For IGE/TD/12 the computation of fatigue stresses is detailed in Section 5.4.4 of that code. This section of the code states: "The principal stress in any plane can be calculated for any set of conditions from the following formula:"
Where:
Sh = Hoop stress
Sa = Axial stress
Sq = Shear stress
"This should be used for establishing the range of stress, due regard being paid to the direction and sign." For all other piping codes in CAESAR II, the fatigue stress is computed as the stress intensity, as follows:
3D Maximum Shear Stress Intensity (Default)
SI = Maximum of:
-
S1OT - S3OT
-
S1OB - S3OB
-
Max(S1IT,RPS) - Min(S3IT,RPS)
-
Max(S1IB,RPS) - Min(S3IB,RPS)
Where:
S1OT=Maximum Principal Stress, Outside Top
= (SLOT+HPSO)/2.0+(((SLOT-HPSO)/2.0)2+TSO2)1/2
S3OT=Minimum Principal Stress, Outside Top
=(SLOT+HPSO)/2.0-(((SLOT-HPSO)/2.0)2+TSO2) 1/2
S1IT=Maximum Principal Stress, Inside Top
=(SLIT+HPSI)/2.0+(((SLIT-HPSI)/2.0)2+TSI2) 1/2
S3IT=Minimum Principal Stress, Inside Top
=(SLIT+HPSI)/2.0-(((SLIT-HPSI)/2.0)2+TSI2) 1/2
S1OB=Maximum Principal Stress, Outside Bottom
=(SLOB+HPSO)/2.0+ (((SLOB-HPSO)/2.0)2+TSO2) 1/2
S3OB=Minimum Principal Stress, Outside Bottom
=(SLOB+HPSO)/2.0- (((SLOB-HPSO)/2.0)2+TSO2) 1/2
S1IB=Maximum Principal Stress, Inside Bottom
=(SLIB+HPSI)/2.0+ (((SLIB-HPSI)/2.0)2+TSI2) 1/2
S3IB=Minimum Principal Stress, Inside Bottom
=(SLIB+HPSI)/2.0- (((SLIB-HPSI)/2.0)2+TSI2) 1/2
RPS=Radial Pressure Stress, Inside
HPSI=Hoop Pressure Stress (Inside, from Lame's Equation)
HPSO=Hoop Pressure Stress (Outside, from Lame's Equation)
SLOT=Longitudinal Stress, Outside Top
SLIT=Longitudinal Stress, Inside Top
SLOB=Longitudinal Stress, Outside Bottom
SLIB=Longitudinal Stress, Inside Bottom
TSI=Torsional Stress, Inside
TSO=Torsional Stress, Outside