19-1 Lesson 19: Reliability of Nondestructive Evaluation (NDE)

19-2 Objectives Identify traditional role of NDE Recognize NDE as integrated engineering element Determine NDE requirements in structural integrity Identify how NDE interfaces with design life assessment Define NDE requirements for flaw detection State how NDE capabilities are quantified Define NDE reliability & sources of variance Recognize need for sound engineering discipline & judgment

19-3 Lesson 19: Reliability of Nondestructive Evaluation (NDE) Ward D. Rummel –Consulting Engineer, NDE Additional contributors: –Dr. Alfred Broz Chief Scientist, NDE FAA –Professor R. Bruce Thompson Director, Center for NDE Iowa State University

19-4 Integration

19-5 Integration

19-6 Integration

19-7 Engineering Disciplines Ignored, Lessons Relearned

19-8 Lessons Learned from Oil Rig Platform Failure Learn from past—you don’t have time to make all mistakes yourself NO random failures –Many random successes Culture has changed, tools have changed Engineering discipline is continually challenged as culture changes & new tools are developed Engineering discipline remains as steadfast requirement

19-9 F-111, December 1969 Failed at 100 hours

19-10 F-111 NDE not possible in some critical locations due to access Wing box is D6AC steel, heat-treated to 220 KSI Use nil-ductility transition temperature as screen aid –Lesson from WWII Liberty ships

19-11 F-111 NDE Apply conventional NDE to accessible areas Proof test cooled down structure (-50 °C)

19-12 Shift to Damage Tolerance & Durability for Structural Integrity Air Force—F-111 return to flight Invoked as requirement for design of: –B-1 bomber –Space Shuttle Fracture mechanics & fatigue analyses

19-13 Revolution in Design Engineering From Safe life –Experienced-based design & 1 st -article test –NDE primarily experience- & procedure-based for quality assurance NDE (QA-NDE) to Damage tolerance/durability –Quantification/demonstration of NDE capabilities required (QNDE)

19-14 Revolution in NDE From QA-NDE: collection of material & procedure- based methods applied in general similar applications (per general public written protocol) To QNDE: performance-based & has shown shortfalls & limitations of classical QA-NDE applications

19-15 Material Fracture Critical crack size varies with stress level –Static case Critical Crack Size PM 19-7

19-16 Dynamic Case (Fatigue) Cracks will grow by cycle loading Thermal / Environment Stresses Impact Damage Wind Loads, etc. PM 19-7

19-17 Fracture Mechanics/RFC Works Crack & fracture at fastener hole in T-38 aircraft wing Stable subcritical crack growth

19-18 Detection Importance Not smallest flaw ever Detected But largest flaw ever Missed

19-19 Crack Initiation Cracks will initiate & grow (fatigue) due to cyclic loading (service operation) PM 19-8

19-20 Retirement for Cause Throw it away before it breaks PM 19-9

19-21 Original NASA Task Approach 118 aluminum panels of 0.060″ & 0.210″ thickness 328 fatigue cracks NDE by 3 different inspectors: –“As-machines” condition –After etching –After proof loading X-radiography, eddy current, ultrasonic & fluorescent penetrant Later added holography & monitored panel fracture by acoustic emission

19-22 Program Sequence Experimental test & analysis sequence PM 19-12

19-23 Actual Crack Micrographs PM 19-13

19-24 How Do We Present Data? Have ~1,000 observations for each method & sequence All cracks of same size are not equally detectable, thus we have a statistical (probabilistic) problem Need to reduce to single number Need to plot data Need statistical rigor in output in tradition of materials properties data (MIL-HDBK-5)

19-25 Data Analysis 95% confidence requires 29 successes out of 29 trials (used for qualification) Order NDE observations from largest to smallest in group Count down 29 observations & plot point estimate results Drop largest flaw in group, count down 29 observations & plot point estimate result Repeat to end of data set This is moving average method

19-26 Development of POD Method PM 19-15

19-27 Origin of 90/95 90% detection value was selected by judgment call (now convention) 90% detection value was consistently near inflexion point of POD curve Supporting data provides 95% confidence in detection capability value reported (MIL-HDBK-5) Dependent on sample set tested 90 % Detection 95% Confidence

/29 Subset of POD After POD value is established: –Additional facilities may be qualified using 29/29 method –Additional operators may be qualified using 29/29 method –NDE procedure may be requalified using 29/29 method –Same or similar flaws must be used to ensure that subset is representative of full POD assessment specimens