Aircraft Structural Integrity Research Articles

Fatigue Life Monitoring Program of RMAF MiG-29

The Royal Malaysian Airforce (RMAF) operates one squadron of MiG-29 which were designed on Safe Life principle. RMAF conducts a fatigue life monitoring program to these airplanes. This activity is conducted based on the experience of having the fatigue life monitoring program to the RMAF F/A-18D. The fatigue life of RMAF MiG-29 is based on the wing-fuselage lug joint structure, and Low Cycle Fatigue (LCF) approach is adopted. The stress spectra of this component, is derived through mapping of g-spectra to the 1-g stress level of the lug. The g-history is obtained from the accelerator installed in the airplane, while the 1-g stress level is obtained by finite element modeling of the wing structure and lug joints. Rainflow cycle counting procedure was then applied. The fatigue characteristics (strain-life) of the lug material was obtained from the laboratory test, using the lug material sample, combined with the empirical formula of strain-life diagram. Notched effect is taken into account using Neuber theory. Mean stress effect is dealt with using Smith-Watson-Topper formula. Miner’s rule is used to calculate the fatigue damage accumulation. A fatigue life prediction software for RMAF MiG-29 which incorporates the above concepts had been developed. Currently, this software is operational with the RMAF MiG-29, and is being used as part of its Aircraft Structural Integrity Program (ASIP). This paper reports on the development of the fatigue life monitoring strategy and software for the RMAF MiG-29.

Damage tolerance demonstration testing for the Australian F/A-18

Over recent years Australia has been involved in a number of full-scale fatigue testing programs in support of the through-life structural integrity of the Royal Australian Air Force’s (RAAF) F/A-18 fleet. It was recognised early in the acquisition cycle that the certification testing conducted by the manufacturer failed to considered damage tolerance requirements and would be unlikely to cover the typically more severe and diverse RAAF operations. Given similar aircraft structural integrity management philosophies, major benefits were to be realised through collaboration with the Canadian Forces (CF). In particular, as fatigue testing under representative CF/RAAF loading was the basis for both countries’ structural integrity management, the International Follow-On Structural Test Project (IFOSTP) was successfully concluded. This paper emphasises the Australian components of IFOSTP, including the damage tolerance testing and demonstration for the aft fuselage that incorporated the simultaneous application of both manoeuvre and dynamic buffet loads. Many of the innovations and consequences of this work program are highlighted, and may be applicable to future fighter aircraft structural integrity programs.

Influence of Advances in Technologies for the Management of Aircraft Structural Integrity within RAAF

The basic concept of an Aircraft Stuctural Integrity Program is to ensure that airframes are adequately managed to ensure safe operation without catastrophic failure, to maximise fleet availibilty and to minimise cost of ownership. In managing these three aspects, a number of conventional and advanced technologies are being adopted and applied within the RAAF. Composites and bonding technology has been developed and transitioned onto various RAAF aircraft over many years in the form of Bonded Patch Repairs to airframe structures (wing and fuselage). Based upon conventional heat treatment behaviour of aluminium alloys, Retrogressive Re-Ageing technology is being transitioned to minimise stress corrosion cracking. From structural mechanics and FEA technologies, Geometric Shape Optimisation methods have been applied to minimise peak stress thresholds within aged airframe structures. To manage structural degredation (fatigue and corrosion) a number sensor-based monitors are being developed and applied on RAAF aircraft. Finally, using the reliability methodology, a proactive management program to assess the extent of corrosion degradation within a whole aircraft is being instituted. This methodology is being articulated through a new paradigm known as “Environmental Degradation Management System – Tool Box” (EDMS-TB). Within RAAF it can be demonstrated that candidate technologies which are adopted have direct and in-direct positive influences within ASIPs to address the key structural integrity parameters of Safety, Availability and Cost of Ownership.

Indirect approaches to individual aircraft structural monitoring

At present, aircraft structural integrity is maintained by equally applying to an entire fleet knowledge obtained during the design phase and/or from a fleet sample and by introducing conservative assumptions, load surveys, costly inspections and maintenance procedures to safeguard airworthiness. Several investigations have highlighted the significant variations in individual aircraft usage and have indicated the inadequacy of applying “conservative” assumptions and updating the assumptions by monitoring a fleet sample. They have, in addition, suggested the importance of individual aircraft tracking (IAT). IAT can be achieved by fitting each aircraft with strain gauges to monitor critical component loads directly, but maintaining a large number of strain gauges entails high operational costs. Affordable IAT can be achieved by analysing recorded flight parameters to indicate usage, loads and fatigue indirectly. This paper reviews the key investigations into indirect IAT structural monitoring methods.

Corrosion Fatigue in 7075-T6 Aluminum: Life Prediction Issues for Carrier Based Operations

Fatigue experiments performed on 7075-T6 Al alloy at various frequencies and stress levels and under alternating wet and dry environments indicate that simultaneous action of corrosion and fatigue substantially accelerates crack initiation and growth rates when compared to pure fatigue (dry air) conditions. In experiments performed under alternating wet and dry conditions, fatigue crack growth rate was observed to increase rapidly in the presence of mildly corrosive (salt solution) and decrease sharply when subjected to noncorrosive dry air. Crack arrest of various durations was observed at transition points between dry and wet cycles. Lowering the frequency of fatigue loading significantly reduced crack initiation and overall life time. S-N curves showed a continuous downward trend without reaching a plateau or threshold. These observations led to the conclusion that aircraft structural integrity can be seriously compromised even under mildly aggressive environments and at subcritical stress levels when cracks are present regardless of changing of environments and/or test frequency.

Intelligent Computational Methods for Corrosion Damage Assessment

Corrosion is one of the damage mechanisms affecting the structural integrity of aging aircraft structures. Various nondestructive inspection (NDI) techniques are being used to obtain images of corroded regions on structures. A computational approach using wavelet transforms and artificial neural networks to analyze and quantify the extent of corrosion damage from the NDI images is described. The wavelet parameters obtained from the images were first used to classify between corroded and uncorroded regions using a clustering algorithm. The corroded regions were further analyzed to obtain the material loss due to corrosion using an artificial neural network model. Experiments were carried out to investigate the developed methods for aircraft panels with engineered corrosion obtained from the Federal Aviation Administration Validation Center in Albuquerque. The results presented indicate that the computational methods developed for corrosion analysis seem to provide reasonable results for estimating material loss due to corrosion damage.

繊維強化複合材料のき裂感受性に関する研究

The aircraft structural integrity should be demonstrated by considering every factors which degrade its strength. Recent helicopter structural components such as rotor hub and blades are manufactured mainly by uni-directional composite materials. The reasons of the usage of the composite material are the low notch sensitivity and high damage tolerance. Aramid composite has extremely high fatigue strength and high toughness. Then the fatigue strength characteristics of a uni-directional glass composite and aramid composite has been studied by high cycle fatigue tests with open hole. The test result of aramid composite showed the high notch sensitivity at high cycle region. In case of the aramid composite, the crack expanded laterally to the load direction from the hole, on the other hand, in case of glass composite the crack expand parallel to the load direction from the hole. Some coupon tests about those materials and FEM analysis have been carried out. From this study, following results were obtained. 1) In case of aramid composite, high stress concentration occurs, because of low shear modulus of the fiber that is almost same as the epoxy matrix. Then, no matrix crack occurs around a hole, so fiber breakage occurs and propagates. 2) In case of glass composite, the stress concentration is relieved by small cracks around a open hole because of shear modulus difference of the fiber and matrix. Then, no fiber breakage occurs. The conclusion of this study is that the appropriate resin system design should be placed on the shear modulus difference between fibers and matrix.

Effects of pitting corrosion on the fatigue behavior of aluminum alloy 7075-T6: modeling and experimental studies

The effects of pitting corrosion on the fatigue behavior of bare 7075-T6 aluminum alloy were investigated. Pitting corrosion decreased the fatigue lives by a factor of about 6 to 8. The fatigue lives were also calculated assuming an equivalent initial flaw corresponding to pits of average and maximum dimensions. The measured fatigue lives generally agreed with the predictions using the average rather than the maximum pit size as the initial crack size. This result could be explained by the pit size distributions offering a significantly larger population of pits near the average size. This work has demonstrated the promise of standardized spray tests for obtaining quantitative measures of corrosion that can be used as inputs in analytical models for fatigue life prediction for evaluating integrity of aircraft structures.

Characterization of trapped hydrogen in exfoliation corroded aluminium alloy 2024

Department of Mechanical and Industrial Engineering, University of Thessaly, Pedion Areos,GR-38334 Volos, Greece(Received September 21, 1998)(Accepted in revised form September 9, 1999)Keywords: Aluminium; Hydrogen; Trapping statesIntroductionStress corrosion and corrosion fatigue represent major threats to the structural integrity of aircraftstructures and have been extensively investigated [1–3]. Among others, Pantelakis et al. [4] observeddramatic degradation of toughness and ductility of Al-Li alloys 2091 and 8090, as well as conventional2024 alloy in several types of accelerated corrosion tests. Based on a comparison of mechanicalproperties (in particular fracture energy density and fracture strain) of naturally and artificially corrodedspecimens, they chose the exfoliation accelerated corrosion test to simulate natural corrosion [5].The above behavior has been attributed by many investigators to hydrogen embrittlement. Hydrogen-produced during corrosion- has been accused of diffusing to the interior, leading to material embrittle-ment through hydrogen-metal interactions [6,7,8]. Despite the lack of a universally accepted hydrogenembrittlement mechanism, a generally recognized common feature is that some critical concentration ofhydrogen must build-up at potential crack sites, for failure to initiate. Thus, the distribution of hydrogeninside the metal and its pattern of migration are of paramount importance in understanding thephenomena and designing alloys with improved behavior.It has been shown [9,10] that lattice defects (vacancies, dislocations, grain boundaries) and precip-itates provide a variety of trapping sites for diffusing hydrogen. Hydrogen traps have mechanisticallybeen classified by Presouyre [11] as reversible and irreversible, depending on the steepness of theenergy barrier needed to be overcome by hydrogen to escape from the trap. For example, during adegassing experiment reversible traps will release hydrogen continuously, while irreversible ones willdo so only after a critical temperature has been reached. This is the temperature at which the probabilityof a single jump out of the steep trap becomes nonnegligible.Reversible and irreversible traps may play different roles during an actual experiment [12]. Inparticular, irreversible traps will always act as sinks for hydrogen, whereas reversible traps may act assinks or sources depending on initial hydrogen charging of the lattice. A uniform distribution ofirreversible traps is believed to provide a beneficial effect in alloy behavior under embrittlingconditions, by arresting diffusing hydrogen and thus delaying its build-up at the crack sites [13].

Influence of fastener holes on residual strength

This paper derives simple formulas to predict the reduction of the residual strength due to rivet holes in stiffened and unstiffened panels of aircraft structure. Results show that the reduction can be 40 percent or more from that of panels without rivet holes. The problem is particularly severe for materials of low yield strength and high fracture toughness and for designs with small rivet spacing. It can have a serious airworthiness implication if the effect of rivet holes is unaccounted for in the assessment of aircraft structural integrity.

53659 USAF research and development programs for aging aircraft

For the pasf thirty-five years the United States Air Force has used the USAF Aircraft Structural Integrity Program (ASIP) to maintain safe and economical operation of aging aircraft. This program has been supported over the years by Air Force laboratory programs in the areas of fracture mechanics, corrosion prevention, flight loads, nondestructive evaluation, human factors, and maintenance and repair. These efforts provided the Air Force with the technology required to support the operational aircraft maintenance programs based on damage tolerance. The Federal Aviation Authority in the United States has also focused much of their attention on aging aircraft issues in the commercial fleet of aircraft. There are close parallels between the military and commercial approaches for damage tolerance derived inspection programs. There has also been a mutual need to develop the capability to estimate the time of onset of widespread fatigue damage that could degrade the damage tolerance capability of aircraft. It is the purpose of this paper to discuss the USAF research and development programs in aging aircraft and to identify the technology areas where the USAF and the FAA are cooperating to find solutions to current problems.

51691 Assessment of aircraft structural integrity by detecting disbonds through ultrasonic scanning

A study of nondestructive evaluation of aircraft bonded joints using the contact scanning technique is presented. Reflected signals acquired through the contacting transducer characterize the test region as being bonded or disbonded. Ultrasonic signals are attenuated more rapidly in the bonded regions when compared to disbonded regions. A peak amplitude based method and an artificial neural network are used to classify the signals. Results obtained using an artificial neural network exhibited significant insensitivity to signal variation when compared to the peak amplitude. Very good agreement is observed between results obtained using the present technique and those obtained using immersion scanning.

ベイズの方法による航空機構造の健全性維持について 第２報 ファジイ情報の導入

This paper describes a Fuzzy-Bayesian analysis for deciding a proper inspection schedule to maintain aircraft structural integrity and compares it with the conventional Bayesian analysis developed in the preceding paper. Although subjective information is often given by inspection results because of fuzzy environment which arises from inspectors' judgement, it is difficult to make use of fuzzy information under conventional statistical theories. In the present study, an atempt is made for investigating fuzzy information expressed in terms of linguistic variable and considering a fuzzified Bayes' theorem in order to obtain posterior probability from the information. According to this approach, a priori uncertain parameters are estimated from data collected during in-service inspection. Numerical examples are presented to demonstrate the effectiveness of fuzzy information as well as that of objective one on continuing aircraft structural integrity.

Method for reducing the fatigue crack growth rate of cracks in the aluminium alloy fuselage skin of an aircraft structure: Parker, E.R. and Parker, W.J. US Patent 5071492 (10 Dec 1991)

Over recent years Australia has been involved in a number of full-scale fatigue testing programs in support of the through-life structural integrity of the Royal Australian Air Force’s (RAAF) F/A-18 fleet. It was recognised early in the acquisition cycle that the certification testing conducted by the manufacturer failed to considered damage tolerance requirements and would be unlikely to cover the typically more severe and diverse RAAF operations. Given similar aircraft structural integrity management philosophies, major benefits were to be realised through collaboration with the Canadian Forces (CF). In particular, as fatigue testing under representative CF/RAAF loading was the basis for both countries’ structural integrity management, the International Follow-On Structural Test Project (IFOSTP) was successfully concluded.This paper emphasises the Australian components of IFOSTP, including the damage tolerance testing and demonstration for the aft fuselage that incorporated the simultaneous application of both manoeuvre and dynamic buffet loads. Many of the innovations and consequences of this work program are highlighted, and may be applicable to future fighter aircraft structural integrity programs.

Getting it together - and keeping it that way: the integrity of aircraft structures

Structural integrity has been a prime concern of every aircraft designer since the Wright brothers. Here the author describes the various technologies underlying this important aspect of-aeronautical engineering.