pipingselfstudy: Importance & Impact of Stress Intensification Factor (SIF): A Presentation

SIF : As per ASME Section III Code SIF is defined as the Fatigue Correlation Factors that compare the fatigue life of Piping Components (Tees, branch connections) to that of girth butt welds in straight pipe subjected to bending moments.

HISTORY:

MARKL FATIGUE CURVE FOR STRAIGHT PIPE (Fig. 1):

Fig. 1: Markl Fatigue Curve for Butt Welded Steel pipe

Markl stress intensification factor : iS = 245,000N^-0.2

i =( CN^-0.2)/S

Where i=Stress intensification factor

C=245,000 for carbon steel materials

N=Cycles to failure

S=Nominal stress amplitude

Markl SIF is based on deflection control, fully reversed, cyclic bending fatigue tests.

APPLICATION:

Section III, for class 2 & 3 piping and B31.1

Calculated stress S = iM / Z

where M = (M_X²+ M_Y² + M_Z²)^1/2

Z=Section Modulus

i= Stress intensification factor

This approach is conservative.

Thermal expansion stress S_E = (S_b² + 4S_t²)^1/2

where

S_b=resultant bending stress

S_t=torsional stress = M_t/2Z

The resultant bending stress is calculated by

S_b= [((i_iM_i)²+ (i_oM_o)²)^½]/Z

where

i_i= in-plane bending stress intensification factor (Refer Fig. 3)

i_o= out-of-plane stress intensification factor (Refer Fig. 3)

M_i= in-plane bending moment (Refer Fig. 2)

M_o= out-of-plane bending moment (Refer Fig. 2)

Fig. 2: Moments in Elbow and Branch Connections

Fig. 3: Table D300 of ASME B 31.3

Eg: For 12” Std schedule long radius elbow.

a)Elbow without flange

R₁= 1.5(12) = 18”

T = 0.375” (Std) Assume same thk for pipe & elbow

r₂= ( OD-T)/2 = 6.1875”

h = (TR₁ )/(r₂)²= 0.176 (From table D-300 of B31.3)

i_i= 0.9 / h^2/3 = 2.86

i_o= 0.75/ h^2/3 = 2.4

b)If one end is flanged the correction factor C₁ = h^1/6= 0.7486

i_i= C₁ ( 0.9 ) / h^2/3 = 0.7486 x 2.86 = 2.14

i_o= C₁ ( 0.75)/ h^2/3 = 0.7486 x 2.4 = 1.797

PRESSURE VS SIF:

Effect of Pressure on SIF:

Fig. 4: Calculation showing effect of pressure on SIF

FEM TECHNIQUES:

The various methods for calculating SIF Are

Analytical Methods defined by Piping Codes
FEM techniques

The user can see that the basic SIF procedure is:

Build the model.
Apply a moment through the nominal attached pipe.
Read the highest stress from the result plot.
Divide by M/Z to get the SIF.

The Fem Softwares widely used for calculating Sif are

FE-pipe
Ansys

SIF Calculations

The basic definition of a stress intensification factor (SIF) is:

SIF = (Actual Peak Stress in Part)/( Nominal Stress in Part)

The nominal stress in the part for a piping component subject to bending loads is M/Z where “M” is the moment that the pipe exerts on the component, and Z is the section modulus of the matching pipe welded to the part being analyzed.

Example Case:

For example, when SIFs are needed for a large D/T “Wye” Fitting, there are usually four SIFs involved.

one for the inplane moment about the wye,
one for the outplane moment
For both the main header and branch sections.

An example, demonstrating this calculation for a 32×0.375 wye fitting is shown in the example.

Markl’s definition of the SIF is the ratio of the actual stress in the part due to a moment “M”, divided by the nominal stress in a girth (circumferential) butt weld due to a similar moment “M”.

B31 sif = Actual Stress in Part (due to M)/ Stress in Girth Butt Weld (due to M)