Francesco Marulo

Low-velocity impact behavior of fiber metal laminates

The low-velocity impact response of a range of fiber metal laminate (FML) panels was investigated through testing and finite element simulations. The objective of this study was to understand the impact-damage resistance of these novel composites, so that they can be designed optimally for impact-resistant aircraft applications. The FML panels were made up of aluminum alloy 7475 T761 and unidirectional S2 glass/epoxy oriented in a cross-ply configuration. Experimental tests were performed using a free-fall drop dart testing machine. The plate specimens were constrained on a circular edge by the clamping fixture. The shape and the nature of the damage inflicted by impact were evaluated using both destructive cross-sectional microphotography and nondestructive ultrasonic techniques. The tests showed that FML laminates are capable of absorbing energy through localized plastic deformation and through failure at the interface between the layers. In particular, delaminations occurred in the back face of the aluminum-alloy sheet and its adjacent fiber-reinforced epoxy layer and in between adjacent fiber-reinforced epoxy layer. The finite element code, LS-DYNA3D, was used to perform numerical simulations of low-velocity impact to predict the complex damage propagations. The computed post-impact deformed shapes and damage patterns were found to be fairly close to experimental results.

Design and testing of a fiber-metal-laminate bird-strike-resistant leading edge

One of the major concerns related to flight safety is the impact of birds. To minimize the risks, there is the need to improve impact resistance of aircraft by developing high-performance materials and better structural design of aircraft structures. Because of their remarkable impact properties, fiber metal laminates with layers of aluminum alloy and high-strength glass-fiber composite are potential candidate materials to be employed for aircraft structures susceptible to bird strikes. This paper describes an experimental and numerical campaign aimed at assessing the bird strike resistance of a fiber-metal-laminate-composite leading edge for the wing of a transport aircraft. Three different fiber-metal-laminate leading-edge structures were designed using advanced finite element simulations; they were manufactured and finally tested to analyze their impact-energy-absorbing capabilities. The finite element models were developed, adopting a Lagrangian approach in such a way to be able to correctly simulate impacts with large deformations and perforations of the structures and to characterize the different inelastic/brittle behaviors and failure modes of the fiber metal laminates. The numerical simulations were generally in good agreement with the experimental values, demonstrating the robustness of the developed finite element simulations in supporting the design of bird-strike-resistant aircraft structures.

Damage detection in wind turbine blades by using operational modal analysis

In this paper a vibration-based approach to identify a crack in a wind turbine blade is described and demonstrated numerically and experimentally. Operational modal analysis has been performed before and after a buckling test, and vibration data, gathered from some accelerometers placed along the blade, was used to monitor the integrity of the structure, since the modal parameters are directly influenced by the physical properties of the structures. Additionally a numerical prediction has been done both with a full-scale model and with a one-dimensional model. The results show that this approach is able to estimate successfully the presence of damage and a good numerical and experimental correlation has been found. Finally, some considerations regarding the rotation of the blade in the undamaged and damaged cases have been done.

Steel Structure Impacting onto Water: Coupled Finite Element-Smoothed-Particle-Hydrodynamics Numerical Modeling

Ditching in emergency conditions is an important issue for a safe transport plane. Because of the different distributions of loads on the structure, the solutions implemented to secure safety during an impact on the ground are often ineffective in the case of an impact on water. To improve the understanding of the impact dynamics between water and a metallic structure, the Italian Center of Aerospace Research started a preliminary experimental campaign aimed to test the behavior of a simple mockup structure during a water impact. The test specimen was instrumentedwith accelerometers and pressure sensors linked to a high-speed data acquisition system able to record the time histories to be used for different numerical simulations. Such numerical simulations have been carried out, with different software, both at the DLR, German Aerospace Center and at the Department of Aerospace Engineering at the University of Naples “Federico II”. The paper presents the results obtained by using the explicit finite element code LS-DYNA in numerically reproducing each experimental test to define the laws of the materials and correctly simulate the structural behavior under study. The fluid region was modeled using the smoothedparticle-hydrodynamics approach, obtaining a satisfactory numerical–experimental correlation.

Innovative Anti Crash Absorber for a Crashworthy Landing Gear

This paper defines an innovative concept to anti-crash absorber in composite material to be integrated on the landing gear as an energy-absorbing device in crash conditions to absorb the impact energy. A composite cylinder tube in carbon fiber material is installed coaxially to the shock absorber cylinder and, in an emergency landing gear condition, collapses in order to enhance the energy absorption performance of the landing system. This mechanism has been developed as an alternative solution to a high-pressure chamber installed on the Agusta A129 CBT helicopter, which can be considered dangerous when the helicopter operates in hard and/or crash landing. The characteristics of the anti-crash device are presented and the structural layout of a crashworthy landing gear adopting the developed additional energy absorbing stage is outlined. Experimental and numerical results relevant to the material characterization and the force peaks evaluation of the system development are reported. The anti-crash prototype was designed, analysed, optimized, made and finally the potential performances of a landing gear with the additional anti-crash absorber system are tested by drop test and then correlated with a similar test without the anti-crash system, showing that appreciable energy absorbing capabilities and efficiencies can be obtained in crash conditions.

Sensitivity Investigation of Aircraft Engine Noise to Operational Parameters

Aircraft noise is associated to two main groups of acoustic sources: those which produce airframe noise and the other ones which produce engine noise. Airframe noise can be numerically simulated once defined aircraft geometry and its intensity depends on aerodynamic configuration of aircraft. Engine noise is due to contribution of several sources according to engine’s type: in several cases these sources are not all well theoretically defined for the difficulty of performing experimental campaigns, and this does not allow a general validation of numerical simulation tools. Sound pressure levels associated to fans, compressors, jets, propellers can be numerically evaluated through commercial codes that require a big amount of input parameters concerning the geometric description of the different engine parts and the engine operational conditions that are well known from a physical standpoint but can be difficult to manage in a preliminary assessment of the noise emission due to a typical civil aircraft engine. The numerical definition of all these parameters requires a deep knowledge of the mechanism of the engine part subject of investigation and, often, also having a clear view of how this part works, the values of operational parameters, during the aircraft engine working, can be not available. For this reason an investigation of the sensitivity of the overall sound pressure levels associated to the different noise sources to the engine operational parameters can be useful to understand how much the noise can change if the working conditions change and to rank the variables from an acoustic standpoint fixing the most influential parameters. The present paper focuses on some of the acoustic sources acting on a typical civil aircraft engine and is based on the use of commercial codes able to simulate numerically the sound pressure levels associated to these sources.

Influence of experimental testing set-up and geometric parameters on damping measurements

The paper presents the activities performed by the authors in order to develop and validate an experimental set-up for measurements of damping characteristics of typical materials employed within aeronautical and industrial field for passive vibrations reduction. These activities have been carried out within the research program funded by the European Commission named “F.A.C.E.” (Friendly Aircraft Cabin Environment). The set-up has been designed to operate through a PC-based acquisition system developed in LABVIEW programming environment. The development of the experimental damping measurement set-up is based on the principle of the “Oberst beam”, and it has been improved to allow the implementation of different approaches with “contacting” sensors and actuators. The influence of the damping on the stability and reliability of the results will be investigated, by evaluating the effect of the beam thickness to the applied damping thickness ratio. The results will be presented for some damping treatments like “constrained layers” as far as for rubbery materials commonly employed within the aeronautical field. These activities are aimed to the implementation of a better damping modelisation of a typical finite element model of light structures.

Mixed FE–MB methodology for the evaluation of passive safety performances of aeronautical seats

ABSTRACT The certification of aircraft seats involves the investigation of the structural performance of their components under emergency landing conditions. . The paper reports the activities related to an experimental sled-test of a single row of two seats placed in front of a stiff fuselage bulkhead, by considering a single anthropomorphic dummy arranged on one of the seats. Tests have been developed at the facility equipped with a sled decelerator testing system compliant with certification requirements from FAR25 for TSO C127a regulations. Four different numerical models have been developed in order to simulate the experimental test: a full-FE modelby LSTC-LsDyna® code, a full-MB modeland a hybrid FE/MB developed using by TNO-Madymo® code and a Coupled FE/MB model. Numerical results achieved by these models have been compared with experimental ones, in order to assess their use for CBA (certification by analysis) purposeto be applied since the preliminary design phase of the seat.

Experimental Tests Analysis of Fiber Metal Laminate under Birdstrike.

The work is based on the experimental activities developed to investigate the behavior of fiber metal laminate (FML) materials used for very critical structural components, such as leading edge panels that must satisfy a series of requirements defined in terms of structural resistance, applying impact loads at different speeds, arising, for example, from birdstrike on wing leading edge. The experimental tests have been carried out on some panels manufactured by Alenia Aeronautica in the context of a national project, that had the aim to identify the material that defines the best solution in terms of weight and performance to realize a leading edge that was able to be resistant at the birdstrike requirements. The first part of work presents an extensive campaign of tests performed to determine stiffness and strength properties on FML (trade name GLARE). The experiments considered the static tests useful to determine the stress-strain curve; the dynamic tests at different strain rates, and different impact tests to evaluate the threshold energy that correspond to a visible impact damage on the coupon. Subsequently, several tests were performed on two different types of joints that were mechanically tested to determine strength properties at a different strain rate. In the second part of this work, the experimental tests have been reproduced numerically with the aim of validating models able to correctly simulate the phenomenon in question, obtaining a satisfactory numerical-experimental correlation. In the present work the guidelines for the future development of similar numerical models have been traced and advantages and disadvantages of birdstrike modeling have been highlighted and numerical activities allowed to define the parameters of the material and to improve the design rules for a leading edge subjected to the birdstrike.

Bird strike assessment for a composite wing flap

ABSTRACT The paper presents the design, analysis and test of the CFRP flap to demonstrate its compliance with the regulation and safety standards toward the bird strike requirement. The purpose of this work is to define a methodology based on the finite element analysis to predict the damage for high-velocity impact of a full-scale aircraft component using the SPH bird impact model, 4 lb, at a velocity of 194 kts (100 m/s). In order to validate the numerical models, materials were initially characterised through an extensive campaign of tests aimed to identify their static and dynamic properties. The bird strike simulation was carried out, first on a rigid plate to correlate the experimental test performed by Barber, and then the results were extended for a selected composite panel to investigate the particles distance for the SPH approach and pressure path distributed on the target, and finally these validated results were applied on the flap structure. The numerical simulations have been used from the initial designing phase as well as for the certification according to the aeronautical requirements, to be able to design a flap structure and the relative fixture reproducing the real test condition. Finally, the flap has been manufactured, tested, certified and installed on the large commercial aircraft.