nanowelding and multiscale modeling and simulation

American Institute of Aeronautics and Astronautics1

AIAA-2008-xxxx

Nanowelding And Multiscale Modeling & Simulation

Bahram Farahmand1

Boeing Aerospace-IDS, 5301 Bolsa Ave, Huntington Beach, California 92647

Reza Shahbazian Yassar 2

Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931

and

G.M. Odegard3

Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931

I. IntroductionComposite materials are used increasingly in engineering applications because of their high specificstrength. A more effective and economical design could be achieved when composites are used incombination with metal structures. A space vehicle, for example, must be manufactured of a material thathas adequate fracture allowables to resist cracking when subjected to a cyclic load environment, and belight enough to reduce payload. Composite-metal structures are susceptible to structural failures due tometal fatigue and viscoelastic creep of the composite, temperature effects due to the coefficient of thermalexpansion mismatch, space radiation, and moisture absorption causing differential strain between the metaland composite. Fig. 1 shows the failure of a pressurized tank during the proof test operation due to poorbonds between the aluminum metallic flange and the composite over-wrapped filaments. Other similarfailures have been reported in the literature where in most cases the failure initiated in the jointed area atthe metal to composite interface. Therefore, strong bonds between the composite material and the metal areneeded in order to avoid structural failure and more importantly loss of life. The proposed concept is anattempt to create interface bonds between metals and composites very much similar to the conventionalwelding: that is, introducing nanofilament material in the composite matrix (resin) that has the same atomicstructure as the metal under consideration. A multiscale modeling and simulation technique, based onmolecular dynamics and a limited amount of quantum mechanics, has been proposed that will assess thesensitivity and integrity of bonds between composites and metals, which are the source of sudden failure asthe result of combined in-plane and out-of-plane dynamic and cyclic load. The strength of using themultiscale modeling and simulation technique is its predictable power to estimate the material propertiesunder investigation. This technique will look at the composite-metal interface bonds from moleculardynamics and quantum mechanics by assuming two extreme cases where interface bonds betweencomposite-metal are with and without functionalization (strong and weak bonds) applied to the polymerand nanofilaments. The intent is to create strong bonds between the metallic nanofilaments and resinthrough the functionalization process to withstand stresses when a load is applied

1 Boeing Technical Fellow, Structural Analysis, H013-C326, Technical Member of the AIAA AdaptiveStructure Committee2 Insert Job Title, Department Name, Address/Mail Stop, and AIAA Member Grade for third author.3 Assistant Professor, Department of Mechanical Engineering – Engineering Mechanics, Senior Member

49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference <br> 16t7 - 10 April 2008, Schaumburg, IL

AIAA 2008-1948

Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


Figure 1. Failure at the interface between metallic flange and composite over-wrapped during prooftest operation

to the interface (Fig 2). The multiscale modeling and simulation technique, with a limited amount ofexperimentation, can be useful to fully assess the bonded region. While post-deformation experiments ofthe interface configuration provide the information necessary to analyze the fracture throughout the regionof interest, it provides little insight into the actual mechanism by which the interface boundaries fail. Usingin-situ TEM testing technique, not only the deformation process in real-time can be observed, but also theforce-displacement behavior at the interface of polymers and nanofilaments can be quantified. Thus, it ispossible to measure the initiation of nanocracks and their local propagation as a function of cycles in thenano/micro scales. This data will provide a validation tools for the molecular dynamics model. The in-situTEM should be able to characterize the crack growth up to the scale of nano/micron. For larger cracks, in-situ SEM testing can be used to quantify the growth up to micron/mm.

II. Background

Weight has been and always will be the driver for aircraft and aerospace structure designs. Depending onthe application, the cost savings associated with the weight reduction of structural parts is estimated torange from $100/lb to $10,000/lb. Aerospace and other industries have made significant investments inNanomaterials in an attempt to modify proven material systems with the intent to have superiormechanical-/thermal/electrical properties. Successes with composite materials have been seen throughoutthe aircraft and aerospace industry. In many occasions it is necessary to join composite materials withmetallic parts. Adequate strength between composite and metallic material interface must be establish inorder to assure the structural safety. The conventional trail-and-error approach is not working and is costlyand time-consuming. The proposed method will reinforce metallic nanofilaments into the polymer suchthat, upon the application of high frequency low amplitude vibrations, strong interface bonds areestablished between metal and composite, as depicted in Figure 2. However, two challenging areas thatmust be addressed are: 1) The bond strength between nanofilaments in the polymer and the metallicmaterial (welding process via high frequency application) and, 2) As equally important, enough strengthbetween nanofilaments and polymer resin. The latter can be accomplished through an appropriatefunctionalization process. These important issues must be assessed through the multiscale modeling andsimulation technique by using the molecular dynamics approach with emphasis on the coarse graintechnique.


Figure 2: Nanowelding concept with nanofilament at the interface of composite and metal

III. ApproachThe proposed study is geared to the question of how to conduct the multiscale simulation of the weldedinterface analysis viable. The technique of reducing the complexity of the modeling simulation is calledthe coarse-graining method [1-10]. There are various so-called coarse-graining methods which have beendeveloped to enhance Nanocomposite simulations. On a larger length scale, dissipative particle dynamics(DPD) and smoothed particle dynamics (SPD) are frequently used [7, 8]. The DPD technique is amesoscale simulation program and will be incorporated into this analysis. The DPD takes account of theinteractions of the particles via a "dissipation force." This force is both velocity (of particles) and distancedependant. This has been shown to reproduce better non-equilibrium behavior of the system (of particles).The scaling sequence of the coarse-graining technique is shown in Fig. 3 where the nanofilaments aremodeled such that they can be identified with one bead per several atoms at the interface region and stepsnecessary to link nano-micro-meso-macro.

The main idea of coarse-grained models is to represent essential parts of the system with a larger-scalerepresentative computational model. One very successful strategy is to reduce the number of particles as inthe bead-and-spring model in the interface region in which a group of atoms is replaced by one unit, Fig. 3.The unit can represent a chemical group of a few atoms, an entire molecule or a monomer unit in a polymercontaining nanofilaments, groups of monomers, or chain segments of various lengths. The AccelrysMaterial studio [11] is powerful software that can be used to properly implement the coarse grain techniqueand moreover, it has the capability of creating the correct potential energy that can address the interactionof atoms at the interface region for the molecular dynamics analysis. Several mesoscale simulations will beconducted using the material studio via the DPD program over long length and time scales. These analysiswill provide useful information regarding the bonds integrity at the interface and key parameters that canhave global effects on the strength between metals and composite region.

The coarse-grained technique minimizes computational time by eliminating the unnecessary details ofmolecular calculations. This speeds up calculations three to four orders of magnitude, allowing one toapproach problems that were previously beyond the range of molecular dynamics. For example, a fullyatomistic simulation, considering all the internal degrees of freedom of the molecules, will require morethan a month to perform a 200-picoseconds simulation of 2500 atoms in Accelrys/Cerius2 [11] with an SGIR10000 microprocessor.


Figure 3: The coarse grain technique for mesoscale simulation of interface region between metal andcomposite

To validate the computational approach, a set of explanatory experiments will be conducted. A few samplesthat represent the interface structure between nanofilaments-polymer resin and nanofilaments-metallicsubstrate will be prepared for the in-situ TEM investigations. The samples will be subjected to cyclic andmonotonic loading, and the failure event of atomic bonding in the interface structures will be recorded inseveral still frame images. The load-displacement data are also recorded simultaneously and a one-to-onecorrelation is made with observed fracture in the nanofilaments-polymer resin and nanofilaments-metallicsubstrate. The collected load-displacement data will be used to validate the proper potential energydefining the interface atomic interactions used in the atomistic simulations.

IV. Analysis Results

Preliminary data on the molecular modeling of the alumina/polymer interface has been collectedfrom the proposed multiscale modeling approach. The interfacial Young’s modulus of the composite wasdetermined for both functionalized and non-functionalized composite systems with 6100 and 5524 atoms,respectively (Figures 4 and 5). For the functionalized interface, the polymer molecules were covalentlybonded to the silane/Al2O3 at 16 locations on the surface over a surface area of 13.9 nm2. For the non-functionalized interface, only van der Waals forces bonded the polymer to the silane/Al2O3. Three differentmolecular RVEs were determined for each of the two composite systems, and the resulting values ofYoung’s modulus were averaged. The ratio of the average interfacial Young’s modulus of the non-functionalized to functionalized systems was 0.47. In other words, the Young’s modulus of thefunctionalized interfacial region was nearly 2 times higher than that of the nonfunctionalized interface.Therefore, this shows a preliminary trend of the stiffening of the polymer/metal oxide interface withincreasing levels of the functionalization. The proposed research will continue to look at the influence ofvarious levels of functionalization on the stiffness and strength of the polymer/metal oxide interface on themolecular level. Also, as described, multiscale techniques will be used to predict the influence of theseresults on the larger-scale nanowelded interface.

V. Summary

The benefits of this analytical technique will have a direct effect on the construction of future spacevehicles and will improve the method of joint repair for existing vehicles. Metallic Nanofilaments and/ornanosize metallic foams will be introduced into the resin of a composite material. Metallic bonds would becreated between the metallic nanofilaments introduced into the resin and a metal strip placed on the surfaceof the composite panel. The static strength level of the bond should be acceptable for the desiredapplication. Acceptable fatigue strength for the desired application should be achieved. The method shouldallow for a NDE inspection that offers a significant defect detectability level prior and during structuralservice usage. The multiscale modeling and simulation using the coarse grain technique can link theatomistic-micro-meso-macro length scale via the Accelrys software. However, proper potential energydefining the interface atomic interactions must first be established.


Figure 4. Schematic of multiscale modeling approach

Figure 5. Mechanical deformation of interface in the molecular model

References

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2. B. Forrest and U. W. Suter, J. Chem. Phys. 102, 7256 (1995).3. I. Carmesin and K. Kremer, Macromolecules 21, 2819 (1988).


4. W. Paul, K. Binder, K. Kremer, and D. Heermann, Macromolecules 24, 6332 (1991).5. J. Baschnagel, K. Binder, W. Paul, M. Laso, U. W. Suter, I. Batoulis, W. Jilge and T. B¨urger, J.

Chem. Phys. 95, 6014 (1991).6. P. Doruker and W. Mattice, Macromolecules 30, 5520 (1997).7. P. Doruker and W. Mattice, J. Phys. Chem. B. 103, 178 (1999).8. R. Groot and P. Warren, J. Chem. Phys. 107, 4423 (1997).9. P. Espanol, M. Serrano, and I. Zuniga, J. Mod. Phys. C 8, 899 (1997).10. R. D. Groot and K. L. Rabone, Biophys. J. 81, 725 (2001).11. Accelrys, Materials Studio Release Notes, Release 4.1, Accelrys Software, Inc.: San Diego, 2006

nanowelding and multiscale modeling and simulation

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