10:50
Session 6: Structural health / loads monitoring
Chair: Thierry Ansart
10:50
20 mins
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Optimization of selecting strain measurement locations for distributed load recovery from strain measurements
Hongna Dui, Dongliang Liu, Lixin Zhang
Abstract: In order to solve the reverse problem of structural load distribution recovery from strain measurements, this paper focuses on how to select strain measurement locations in the influence coefficient method on the basis of the previous research. An optimization procedure of selecting strain measurement locations based on basis strains selection method is proposed. It is matched with the Euclidean space method based on Schmidt's orthogonalization of the maximum vertical distance proposed in Ref[1,2]. The optimization steps are as follows:
a) Construct load column space according to design load cases, and stepwise select load basis cases based on Schmidt's orthogonalization;
b) Construct strain row space according to candidate strain set, and stepwise select basis strains based on Schmidt's orthogonalization;
c) Construct strain column space according to basis strains matrix, and stepwise select strain basis cases based on Schmidt's orthogonalization, then influence coefficient matrix [A] is determined;
d) Calculate the determinant of [A]T[A];
e) Change the initial basis strain in step b), repeat step b) ~d), and finally select [A] with the maximum det([A]T[A]), then the optimal influence coefficient matrix and basis cases are determined.
Taking the load rams and optical fiber sensor data in a certain full-scale wing fatigue test as a case study, the feasibility of the optimization procedure of selecting strain measurement locations is verified under the condition that the number of strain measurements is limited. The load prediction accuracy under different numbers of strain measurements and different strain measurement errors is compared and analysed. It is verified that basis strains selection method combined with the load distribution recovery based on Euclidean space method can provide load predictions of quite high accuracy and robustness. The load prediction accuracy is higher than traditional load calibration program based on strain bridges. With the decrease of the number of strain measurements, the precision of load recovery is reduced. It is suggested that the number of strain measurements should be at least twice the number of basis load cases in practical application.
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11:10
20 mins
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The development of national MRO capability for a basic trainer aircraft: flight and ground tests
Tuomas Korteniemi, Jouni Pirtola, Antero Miettinen, Sauli Liukkonen, Aslak Siljander, Olli Orell, Jarno Jokinen, Mikko Kanerva
Abstract: Patria has an ongoing program to develop a partial national MRO capability for the new basic trainer aircraft of the Finnish Air Force (FINAF). The capability is developed since no original design data is available for Patria. This paper presents the research done on the flight tests and static ground tests of individual components as two subprojects of the MRO capability program.
The flight test subproject, including ground calibrations followed by 22 flights covering the whole flight envelope with one instrumented aircraft, was carried out in cooperation between Patria, FINAF and VTT. The data from the instrumentation suite including 40 strain channels, 8 temperature sensors and various flight parameter sensors was successfully captured for later analyses. From the flight test data, control surface loads were directly calculated using calibration results. Global loads were calculated using a neural network analysis. Flight test results have already been utilized when designing repairs and modifications for the aircraft.
The ground test subproject was carried out in cooperation between Patria, VTT and TAU. For the ground tests, the instrumentation of the aircraft components was extended to a total of 260 strain channels. In addition, 6-8 displacement sensors were used per component. The full-field deformation data from the areas of interest of the components was captured simultaneously using Digital Image Correlation (DIC). During the ground tests, all major components of the aircraft were individually subjected to 11-35 static load cases in a test jig and structural responses (loads, strains, and displacements) were measured. In this paper, the setup, the load cases, and the results of one or two significant components are discussed. Results obtained with different techniques are compared to assess the applicability of DIC also in the future. The ground tests provided useful data since linear responses were measured from the components. The ground test results can be used later in the MRO capability program as a global FE-model of the aircraft is modelled and material properties of the aircraft components are determined with a combination of testing and analysis.
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11:30
20 mins
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Study on the clustering method of flights and life prediction for Individual Aircraft Tracking(IAT)
Yanjun Zhang
Abstract: Life consumption assessment of in-service aircraft is an indirect method to develop the relative relationship between the severity of damage to an individual aircraft, and it is also the most important basic data for Individual Aircraft Tracking (IAT) and life management.
Firstly, combining the structural characteristics, material properties and load/environment spectrum of critical fatigue parts of a certain aircraft, a full-scale fatigue test lasting for four times of the Design Service Goal (DSG) was carried out based on the load spectrum derived from measured data of flight test. Secondly, using the load spectrum for full-scale test as the reference spectrum, combined with various damage equivalent calculation formulas, the equivalent damage equation was determined through numerical calculation based on the center of gravity overload suitable for the aircraft. Then, sampling rate and filtering threshold value were comprehensively analysed and finally given in aircraft life calculation, which provides basic input for the implementation of individual aircraft tracking of the fleet. Next, the reference equivalent damage per flight hour (Standard Equivalent Damage Rate) was determined through the calculation of the reference load spectrum and the DSG. It was followed that the equivalent damage per flight hour was then obtained according to the flight parameters for the actual data of each flight, which establishes the relationship between the actual flight data and the fatigue test load spectrum of the aircraft. Then, based on the measured load spectrum, the operational load prediction adopting machine learning was developed, and the accuracy meets the needs of engineering. Finally, the life consumption assessment of the aircraft fleet was given by calculation, including the individual aircraft life consumption of the fleet, the annual life consumption, and the equivalent damage statistics of typical missions and the statistical assessment of life ratios.
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11:50
20 mins
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Approach to structure monitoring of the Mi-24 helicopter
Krzysztof Dragan, Michal Dziendzikowski, Artur Kurnyta, Piotr Synaszko, Marcin Kurdelski
Abstract: Mi-24 helicopters have been introduced to Polish Army in late 1970s. Due to the necessity to extend their service life and planned modernization the Ministry of Defence approved preparation and execution of the complex Service Life Extension Program for these helicopters.
Basing on the previous proven track of test delivery, the Air Force Institute of Technology (AFIT) was selected to prepare concept, control and supervise the Full Scale Fatigue Test (FSFT) concept of the Mi-24 helicopter structure as well as introduction of the onboard loads and vibration monitoring system (SMST) in order to extend the service life based on the monitoring the actual health.
The FSFT run was delivered by the PZL-Świdnik (a LEONARDO HELICOPTERS company) under the AFIT supervision and control. Consecutively for the safety structure operation the health and condition monitoring system (SMST) was developed.
The SMST is a system incorporating strain measurements in selected regions (for operational load monitoring), as well as vibration sensors monitoring the power transmission system and corrosion as well as structure monitoring sensors. The strain and corrosion measuring system was already installed on one Mi-24 helicopter designated for test flights, which provided load data for FSFT load spectrum definition. Structure under FSFT was also instrumented with partial SMST system for the purpose of the crack and failure mode monitoring as early as possible due to the remote AFIT supervision. At present preparation for the design and implementation fully functional SMST system onboard of the Mi-24 helicopter for operational structure monitoring has been provided at AFIT. Article will present effort to FSFT delivery, development of the SMST system, results obtained for in flight tests as well as for FSFT. Article will also provide with description of the SHM technologies developed at AFIT and used for aircraft operation. Finally future work plans will be presented for Mi-24 helicopter.
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12:10
20 mins
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VIBFAT – Vibration-Induced Fatigue life estimation of the Vertical Tail of the F/A-18 aircraft using virtual sensing
Vesa Nieminen, Tomi Viitanen, Keijo Koski, Risto Laakso, Mikko Savolainen
Abstract: Unmeasured quantities and responses at selected locations in structural monitoring can be estimated by different virtual sensing techniques. Virtual sensing enables estimation of operational response of the structure at any location based on a limited set of measurements and the numerical model of the structure. This paper presents the study for expansion of sparse response data by means of a component simulation model to enable fatigue analysis anywhere in the structural component or subassembly under question.
The object of this case study is a high-performance F/A-18 Hornet fighter aircraft. It is extremely manoeuvrable thus capable of flying at high angles-of-attack. In those flight regimes the high-energy vortices generated by the inner wing leading edge extensions induce severe cyclic loading to the downstream structure by exciting the resonance frequencies of the empennage. The broad band dynamic loads, i.e. buffet loads, together with the static manoeuvring loads, contribute to the fatigue of the Vertical Tails (VT) of the aircraft. The Finnish Air Force (FINAF) has been running the Hornet Operational Loads Measurement (HOLM) program since 2006 to quantify the effects of operational usage on the structure of the F/A-18C/D aircraft. The HOLM program employs two intensively instrumented aircraft without operational restrictions. Both Vertical Tails of the HOLM aircraft include three strain sensors at the Vertical Tail Stubs and two acceleration transducers at the tip of the Vertical Tail, enabling to detect all the significant loading events locally in the VT.
The virtual sensing applied in this study is based on bandpass filtering the measured acceleration data to get modal-specific responses for the primary VT modes. Component FE-model of the VT is then used to create modal-specific acceleration-to-strain conversion functions for the structural details of interest. For each mode individual virtual strain signal will be generated at the chosen structural location. All the signals of the primary modes at the same location are combined by the superposition principle to achieve the total virtual strain, which then can be used as an input in the typical fatigue analyses.
Research focus of this study is applicability of the local/component FE-models for the virtual sensing. Research question is to investigate the limitations of the method when applying only a local/component FE-model and minimum number of sensors for the virtual sensing. The results demonstrated applicability of the local component FE-model and only a single acceleration measurement for virtual sensing in cases where dominant modes are well separated.
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