Procedia Engineering 86 ( 2014 ) 375 – 383
1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research doi: 10.1016/j.proeng.2014.11.051
ScienceDirect
1st International Conference on Structural Integrity, ICONS-2014
Nondestructive Evaluation – A Critical Part of
Structural Integrity
Ward D. Rummel
D&W Enterprises, LTD, 8776 W. Mountain view Lane LITTELTON, COLORADO, 80125, USA
E-mail ID: [email protected]
Abstract
The structural integrity of modern structures and systems depend on the use and capabilities of nondestructive evaluation (NDE) for quality assurance in production; condition monitoring and repair assessments in service; risk analysis; and service/lifetime retirement. NDE is an ancient technology that has been applied, in many forms, since the beginning of time. The development and application of fracture mechanics was a revolution in design and structures management; imposed new and demanding requirements for NDE applications; and incorporated NDE as a critical part of design, production and management of critical structures and systems.
New NDE requirements included quantification of capabilities and supporting considerations and technologies for development, validation, application and assessment of NDE capabilities and practices. NDE methods provide indirect measures / assessments of a desired property or condition and involve complex integration of parameters to produce the desired end result. Assessments of the capability and reliability of NDE procedures is continually evolving as an NDE engineering technology that is generally described in terms of a “probability of detection” (POD) metric. This paper summarizes the origin of the POD metric; various approaches to addressing POD requirements; considerations in NDE engineering; and suggested paths forward.
© 2014 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the Indira Gandhi Centre for Atomic Research.
Keywords: Traditional NDE; Damage Tolerance; Probability of Detection (POD); POD Modeling; Procedure Development and Application
1. Introduction
Nondestructive Evaluation (NDE) has been an essential quality assurance tool in the development and advancement of modern industry. The same procedures have often been used in periodic / maintenance operations to add confidence in the continuing fitness for purpose of critical components, structures and systems. The NDE procedures used were often prescriptive and the damage detection capabilities were unknown, but were accepted © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
based on damage detected and on continuing useful service on components that had been examined and evaluated in specific industries. The basis for success was often due to substantial design margins that were applied to address many uncertainties in design, manufacturing, acceptance and service life estimate processes. NDE procedures evolved and were often validated by trial and error as design practices and specific industry applications evolved.
New design challenges included weight reduction and margins were reduced to accommodate demands for improved structures efficiencies. Confidence in reduced margins and performance were usually addressed by increasingly stringent and extensive structures testing. Assumptions were that the test articles were representative of the production processes including fitness for purpose as supported by applied NDE procedures in production quality assurance. The general perception was that components, that had been subjected to “quality assurance NDE (QANDE) procedures, were “flaw / damage” free. Indeed, many component drawings included direction for “inspection (NDE) with a “no flaws / defects” acceptance criteria. In the event of a failure, it was easy to rationalize the cause as “(NDE) operator error”. The need and challenges to reduce weight and margins were a priority in the aircraft and space industries. Aircraft design practices included attention to materials selection, service loads analyses and environment and specific structures tests. Collectively, the design practices were known as “SAFE LIFE” and many very durable and successful aircraft were produced and served well.
Advancements in materials science were incrementally incorporated in specific applications, but were not integrated nor considered as requirements to change design practices. The failure a new, F111 aircraft, with 100 hours flight time, prompted attention to the potential of damage tolerance and fracture mechanics in aircraft structures design and life cycle management. The failure site is shown in Figure 1. A United States Air Force (USAF) team, led by Dr. John W. Lincoln [1], developed the direction, engineering protocol and support technology to implement “DAMAGE TOLERANCE” in the design, acceptance and life-cycle management of aircraft structures. In 1984, the work was shaped into an annual conference format that is known the Aircraft Structural Integrity Program (ASIP) [2]. ASIP continues to provide a forum for the technical interchange of information between personnel responsible for structural integrity, including design, analysis, testing, manufacture, certification, nondestructive evaluation/inspection, maintenance, repair, safety, risk assessment and mitigation, durability and life management.
The first extensive applications of damage tolerance principles and practices were incorporated into the National Aeronautics and Space Administration (NASA) “Space Shuttle” and the USAF “B1-Bomber” programs. We have learned much from this early work. Damage tolerance science / technology has grown and is now a part of wide spread industry applications through out the world.
Figure 1. USAF F111 Aircraft Wing Failure
Figures 2 and 3 show initiation and slow crack growth in an aircraft structure. This graphically illustrates a basic principle of damage tolerance. The crack initiated at the root of the chamfer and grew stably in service until final fracture.
Figure 2. Crack in an aircraft fastener hole Figure 3. Stable crack growth
2. Damage Tolerance
Damage tolerance was a revolution in aircraft structures design and life management and was a vital program in prolonging the life and ensuring the structural safety of all aircraft (including aging aircraft). Requirements for implementing damage tolerance also precipitated a revolution in nondestructive evaluation / testing / inspection (NDE) technology. Assumptions that materials and structures were “flaw free” after application of QANDE procedures were shown to be invalid as a basis for design. Damage tolerant structures were now assumed to contain flaws at the time of entry into service and flaw growth properties were now use as a part of life prediction. Transition considerations are shown in Table 1.
Table 1. Transition from SAFE LIFE to DAMAGE TOLERANT
SAFE LIFE STRUCTURES DAMAGE TOLERANT STRUCTURES
1. Materials selection based on experience in use. 1. Materials selection based on quantified damage tolerance properties and experience in use. 2. Calculated loads, environment and use analyses based
on structure propertied and experience in use.
2. Quantified loads and use analyses combined with quantified damage tolerance analyses, life analyses and experience in use.
3. Material integrity based on “flaw free” assumptions resulting from production practices, quality assurance NDE and use
3. Materials integrity based on the presence of calculated “critical initial flaw” sizes and calculated growth in service use.
4. “NO FLAWS” NDE acceptance criteria 4. NDE acceptance criteria based on; • Flaw / damage type • Flaw / damage size • Location
• Orientation • Nearest neighbor
5. Prescriptive NDE procedures validated by previous experience and use in industry.
For new applications – based on “EXPERT” engineering judgment
5. Engineered and validated procedures characterized by:
• Applicability • Reproducibility • Repeatability
• Capability validated by quantified data Major assumptions for the capabilities of QANDE were shown to be invalid and requirements to quantify NDE detection capabilities were a major challenge to NDE practices and analyses. NDE was integrated as an integral part of design, life and risk analyses, maintenance schedules and support of quantified bases for structures “retirement for cause.” A schematic illustrating the new, integrated role of NDE is shown in Figure 4. Note that all locations within a structure are not “fracture critical” and may not be included in damage tolerance analyses. Integrated structures life, continuing fitness for service, and operational risks are controlled by the damage content and severity. The damage detection capabilities of unvalidated quality assurance NDE procedures are unknown and are inadequate for damage tolerance management. NDE procedures that are integrated into structures management are required to be reproducible, repeatable and capable. Quantitative data that support the detection capabilities of NDE procedures are a new and essential requirement for initial and continuing damage tolerance acceptance. New challenges in NDE procedure development, validation and application have been added to expanding NDE technology applications.
Figure 4. NDE is integral to structural integrity
3. The NASA Space Shuttle Program and Probability of Detection (POD)
NASA incorporated damage tolerance design in the development and management of the “Space Shuttle Program”. In preparation, a pioneering program was initiated to support implementation of fracture mechanics technology on the Space Shuttle program [3]. 2219 aluminum alloy had been selected as a primary structures material. Tightly closed fatigue cracks were selected as a potential damage mechanism. Two representative thicknesses were cut, with the grain rolling in both directions, to fabricate representative test panels. 318 fatigue cracks were grown in random locations on both sides of 119 panels. The cracks had multiple aspect ratios and were distributed in size from 0.004 to 0.500 inches. Starter notches were removed by machining to produce representative surfaces finishes. The test panels were then inspected by three different, skilled operators using production X-radiography, ultrasonic shear wave, eddy current, and high sensitivity fluorescent penetrant procedures. Detection results were recorded as “HIT / MISS” by panel number, and location on the panel.
All panels in the test set were then lightly etched to remove residual machining effects. Inspections of all panels were then repeated and documented (three operators, all NDE procedures). All panels in the test set were proof loaded to 70% of the nominal yield strength. Inspections of all panels were then repeated and documented (three operators, all procedures). All cracks in all panels were then broken open, their size (length and depth) measured, and all data tabulated. The program sequence is shown schematically in Figure 5.
Det ect io n Pro b a b ilit y Figure5. Program It was recognized that NDE procedure It was necessary to provide a single, quantitativ method that we selected was to order the data number of observations by counting down from estimate calculation (hits divided by detection o the largest size crack in the group; repeat the p exhausted. The plotted result was a probability confidence was then calculated for each sample Visual examination of the various curves show curve had an inflexion point at the 90% level an damage tolerance / fracture control analyses wa plot passed through the 90% point. This desc frequently referenced in the literature. An examp
Figure 6. A POD PLOT FROM THE O This method is rigorous and provides a quantita set. It is not a constant and varies with variation the development and validation of procedures NDE methods and procedures. It is a measure o assessments and in damage tolerance analyses.
Precrack Machine
Sequence for the Development of POD
s are the product of multiple variables and produces varia ve output value that could be used in fracture control an a from the largest flaw in the data set to the smallest; c m the largest flaw to the smallest flaw in the sample group pportunities (binomial method); plot the calculated detect process for the next sample group and continue until the y of detection (POD) curve as a function of flaw size. A e group data point using the methods described in MIL H wed that the data were reasonably constant for larger flaw
nd then rapidly decreased. The value selected to use for as the calculated lower 95% value at the location were th criptor was adopted by convention and is the 90/95 p ple plot from the original data is shown in Figure 6.
ORIGINAL NASA PROGRAM
ative metric for the capability of the applied NDE proced ns in test specimens, applied procedures and protocol. It
/ NDE systems and in comparison of the performance f detection capabilities and is an essential element in NDE
NDT NDT NDT Etch Proof Load Load to Failure Fracture Analysi able results. nalysis. The combine the p; do a point tion point at e data were lower 95% HDBK 5 [4]. ws and each purposes of e POD data point that is
dure and test is useful in of different E reliability e
4. Reaction / POD Response
Disclosure and publication of the original POD work generated multiple responses. The first responses were disbelief and attack. The work disproved many long held myths, beliefs, and misconceptions concerning NDE capabilities (NO FLAWS MIND SET). Questions on many traditional and special purpose NDE procedures were opened. The challenge was and is to understand the problem to be addressed and to integrate methods, technologies and skills that were outside of traditional NDE applications. The integrated elements / “short falls” have continued to emerge as “NDE Engineering” challenges.
POD provides a rigorous metric that characterizes the end to end output a complex, multiple parameter assessment. The output incorporates the following identifiable variances:
• Flaw (Artifact) Variables • Test Object Variables • NDE Method Variables • NDE Materials Variables • NDE Equipment Variables • NDE Procedure Variables • NDE Process Variables • Calibration Variables
• Acceptance Criteria / Decision Variables • Human Factors
Most of the variances are controllable in developing and validating NDE procedures. However, flaw variables and test object variables must be addressed on a case by case basis.
Rigorous POD assessment is beyond the practical capacities of many NDE applications and various methods have been developed to reduce the requirements for large numbers of test specimens and analysis of large data sets. A subset of a full POD was adopted by NASA that is known as the 29/29 method [5]. It uses the same sampling rigor for a flaw size near the engineering acceptance limit and provides a single data point on an expected POD curve (plot). All 29 flaws must be detected to provide a 95% confidence in detecting flaws at the selected flaw size. Data on detection of smaller flaw sizes is not provided by this method, but the procedure is validated to meet the engineering limit, if the flaws are representative of the population of flaws to be addressed. The method is applicable to smaller sample sizes and may be used in initial procedure development with corresponding reductions in confidence levels.
The merger of two groups of normally distributed data may be described by a “log logistics” model and was proposed for analysis of POD data by Berens [6]. Use of this model significantly reduces data requirements and has been widely used in NDE data analyses. It was validated using some of the original NASA data and has evolved as the basis for MIL STD 1823 [7]. Its use is applicable to data that conforms to the data form, constraints and other assumptions and requirements, including:
• An increasing (near linear) signal response with increasing flaw size (below a saturation value) • A near constant baseline noise
• Selection, application and documentation of a fixed NDE discrimination level
A typical data output form is shown schematically in Figure 7 and a POD result is shown in Figure 8. The aNDE value is the value that is input to damage tolerance analyses. The Berens model spawned a large number of
models and model types and uses, including: • Ray tracing in procedure development • Data analysis models
• Capability predictive models [8]and • Model assisted POD (Thompson)[9 ]
Close attention must be paid to data form, model constraints and limits in applying all models and analysis routines. All models must be validated for applicability.
Figure 7. Data Form for the Berens Model Figure 8 Output Using the Berens Model 10]
5. NDE Procedure Development and Application
The greatest potential for advancements in quantitative NDE is in NDE procedure development and validation. The combined requirements are complex and the range of knowledge, expertise and skills are beyond the capabilities of most NDE craftsmen. The integrated task may be described as “NDE Engineering”. Quantitative NDE procedures must be:
• Applicable • Reproducible • Repeatable and • Capable
The same NDE output results are expected not only in initial structures acceptance, but also at incremental “maintenance” intervals throughout its life.
1. Applicability of an NDE procedure integrates a combination of science and engineering with documented requirements and limits for use.
2. Reproducibility of a procedure requires control of NDE system set up and “calibration”. A multiple point, system calibration is necessary to support producing the same measurements each time a procedure is applied. (This is usually not a protocol in QANDE procedures.)
3. Repeatability of a procedure is assessed by producing consistent measurement results on representative test artifacts as applied in the intended application environment. Repeatability may include successive measurements that have not been a part of tradition quality assurance NDE applications. For example, verification of consistent signal levels from multiple, representative artifacts; measuring and documenting “noise levels” in intended application locations; documenting NDE threshold signal discrimination levels; and documenting signal and noise signal levels over the range of expected flaw sizes.
0 10 20 30 40 50 60 70 80 90 100 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75
ACTUAL CRACK LENGTH - (Inch)
P RO B AB IL IT Y O F DE T E CT IO N ( % ) 0 10 20 30 40 50 60 70 80 90 100
Data Set: ETA1001A Test Object : Aluminum / Flat Panel Condition: As Machined Method: Eddy Current - Hand Scan Operator: A
Opportunities = 311 Detected = 208 90% POD = 0.196 in. False Calls = Not Documented
--- PRED. POD X HIT / MISS DATA
a
(NDE)4. POD capability assessments may be initiated with a few measurements to assess control and documentation of the known variables and variable levels and the form of the data output. Model assisted POD may be applicable for variations in previously validated applications.
In addition to documenting procedure application requirements, important measurements should be made for each application. These include:
• Noise level (Assumed to be constant) but should be measured and documented for both procedure validation and each application
• Signal level at target flaw / damage size • NDE threshold acceptance level • Signal form (validation)
• Signal to noise level over the target range of measurements (validation)
The additional measurements are useful in assessing the applicability and stability of a procedure and may be useful in identifying and adjusting outputs to accommodate materials properties changes, repairs and configuration modifications and in linking measurements to prior POD data as in the example in Figure 9. Note that the data form must also be linked to maintain confidence in the results.
Further adjustments may be required on a case by case basis to account for uncontrollable , but known variable such as flaw types; applied loads during NDE measurements; temperature variances affecting material signal and noise responses; surface condition , coatings variances, etc. Adjustments may be aided by results from laboratory test samples to bound the magnitude of the impact on detection. Additional design margins may be required to accommodate some conditions found in service applications. [11 -15]
Figure 9. Linking Measurements to Prior POD Data
Summary
NDE evolved primarily as an applied procedural art due to the indirect, multi parameter nature, of the methods and the diverse and broad applications. It has a long history of applications and the benefits have been validated (but capabilities not quantified) in large part, by trial and error. In many applications the same methods
and practices are similar to that involved in the medical industry. Practice as an art is expected to continue, but great benefits of procedure quantification are expected to develop in parallel.
The introduction of damage tolerance imposed requirements for quantified NDE assessments and the integration of NDE into life cycle management of structures during service. The economic benefits of integrating reliable and quantified NDE into structures management are enormous. The task of transitioning traditional, non quantified, quality assurance procedures to meet quantified damage tolerance requirements is also enormous.
In modern industries, new developments usually involve new science and proof of principle; new engineering to reduce principles to practice, and transfer to skilled craftsmen to apply the new technology. An engineering branch of NDE is required and is expected to develop to meet the demands of damage tolerance design principles and management practices.
Probability of detection (POD) assessment procedures and metrics that were developed to support NASA and other programs have provided a basis for assessing the capabilities of NDE procedures, a framework for procedures integration and support of NDE engineering as a new division of NDE technology. POD has spawned much work on procedure quantification. This work has helped to bridge the gap between new engineering requirements and implementation. Much work remains. NDE is indeed integral to, and a critical part of structural integrity in modern engineering structures.
References
1. DOD, MIL-A-83444, USAF Damage Tolerant Design Handbook:Guidelines for the Analysis and Design of Damage Tolerant Aircraft Structures, Department of Defense, Washington, D.C., 1974.
2. Aircraft Structural Integrity Program (ASIP) – (http://asipcon.com)
3. Rummel, W.D., P.H. Todd, Jr., R.A. Rathke and W.L. Castner, “The Detection of Fatigue Cracks by Nondestructive Test Methods,” Materials Evaluation, Vol. 32, No. 10, October 1974, pp. 205–212.
4. DOD, MIL-HDBK-5J, DEPARTMENT OF DEFENSE HANDBOOK: METALLIC MATERIALS AND ELEMENTS FOR AEROSPACE VEHICLE STRUCTURES (31 JAN 2003)
5. NASA, NASA-STD-5009, Nondestructive Evaluation Requirements for Fracture Critical Metallic Components, National Aeronautics and Space Administration, Washington, D.C., 2008.
6. Berens, A.P., “NDE Reliability Data Analysis, Nondestructive Evaluation and Quality Control, Quantitative Nondestructive Evaluation,” ASM Metals Data Book, Vol. 17, 5th ed., ASM International, Materials Park, Ohio, December 1997, pp. 689–701.
7. DOD, MIL-HDBK-1823, Nondestructive Evaluation (NDE)System Reliability, Department of Defense, Washington, D.C., 30 April 1999.
8. Maleo, N., “Project Overview,” PICASSO (Improved Reliability Inspection of Aeronautical Structure through Simulation Supported POD), Moissy-Cramayel, France, 2010, www.picasso-ndt.eu/project-overview.
9. Thompson, R.B., “A Unified Approach to the Model-assisted Determination of Probability of Detection,” Materials Evaluation, Vol. 66, No. 6, June 2008, pp. 667–673.
10. Rummel, W.D. and G.A. Matzkanin, Nondestructive Evaluation (NDT) Capabilities Databook, 2nd ed., Nondestructive Testing Information Analysis Center, November 1997.
11. Rummel, W.D., “Transfer of POD Performance Capabilities from Simple Shapes to Complex Shapes,” Review of Progress in Quantitative Nondestructive Evaluation, Vol. 18, 1999, pp. 2305–2310.
12. Rummel, W.D., “A Path Forward for NDE Reliability, Transactions, Prepared for presentation to the 5th European-American Workshop on Reliability of NDE, October 7-10, 2013 - Berlin, Germany, IN PRESS.
13. Corbly, D.M., P.F. Packman and H.S. Pearson, “The Accuracy and Precision of Ultrasonic Shear Wave Flaw Measurements as a Function of Stress on the Flaw,” Proceedings of the American Society for Nondestructive Testing Fall Conference, October 1969.
14. Wooldridge, A.B., “The Effects of Compressive Stress on the Ultrasonic Response of Steel-steel Interfaces and Fatigue Cracks,” Report NW/SSD/RR/42/79, Central Electricity Generating Board, London, United Kingdom, April 1979. 15. Wooldridge, A.B. and G. Steel, “The Influence of Crack Growth Conditions and Compressive Stress on the Ultrasonic
Detection and Sizing of Fatigue Cracks,” Report NW/SSD/RR/45/80, Central Electricity Generating Board, London, United Kingdom, April 1980.
