Chair: Abraham Brot
DEVELOPMENT OF “LOW STRESS CRITERIA” THAT ELIMINATE A LARGE PORTION OF THE AIRCRAFT FROM THE NEW PART 26 FEDERAL REGULATION THAT REQUIRE DAMAGE TOLERANCE BASED MAINTENANCE PROGRAM FOR STRUCTURAL REPAIRS|
Abstract: The main purpose of the paper is to outline major criteria developed for new Part 26 Code of Federal Regulation (CFR) that require aircraft manufacturer like Boeing to support operator compliance to damage tolerance based maintenance program for aircraft structural repairs and alterations. The paper focuses on the newly developed “low Stress Criteria” that eliminates large areas of the aircraft from damage tolerance evaluation and the resulting damage tolerance inspections (by the operators) if stresses are below a certain level.
For past 43 years airworthiness standards for transport category airplanes has been governed by 14 CFR (Code of Federal Regulation) Part 25. Specifically, § 25.571 covered Damage Tolerance and Fatigue Evaluation of Structure which requires that catastrophic failure due to fatigue, corrosion, manufacturing defects, or accidental damage, will be avoided throughout the operational life of the airplane.
A new Part 26 rule and AC was published in the Federal Register on December 12, 2007 and becomes effective January 11, 2008 that directs Type Certificate Holders (TCH) like Boeing to support operator compliance to the operational rule 14 CFR 121/129 that directs operators to have a damage tolerance based maintenance program for all structural repairs and alterations to the Fatigue Critical Baseline Structure (FCBS).
In order to comply with this Aging Airplane Safety Rule (AASR) Boeing is required to provide damage tolerance data for repairs and alterations to the operators so that they can develop inspection program to maintain airworthiness of their aircrafts. As the focal of this activity, Structural Damage Technology (SDT) group of Boeing is actively engaged in developing methods and criteria to support the Part 26 rule.
This paper describes in detail one of the major criteria, the Low Stress Criterion, that has been developed recently that eliminate large areas of the aircraft from damage tolerance evaluation by Boeing and resulting inspections by the operators if stresses in the repaired structure are below a certain level.
Low Stress Criteria is based on both fatigue crack initiation and propagation considerations. Boeing over the years has demonstrated to its customers that there will be minimal fatigue cracking within the Design Service Objective (DSO). This is achieved by using appropriate fatigue design methods validated by large number of tests including full-scale fatigue tests. Repairs are designed following similar design principles. Often, in certain fatigue prone areas, stresses are so low that the fatigue lives turn out to be several multiple of DSO. From crack initiation perspective, this means that fatigue crack initiation could be predicted to be long after an airplane is retired. Using this concept a low stress level is determined by using standard Boeing analysis methods. Large number of analysis is conducted on many typical repairs covering most of the major component of the aircraft and covering several major models.
Low Stress Criteria is also looked from the point of view of the repeat inspection requirements to maintain the structural safety of the aircraft. Repeat inspections are based on fracture mechanics based damage tolerance analysis. The DTR method developed by Boeing evaluates the goodness of repeat inspections by obtaining the DTR for same and comparing them with the required DTR approved by the FAA. The same DTR method is applied for developing Low Stress Criteria for the repairs. Damage tolerance analysis is conducted on the same repairs that underwent fatigue analysis and DTR plots are developed. This is done for each repair for various stress levels gradually reducing the stress. The stress level at which a routine inspection provides many times the required DTR was established as the Low Stress value.
Finally, lower of the fatigue based low stress and damage tolerance based low stress is used to develop the Low Stress Criteria. The paper describes in detail how this is dependent on type of structure and material used. Recently, the criteria have been presented to the FAA specialists who after review raised no major concerns.
The paper describes in a comprehensive manner how the criteria has been developed which in the long run will save the world airlines significant maintenance cost by eliminating inspections on repairs in less critical low stress areas, thereby allowing them to focus on more critical structural repairs.
DAMAGE TOLERANCE PHILOSOPHY FOR BONDED AIRCRAFT STRUCTURES|
Calvin Rans, Rene Alderliesten
Abstract: As the demand for lower aircraft operating costs increases, the improvements in structural weight and operational maintenance offered by adhesive bonding technologies over mechanical fastening technologies becomes an important factor in aircraft design.
The increase in service life offered by a bonded structure over that of a riveted structure makes adhesive bonding technology desirable for aircraft structures. The assessment of the structural integrity of an aircraft, however, is not simply defined by service life. To ensure the structural integrity of an aircraft structure, its response to both incidental (fatigue, limit/ultimate loads) and accidental (overloads, impact damages, manufacturing flaws, etc.) need to be assessed.
Currently, the widespread use of mechanically fastened technologies continues due to the relative ease of predicting the types of damages and assessing the response in the presence of damages within such structures. The so called damage tolerance analysis of mechanically fastened structures is well established. In order to further advance the use of adhesive bonding technologies in the aircraft industry, an equally simple and effective means of assessing the damage tolerance of bonded structures is needed.
Assessing the damage tolerance of bonded structures is not a trivial matter. The damage tolerant behaviour of such a structure is dependant on both the integrity of the bond line and the bonded adherents. Most approaches for failure prediction of bonded structures available in the literature examine either bond line or adherent failure. However, in many bonded structures, such as bonded patch repairs, both aspects need to be treated simultaneously. Furthermore, existing models for bonded structures tend to consider static failure only. This is insufficient for the application of a damage tolerance philosophy where knowledge about static and fatigue behaviour are required.
Based on experience gained through the development of damage tolerance assessment tools for crack growth in Fibre Metal Laminates (FMLs), the application of a fracture mechanics based approach to delamination growth has proven quite successful. Additionally, the simultaneous treatment of fatigue cracking in the metallic layers and delamination growth between metal and fibre layers necessary in FMLs has resulted in an analysis approach suitable for simultaneous assessment of bond line and adherent failure.
This paper describes a general approach for assessing the damage tolerant behaviour of bonded structures based upon the experience and capabilities acquired through FML development and research. The overall approach for dealing with adherent damage growth, bond line damage growth, and the interaction between the two will be described. The necessary experimentation and analysis required for the approach will be described. Additionally, methods for treating influences, such as residual stresses and thermal environment, will be presented and discussed.
The potential of the presented approach will be demonstrated using several case studies. These case studies focus on metal-to-metal and metal-to-composite bonded structures; although the concept is envisioned to be applicable to a wider range of bonded structures, including composite-to-composite bonded structures.
Finally, the limitations of the proposed methodology will be discussed. Although the overall approach is sound, there remain gaps in our understanding and predictive capabilities for certain damage scenarios. These limitations will be discussed and provide a focus for future research efforts.
FIRST DIAMOND - 'DAMAGE TOLERANCE' FOR THE STRUCTURAL HONEYMOON|
Abstract: This paper is the third in the ‘diamond’ series on damage tolerance. The first was Rough Diamond, in 2005. It introduced the ‘diamond’, a simple division of damage tolerance into five essential ‘facets’: site, scenario, detectable, dangerous and duration.
The second was Rusty Diamond, in 2007. It argued the diamond’s applicability to corrosion (or any progressive damage), not just fatigue.
Now, this is First Diamond. In the same tight, clear style, it tackles the thorny issue of the threshold: the end of the structural ‘honeymoon’, the beginning of the inspection program.
Thorny? Yes, the threshold is easily the most contentious aspect of damage tolerance. It is even contentious for the two biggest airworthiness authorities and the two biggest aircraft manufacturers. Airbus’ criticism of FAA policy at ICAF 2005 is just one example.
Contention is not surprising. First, it is the money at stake. Late thresholds save maintenance for airlines and boost sales for manufacturers.
Second is the sensitivity to manufacturing quality. Flaws only have to increase by millimetres to decrease thresholds by thousands of hours.
Third is the obsession with prescription. Prescribing, exactly, manufacturing quality (type and size of flaw) and analysis method (like fracture mechanics) frustrates those who could get the same safety outcome, with a better threshold, in other ways.
On ‘safety’, if we prescribe manufacturing quality, as just described, what is the incentive to improve it? Or, to consider other flaws or damage, possibly more likely or more dangerous?
First Diamond offers three ideas to improve thresholds:
• Damage tolerance
use the ‘diamond’ exactly as for intervals
to improve simplicity and consistency
• Risk management
consider all risks (not just fatigue), prioritised by chance and consequence
to improve safety and efficiency
• Human factors
to temper conservatism, which is counter-productive if excessive
to improve credibility and enforceability
• Outcome-based rules
brief, broad safety outcomes
to improve clarity and flexibility
The author argues and illustrates his ideas with examples drawn from 25 years regulating structural maintenance programs (including thresholds) for one of the world’s oldest and hardest-working fleets.
He also reviews the work of others, especially the USAF and the promoters of HOLSIP.
He then considers special cases:
• Escalated thresholds
Are ‘nil findings’ relevant?
• Unnecessary thresholds
Could a threshold cost more than it saves?
• Unpublished thresholds
Is it safe to be secretive about post-DSG (Design Service Goal) thresholds?
• Enhanced thresholds
When are cold working and other fatigue counter-measures creditable?
• Combined thresholds
Should thresholds for fatigue and corrosion be separate or combined?
• Composites thresholds
How are they different?
• Healthy thresholds
How will Structural Health Monitoring affect thresholds?
• Life limits and thresholds
How are they alike and what are the implications?
The paper hopes to promote safety, economy and harmony in our use of thresholds.