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Fatigue+ is a fatigue life analysis software for pressure vessels, boilers, heat exchangers, mechanical components at elevated temperatures, as well as other structures subjected to cyclic loading. Fatigue+ is a design code compliance software. Fatigue involves complex phenomena and mechanism, and the design codes constantly change as researches, practices and experiences evolve. Using Fatigue+ the user can minimize risk of failure from cyclic loading and ensure design code compliance by applying automated fatigue life analysis, making the complicated tasks easy. In addition, Fatigue+ provides a generic fatigue life analysis module that incorporate Basquin model, Basquin-Coffin-Manson model, Kandil-Brwon-Miller model, Fatemi-Socie model and Matake Criterion.
Fatigue+ can handle cyclic loading from as simple as a single stress value, to component stresses of large number of nodes/elements in large number of loading steps generated by ANSYS. Stress history files generated by other FEA packages can also be imported and processed. Data input can be generated by manual entry, by importing ANSYS .rst files, or by importing stress history files. The loading can be of constant amplitude or variable amplitude, proportional or non-proportional. For non-proportional loading conditions, the program scans for critical plane that generates the highest fatigue damage ratio.
Fatigue+ calculates fatigue damage ratios for each loading cycle, sums cumulative damage ratios using Miner's Rule, and counts the number of loading cycles using rainflow counting algorithm following ASTM 1049 procedure. In addition to reporting calculations, the graphical charts of stress range time history, stress range histogram and fatigue damage ratios are also generated.
Fatigue+ consists of different modules for different design codes, including the following:
1.Generic Fatigue Life Analysis Module.
2.EN 12952-3, Water-tube boilers and auxiliary installations — Part 3: Design and calculation for pressure parts of the boiler.
3.EN 13445-3, Unfired pressure vessels Part 3: Design.
4.ASME Section VIII, Rules for Construction of Pressure Vessels Division 2 — Alternative Rules.
5.ASME Section VIII, Rules for Construction of Pressure Vessels Division 3 — Alternative Rules for Construction of High Pressure Vessels.
6.ASME Section III — Rules for Construction of Nuclear Facility Components — Appendices.
The user can select a material from ASME Secion II Part D Properties, EN 10028 flat products, EN 10216 seamless tubes, EN 10217 welded tubes, EN 10213 castings, EN 10222 forgings, EN 10269 fasteners, or EN 10273 weldable bars. The Generic Fatigue Life Analysis Module also contains a material database with common metals such as ASTM A36, SAE-AISI 1035 etc. in which the user can edit material, add material or delete material; the user can also export or import the material database.
Generic Fatigue Life Analysis Module
This is a general purpose fatigue life analysis module that includes the following models:
1.Basquin's Equation, which describes high-cycle low-strain behavior. Mean stress can be considered using Soderberg criterion, Goodman criterion, Gerber criterion, Morrow criterion, Smith-Watson-Topper criterion, or Walker equation.
2.Basquin-Coffin-Manson model, which incorporates Basquin's Equation and Coffin-Manson Relation. While Basquin's Equation describes high-cycle low-strain behavior, Coffin-Manson Relation describes low-cycle high-strain behavior. Basquin-Coffin-Manson model, also known as Morrow Design Rule, combines elastic strain and plastic strain into a total strain relationship. Mean stress can be considered using Morrow criterion, Smith-Watson-Topper criterion, Walker equation, or Manson-Halford criterion.
3.Kandil-Brwon-Miller criterion, which adopts a critical plane approach, justified by experimental observations of the nucleation and growth of cracks during loading: depending on the material, stress state, environment, and strain amplitude, fatigue life is usually dominated by crack growth along either shear planes or tensile planes. Kandil, Brown and Miller proposed that both the cyclic shear and normal strain on the plane of maximum shear must be considered due to the fact that cyclic shear strains help to nucleate cracks, while normal strains contribute in their growth. This programs uses Kandil-Brwon-Miller criterion with Wang-Brown modification on normal strain excursion.
4.Fatemi-Socie Criterion, similar to those by Brown and Miller, that is, the normal strain in the plane of maximum shear strain accelerates the fatigue damage process through crack opening. Crack opening (by maximum normal stress) decreases the friction force between slip planes. This model was born as a modification of the Brown-Miller’s critical plane model, considering mainly the maximum shear strain amplitude and the maximum normal stress on the maximum shear strain amplitude plane. This program provides an option to consider compressive normal stress benefit on fatigue life, even though it is not the intention of this model. It is up to the User's discretion to take advantage of this option.
5.Matake criterion, assuming that fatigue damage of material is due to the maximum shear stress with a higher normal stress value.
EN 12952-3, Water-tube boilers and auxiliary installations — Part 3: Design and calculation for pressure parts of the boiler
Water-tube boiler pressure parts are designed in accordance with the requirements of this European Standard. Boiler components are deemed to be exposed to cyclic loading if the boiler is designed for more than 500 cold start-ups. The design rules presented in Annex B, Fatigue cracking – Design to allow for fluctuating stress, apply to the design of pressurized components of boilers made from ferritic and austenitic rolled or forged steels. These rules allow for the fluctuating stresses occurring at the most highly stressed points as a result of internal pressure and differences in temperature and/or the addition of external forces and moments. The maximum shear stress theory shall be used in the determination of the decisive cyclic stress amplitude and the mean cyclic stress. Stress intensity, which is two times of shear stress, is also used in the analysis. The controlling stress range is divided into elastic range, partly elastic range, and fully plastic range.
Cyclic stress range and mean cyclic stress shall be increased to account for the notch effect (micro notch effect) associated with surface and weld influences. Here, the governing factor in each case is the final state following manufacture.
In the case of a load-cycle temperature t* ≥ 100 °C, the reduction in the fatigue strength caused by the temperature shall be taken into account by means of a correction factor Ct* for ferritic and austennitic alloys.
EN 13445-3, Unfired pressure vessels Part 3: Design
Clause 18, detailed assessment of fatigue life, specifies requirements for the detailed fatigue assessment of pressure vessels and their components that are subjected to repeated fluctuations of stress. The Tresca criterion (associated with stress intensity) is applied in this clause but use of the ‘von Mises’ criterion (associated with von Mises equivalent stress) is also permitted.
A fatigue assessment shall be made at all locations where there is a risk of fatigue crack initiation. It is recommended that the fatigue assessment is performed using operating rather than design loads. In fatigue, welds behave differently from plain (unwelded) material. Therefore the assessment procedures for welded and unwelded material are different. Plain material might contain flush ground weld repairs. The presence of such repairs can lead to a reduction in the fatigue life of the material. Hence, only material which is certain to be free from welding shall be assessed as unwelded.
ASME Section VIII, Rules for Construction of Pressure Vessels Division 2 — Alternative Rules
ASME BPVC.VIII.2 contains mandatory requirements, specific prohibitions, and nonmandatory guidance for the design, materials, fabrication, examination, inspection, testing, and certification of pressure vessels and their associated pressure relief devices. Pressure vessels are containers for the containment of pressure, either internal or external. This pressure may be obtained from an external source or by the application of heat from a direct or indirect source as a result of a process, or any combination thereof. These vessels shall be designated as either a Class 1 or Class 2 vessel in conformance with the User’s Design Specification.
Class 1 Vessel – a vessel that is designed using the allowable stresses from Section II, Part D, Subpart 1, Table 2A or Table 2B.
Class 2 Vessel – a vessel that is designed using the allowable stresses from Section II, Part D, Subpart 1, Table 5A or Table 5B.
Fatigue provisions are in Part 5 – Design by Analysis Requirements, provides requirements for design of vessels and components using analytical methods. A fatigue evaluation shall be performed if the component is subject to cyclic operation. The evaluation for fatigue is made on the basis of the number of applied cycles of a stress or strain range at a point in the component. The allowable number of cycles should be adequate for the specified number of cycles as given in the User’s Design Specification.
ASME Section VIII, Rules for Construction of Pressure Vessels Division 3 — Alternative Rules for Construction of High Pressure Vessels
The rules of ASME BPVC.VIII.3 constitute requirements for the design, construction, inspection, and overpressure protection of metallic pressure vessels with design pressures generally above 70 MPa (10 ksi). Pressure vessels within the scope of this Division are pressure containers for the retainment of fluids, gaseous or liquid, under pressure, either internal or external. This pressure may be generated by: an external source; the application of heat from direct source or indirect source; a process reaction; or any combination thereof.
Article KD-3 Fatigue Evaluation presents a traditional fatigue analysis design approach. In accordance with KD-140, if it can be shown that the vessel will fail in a leak‐before‐burst mode, then the number of design cycles shall be calculated in accordance with this Article.
Cyclic operation may cause fatigue failure of pressure vessels and components. While cracks often initiate at the bore, cracks may initiate at outside surfaces or at layer interfaces for autofrettaged and layered vessels. In all cases, areas of stress concentrations are a particular concern. Fatigue-sensitive points shall be identified and a fatigue analysis made for each point. The result of the fatigue analysis will be a calculated number of design cycles Nf for each type of operating cycle, and a calculated cumulative effect number of design cycles when more than one type of operating cycle exists. The resistance to fatigue of a nonwelded component shall be based on the design fatigue curves for the materials used. Fatigue resistance of weld details shall be determined using the Structural Stress method, which is based on fatigue data of actual welds.
The theory used in this Article postulates that fatigue at any point is controlled by the alternating stress intensity Salt and the associated mean stress σnm normal to the plane of Salt. They are combined to define the equivalent alternating stress intensity Seq, which is used with the design fatigue curves to establish the number of design cycles Nf.
ASME Section III — Rules for Construction of Nuclear Facility Components — Appendices
ASME BPVC.III.A XIII-3500 Analysis for Fatigue Due to Cyclic Operation covers fatigue life analysis. The design fatigue curves used in conjunction with XIII-3500 are those in Mandatory Appendix I. When more than one curve is presented for a given material, the applicability of each is identified. Only the stress differences due to service cycles as specified in the Design Specifications need be considered. Stresses produced by any load or thermal condition which does not vary during the cycle need not be considered, since they are mean stresses and the maximum possible effect of mean stress is included in the fatigue design curves.