Nanowelding and Multiscale Modeling and Simulation

AIAA 2008-1948

49th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference
16t 7 - 10 April 2008, Schaumburg, IL

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. Introduction Composite materials are used increasingly in engineering applications because of their high specific strength. A more effective and economical design could be achieved when composites are used in combination with metal structures. A space vehicle, for example, must be manufactured of a material that has adequate fracture allowables to resist cracking when subjected to a cyclic load environment, and be light enough to reduce payload. Composite-metal structures are susceptible to structural failures due to metal fatigue and viscoelastic creep of the composite, temperature effects due to the coefficient of thermal expansion mismatch, space radiation, and moisture absorption causing differential strain between the metal and composite. Fig. 1 shows the failure of a pressurized tank during the proof test operation due to poor bonds between the aluminum metallic flange and the composite over-wrapped filaments. Other similar failures have been reported in the literature where in most cases the failure initiated in the jointed area at the metal to composite interface. Therefore, strong bonds between the composite material and the metal are needed in order to avoid structural failure and more importantly loss of life. The proposed concept is an attempt to create interface bonds between metals and composites very much similar to the conventional welding: that is, introducing nanofilament material in the composite matrix (resin) that has the same atomic structure as the metal under consideration. A multiscale modeling and simulation technique, based on molecular dynamics and a limited amount of quantum mechanics, has been proposed that will assess the sensitivity and integrity of bonds between composites and metals, which are the source of sudden failure as the result of combined in-plane and out-of-plane dynamic and cyclic load. The strength of using the multiscale modeling and simulation technique is its predictable power to estimate the material properties under investigation. This technique will look at the composite-metal interface bonds from molecular dynamics and quantum mechanics by assuming two extreme cases where interface bonds between composite-metal are with and without functionalization (strong and weak bonds) applied to the polymer and nanofilaments. The intent is to create strong bonds between the metallic nanofilaments and resin through the functionalization process to withstand stresses when a load is applied

1

Boeing Technical Fellow, Structural Analysis, H013-C326, Technical Member of the AIAA Adaptive Structure Committee 2 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 1 American Institute of Aeronautics and Astronautics 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 proof test operation to the interface (Fig 2). The multiscale modeling and simulation technique, with a limited amount of experimentation, can be useful to fully assess the bonded region. While post-deformation experiments of the interface configuration provide the information necessary to analyze the fracture throughout the region of interest, it provides little insight into the actual mechanism by which the interface boundaries fail. Using in-situ TEM testing technique, not only the deformation process in real-time can be observed, but also the force-displacement behavior at the interface of polymers and nanofilaments can be quantified. Thus, it is possible to measure the initiation of nanocracks and their local propagation as a function of cycles in the nano/micro scales. This data will provide a validation tools for the molecular dynamics model. The in-situ TEM should be able to characterize the crack growth up to the scale of nano/micron. For larger cracks, insitu 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 on the application, the cost savings associated with the weight reduction of structural parts is estimated to range from $100/lb to $10,000/lb. Aerospace and other industries have made significant investments in Nanomaterials in an attempt to modify proven material systems with the intent to have superior mechanical-/thermal/electrical properties. Successes with composite materials have been seen throughout the aircraft and aerospace industry. In many occasions it is necessary to join composite materials with metallic parts. Adequate strength between composite and metallic material interface must be establish in order to assure the structural safety. The conventional trail-and-error approach is not working and is costly and time-consuming. The proposed method will reinforce metallic nanofilaments into the polymer such that, upon the application of high frequency low amplitude vibrations, strong interface bonds are established between metal and composite, as depicted in Figure 2. However, two challenging areas that must be addressed are: 1) The bond strength between nanofilaments in the polymer and the metallic material (welding process via high frequency application) and, 2) As equally important, enough strength between nanofilaments and polymer resin. The latter can be accomplished through an appropriate functionalization process. These important issues must be assessed through the multiscale modeling and simulation technique by using the molecular dynamics approach with emphasis on the coarse grain technique.

2 American Institute of Aeronautics and Astronautics

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

III. Approach The proposed study is geared to the question of how to conduct the multiscale simulation of the welded interface analysis viable. The technique of reducing the complexity of the modeling simulation is called the coarse-graining method [1-10]. There are various so-called coarse-graining methods which have been developed 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 a mesoscale simulation program and will be incorporated into this analysis. The DPD takes account of the interactions of the particles via a "dissipation force." This force is both velocity (of particles) and distance dependant. 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 are modeled such that they can be identified with one bead per several atoms at the interface region and steps necessary to link nano-micro-meso-macro. The main idea of coarse-grained models is to represent essential parts of the system with a larger-scale representative computational model. One very successful strategy is to reduce the number of particles as in the 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 polymer containing nanofilaments, groups of monomers, or chain segments of various lengths. The Accelrys Material studio [11] is powerful software that can be used to properly implement the coarse grain technique and moreover, it has the capability of creating the correct potential energy that can address the interaction of atoms at the interface region for the molecular dynamics analysis. Several mesoscale simulations will be conducted using the material studio via the DPD program over long length and time scales. These analysis will provide useful information regarding the bonds integrity at the interface and key parameters that can have global effects on the strength between metals and composite region. The coarse-grained technique minimizes computational time by eliminating the unnecessary details of molecular calculations. This speeds up calculations three to four orders of magnitude, allowing one to approach problems that were previously beyond the range of molecular dynamics. For example, a fully atomistic simulation, considering all the internal degrees of freedom of the molecules, will require more than a month to perform a 200-picoseconds simulation of 2500 atoms in Accelrys/Cerius2 [11] with an SGI R10000 microprocessor.


Figure 3: The coarse grain technique for mesoscale simulation of interface region between metal and composite To validate the computational approach, a set of explanatory experiments will be conducted. A few samples that represent the interface structure between nanofilaments-polymer resin and nanofilaments-metallic substrate will be prepared for the in-situ TEM investigations. The samples will be subjected to cyclic and monotonic loading, and the failure event of atomic bonding in the interface structures will be recorded in several still frame images. The load-displacement data are also recorded simultaneously and a one-to-one correlation is made with observed fracture in the nanofilaments-polymer resin and nanofilaments-metallic substrate. The collected load-displacement data will be used to validate the proper potential energy defining 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 collected from the proposed multiscale modeling approach. The interfacial Young’s modulus of the composite was determined 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 covalently bonded to the silane/Al2O3 at 16 locations on the surface over a surface area of 13.9 nm2. For the nonfunctionalized interface, only van der Waals forces bonded the polymer to the silane/Al2O3. Three different molecular RVEs were determined for each of the two composite systems, and the resulting values of Young’s modulus were averaged. The ratio of the average interfacial Young’s modulus of the nonfunctionalized to functionalized systems was 0.47. In other words, the Young’s modulus of the functionalized 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 with increasing levels of the functionalization. The proposed research will continue to look at the influence of various levels of functionalization on the stiffness and strength of the polymer/metal oxide interface on the molecular level. Also, as described, multiscale techniques will be used to predict the influence of these results on the larger-scale nanowelded interface.

V. Summary The benefits of this analytical technique will have a direct effect on the construction of future space vehicles and will improve the method of joint repair for existing vehicles. Metallic Nanofilaments and/or nanosize metallic foams will be introduced into the resin of a composite material. Metallic bonds would be created between the metallic nanofilaments introduced into the resin and a metal strip placed on the surface of the composite panel. The static strength level of the bond should be acceptable for the desired application. Acceptable fatigue strength for the desired application should be achieved. The method should allow for a NDE inspection that offers a significant defect detectability level prior and during structural service usage. The multiscale modeling and simulation using the coarse grain technique can link the atomistic-micro-meso-macro length scale via the Accelrys software. However, proper potential energy defining the interface atomic interactions must first be established. 4 American Institute of Aeronautics and Astronautics

Figure 4. Schematic of multiscale modeling approach

Figure 5. Mechanical deformation of interface in the molecular model

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