Back to Basics
Nondestructive Inspection Quantification
and Aviation Safetyby Ripudaman Singh*and J. Steve Cargill+
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Many of us envision NDT as the knight on a white horse riding in to save the day (those of us in Texas may think of the Lone Ranger instead). Whatever you think of, you must have the means to accomplish the job. Here is an introduction to what it takes to do the job.
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Introduction
God created a flying life form, the bird, and gave it a certain life. Nature permits it to reproduce before it finishes with the allotted quota of breathing cycles and ends up in a definite discrete event called death. Engineers have created a flying machine which cannot reproduce. Designers gave it a certain life, at the end of which it does not really die: it degrades slowly and at some point is retired from service. The competitive business environment and shrinking defense budgets are forcing operators to push the service life of these flying machines beyond their original design life. The operators are willing to invest in life extension programs and accept a calculated risk of failure up to a certain level. This is a daunting engineering challenge, requiring a multitude of technologies and processes.
Role of Nondestructive Inspection in Damage Tolerance
The deterioration and degradation of airframes and aerospace engines is primarily classified into four categories:
fatigue cracks from actual service usage based on flights and flight hours corrosion based on duration of exposure and hostility of the environment accidental damage wear out mode - a general category of degradation from creep, erosion, mechanical rubbing and various other phenomena. One key to continued safe flying lies in the timely detection of cracks and corrosion which are capable of causing catastrophies in flight failures (Figure 1). This makes inspection programs, that are tailored to damage growth prediction, the most significant technology in assuring flight safety in programs of life extension of airframes.
The indication of the mere presence or absence of a crack is not enough.
Safety through inspection was recognized long ago and resulted in damage tolerance based design requirements. A structure is termed damage tolerant if it has a reasonable damage growth life such that the damage can be detected during one of the scheduled inspections before it can precipitate a failure. Figure 2 depicts the damage tolerance concept under deterministic and probabilistic scenarios. Quantification of inspection capability is important in obtaining a realistic estimate of damage growth life and deriving the inspection interval. A small change in assumed detectable size translates into a much larger change in opportunity for damage detection. In the damage tolerance philosophy, the inspection intervals are typically half of the duration a crack would take to grow from detectable to critical size. This provides two opportunities for crack detection during scheduled inspections. The critical size is determined by the structural design. This cannot be changed once a structure is built unless the operational conditions change. The detectable size depends upon an inspection system consisting of nondestructive inspection equipment, the process used, the materials involved, the environment and the inspector. Obviously, nondestructive inspection system performance, quantified in terms of crack sizes and detectable with a certain probability and confidence, is the premise for developing an inspection schedule. The indication of the mere presence or absence of a crack is not enough. This requirement drives the concept of 90/95 crack size, where the estimated probability of detection of the crack is 90%, with a confidence level of 95%.
Figure 1 - The Aloha Airlines disaster of April 1988: could this dramatic instance of catastrophic failure have been averted through nondestructive inspection?
Figure 2 - Qualitative representations of the concept of damage tolerance, where opportunity for detection is during the period when the damage grows from a detectable level to a failure level: (a) deterministic scenario; (b) probabilistic scenario.
Impact of Nondestructive Inspection Quantification
An optimum inspection program is one where a combination of inspection systems and the corresponding inspection intervals provide the best value in terms of lower life cycle cost and operational risks for a desired service life. The capability to detect smaller cracks permits extending the inspection intervals, which may lead to substantial savings in maintenance costs and increased availability of personnel at the cost of better inspection equipment or personnel training. At other times, lower cost and more frequent inspections with poorer detection capability may be more economical over the life cycle of the structure.Once an inspection program is optimized and defined in terms of system and intervals, it becomes important to follow it. Generally, the inspection intervals are adhered to very well, but the reliability of the program to detect the crack sizes for which the program was defined is rarely examined. If the inspectors are able to find cracks smaller than originally established, then the inspections are occurring more frequently than required, leading to cost penalty. On the other hand, if the inspectors are not able to detect at assumed levels, then there may not be adequate opportunity to detect the rogue cracks that can cause inservice failure, leading to risk penalty. The concepts of damage tolerance and optimum inspection programs lose meaning if the actual field nondestructive inspection performance levels do not match with the originally assumed levels. Thus, it is extremely important to assure that the inspection performance is quantified periodically and compared with expected levels. Based on the comparison, the inspection intervals and processes can be adjusted to continue with safe and economical operation.
Rotating components in aerospace turbine engines present a particularly critical need for damage tolerance definitions and well characterized nondestructive inspection. Although commercial aircraft do employ containment technology to prevent broken rotating hardware from penetrating the aircraft, there is so much kinetic energy associated with a failed disk that pieces are not contained. In fighter aircraft, there is no containment, so both examples present strong arguments for taking the time and expense to fully employ and characterize damage tolerance in disks, spacers and some blades. The US Air Force has developed a military standard, MIL-STD-1783 (Department of Defense, 1997), to address damage tolerance, nondestructive inspection and other issues associated with procuring and maintaining fully proven engines for all of their aircraft. The US Federal Aviation Administration has met with those who are responsible for the administration of the military standard to determine to what degree the application of the standard would be beneficial to commercial aviation. Much of the advanced eddy current inspection technology that has been made available today has its roots in nondestructive inspection of critical turbine engine parts. High reliability has been shown with automated systems and with some of the advanced flexible array technology to detect fatigue crack sizes as small as 0.1 mm (5 x 10-3 in.) in depth.
Conclusion
The safety of damage tolerant structural systems with predefined inspection intervals is governed by the reliability to detect cracks of the sizes established during the determination of the inspection interval. Nondestructive inspection capability and field performance quantification is a major component of continued safe fleet operations.
References
Department of Defense, MIL-STD-1783, Engine Structural Integrity Program, 1997.
* Karta Technologies, Inc., 5555 Northwest Pkwy., San Antonio, TX 78249; (210) 582-3326; fax (210) 681-9198; e-mail <rsingh@karta.com>.
+ Aerospace Structural Integrity, 8637 SE Sharon St., Hobe Sound, FL 33455; (561) 546-7718; fax (561) 881-4675; e-mail <cargill2@juno.com>.
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