Research at the Laboratory of Technology and Strength of Materials

Citation

Pantelakis, P.S. (2011), "Research at the Laboratory of Technology and Strength of Materials", International Journal of Structural Integrity, Vol. 2 No. 2. https://doi.org/10.1108/ijsi.2011.43602bab.001

Research at the Laboratory of Technology and Strength of Materials

Article Type: Technical paper From: International Journal of Structural Integrity, Volume 2, Issue 2

Introduction

The Laboratory of Technology and Strength of Materials (LTSM) belongs to the Department of Mechanical Engineering and Aeronautics at the University of Patras. The mission of LTSM is to offer undergraduate and post-graduate education on the science and technology of materials, strength of materials and components of engineering structures, as well as to conduct fundamental and applied research on the above scientific fields.

Currently, there are 24 staff members at the LTSM, five of which are academic personnel, i.e. Professor Sp. Pantelakis (Director of LTSM), Emeritus Professor Th. Kermanidis (Director of LTSM from 1975 until 2007), Associate Professor G. Labeas, Assistant Professor Ch. Apostolopoulos and Lecturer K.I. Tserpes.

LTSM has established a national and international reputation in the research area of aeronautical materials and structures through participation in more than 80 research projects. Recent activities of LTSM extend to the following scientific areas:

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  science and technology of materials (development of new materials and their production technologies, materials characterization and certification, experimental characterization and simulation of the mechanical behavior of materials and structures);

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  strength of materials and structural components (determination of strength of metallic and composite materials, nano-materials and structural parts made of the pre-mentioned materials);

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  structural analysis (stress analysis, evaluation of structural integrity and impact strength, structural optimization);

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  design of aeronautical structures based on damage tolerance;

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  fracture mechanics; and

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  damage mechanics.

Research in the above areas is mainly conducted in the frame of European competitive research programs. Some of the recent programs, completed or in progress, are:

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  Development of Short Distance Welding Concepts for Airframes (WELAIR), EU FP6, 2003-2006.

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  Aeronautical Application of Wrought Magnesium (AEROMAG), CEC, STREP, 2005-2007.

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  Non-linear Multiscale Analysis of Large Aero-structures (MUSCA), CEC, STREP, 2005-2007.

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  Modular Joints of Aircraft Composite Structures (MOJO), EC, 2006-2009.

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  Cellular Structures for Impact Performance (CELPACT), EC, 2006-2009.

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  Cost-effective Integral Metallic Structures (COINS), EU FP6, 2006-2010.

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  More Affordable Aircraft through Extended, Integrated and Mature Numerical Sizing (MAAXIMUS), EU FP7, 2008-2013.

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  Simulation-based Solutions for Industrial Manufacture of Large Infusion Composite Parts (INFUCOMP), EU FP7, 2008-2013.

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  Cost-Effective Reinforcement of Fastener Areas in Composites (CERFAC), EU FP7, 2010-2014.

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  Extended Non-destructive Testing for Composite Bonds (ENCOMB), EU FP7, 2010-2014.

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  Innovative Manufacturing of Complicated Ti Components (INMA), EU FP7, 2010-2014.

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  Smart Aircraft in Emergency Situation (SMAES), EU FP7, 2010-2014.

Research results obtained at LTSM are published in peer review scientific journals and presented at international conferences. During the last decade, more than 180 publications were made by LTSM members, while in the last two decades, more than 20 doctoral theses have been completed.

Since 2002, LTSM is chairing the European Aeronautics Science Network (EASN). Subject of EASN is to support and upgrade research activities of the European Aeronautics Universities, as well as to facilitate them to respond to their key role within the European aeronautical research community in incubating new knowledge and breakthrough technologies. Members of this network are professors of the most significant universities across Europe.

Research areas

There are a number of research areas at the LTSM, some of them are as follows.

Mechanical behavior of materials

Plate 1 The in-house developed split Hopkinson bars device for performing high strain rate mechanical tests

Experimental characterization of the mechanical behavior of metallic and composite materials is one of the major research areas of LTSM. The testing capabilities in this sector comprise testing under quasi-static loads (tension, compression, bending, torsion, buckling, fracture toughness, etc.), fatigue testing, high-strain rate mechanical testing (split Hopkinson bars, Plate 1), non-destructive testing, metallographic characterization, high-temperature testing and corrosion testing of aeronautical alloys (Figure 1).

Figure 1 Effect of salt spray corrosion exposure on the tensile behavior of the rolled AZ31 magnesium alloy

Stress analysis of engineering structures

LTSM has accumulated significant experience in the stress analysis of engineering structures, and especially aeronautical structures, by means of analytical and numerical methods. Some of the activities in this area are:

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  analysis of wings, frames and parts of aircraft fuselage;

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  numerical analysis by means of finite element method (FEM), boundary element method and transfer-matrix method;

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  development of numerical tools for the buckling analysis, bearing-by-pass stress analysis and preliminary design;

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  non-linear analysis (geometry and materials) of large-scale aerostructures by means of adaptive progressive damage modeling (Figure 2); and

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  buckling of reinforced fuselage and wing panels.

Figure 2 Global FE model for the non-linear analysis of a generic shear joint

Impact and crashworthiness

Plate 3 Impact test in a cellular solid

The laboratory has the knowledge and the equipment for studying impact and crashworthiness of materials and structures under impact loads of different types. To this end, impact tests are carried out using an Dynatup drop tower (Plate 2), while impact FE simulations are conducted by means of the ANSYS, LS-DYNA and PAM-CRASH FE codes. The main activities in this area comprise impact in sandwich materials and cellular solids (Plate 3), low- and high-velocity impact (Figure 3) and debris impact in aircraft wings.

Plate 2 The Dynatup drop tower

Figure 3 FE simulation of high-velocity impact

Simulation of advanced manufacturing technologies

Simulations of the forming and joining processes of metallic and composite structural parts using numerical methods are conducted in LTSM. Specific activities involve laser welding of metals and polymers, laser forming (Figure 4) of metallic sheets and friction stir welding in metallic sheets. In-house numerical models have been developed for the optimization of these processes with regard to the residual thermal stresses and deformation of the components.

Figure 4 Prediction of deformation and buckling angle in metallic sheets due to laserforming

Maintenance and ageing aircraft

The structural integrity of a component represents the aggregation of the critical conditions that the component must fulfill in order to be capable to efficiently carry service loads. “Ageing aircraft” is a technical term revealing that the aircraft has reached or it is about to reach the service time for which it has been initially designed for. Ageing aircrafts are prone to widespread fatigue damage or multiple site damage. LTSM has developed an integrated FE code for the computation of multiple site damage evolution and the determination of inspection intervals for the maintenance of aircrafts. The code comprises the following two components:

1. 1.

   stress analysis of aircraft structures:

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     super-element methodology (Figure 5); and

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     analysis of corner cracks (Figure 6) and through-width cracks.

2. 2.

   prediction of crack initiation:

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     using the equivalent crack length concept; and

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     considering the stochastic nature of the phenomenon.

Figure 5 FE simulation of a joint using super elements

Figure 6 FE analysis of a corner crack

Optimization of composite materials

Analysis and optimization of small- and large-scale composite aerostructrures is an important research area of LTSM. Basic tool in this process is the FEM. In the laboratory, a progressive damage modeling method has been developed for predicting the mechanical performance and strength of composite parts by simulating progression of all damage types in composite materials. The method has been applied to both unidirectional composite laminates and textile composites. Some of the applications that have been made in this area are: strength prediction of composite bolted joints, optimization of composite bonded joints, analysis of the composite adhesively bonded flap-track beam of the Airbus A400M (Figure 7) and analysis and optimization of a composite multi-spar wing (Figure 8).

Figure 7 (a) The flap track of the Airbus A400M, (b) FE model of the flap-track beam

Figure 8 FE models of multi-spar wings with different numbers of reinforcing

Strength and mechanical performance of CNTs and nano-reinforced composites

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure consisting of pentagon rings. Since their discovery, CNTs have triggered the interest of scientific community due to their excellent physical properties. Owing to their extraordinary mechanical properties (stiffness around 1 TPa and strength around 120 GPa) and fiber-like structure, CNTs are contemplated as potential reinforcements in polymers and CFRP laminates. LTSM has started working on CNTs since 2005, mainly in the:

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  development of atomistic-based continuum models for the strength prediction and mechanical performance simulation of CNTs (Figure 9); and

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  development of multi-scale methods for the simulation of the mechanical performance of CNT-reinforced polymers (Figure 10).

Figure 9 Modeling of a CNT ass a space frame

Figure 10 Multi-scale analysis of CNT-reinforced polymer

Cost analysis of materials and processes

In LTSM, cost analysis of production, forming and joining process of composite parts is performed using the activity-based costing methodology (Figures 11 and 12). In this frame, the LTSM-OPT software tool which can be also used for metallic materials. To the date, the software tool has been applied to the following cases:

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  high-speed lay-up process;

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  diaphragm-forming process;

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  RTM low-pressure process;

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  roll-forming process for thermoplastics;

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  continuous manufacturing process of contoured profiles;

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  laser beam-welding process for thermoplastics; and

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  non-crimp fabrics.

Figure 11 Flowchart of the activity-based costing methodologys

Figure 12 Cost allocation to different sub-processes

For more information on the LTSM, please see the web site: www.mech.upatras.gr/∼ltsm

Professor Sp. PantelakisLaboratory of Technology and Strength of Materials, Department of Mechanical Engineering and Aeronautics, University of Patras, Patras, Greece