Crack initiation transients

With the reference toughness curve approach, the RT m index first came into use as the reference nil-ductility temperature which is determined in accordance with the ASME Code, Section III, Subsection NB-2331. The reference toughness Km curve and the Ki curve for static crack initiation later came into use as part of Section XI where the Km curve was called the crack arrest Xia curve. Thus, the RTndt reference temperature index has become the key material parameter in determining the allowable (P-T) limits for plant operation and for evaluating RPV integrity as the result of extreme transients such as PTS. Note that several years ago, the concept of a different, directly measured fracture toughness Master Curve approach was accepted in the ASME Code based on the index parameter RTjq. This development is covered in detail in Chapter 10. 

The CPI is computed for each flaw in a distribution of flaws in a trial vessel. Flaws that have a non-zero probability of extending under the applied loading conditions are then evaluated to compute the probability that the flaw will grow through the vessel wall. The CPI and CPF computations are repeated for a large number of trial vessels to obtain CPI and CPF distributions. The CPI and CPF distributions are combined with frequency distributions for each transient included in the analysis to obtain distributions of crack initiation and TWCF. 

The performances of the 3D CZM model were compared with a previously developed 2D model and analytical solutions on mode I, mode II and mixed-mode I/n loaded cracks in bonded aluminum or composite assemblies. The results are in an overall good agreement with each other, with the exception of the first instants of propagation of the 3D model, where the CZM process zone has to shape up. Next step should to assess this initial transient of 3D process zone formation, leading to a higher predicted number of cycles with respect to 2D simulation, by experimental evidence. 

A fatigue assessment is conducted in the design phase in order to prevent any crack initiation. This assessment is made by using the cyclic stresses and number of cycles given in the RPVI design report. These values are determined using the estimates of the type and number of transients provided by the NSSS vendors. 

We solved the transient hydrogen diffusion initial/boundary-value problem coupled with the large strain elastoplastic boundary value problem for a pipe of an outer diameter 40.64 cm, wall thickness h = 9.52 mm, and with an axial crack of depth 0.2/i on the inner wall-surface. We obtained the solutions under hydrogen gas pressure of 15 MPa, material properties for an X70/80 type steel, and... 

When the applied stress a is less than Su, creep of the matrix will commence after application of the load. During this creep, the matrix will relax and the stress on the fibers will increase. Therefore, further fiber failure will occur. In addition, the process of matrix creep will depend on the extent of prior fiber failure and, as mentioned previously, on the amount of matrix cracking. The details will be rather complicated. However, the question of whether steady-state creep or, perhaps, rupture will occur, or whether sufficient fibers will survive to provide an intact elastic specimen, can be answered by consideration of the stress in the fibers after the matrix has been assumed to relax completely. Clearly, when the matrix carries no stress, the fibers will at least fail to the extent that they do in a dry bundle. It is possible that a greater degree of fiber failure will be caused by the transient stresses during creep relaxation, but this effect has not yet been modeled. Instead, the dry bundle behavior will be used to provide an initial estimate of fiber failure in these circumstances. 

Fig. 10.5 (a-f) Normalized crack growth velocity A a/K] l versus normalized crack extension A = ball for transient SSC crack growth under constant / loading based on a model for crack growth by grain boundary cavitation (taken from Ref. 45). The parameters K, r A0, and Ass are normalized values of, respectively, the stress intensity factor (which is held constant), the crack growth initiation time, the initial crack velocity, and the steady-state crack velocity. 

The transient thermal stress without any crack present, at the center of the surface of the specimen corresponding to the transient temperature distribution is shown in Figure 5. Initially the surface is in residual compression. As the heating initiates, the compression increases, peaks and decreases to come to some steady-state value. The subsequent transient cooling causes a transient tensile stress on the surface. 

All design data such as pressures and temperatures are regarded as mandatory conditions that must be satisfied. Upset and transient operating conditions are sometimes governing. Eor example, the maximum design pressure and coincident temperature will determine the wall thickness of a carbon steel process vessel containing dry H2S. However, even traces of liquid water in the presence of H2S can initiate sulfide stress corrosion cracking in carbon steel. This should add the requirement of PWHT. 

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