Associating SCFs with Points

Like SN curves a set of stress concentration factors (SCFs) must be associated with each fatigue point. The details differ based on the type of fatigue being computed, but in all cases it is accomplished with an -SCF option. For tubular joint fatigue, the SCFs will normally be computed based on the joint geometry and the load path and this will be discussed below.

For elements there are three ways to define them: on a &DEFAULT command, on the class definition command, or on the element definition command. In any case failure to include a definition results in the last definition being used; i.e.

• For intermediate fatigue points between tubular sections, an SCF will be computed based on the algorithm specified on the -IN_SCF option on the &PARAMETER command,
• For intermediate fatigue points between tubular and conical sections, an SCF will be computed as specified in API RP2A,
• For other cases, the -SCF specified on the element command is used,
• If no -SCF is specified on the element command, then that specified on the class definition is used,
• If no -SCF is specified on the class definition, then that defined on the &DEFAULT is used.

To define the "inline" SCF between two tubular sections, then the

     -IN_SCF, TYPE

option of &PARAMETER is used. There type must be either DNV or a number. The number if the exponent, e, used in computing the SCF according to:

     SCF = 1 + A * B
 
     A   = 3 ( T2 - T1 )/ T1
 
     B   = T1**e / ( T1**e + T2**e)

If DNV is specified a value of 2.5 will be used for e. Here T2 is greater than T1.

For the &DEFAULT command, the default SN curves at various points on and element are defined with the option

     -SCF, TYPE(1), SCF(1), ....

The -SCF option defines default stress concentration factors for different types of section. Here, TYPE(i) defines a section type and must be either: BOX, PRI, BU, W, M, S, HP, WT, MT, ST, L, C, MC, WBOX, DL, LLEG, or PLATE. The values SCF(i) are the stress concentration factor for the corresponding type. Please notice that one cannot define a default SCF for a tube. This would defeat the normal computation of SCF's for joint fatigue. If you have a tube, want to do beam fatigue, and want an SCF other than 1, you need to manually do it on the command which defines the beam.

For class definition commands, we have the option:

     -SCF SCF_BEG, SCF_END

Here, SCF_BEG is the SCF which will be used at the beginning of the segment and SCF_END is the curve used at the end of the segment. If one uses two SCFs for each segment, the one in the middle will be defined twice. If this is done, the second definition will actually be used; i.e. the first SCF specified will, in fact, be used for the beginning of the segment. Also, SCF_END needs to be specified only on the last segment.

The SCF definition for elements is accomplished with the option:

     -SCFi, B_SAD(i), B_CROWN(i), B_IPB(i), B_OPB(i), \
     C_SAD(i), C_CROWN(i), C_IPB(i), C_OPB(i)

The stress concentration factors for an element depend on the section type. For a tube, they are generally computed based on the load path and the formulae specified (See section on Tubular Joints for more information). For other type sections, they are input. For non-tubes, if no -SCF option is present, then the default SCF defined for this class will be used. If one wants to change it, he should use the -SCFi option with a single number following. In other words, for non-tubes, only a single SCF per end will be used. Here, -SCFA will change the values at the first end of the beam -SCFB will change them at the other end, and -SCF1 will change them at the intersection between the first segment and the second, etc. If -SCF is used, then the stress concentration factor will be the same at both ends all fatigue points. For tubes one can override the computed values by input of eight values. Here, B_SAD(i), B_CROWN(i), B_IPB(i), and B_OPB(i) correspond to the stress concentration factors for axial at the crown points of the brace, the saddle points of the brace, inplane bending of the brace, and out of plane bending of the brace. The values C_SAD(i), C_CROWN(i), C_IPB(i), and C_OPB(i) are for the points on the chord corresponding to the points on the brace.