SC4.1 Basic assumption. The general procedure in fatigue evaluation is based on the use of strain controlled fatigue data. The resulting fatigue design curve is a composite curve. In the low cycle range the stress amplitude is primarily a function of the true fracture strain multiplied by the elastic modulus. In the high cycle range the stress amplitude is, as a matter of convenience, made equal to the endurance limit.
This method should result in families of curves rather than the curves in Figures SC1.2.1 and SC1.2.2 which are based on a large number of tests carried out on steels having diverse material properties. However, owing to the very wide scatter of test points it was found that only a lower limit curve could be justified.
The method, therefore, discriminates against a number of steels, particularly against those in the high strength range. To permit a better evaluation of the fatigue strength of such materials, or where it is desired to use higher peak stresses than can be justified by the methods of the previous rules, the adequacy of a part to withstand cyclic loading may be demonstrated by means of a fatigue test. However, the fatigue test shall not be used as justification for exceeding the allowable values of primary or primary-plus-secondary stresses.
SC4.2 Testing of component. The test component or that portion to be tested shall be constructed of material having the same composition as the vessel component and shall be subjected to the same mechanical working and heat treatment so as to produce mechanical properties equivalent to those of the material in the prototype component. Geometrical similarity must be maintained, at least in those portions whose ability to withstand cyclic loading is being investigated and in those adjacent areas which affect the stresses in the portion under test.
The test component or portion thereof shall withstand the number of cycles as specified below without failure. Failure is defined herein as a propagation of a crack through the entire thickness, such as would produce a measurable leak in a pressure-retaining member.
The minimum number of cycles (hereinafter referred to as ‘test cycles’) which the component must withstand, and the magnitude of the loading (hereinafter referred to as ‘the test loading’) to be applied to the component during the test, shall be determined by multiplying the design service cycles by a specified factor Kn′, and the design service loads by Ks′. Values of these factors shall be determined from a composite fatigue curve constructed in accordance with Paragraph SC4.3.
SC4.3 Construction of test fatigue curve. The fatigue test curve is drawn from the applicable original fatigue curve. It is less conservative than the original curve in order to compensate for the higher allowable stresses.
(a) Construct the test fatigue curve by multiplying the values of Saof the original curve by the factor Ksand draw a new fatigue curve Sas through these points, as shown in Figure SC4.3. Next, construct a second fatigue curve by multiplying the values of N in the original curve by Knand draw a second fatigue curve San through these points, as shown in Figure SC4.3. The test fatigue curve Sa′is constructed using the higher segments of Sas and San, as shown in Figure SC4.3. (See Paragraph SC4.3 (c) for notation.)
(b) Assume the service condition for the prototype vessel to be Saand N, defined by point A on the original curve. Project point A vertically and horizontally to points D and C on the test curves Sa′. The segment of Sa′between the two points C and D embraces all allowable combinations of Ks′and Kn′. The values for a point B determines the following corresponding values of Ks′and Kn′:
Ks′= Kn′=
Test loading Pt= Ks′ ×design service loading Test cycles Nn= Kn′ × design service cycles
The designer therefore has available a choice of test cycling conditions ranging from — Point D,
where Ks′ = ordinate A; Kn′= 1, signifying a maximum increase of load amplitude and no change in number of cycles,
to — Point C,
where Ks′ = 1; Kn′= abscissa D/abscissa A, signifying an increase of number of cycles and no change of load amplitude, when compared with the fatigue design requirements of the prototype vessel.
(c) The values of Ksand Knare the multiples of factors which account for the effects of size, surface finish, cyclic rate, temperature, and the number of replicate tests performed. They shall be determined as follows:
Ks = Ksl ×Ksf×Kssbut shall never be allowed to be less than 1.25 K = Ks4.3but shall never be allowed to be less than 2.6
Ksl = factor for the effect of size on fatigue life = 1.5 – 0.5 (LM/LP), where LM/LP is the ratio of linear model size to prototype size
FIGURE SC4.3 CONSTRUCTION OF TEST FATIGUE CURVE
Ksf = factor for the effect of surface finish = 1.75 - 0.175 (SFM/SFP) where (SFM/SFP) is the ratio of the model surface finish to the prototype surface finish expressed in arithmetic average
Kss = factor for the statistical variation in test results
= 1.220 - 0.44×number of replicate tests
No value of Ksl, Ksf or Kssless than 1.0 may be used in the calculation of Ks. (d) Additional K-factors can be found in technical literature, covering other conditions. The
designer will also be faced with conditions for which data are not available, in which case the K-factors must be developed from tests in accordance with Paragraph SC5.
SC5 DETERMINATION OF FATIGUE STRENGTH REDUCTION FACTORS. The following criteria are applicable in the determination of fatigue strength reduction factors:
(a) A reduction in fatigue strength of a component may be due to the presence of a notch, a ‘notch’ for the purpose of this Supplement being an actual notch or an abrupt change in cross-section, or a transition section of differing curvatures, or attachments for supports, or penetrations into shells, e.g. drill holes and welded nozzles with varying diameters and corner radii.
(b) The fatigue strength reduction factor shall be determined by tests on ‘notched’ and
‘unnotched’ specimens and calculated as the ratio of the ‘unnotched’ stress to the
‘notched’ stress for failure or other equivalent methods.
(c) The test part shall be fabricated from the same material and shall be subjected to the same heat treatment as the component.
(d) The stress level in the specimen shall not exceed the limit given in Paragraph SH2.4.3(d), Appendix SH, and shall be such that failure does not occur in less than 1000 cycles.
(e) The configuration, surface finish, and stress state of the specimen shall closely simulate those expected in the components. In particular, the stress gradient shall not be more abrupt than expected in the component.
(f) The cyclic rate shall be such that appreciable heating of the specimen does not occur, nor shall it exceed 100 Hz.
SC6 CYCLIC THERMAL STRESSES. The following requirements are applicable to cyclic thermal stress conditions:
(a) Pressure vessels which operate at elevated or subzero temperature should be heated or cooled slowly, and should be efficiently lagged to minimize temperature gradients in the shells. Rapid changes of shell temperature should be avoided during service.
(b) The vessels should be able to expand and contract without undue restraint.
(c) Provided that the conditions in (a) and (b) above and those of Paragraph SC3 are observed, estimates of thermal stresses due to temperature changes need not be specially considered.
(d) The use of pad-type reinforcement or partial penetration joints is not suitable for cases where there are significant temperature gradients, especially where these are of a fluctuating nature.
SC7 FORCED VIBRATIONS. Pulsations of pressure, wind-excited vibrations or vibrations transmitted from plant, e.g. rotating or reciprocating machinery, may cause vibrations of piping or local resonance of the shell of a pressure vessel. In most cases these cannot be anticipated at the design stage. It is therefore advisable to make an examination of plant following initial start-up. If such vibration occurs and is considered to be excessive, the source of the vibration should be isolated, by stiffening, additional support or damping which should be introduced at a location local to the vibration.
SC8 CORROSION FATIGUE. Corrosion conditions are detrimental to the endurance of carbon steels, carbon-manganese steels, and ferritic alloy steels. Fatigue cracks may occur under such conditions at low levels of fluctuation of applied stress. Since the tensile strength of a steel has little or no effect upon the fatigue strength under corrosive conditions, the use of high strength steels in severe corrosion fatigue service will offer no advantage unless the surface is effectively protected from the corrosive medium. Where corrosion fatigue is expected, it is desirable to minimize the range of cyclic stresses and carry out inspection at sufficiently frequent intervals to establish the pattern of behaviour.
APPENDIX SD
RECOMMENDED CORROSION PREVENTION PRACTICE
Appendix D of AS 1210 applies.
APPENDIX SE
INFORMATION TO BE SUPPLIED BY THE PURCHASER TO THE MANUFACTURER
Appendix E of AS 1210 applies, with the following addition:
It is the purchaser’s responsibility to specify or cause to be specified (see Clause S3.1.2) the following:
(a) Information in sufficient detail that the need for a fatigue analysis can be determined and that any required analysis can be carried out.
(b) Whether or not a corrosion (and erosion) allowance is required, and if so the amount.
(c) Whether or not the vessel will contain lethal material (see Clause 1.7.1).
APPENDIX SF
INFORMATION TO BE SUPPLIED BY THE MANUFACTURER Appendix F of AS 1210 applies.
APPENDIX SH
DESIGN REQUIREMENTS FOR LOADINGS AND COMPONENTS NOT COVERED BY SECTION 3
(This Appendix forms an integral part of this Supplement.)
Appendix H of AS 1210 does not apply to this Supplement and the following shall be substituted:
SH1 GENERAL. This Appendix specifies design criteria for stress systems resulting from the application of loads or the use of components of types not covered explicitly by Section S3.
The intention is to ensure that the design basis for these components and loadings shall be consistent with that underlying the specific requirements given in Section S3.
Formal analysis in accordance with this Appendix is required only in the case of significant additional loadings, or for components significantly different from those dealt with in Section S3. Relevant experience of similar designs may be considered in deciding whether an analysis is required.