NASGRO Crack Propagation Analysis

Pressurized Pipe — TC07 Critical Crack Size

A thin-walled aluminum pipe (Al 2024-T3) under internal pressure was analyzed using NASGRO geometry TC07 — a through crack in a pressurized cylinder. With R = 2 in, D = 4 in, t = 0.1 in, and a fracture toughness of K_IC = 22,000 psi√in, a factor of safety of 1.4 was applied, reducing the effective toughness to K_cr = 15,714 psi√in. NASGRO iterated using the Regula Falsi method to converge on a critical crack size of c = 6.667 × 10⁻³ in, confirming the pipe operates well below its fracture threshold at the applied stress state.

TC07 geometry schematic · NASGRO iteration output — CCS = 6.667×10⁻³ in

Flat Plate with Loaded Hole — SC11 Flaw Acceptability

A flat plate (Al 2024-T3, t = 0.1 in, W = 20 in) with a loaded hole and surface flaw (a/c = 0.5, geometry SC11) was evaluated for flight acceptability. The existing flaw size is c = 0.5 in with a bearing load of 1000 lbs (S₀ = 20 ksi, S₃ = 50 ksi). The initial hole diameter D = 0.5 in triggered a NASGRO geometry error (D/t ratio out of range), requiring a conservative adjustment to D = 0.2 in to proceed.

NASGRO computed a critical crack size of a = 1.281 × 10⁻² in, c = 2.503 × 10⁻² in. Since the existing flaw (c = 0.5 in) far exceeds this CCS, the answer is clear: the part is not safe for flight and must be rejected or repaired.

SC11 geometry · Geometry boundary error (D/t) · NASGRO CCS output — flaw unsafe for flight

Valve Fatigue Life — EC01 Crack Growth to Failure

A cyclic fatigue life analysis was performed on an Al 7075-T6 valve body using NASGRO geometry EC01 — an embedded elliptical crack in a finite plate under uniaxial stress. With t = 0.1 in, w = 0.56 in (from 2c/W ≤ 0.5 constraint), and an NDE-detectable initial flaw consistent with radiographic inspection (X-ray 5009), a proof factor of 1.5× was applied.

NASGRO predicted failure at cycle 10,511, with a final crack size c = 0.210 in. The valve service life requirement is 10,000 cycles, meaning the part just barely exceeds its target — but applying a 4× factor of safety reduces the allowable life to 2,500 cycles, confirming the current design does not meet safe-life requirements under fracture control criteria.

EC01 geometry · NASGRO fatigue output — failure at cycle 10,511, below FOS-adjusted safe-life limit of 2,500 cycles

Pressurized Fuselage Skin — TC16 Crack Growth Life

A pressurized aircraft fuselage skin was analyzed using NASGRO geometry TC16 — a through crack in a thin curved stiffened panel with bulging correction. The aircraft flies at 40,000 ft with a cabin pressurized to 7,000 ft equivalent, generating a hoop stress S₀ = PR/t = 8.62 ksi. Material is Al 7075-T6 (K_IC = 25,000 psi√in). A 1-bay stiffened configuration with Chen + Schijve bulge factor and R+C correction was applied.

An initial target of 3,000 cycles produced a NASGRO convergence error — the target life was too low for the crack size to propagate within bounds. Adjusting to a more realistic target, NASGRO converged to an initial crack size of c(init) = 3.569 in, with an actual fatigue life of 27,026 cycles — well beyond the typical aircraft inspection interval, confirming structural adequacy for the given load spectrum.

TC16 geometry setup · Convergence error at 3,000 cycle target · Final solution — 27,026 cycle fatigue life