19-1 POD Is Not Reliability Reproducibility –Achieved by rigid calibration Repeatability –Achieved by rigid process control Capability –First principles of detection –POD is a measure of detection capability

19-2 QNDE Is a Measurement Process Detection requires discrimination of signal from noise PM 19-19

19-3 Detection Requires Separation of Signal from Noise Probability density distribution Probability density distribution Increasing discrimination Probability density distribution Flaw size POD PM 19-20

19-4 Berens & Hovey Model To reduce number of required flaws, Berens & Hovey developed model Crack size Signal response level Threshold signal acceptance level Assumes linear (log-linear) relationship between crack size & signal response

19-5 Berens & Hovey Model (cont.) Cumulative lognormal distribution function is approximated by log-odds model & data may be described by: –Where a = crack length Maximum likelihood is used to estimate  &  parameters of model (MIL-STD-1823—see references for further information) exp [  ln  POD(a) = 1 + exp [  ln 

19-6 Probability of Detection Curve PM 19-21

19-7 Design of Experiments Validating POD Capability of Inspection Systems (DOEPOD) Exciting new tool under development at NASA LaRC PM 19-22

19-8 NDE Requirements Specification of flaw: –Type –Size –Location –Orientation –Nearest neighbor All parts are not fracture-critical All areas of fracture-critical parts are not fracture-critical

19-9 NDE Application Access Location –Where to look! What to look for What is critical

19-10 Fracture-Critical by Definition Pressure vessels Large moving parts with inertia Components whose failure may impact critical part Single load path where failure may cause loss of critical part Single critical function materials/components, such as heat shields Failure may cause loss of life or mission

19-11 What Flaw Size to Use? Experience with QA-NDE rules of thumb: –Use flaw size that is 3× value provided using cracks for “calibration” –Assume flaw size that is 6× value provided using slots for “calibration” –Validate assumptions by assessment Lessons Learned If QA-NDE is inadequate, QNDE is required

19-12 QNDE Knowledge from Prior Lessons Learned Prior applications that have been characterized –MSFC 1249 –NASA-STD-5009 –NDE Capabilities Data Book –Prior applications & experience Characterized by POD method –Use 29/29 to prove similarity or assumptions

19-13 Who Does NDE? We depend on highly-trained & skilled “craftsmen” Training guidelines have been provided –American Society of Nondestructive Testing (ASNT) Knowledge & skilled performance are responsibility of employer Most skilled are designated as “Level III” Lessons Learned Skilled craftsman required

19-14 NDE Engineer Skills Materials engineering Structures/mechanical engineering Production/process engineering Maintenance Basic sciences –Physics & chemistry NDE applications Rarely “Level III” Lessons Learned NDE engineering skills required

19-15 Where Do We Go? New materials, processes & applications Requirement to understand & define damage modes “Crack equivalent” quantifier is required NDE requirements—flaw/damage –Type –Size –Location –Orientation –Nearest neighbor

19-16 Sioux City IA, DC-10 United Airlines Flight 232, McDonnell-Douglas DC- 10, July 1989 Event –Uncontained engine explosion in flight –Aircraft controls damaged

19-17 Sioux City IA, DC-10 (cont.)

19-18 Sioux City IA, DC-10 (cont.)

19-19 Sioux City IA, DC-10 (cont.) Inspection history –Engine underwent overhauls, requiring fan piece part inspections, 6 times –Fluorescent penetrant inspection required –Crack size estimated to have been ½″ long at 760 cycles inspection prior to accident, but was undiscovered (penetrant residue found in crack) –NTSB concluded that disk crack was of sufficient size that penetrant inspection, if performed properly, should have discovered the crack

19-20 Sioux City IA, DC-10 (cont.) Inspection history PM 19-31

19-21 Sioux City IA, DC-10 (cont.) Event –In flight, uncontained disk burst Root cause –Material processing –Management/inspection maintenance Interim action –Focused inspection/improvements –Failure analysis Corrective action –Changes in process control & analysis –Focused inspections