ASME NTB-4:2021 pdf free download

ASME NTB-4:2021 pdf free download.Background Information for Addressing Adequacy or 0ptimization of ASME BPVC Section lll，Division 5 Rules for Nonmetallic Core Components.
The declared target or POF value. determined from the component classification and loading level assessment, are then compared with the detennirsed POP for the component. based on any one of the three design approaches discussed in the design rules in IIIIA-3(XX) 1221. The design process can be suinnrarited as tbllows:
• Classify the part, then establish and categorile the loads that are to be applied. Determine the rclbhility targets for thc parts for the various load conditions.
• Calculate the stresses (expressed as an equivalent stress — as explained in Section 4) in the part when it is exposed to the various load combinations.
• Compare the stresses in the part with the allowable limits by one of two methods:
– Simple assessment 1311; Compare the predicted stress statefor the load case) with stress limits derived from the allowable POF and the material test data, as demonstrated by the cumulaiivc density functions (CDF) in Figure 5 1201. The design margin is incorporated through the use of the lower 95% confidence bound on the Weibull parameters (according to the procedures and requirements established in 161. 1321) and the selection of the target reliability value. The simple assessment assumes that the failure distribution is the same at all pans of the component. This also means that the calculated allowable (or equivalent) stress. Sg. (Eq. (4)) for a given POE, IS the same stress at all parts of the component arid equal to the overlap area referenced in the plot (Figure 5). The allowable stress is compared with the peak equivalent stress. This two-parameter Weibull method has been demonstrated to be conservative for graphite components with low POFS.
The material strength depends on inherent defects like pores. inclusions, cracks, and niicrostructural 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 1471. Additionally, experimental data for the behavior of graphite showed that for specimens of the same material of similar sire, failure stresses for compression and bending were both higher than stresses for tension 14X1, 1491. For small sample sires (close to the tiller particle sire), it was demonstrated that the strength of graphite was independent of the volume 1501, 1511.
Because graphite’s tensile strength is less than its compression cc bending strength. only tensile specimen test data were used to calculate the codc-delmned Weihull failure model of the different modeled geometries, applying the full assessment appniach. 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 181. 1511.
Weibull’s theory is most commonly used for load-bearing structures of brittle material, as it is assumed that the strength of a brittle solid is controlled by its flaws 1JOI It was previously discussed 1501 that Weibull assumes that the POF increases with increasing volume (more flaws or unfavorable defects in a larger volume) and, subsequently, increasing stress will increase the POF, This has been demonstrated to be true for experimental test results for nuclear graphite observing the bending strength. However, this volume effect does not support the tensile behavior. The tensile strength appeao. to he independent at small volumes, but it was observed that volume does allect small specimens in which the volume approaches the grain sire. Thus. Weibull’s volume theory is consistent, as it predicts that specimens under bending will fail at higher stresses than specimens under tension (because of the difference in the highly stressed volume of the material). Still, it is inconsistent with the experimental results of small tensile specimen tests, in which the strength of a tensile specimen decreased as the gauge diameter decreased and approached the grain size. Therefrire. the standard Weihull approach poody represent nuclear graphite. hut the characteristics of nuclear graphite have been studied and a Weibull modification was proposed. Denninghofs modified-volume, normalired Weihull weakest-link failure criterion approach, as reported by 1-lindley 1341. describes the POF cia graphite component under a stress state. This approach is adapted in the ASME design code 1331 as the full assessment method. The l&mulation of the probability of survival (POS) and the POF are shown in Eqs. (5) and (6) respectively.