PSI - Issue 60

Sreerag M N et al. / Procedia Structural Integrity 60 (2024) 20–35 Sreerag M N/ Structural Integrity Procedia 00 (2023) 000 – 000

22

ksc L/s C/s RM R0 R1 R2 R3

kilogram per centimetre square

Long seam Cir-seam Retro motor First repair Second repair Third repair

Zero Repair (Virgin weld)

FEM PPT

Finite Element Method Proof Pressure Test Radiographic Test

RT

2. Significance of no-flaw burst pressure for estimating the failure pressure of Maraging steel motor The main alloying elements of M250 Maraging steel material are 18% nickel, 8% cobalt and 5% molybdenum. Material has good characteristics like unique combination of strength-toughness, easy fabrication and heat treatment with minimal dimensional distortion and good weldability. The properties of the material used for the fabrication of 3.2 m diameter motor is summarized in the table 1 below.

Table 1. Properties of M250 maraging steel Nageswara Rao (1993). Parameter

Value ≥90 MPa√ 1725MPa

Plane strain fracture toughness

Yield strength (0.2% PS) Ultimate tensile strength

1765MPa

Weld efficiency (up to 1 st repair)

≥90% on UTS

Damage-tolerant and safe-life approach is used for the selecting the shell thickness of 3.2m diameter motor cases. In this approach, one has to assess the failure strength of a component with an assumed defect size. The defect size is decided such that the NDT technique can detect the defect with required confidence level and repeatability. The failure pressure in presence of this defect is estimated using fracture mechanics principle. A 3-parameter elastic plastic fracture mechanics is used to estimate the failure pressure in presence of a defect. While estimating this no flaw burst pressure plays a significant role. Stress intensity factor is a function of load, geometry and defect size. Nageswara Rao and Acharya derived a relation between stress intensity factor at failure (k max ) and failure stress (σ f ) using a three parameter (K F , m and p) by correlating the fracture data of cracked specimen. Nageswara Rao (1997) The relationship between stress intensity factor at failure (k max ) and failure stress (σ f ) is given below:





p

f  

f  

 

 

1                  (1 ) m u u

max k K m  F

(1)

For cylindrical pressure vessel, σ u is the hoop stress at the burst pressure of unflawed cylinder. σ f is the hoop stress at the burst pressure of cylinder with the defect.

a

Q 





k

H    

FS

Neuman(1979)

(2)

max

m

b