Downloaded from https://www.everyspec.com ADS-79D-HDBK i. Requires establishing probability of detection (POD) curve for each failure modes. The PODs may be established using sub-element of structures. ii. Evaluate frequency of inspection based on worst case field spectrum, 90% POD/95% confidence flaw size, and requires damage tolerance inspection interval factor for structural reliability improvement d. Validation - Validation of the system should demonstrate that the integrated system is at least as reliable at detecting the structural damage as the legacy techniques.
If the goal of implementing the integrated system is to eliminate complex inspection procedures, the output of the integrated system should be compared against the output of the current system to determine efficacy. i. Validation of the system as applied to particular structures should include the effect of geometry and, as applicable, the nearby presence of buildup structure, joints, or fittings. In this example, two tailboom configurations exist: one 0.040‖ skin, the other with 0.063‖ skin. Implementation of the integrated NDI system on the tailboom for crack detection would require evaluation of the skin thickness effect.
Also, response waveform of the embedded sensor system may be sensitive to degree of interference fit on joints. Implementation of the integrated NDI system on joint or fittings would require evaluation of the interference fit effect. ii. The analysis should include the identification of deficiencies that could cause performance shortfalls e. Durability of the system should be demonstrated. C.5.2.2 Example Inspection Interval Adjustment.
Although current engineering practice commonly used in rotorcraft industry doesn‘t require damage tolerance based approach for fatigue life design of dynamic components, the concept of damage tolerance has been employed from time to time to establish appropriate inspection intervals for components subject to potential damage beyond their intended fatigue initiation stage. The concept of a widely adopted inspection planning methodology for in-service damage detection is depicted in Figure C-1. In this approach, a fatigue crack growth analysis is performed first.
The typical material data, in terms of estimated value of equivalent initial flaw size, expected value of crack growth rate, anticipated usage and applied load are used in the analysis. The average crack growth behavior is predicted as the outcome of the analysis. To incorporate effect of NDE, a characteristic value representing inspection capability, a NDE , is considered. In general, the a NDE intends to represent high reliability of detection. If a POD model for the NDE exists, a 90/95 is often used. The a 90/95 represents defect size with which there is 90% of chance of detection with 95% of confidence.
Otherwise, a conservative value of estimated crack size with very high detectability is used. Similar to the concept of P-F interval, a damage growth life is defined as the amount of time that a crack growth from a NDE to a pre-determined critical crack size. The damage growth life represents the window of opportunity to reliably detect damage before it reaches unstable growth and yields final failure. Due to the inherent randomness associated with damage progression, the damage growth life also fluctuates. To address the associated variability, the estimated average damage growth life is further divided by a damage tolerance inspection interval factor.
The adjusted value is defined as the inspection interval. In theory, the first inspection should be started at time corresponding to a NDE . In practice, the inspection begins as the induction of the inspection plan. Some applications require quarter life or half life as the time of performing the first inspection. 66