A Methodology to Evaluate the Feasibility of Descoping Nondestructive Evaluation (NDE) Fracture Control Requirements in NASA-STD-5019A

A new methodology is proposed to evaluate the risk of structural failure if NDE flaw screening requirements are relaxed, which is referred to as an NDE descope. The proposed method estimates the probability of a critical initial flaw size (CIFS) existing in a part based on a historical record of NDE inspections that are predictive of flaw existence in future parts fabricated under the same time-invariant process control. The methodology conservatively assumes that the CIFS is equal to the NDE detectable flaw size, and it further assumes that if a CIFS defect exists it will lead to structural failure during the part’s operational lifetime. While conservative, these assumptions are necessary to utilize historical NDE databases that do not include flaw size characterization.

The proposed risk evaluation method is based on the premise that if the probability of CIFS defect existence is considered sufficiently low, leading to an acceptably low risk of primary structural failure, then rationale for descoping NDE may exist. Acceptable risk of structural failure is application/system specific. Descoping NDE is shown to result in an elevated risk acceptance posture even if a sufficiently large NDE database is available and there are no flaw detections in the inspection history. The method depends on time-invariant process control and assumes that the part was initially damage tolerant prior to descope.