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RTFI Process Assurance

RTFI Process Assurance. Assuring On Target Performance Through Reliability Metrics. IC. AC. HF. Reliability. Flaw Size. Lloyd A Schaefer - NDTU. RTFI Process Assurance - Agenda. Radiographic process Goals and Objectives RTFI in the Product Design process

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RTFI Process Assurance

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  1. RTFI Process Assurance Assuring On Target Performance Through Reliability Metrics IC AC HF Reliability Flaw Size Lloyd A Schaefer - NDTU

  2. RTFI Process Assurance - Agenda • Radiographic process Goals and Objectives • RTFI in the Product Design process • Defining and Quantifying Reliability • Human Factors • Methods & tools to ensure On Target reliability • In process metrics to track variation • Conclusions and benefits • Questions

  3. Radiographic Process Objectives Why do we perform Radiographic Inspection? • To find all defects? • To find the “good” parts? • To find the “bad” parts? When we perform RTFI, have we; • Found all of the defects? • Found most of the defects? • Found all of the rejectable defects according to the code? Examination of Engineering Design can provide some answers

  4. Engineered Product Design Process Welds, and other products, examined by RT; • Are imperfect structures with variable integrity • Are the result of design efforts to; • Application optimize products through knowledge of material, engineering, fabrication & cost, variables and behavior Design variables which influence product integrity: • Material specification based on properties & behavior • Fabrication process parameters & capability • Radiographic Inspection process capability RTFI’s role: Describe material & fabrication results Need a way to integrate the inputs & quantify the result

  5. Integrated Results of Design, Fabrication and Inspection Flaw and Detection Distributions n/1 Acceptance Criteria Number of flaws/ Probability of Detection 0 Flaw/Defect size

  6. Domains & Definitions Flaw and Detection Distributions n/1 Acceptance Criteria Number of flaws/ Probability of Detection Flaws Detected at [ ] Probability & Rejected 0 Detected and accepted Flaw/Defect size

  7. Material, Process & Inspection Dynamics • Flaw distributions change via Material Properties & Process Capability n/1 Acceptance Criteria Number of flaws/ Probability of Detection 0 Flaw/Defect size

  8. Material, Process & Inspection Dynamics • Detection results change via technology and performance, parameters & variation n/1 Acceptance Criteria Number of flaws/ Probability of Detection 0 Flaw/Defect size

  9. Material, Process & Inspection Dynamics • For a given process/inspection, the Acceptance Criteria determines net distribution of flaws n/1 Acceptance Criteria Number of flaws/ Probability of Detection 0 Flaw/Defect size

  10. Goal for RTFI Process Reliability:Stay on Target! • Limit POD variability to ensure design estimate of part imperfection distribution is accurate 1 Probability of Detection (POD) RTFI Process Target Historical design assumption for RT/RTFI process 0 Flaw/Defect size

  11. What is RTFI, or any NDE, Reliability? NDE processes can be defined as: • A physical process with intrinsic limits (IC) • Limited and varied by a specific sets of application parameters, (AC) • Further varied by human factors during the performance of the inspection (HF) Conceptual Model for NDE Reliability: R=f [AC,HF] < IC and, AC = f (AP,HF) < IC, where HF=0

  12. Graphical Representations of NDE Reliability IC Phy. limit AC HF R Predicted Reliability … or in terms of probability IC AC 100%------ HF R Flaw size, a, or condition measured

  13. Measuring RTFI Process Reliability RTFI is a Human Factors (HF) intensive process • Specific measurements of reliability, are not routinely made for RTFI • Reliance upon a controlled process and trained, certified inspectors to meet legacy performance assumptions Possible outcomes from an RTFI activity:

  14. Measuring RTFI Process Reliability Receiver Operating Characteristic (ROC) • A method for describing reliability via the 4 possible outcomes

  15. Measuring RTFI Process Reliability • The ROC can be related to the POD curve by plotting the true positives proportions as a function of flaw size 1 Probability of Detection RTFI Process Target 0 Flaw/Defect size

  16. Methods for keeping RTFI on Target Round Robin Testing (RRT) identifies keys: • Training and certification • Years of experience (ref. Fucsok et. al.) • Examined ROC vs. years of experience; 0-5, 6-22, 23-35 • True Positive performance maximized in the 6 to 22 year experience group tested • False call performance maximized in the 23-35 year group • More data needed to validate findings • Consensus, or peer review • Strong & consistent influence on accuracy improvement in multiple Probability of Detection Studies • Rummel • Suggs • Stanley

  17. Effects of Peer Review on RTFI Reliability Peer Review Illustration: • Statistically significant series of Radiographs shown to several certified inspectors • Both flaws and unflawed samples used with at least 3:1 ratio of flawed to unflawed areas • Objective: To determine correct interpretation & consensus reliability • Which welds are acceptable? Which need to be repaired? 24A/6R 21A/9R 16A/14R

  18. Effects of Peer Review on RTFI Reliability • Intersection of decisions produces most correct results via physical/metallurgical validations 24A/6R 16A/14R 21A/9R Peer Review fever: Catch it!

  19. Other Tools/Influences on RTFI Reliability Digital/Quantitative Feedback Training • Densitometer as density training aid • Usage results in increased accuracy of film density estimates • Film Digitization/Profiling • Improves quantitative skills PM Weld

  20. Example of Metrics to Observe RTFI Variation

  21. Other Tools/Influences on RTFI Reliability Identification of Inspector Potential • EPRI developed Dynamic Inspector Aptitude Testing, DIAT • Testing motivated to quantify human factors which influence inspector performance in POD studies • Five (5) aptitudes measured: • General cognitive ability • Abstract reasoning • Spatial visualization • Pattern recognition • Stress tolerance • Saudi Aramco can participate in this testing via its association with EPRI through Henry Stephens

  22. Conclusions & Benefits of on Target RTFI Reliability • Variation in the RTFI process should be managed • Key variations & effects in the RTFI process: • Type II errors, “misses”, produce a right shifted POD curve • Unique improvements, or creeping elegance, which can result in a left shifted POD curve, but which are not uniform in effect • Produces an increase in Type I errors and repairs • Leads to an increase in the amount of inherent flaws • When the RTFI Process is On Target: • Manufacturing has optimum knowledge of process health • Accurate predictions can be made of the condition and life of the structure • Risk management is improved through accurate flaw distribution inputs to the Risk Based Assessment modules Reliability concepts should be extended to all NDE

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