Determination of fatigue characteristics of NBG18 graphite

Abstract

The topic of this research thesis is the determination of the fatigue characteristics of NBG18 graphite, which is the preferred material for the graphite structures of a high temperature gas-cooled nuclear reactor (HTR). This is a newly developed fine grained (maximum grain size of 1,6 mm) near-isotropic graphite grade, that was developed by SGL Carbon for Pebble Bed Modular Reactor company (PBMR), for the application mentioned. SGL Carbon is an overseas company who develops and manufactures graphite materials for nuclear and other applications. PBMR is a South African company who is pioneering the German invented, pebble bed type, high temperature gas-cooled nuclear reactor for power generation purposes. A new test specimen geometry, complete with associated test machine grips, is designed for the determination of the uniaxial fatigue properties of this material across a wide spectrum of load ratios. The geometry of this specimen, called the S70 specimen, is based on the cylindrical ASTM E466 specimen. Tests are conducted for load ratios that range from the static tensile tests, R = +1/+1 to the static compression tests, R = -1/-1. The fatigue cycle ratios are R =+0.5, 0, -0.5, -1, -2, -oo and +2. The stress-life curve (S-N diagram) is determined by testing eight specimens per load with four loads for each load ratio. The Maximum Principal stress is the stress basis for these load ratios. Constant-life charts are compiled for NBG18 from the logarithmic regression equations of the S-N curves. The results of the static strengths compare well with the values quoted by the supplier for NBG18. The uniaxial fatigue characteristics of NBG18 are superior to those of V483T2 for all load ratios. The fatigue characteristics of V483T2 were determined before by Schmidt (2001a). The latter material, which is also a product of SGL Carbon, was the German industry standard material for this application. A novel multiaxial fatigue test specimen, called the S60B specimen, complete with hydrostatic test equipment, is designed for the determination of the multiaxial fatigue characteristics of NBG18 for the same spectrum of load ratios as for the uniaxial fatigue tests. The geometry of the specimen approaches that of a hemisphere. The main features of the new test specimen are its low cost, low cost of associated test equipment and its relative insensitivity to orientation with regard to the grain direction of the graphite. The Maximum Principal stress is the stress basis for the load ratios, similar to the uniaxial tests. Constant-life charts are compiled for NBG18 using the regression equations of the S-N diagrams. The static tensile strength compares reasonably well with the results of the

manufacturer. The static compressive strength subject to this multiaxial loading mode is much higher than expected. This may be attributed to internal friction that is created by the tangential principal stresses. The modified Maximum Deformation Strain Energy stress (MDE) that is prescribed as the stress basis in the KTA 3232 design code, does not yield satisfactory predictions of the multiaxial static failure of the S60B specimen, manufactured from NBG18 graphite. Possible damage due to low pressure (10 MPa), isostatic loading of the graphite, is too small to be conclusive. The measured reduction in strength is less than 3% for all static strengths and a fatigue life of 100 cycles. This research establishes the capability to perform uniaxial and multiaxial fatigue tests in South Africa. The technology is established at the Nelson Mandela Metropolitan University in Port Elizabeth and the North-West University in Potchefstroom respectively.