PSI - Issue 71

Karthik K R. et al. / Procedia Structural Integrity 71 (2025) 210–217

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proof pressure test (AIAA (2018)). As the design is getting improved, more types of failure began to show up, one of such failure would be fracture by means of undetected cracks in the hardware (Stephan et al. (1976)). Even though these hardware undergo thorough non-destructive examination (NDE), some of the cracks may escape inspection and cause trouble later. Cracks have to be considered during the design by conducting a fracture assessment. This design methodology is widely known as damage tolerant design. Damage tolerant design approach starts with the selection of damage tolerant material. AA2219 is such a material which possesses good fracture toughness, high specific strength, and good weldability (Ch. et al. (2014)). Tanks are constructed by welding. A typical tank design is shown in Figure 1.

Figure 1: Typical design of propellant tank.

After the construction, they undergo different NDEs. Parent metal region undergoes ultra-sonic testing (UT) and die penetrant inspections (DP). Weld region is inspected only by RT and DP techniques as UT cannot be performed due to the presence of the weld bead. Depending on the inspection technique, flaw size screened during inspection is varied. Advanced NDE techniques like ultrasonic testing have better flaw screening capabilities. Minimum detectable crack size or the maximum flaw size of surface cracks that can be missed during various inspections, according to NASA STD 5009b (2019) is given in table 1. Table 1 . Minimum detectable crack sizes (NASA (2019)) Inspection technique Crack size (2c x a), mm Ultrasonic Testing

7.6 x 0.76 3.3 x 1.65 1.4t x 0.7t 6.36 x 0.34 3.82x 1.91

Radiographic testing Die penetrant testing

Welds are more vulnerable to defects such as inclusions, porosities, embedded cracks, surface cracks, etc. than parent metal, also they undergo only RT and DP inspection. Among all other defects, surface cracks are more dangerous due to the free surface effect. Surface crack geometries are idealised as semi-elliptical as shown in Figure 2.

Figure 2: Details of surface crack (NASA, 2019))

Since these defects are difficult to avoid completely, integrity of the structure against them needs to be assessed.