fracture from happening. Preventing the insulators from any damage at any stage, storage, transportation or installation can prevent brittle fracture as well.
Since the probability of brittle fracture increases with the increment of the line's voltage, it is fair to say that the higher the E-Field around the insulator, the environment is more suitable for the initiation of corona and therefore, for brittle fracture to occur. Hence, though a corona camera can't predict this type of failure it can reveal it at an earlier stage.
To conclude, this critical type of damage in NCI is subjected to the combined effects of mechanical, electrical and environmental stresses. In most cases it is hard to predict, particularly when the fault lies in the manufacturing stage, but once detected by a corona camera full attention must be given.