Materials Degradation and Strength Reduction Factors of CFRP and Their Applications in Repair of Nuclear Safety Related Piping

Abstract

Abstract Carbon Fiber Reinforced Polymer (CFRP) composites have been used for decades in various industries such as aerospace, oil and gas, mainly due to their high strength-to-weight-ratio and excellent corrosion resistance. However, the use of CFRP in nuclear industry is very limited. Recently, a new ASME Boiler and Pressure Vessel (BPV) Code Case N-871 has been accepted for internal repair of buried Class 2 and 3 nuclear safety related piping using CFRP for Service Levels A, B, C and D. The ASME BPV Code Case N-871 allows the use of both Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) methods to design the CFRP repair system. The USNRC has not yet accepted this code case. In 2016, a relief request was submitted to US Nuclear Regulatory Commission (US-NRC) by Surry Power Station, Virginia Electric and Power Company to perform internal repair of degraded ASME Class 2 and 3 safety related circulating and service water buried piping using LRFD method and CFRP, which was approved by US Nuclear Regulatory Commission (NRC). It is documented in the literature that CFRP materials experience property degradation when exposed to certain environments and creep behavior under sustained loading. From a limited number of tests, it has also been found that the overall strength of CFRP repair decreases with an increase in the number of plies, however, the extent of the strength reduction is not well-supported with test data. The statistical values of CFRP strength can be determined in three ways. The ASME BPV Code Case N-871 recommends using the characteristic value of CFRP strength at a 5th percentile value with 80% confidence, whereas the A-basis (1st percentile with 95% confidence) and B-basis (10th percentile with 95% confidence) values of strength are used in other industries such as in aerospace applications. For a safety related nuclear application, it is therefore important to evaluate how these values compare to one another. The property degradation due to environmental exposure, creep behavior, multi-ply laminate as well as difference in various statistical analysis can be given a general term as strength reduction factor and These factors will have a direct effect on the safety margin of any CFRP repair using the ASD method and hence, it is essential to confirm these factors with an adequate number of experiments. In this work, mechanical testing and statistical analyses have been conducted to identify specific degradation mechanisms in CFRP as well as other design considerations that may affect the effective factor of safety of the repair system. These include the effect of multi-ply laminates, misalignment angle, width of the specimen as well as the characteristic values of the ultimate strength. Finally, the strength reduction factors are determined from test results followed by a discussion of how these factors affect the overall effective factor of safety of the repair system.