PSI - Issue 14

V. Viswanath et al. / Procedia Structural Integrity 14 (2019) 442–448 V Viswanath/ Structural Integrity Procedia 00 (2018) 000 – 000

446

5

4.1. Crack sizes assumed for analysis

No defects have been reported at the weld locations for this propellant tank. However, for fracture analysis, cracks are a necessity as there is no fracture analysis in absence of any defect. Therefore, the minimum detectable crack sizes specified by NASA STD 5003 (2005), standards corresponding to ultrasonic and radiographic inspections are used for evaluation purposes are used for analyses. 4.2. Reference stress used for evaluation The longitudinal residual stress assumed at this location is 95 N/mm 2 . The stress corresponding to internal pressure of 0.46MPa (absolute) is to be known before fracture mechanics based analysis can be attempted. This is evaluated from the true stress strain curve of the tank material given in Fig 5.

300

Stress at total strain of 12771µ ε is 175N/mm 2

250

200

150

Stress due to internal pressure of 0.46MPa (abs) = 80N/mm 2

100 Stress, N/mm2

Residual stress of 95N/mm 2 corresponding to strain of 1400µ ε

50

Welding residual stresss = 95N/mm 2

0

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Strain, mm/mm

Fig. 5. True stress strain curve of tank weld material at RT.

From the Fig 6 the strains corresponding to stress of 95N/mm 2 is 1400  . Adding the peak strains measured during the test to these residual strains, the total strain is 12771  and stress corresponding to this strain is 175N/mm 2 . Therefore, the stress due to tank pressurization is the difference of the two i.e. 80N/mm 2 viz the primary stress.

4.3. Fracture toughness of weld material

With lack of test data on fracture toughness for the tank weld material, the data from NASA FLAGRO (2005) material database is used for analysis. Plane strain fracture toughness of 23.08MPa  m and 25.27MPa  m at RT and cryogenic temperature are considered.

4.4. Failure Assessment Diagram (FAD)

FAD is the most widely used approach for Elasto Plastic Fracture Mechanics (EPFM) based analysis of structural components as discussed in Zerbst et al (2007),Anderson (2005) and British Standard 7910: 2005 (2005). FAD is derived from elastic-plastic J-integral solution. A generalized approach, using two parameter formulations, is used in this paper. The two parameter FAD consists of Failure Assessment Curve (FAC) and Failure Assessment Points (FAP). The axes of the FAD are the non dimensional ratios Lr (plastic collapse ratio) on the x-axis, and Kr (brittle fracture ratio) on the y-axis. FAC is the limiting envelope of FAD. For the FAP lying below FAC, the structure would be safe and unsafe otherwise. The generic expression to draw FAC for L r  L r (max) is given below:         6 0.65 2 0.3 0.7 1 0.14 Lr r r e L K     (2) The maximum L r cutoff value should also be applied to the computed FAD curve to determine the plastic collapse limit. Cut off value of L r is given by the expression:







 

y

u

r L



(3)





max

2





y