Complex Aircraft Structures Research Articles

Phase/frequency analysis of diffuse Lamb-wave field for fatigue-crack detection in an aluminium multi-riveted strap joint aircraft panel

A diffuse Lamb-wave field is utilised for fatigue-crack detection in a multi-riveted strap-joint aircraft panel. The work involves an extensive experimental programme of fatigue testing. Ultrasonic Lamb-wave responses are captured using a network of low-profile, surface-bonded piezoceramic transducers. Various phase/frequency characteristics of Lamb waves are used to detect and monitor fatigue cracks. An outlier statistical analysis – based on these characteristics - is also performed for damage detection. The results show that phase analysis can be used successfully for damage detection in such structures. In contrast, frequency analysis does not offer reliable damage detection. The results demonstrate the potential and limitations of the method for reliable fatigue-crack detection in complex aircraft structures.

Residual stress and long fatigue crack growth behaviour of laser powder bed fused Ti6Al4V: Role of build direction

Laser powder bed fused (LPBF) Ti6Al4V alloy has a great prospect to manufacture complex aircraft structures. The anisotropic fatigue crack growth behaviour and the residual stress effects are two main concerns on the damage tolerance design of aircraft structures, and should be studied further. In this paper, the compact tension (CT) specimens in four off-axis build directions (15°, 30°, 60° and 75°) were designed and manufactured. Residual stresses of the as-built and the stress relieved samples were measured. Fatigue crack paths and fracture surfaces were analyzed and compared. It was shown that the residual stress along the loading direction in the 15° as-built sample was about 3.5 times larger than that perpendicular to the loading direction, while that of the 75° as-built sample was opposite. The crack deflection angle of the 60° as-built sample was the largest, and it was about 22.4°. Fatigue crack growth rate in the as-built sample fluctuated. After post-treatment, the anisotropy of residual stress state and fatigue crack growth behaviour decreased.

Certification of Testing Laboratories – The Basis of Reliability Among Research Vendors in Aviation

Abstract Material characterization and assessment is a crucial stage in most of aviation and aeronautical research and a basis for further design and testing of more complex aircraft elements and structures. Material test’s reliability can only be guaranteed by conducting them at independent and reliable laboratories, operating based on a management system assessed by a third-party such as the accreditation according to the ISO / IEC 17025 or NADCAP or having the qualification of the second-party based on specific customer requirements. This paper introduces basic requirements for material testing laboratories according to accreditation systems and describes its responsibilities as qualified and reliable testing suppliers.

Acoustic emission source location in complex aircraft structures using three closely spaced sensors

The use of three piezoelectric sensors in a small array has previously been investigated to assess its ability to locate Acoustic Emission (AE) sources in simple aluminium and composite structures. It has been theorised that this approach could be used within wireless systems to monitor AE in aircraft, as the close spacing of the sensors removes the need for power-intensive time synchronisation between nodes and reduces excessive cabling. There are, however, two major limitations to this, primarily that the localisation techniques used are only viable within simple structures and additionally that the high computational requirements of the continuous wavelet transform which the technique utilises is infeasible for a low power system. This paper presents modifications to this approach, including the novel second differential method for single sensor modal analysis, which enables the technique to locate AE events within complex structures. A mapping technique is also presented, which accounts for complexity and varying wave velocity in composite structures. Testing is presented which uses artificial sources on a range of structures, from simple plates to an Airbus A350 aircraft wing, with a high level of accuracy and repeatability shown.

A new beam element which blends the Euler-Bernoulli beam theory with idealised transverse shear flows for aircraft structural analysis

The main aim of this work is to develop a new beam element formulation that would allow the modelling of aircraft structures by blending the Euler-Bernoulli beam theory with transverse shear flows from structural idealisation techniques. It would be demonstrated that the modelling of complex aircraft structures with shell elements can be replaced with this new beam element formulation. The meshing of complex aircraft geometries can be avoided as well as the bonding of the shell elements that belong to different parts of the aircraft, such as the bonding of spars with the wing skin, ribs with skin and stringers with skin. Everything will be lumped into “booms” that are part of the cross-section of the new beam element. Besides the advantages already mentioned, this new beam element will allow savings in computational cost, will have the transverse shear flows (and shear stresses) embedded in an Euler-Bernoulli formulation without any locking pathology associated.

A new synthesis aperture-MUSIC algorithm for damage diagnosis on complex aircraft structures

Abstract Multiple signal classification (MUSIC) algorithm has gradually been applied to guided wave based structural health monitoring (SHM) as a novel promising method, which is able to realize directional scanning and has the potential to monitor multiple signal sources. However, on complex structures, the low signal-to-noise ratio (SNR) of damage scattered signals and structural anisotropy usually result in the poor localization performance of MUSIC algorithm based damage diagnosis method. To solve these problems for applying the MUSIC algorithm on complex structures, this paper presents a new synthesis aperture-MUSIC algorithm for damage diagnosis with array error compensation. This method enhanced the damage scattered signals and compensate the phase delay perturbations caused by structural anisotropy, which realized damage localization on complex structures. The proposed method is verified on an aircraft structure with T-stiffeners, ribs and rivets to detect corrosion damages with five severity levels. Application results show the advantage and effectiveness of the presented method.

SPH-FEM simulation of impacted composite laminates with different layups

Composite laminates offer the potential for reducing the weight of civil aircrafts. However, a major hazard to civil aviation is bird-strike impact damage. In this paper, bird-strike impact damages of composite laminates are numerically investigated by means of Smoothed Particle Hydrodynamics (SPH) and finite element method (FEM) analysis. We mainly focus the influence of laminates' layups on damage modes and energy variations of bird impact on a square laminated plate. The SPH method and an equation of state were employed for the bird projectile. A Continuum Damage Mechanics approach in FEM was applied to simulate failure initiation and damage evolution in composite laminates. Damage maps were calculated with respect to three different layups, i.e. 8 plies, 16 plies and 24 plies, indicating localized damage area as the ply number increased. Deformations of laminates and projectile were studied during the impact damage process. The energy variations (kinetic, strain and damage dissipations) were also comparatively investigated. Results can serve as design guideline in future bird-strike study of composite laminates in complex aircraft structures.

Effect of the attitude fine-adjustment of compaction roller on automated fiber placement defects and trajectory

Automated fiber placement shows great potential for fabricating complex aircraft structures. During AFP process, the process-induced defects and placement trajectory have important effect on the processing quality. These defects and trajectory are influenced by many factors, where the attitude of compaction roller is an important factor. By the attitude fine-adjustment of compaction roller, the placement defects and trajectory can be improved. It is necessary to study the effect of the attitude fine-adjustment of compaction roller on placement defects and trajectory. The attitude includes yaw and roll of compaction roller, and the placement defects mainly refer to the fitness degree and fiber pull-up. Firstly, the placement ability of compaction roller is defined by making full use of the allowable distance. Then, the yaw angle and position of compaction roller are calculated by differential geometry. Meanwhile, the yaw distance and placement width are given. Secondly, the fiber pull-up is defined and described as planar sinusoidal based on the fiber waviness theory. Then the roll angle and position of compaction roller are given. Finally, a series of experiments and simulations had been conducted to verify the feasibility of the developed method. Results demonstrate that the method can improve the placement defects and trajectory.

An infrared thermography-based method for the evaluation of the thermal response of tooling for composites manufacturing

The manufacture of large complex aircraft structures made of advanced composites is done by heating the parts on complex, thermally massive tools using convective heating inside autoclaves. In recent years, numerical simulation of the process has shown great value, but lack of knowledge of the convective heat transfer boundary conditions remains a major obstacle to widespread adoption. An infrared thermography method is presented, suitable for evaluating the thermal response of these processing conditions. The method is based on increasing the emissivity of a tool surface with a painted vacuum bag before thermal imaging. Accurate readings with an average temperature difference of 1.1℃ compared to thermocouple data were achieved. The benefit of the thermography method is the highly detailed surface temperature map. Three tools with very similar geometries but made of Invar, aluminum and carbon fibre composite, respectively, were tested, and results interpreted using analytical solutions for the different tooling feature and convective boundary condition combinations. Analytical simulations, which were validated by comparison to numerical models, explain well the effect of autoclave airflow, tooling material and sub-structure variation on the temperature profiles measured by this infrared thermography method.

Elliptical ring distribution probability-based damage imaging method for complex aircraft structures

In engineering applications, the robustness and effectiveness of damage diagnostic imaging for guided wave-based structural health monitoring could be affected by the complexity of structures. In this study, an elliptical ring distribution probability-based diagnostic imaging algorithm is proposed to mitigate this effect using the estimated wave velocity and damage index. This algorithm improves the ability of damage localization by modifying the defect distribution probability of probability-based diagnostic imaging. The elliptical ring distribution probability of the presence of defect is used for each sensing path in the algorithm. The width of the elliptical ring distribution probability is determined by the range of estimated wave velocity. The amplitude of the elliptical ring distribution probability is determined by the damage index. The damage location is determined by the cross region of different elliptical rings for different sensing paths. The capability of the algorithm is validated by identifying damages at different locations on a complex composite fuselage panel. The results show that the proposed algorithm can identify a single damage accurately and it can identify multiple damages effectively as well.

An approach for the assessment of the statistical aspects of the SEA coupling loss factors and the vibrational energy transmission in complex aircraft structures: Experimental investigation and methods benchmark

In the application of Statistical Energy Analysis “SEA” to complex assembled structures, a purely predictive model often exhibits errors. These errors are mainly due to a lack of accurate modelling of the power transmission mechanism described through the Coupling Loss Factors (CLF). Experimental SEA (ESEA) is practically used by the automotive and aerospace industry to verify and update the model or to derive the CLFs for use in an SEA predictive model when analytical estimates cannot be made. This work is particularly motivated by the lack of procedures that allow an estimate to be made of the variance and confidence intervals of the statistical quantities when using the ESEA technique.The aim of this paper is to introduce procedures enabling a statistical description of measured power input, vibration energies and the derived SEA parameters. Particular emphasis is placed on the identification of structural CLFs of complex built-up structures comparing different methods. By adopting a Stochastic Energy Model (SEM), the ensemble average in ESEA is also addressed. For this purpose, expressions are obtained to randomly perturb the energy matrix elements and generate individual samples for the Monte Carlo (MC) technique applied to derive the ensemble averaged CLF. From results of ESEA tests conducted on an aircraft fuselage section, the SEM approach provides a better performance of estimated CLFs compared to classical matrix inversion methods. The expected range of CLF values and the synthesized energy are used as quality criteria of the matrix inversion, allowing to assess critical SEA subsystems, which might require a more refined statistical description of the excitation and the response fields. Moreover, the impact of the variance of the normalized vibration energy on uncertainty of the derived CLFs is outlined.

Active Control of Counter-Rotating Open Rotor Interior Noise in a Dornier 728 Experimental Aircraft: Optimised Sensor Placement

For the next generation of aircraft, new engine and fuselage concepts are investigated because of the increasing demand for fuel efficiency and the CO2/NOx restrictions declared by the European Union. A lightweight design of civil aircraft combined with efficient engines like the counter-rotating open rotor (CROR) is a key technology to reach these ambitious goals. Especially in the lower frequency range, where the CROR blade-passing frequencies (BPF’s) are dominating the acoustic response, the transmission loss of lightweight structures is typically poor. Active control techniques was tested on plates and beams to increase the system performance in terms of global vibration or sound power reduction. Yet to the authors’ best knowledge, only very little work has been conducted to establish sensor or actuator position optimisation on a complex aircraft structure. The presented paper investigates the potential of a sensor position optimisation on a Dornier 728 experimental aircraft of the German Aerospace center (DLR). This experimental aircraft provides a real fuselage structure and the multi-sinusoidal CROR excitation is induced by a 112-channel loud speaker array. The system model of a fuselage segment is experimentally identified, a sensor optimisation is established and compared to empirically placed sensor configurations. The sensor optimisation includes the simulation of an multiple-input multipleoutput (MIMO) active feedforward vibration controller and the resulting sound radiating vibration patterns. It is shown in the experiments that the feedforward controller using optimised sensor locations reduces the radiated sound power by 7 dB, whereas the empirically placed sensors achieve only 4.5 dB. Furthermore, the potential of the proposed optimisation method regarding sound power reduction performance and the number of required sensors is shown for a real aircraft structure and a complex acoustic excitation.

Analysis on Deformation Process in Deep Drawing of Sheet Metal Part by FEM

Finite element analysis is a simulation technique which evaluates the behavior of components, equipments and structures for various loading conditions. It is a computerized method for predicting how a real object will react to forces by mesh of simpler interlocking structures, the simpler structures or finite elements being agreeable to mathematical analysis. The finite element method is originally developed to study the stresses in complex aircraft structures. Minimization of response times and costs, maximization of the efficiency and quality of product are very important for survival in the competitive manufacturing industry. Sheet metal forming is a widely used and costly manufacturing process, This work will be done to study the finite element (elastic-plastic) analysis of sheet metal forming process using the finite element software. ANSYS simulation is carried out to gain accurate and critical understanding of sheet forming process.

Multi-objective design of complex aircraft structures using evolutionary algorithms

In this article, a design methodology for complex composite aircraft structures is presented. The developed approach combines a multi-objective optimization method and a parameterized simulation model using a design concept database. Due to the combination of discrete and continuous design variables describing the structures, evolutionary algorithms are used within the presented optimization approach. The approach requires an evaluation of the design alternatives that is performed by parameterized simulation models. The variability of these models is achieved using a design concept database that contains different layouts for each implemented structural part. Due to the complexity of the generated aircraft structures, the finite element method is applied for the calculation of the structural behaviour. The applicability of the developed design approach will be demonstrated by optimizing two composite aircraft fuselage examples. The obtained results show that the developed methodology is useful and reliable for designing complex aircraft structures.

Pulse phase thermographic non-destructive evaluation of composite aircraft structures

The emergence of composite materials as a strong contender to metal alloys for use in aircraft structures has been remarkable and their large-scale use in the development of new generation aircraft is quite phenomenal. This trend in the application of composites for aircraft structures indicates their proven capability, reliability and confidence levels in these materials. Consequently, the development of large primary aircraft structures is being dominated by composite co-curedlco-bonded assemblies. However, the quality assurance and structural integrity of these large and complex composite aircraft structures are critical and demand improved and reliable non-destructive evaluation (NDE) methods. In the domain of thermal NDE, pulse phase thermography (PPT) is an advanced technique when compared to conventional pulse and lock-in thermography. The development of PPT was initiated with a view to enhance the thermal contrast and defect definition and to overcome the limitation of pulse thermography to detect deeper defects. PPT was successfully applied to identify a variety of defects in composite aircraft components. This paper demonstrates the potential of PPT for improved defect detection in composites when compared to pulse thermography.

Numerical simulation of bird strike damage prediction in airplane flap structure

Numerical analyses of bird impact damage in complex aircraft structures have been performed using ABAQUS/Explicit. A Lagrangian formulation was used for the bird model in combination with various material models. Several failure and damage modes have been considered for different material models used in the inboard flap of a typical large transport aircraft. A submodeling approach has been used to reduce computational time. Parametric analyses have been performed using different bird sizes, impact locations and velocity vectors.

Editorial

This Special Issue of JCISE is drawn from outstanding papers presented at the 2002 ACM Solid Modeling symposium. This is the second year that JCISE and ACM have collaborated to produce such a Special Issue, in what promises to become an annual tradition. Previously, JCISE V1, N4 (December 2001) contained selected papers from the 2001 symposium. The ACM/SIGGRAPH Symposia on Solid Modeling and Applications provide a premier international forum for the exchange of original research results and important practical applications in all areas of solid modeling and its applications. The symposia series brings together leading researchers, practitioners and graduate students working in this field. The seventh ACM Symposium on Solid Modeling and Applications was held at the Max Planck Institute for Informatics in Saarbruecken, Germany on June 17–21, 2002, the first to be held outside North America. The symposium was co-chaired by Hans-Peter Seidel of the Max Planck Institute and Vadim Shapiro of the University of Wisconsin, Madison, USA. The international program committee was co-chaired by Kunwoo Lee of Seoul National University, Seoul, Korea and Nicholas M. Patrikalakis of the Massachusetts Institute of Technology, Cambridge, Mass., USA. The program committee comprised of 25 leading experts on solid modeling theory and its applications from academia and industry working in 10 different countries from around the world. Previous symposia in this series were held in Austin, Texas, 1991; Montreal, Canada, 1993; Salt Lake City, Utah, 1995; Atlanta, Georgia, 1997; Ann Arbor, Michigan, 1999 and 2001. Starting with 2002, the Symposium is expected to be held annually, alternating in location between the USA and other countries within a two year period. Ninety-three papers were submitted to the 2002 symposium and reviewed by one member of the program committee and at least three additional referees. Based on these review results, the program co-chairs selected 26 papers for plenary presentation and 17 papers for poster presentation at the symposium. From the 26 plenary papers, the program co-chairs selected 9 papers for this special issue of JCISE. The selection was based on quality and engineering relevance. All selected papers were updated and extended since the version published in the conference proceedings, and were reviewed again by at least two additional referees and revised accordingly. The topics of the papers selected cover the following solid modeling areas: editing of free-form surfaces with haptic methods; reverse engineering of surface models; meshing of implicit surfaces; topological frameworks for part families; free-form surface design; design of transport elements in complex aircraft structures; collaborative framework for part and assembly modeling; modeling of heterogeneous solids; and iso-surface extraction methods. We wish to thank the authors for revising and extending their papers for this special issue of JCISE and the referees for their valuable comments, which improved the quality of the papers included. We hope that you the readers of JCISE will find these papers stimulating and interesting and that this will encourage you to participate in future solid modeling conferences and submit papers on this important area to these conferences and to regular issues of this ASME Transactions, JCISE, which also officially co-sponsored by ACM.

Localization of acoustic emissions in aerospace structures

Health Usage and Monitoring Systems (HUMS) offer the potential to revolutionize the civil aircraft industry’s approach to routine scheduled inspections. Developments in acoustic emission measurements allow the direct detection and localization of fatigue damage in complex primary aircraft structures. This information has been used to successfully guide inspections and to locate damage in regions inaccessible to conventional NDT methods. The use of such a system in flight will lead to inspection costs being reduced significantly and will promote confidence in the structural integrity of next generation civil aircraft. This paper will examine the question of the optimum positioning of sensors for a given sample using methods such as genetic algorithms and simulated annealing. The use of dimensional reduction projection techniques is also discussed. In an aerospace structure the propagation of sound will not be isotropic because of the variation in the thickness of the structure. This anisotropy will limit the accuracy of localization unless accounted for. The magnitude of this effect is determined empirically for an aircraft wing panel, and a method for calibrating acoustic emission measurements to minimize the effect of this anisotropy is suggested.

Robust control for a noncollocated spring-mass system

Robust control laws are presented for an undamped pair of coupled masses with a noncollocated sensor and actuator. This simple problem captures many of the features of more complex aircraft and space structure vibration control problems. The control problem is formulated in the structured singular value framework, which addresses the stability robustness to parameter variations directly. Controllers are designed by D-K iteration (commonly called /* synthesis), and the resulting high-order controllers are reduced using Hankel model reduction. Design specifications such as settling time, actuator constraints, insensitivity to measurement noise, and parameter uncertainty are achieved by the resulting controllers.

H-infinity synthesis using a bilinear pole shifting transform

An H-inifinity control law design is presented for a benchmark problem consisting of an undamped pair of spring-coupled masses with a sensor and actuator that are not collocated. This simple mechanical system captures many of the salient features of more complex aircraft and space structure vibration control problems. The H-infinity problem formulation enables the issue of stability robustness in the face of large mass and spring constant variation to be directly addressed. Constraints on closed-loop dominant pole locations and settling time are accommodated via a simple s-plane bilinear transform. The transform parameter can give direct control of closed-loop disturbance settling time and controller open-loop pole-zero locations. A four-state H-infinity minimum phase controller was found to meet the given specifications of each design. Detailed analysis on the tradeoffs of sensor noise vs control energy are also presented.