ASME NTB-4-2021 pdf download.Background Information for Addressing Adequacy or Optimization of ASME BPVC Section III, Division 5 Rules for Nonmetallic Core Components
3 GRAPHITE STRENGTH The ASME graphite design code is essentially a design methodology that compares the stress on a graphite component with the material’s strength distribution. The major factors affecting graphite strength are presented in this section, along with the relevant sections of the ASME graphite code. The bond anisotropy of single-crystal graphite (in-plane strong covalent bonds, between-planes weak van der Waals bonds) contributes to the inherent anisotropy of the polycrystalline material, even though the behavior is more isotropic than single-crystal graphite. Moreover, the manufacture of polycrystalline graphite gives rise to the distribution of the size, location, and orientation of porosity in a graphite [36]. The variability in the porosity in proportion to the length scale of the filler particles of a specific size (and thus density) influences the strength of the material, with higher-density graphite exhibiting higher strength. The structure, and hence the strength, of a graphite is a strong function of the exact manufacturing route employed. Several manufacturing features can be identified which impact the structure and properties of the material: · Filler particle type and size · Forming method (extrusion, molding, isostatic pressing) · Process variables (filler particle type, impregnation type, processing temperatures) Depending upon the extent of the materials anisotropy, the strength may be quoted as pertaining to the with- grain (WG) or against grain (AG) orientation. Generally, the WG strength is greater than AG strength. Because of the many potential variations in production of different grades of graphite, each grade will have different properties. It is important, therefore, that the graphite grade be carefully specified. The ASME graphite code [37] requires that the graphite be compliant to either ASTM D7219 [38] or ASTM D7301 [39].
4 CODE VERIFICATION The behaviors of nuclear graphite and design methods are described in Section 2 and Section 3. This section discusses the work that was done to verify the basis of the code. The code uses a probabilistic approach because of the variability in strength data and graphite grades [17], [18]. The material strength depends on inherent defects like pores, inclusions, cracks, and microstructural irregularities that are common to graphite. These defects act as stress-concentrating features that may not sustain low loads and may result in fracture. There is also a large scatter in material strength test measurements due to the variability associated with the defects in the material. When the material is loaded, the damage accumulates until a critical damage level is reached. This was earlier demonstrated by the conceptual and mathematical fracture model proposed by Burchell [47]. Additionally, experimental data for the behavior of graphite showed that for specimens of the same material of similar size, failure stresses for compression and bending were both higher than stresses for tension [48], [49]. For small sample sizes (close to the filler particle size), it was demonstrated that the strength of graphite was independent of the volume [50], [51]. Because graphite’s tensile strength is less than its compression or bending strength, only tensile specimen test data were used to calculate the code-defined Weibull failure model of the different modeled geometries, applying the full assessment approach. The results were then compared with results of experimental testing of various geometries to validate the applied methodology. The variability of the material strength lends itself to the use of a probabilistic design approach, in which a PDF is applied to describe the reliability of the material [8], [51].ASME NTB-4 pdf download.