Fatigue ratchetting

Thermal stresses are secondary stresses because they are self-limiting. That is, yielding or deformation of the part relaxes the stress (except thermal stress ratcheting). Thermal stresses will not cause failure by rupture in ductile materials except by fatigue over repeated applications. They can, however, cause failure due to excessive deformations. 

Repeated cycling may cause different type of damages ratcheting, creep-fatigue, buckling. 

Elastic Stress Analysis - Simplified Elastic-Plastic Method. This method may be used in the case where the method indicated above shows the Pr + Pb + Q stress limits are not satisfied, but indicates that the Pl + Pb + Q range and excluding thermal effects must be less than Sps- Additionally, the effective alternating equivalent stress amplitude must include the fatigue penalty factor, ICej, which is based off of the simplified elastic-plastic criteria from the pre-2007 Section VIII, Division 2. Finally, a thermal stress ratcheting assessment must be made. 

The pathetic ease and gullibility with which the mass media are lured into conventional moral panics may be contrasted to the deep denial behind their refusal to sustain a moral panic about torture, political massacres or social suffering in distant places. Public and media indifference are even attributed to deep states such as compassion fatigue . Moeller describes a cogni tive and moral stupor in which attention thresholds have risen so rapidly that the media try even more desperately to ratchet up the criteria for stories to be covered. In the hierarchy of which events and issues will be covered, a footballer s ankle injury will get more media attention than a political massacre. 

The number of repetitions of testing or cycles of loading per test is dependent on the application, but the accumulation of damage of various types associated with fatigue or ratcheting phenomena should be taken into account for the evaluation of the results and to permit qualification for the service life of the item. 

Ratcheting is produced by a combination of a sustained extensional load and either a strain-controlled cyclic load or a cyclic temperature distribution that is alternately applied and removed. This produces cycling straining of the material which in turn produces incremental growth (cyclic) leading to what is called an incremental collapse. This can also lead to low cycle fatigue. 

The RCC-MRx code [5] has incorporated Mod.9Cr-lMo steel since its 2007 Edition (at that time, the name of the standard was RCC-MR). Since that time, a large amount of studies concerning the use of the Mod.9Cr-lMo steel have been performed, in the frame of the French Astrid project [30] but also in the frame of the European MATTER project [31]. These developments led to improve the knowledge and standardization of the steel but also highlighted the need for specific mles to have a better description for cyclic behavior (ratcheting, fatigue, creep-fatigue) [32,33]. 

Fig. 2.20 High temperature fatigue failure under axial loads of a valve stem of 21-2 valve steel [24]. Note the ratchet marks around the circumference that denote the presence of multiple initiation sites (indicated by arrows)...

High temperature fatigue failure under axial loads of a valve stem of 21-2 valve steel (21 % Cr, 2 % Ni, 8 % Mn, 0.5 % C, 0.3 N) in solution-treated and aged condition and faced with stellite 12 alloy (30 % Cr, 8 % V, 1.35 % C, rem Co), Fig. 2.20 [24]. Note the ratchet marks around the circumference that denote the presence of multiple initiation sites (indicated by arrows). The wavy shape of beach marks is indicative of oflF-axis load that has introduced a bending component. 

Fig. 2.23 Reversed bending failure in 1046 steel [27]. Rubbing has obliterated the early stages of fatigue cracking. Ratchet marks along circumference indicate locations of crack initiation...

When subjected to cyclic strains, ductile materials are found to fail either by Low Cycle Fatigue (when the life is given by the well known Coffin-Manson equation) or by Ratcheting Failure (when the life can be estimated by dividing the strain to failure by strain accumulation per cycle). Both LCF and RF act on the same material and so whichever occurs first causes failure. In this competitive mode of failure, the actual... 

Creep-fatigue interaction, strain ratcheting, irradiation effects, high temperature effects... 

Comfortable margins on the primary stresses, very low accumulated creep-fatigue damage, absence of ratcheting, and crack initiation even after application of more than -500 simulated plant load cycles, and extensive experimental validation of finite element results, assures the structural integrity of primary pipe under all the normal and severe loading situations. Analysis shows that the maximum temperature rise of the core outlet sodium temperature (< 0.2 K), due to the leak through critical crack size, cannot be detected. Therefore, LBB demonstration by this mean is not possible. 

Table 9-24 below, presents several wall thickness values for varying bend radii and pipe operating parameters (temperature and pressure) which would have been experienced in the piping assembly for a mission lifetime of 15 yrs ( 132,000 hrs). It should be noted that these values are minimum wall thicknesses and additional phenomenon may occur which would could have required the wall thicknesses to increase, such as creep-fatigue interactions, irradiation, and thermal ratcheting. 

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