Multiphysics Modeling and Simulations of Mil A46100 Armor-Grade Martensitic Steel Gas Metal Arc Welding Process

A multiphysics computational model has been developed for the conventional Gas Metal Arc Welding (GMAW) joining process and used to analyze butt-welding of MIL A46100, a prototypical high-hardness armor martensitic steel. The model consists of five distinct modules, each covering a specific aspect of the GMAW process, i.e., (a) dynamics of welding-gun behavior; (b) heat transfer from the electric arc and mass transfer from the electrode to the weld; (c) development of thermal and mechanical fields during the GMAW process; (d) the associated evolution and spatial distribution of the material microstructure throughout the weld region; and (e) the final spatial distribution of the as-welded material properties. To make the newly developed GMAW process model applicable to MIL A46100, the basic physical-metallurgy concepts and principles for this material have to be investigated and properly accounted for/modeled. The newly developed GMAW process model enables establishment of the relationship between the GMAW process parameters (e.g., open circuit voltage, welding current, electrode diameter, electrode-tip/weld distance, filler-metal feed speed, and gun travel speed), workpiece material chemistry, and the spatial distribution of as-welded material microstructure and properties. The predictions of the present GMAW model pertaining to the spatial distribution of the material microstructure and properties within the MIL A46100 weld region are found to be consistent with general expectations and prior observations.     

Atomistic simulation of thermally activated glide of dislocations in Fe---Ni---Cr---N austenite

Thermally activated glide of edge and screw dislocations with the Burgers vector b = a/2110 in Fe---Ni---Cr austenite without and with nitrogen has been analyzed using the conjugate gradient method to minimize the potential energy of the crystal (subject to appropriate constraints) and the embedded-atom method to quantify the atomic interactions. The results of the analyses were compared with their experimental counterparts. In austenite without nitrogen, the variation of the crystal energy with the dislocation position gives rise to a Peierls frictional stress. However, the magnitude of this stress is relatively low and both edge and screw dislocations can readily overcome it by the assistance of thermal activation. The presence of nitrogen, on the other hand, causes the core of a screw dislocation to dissociate onto two {111} planes containing the dislocation line. This renders the screw dislocation sessile. Such changes in the core structure of an edge dislocation were not observed. Instead, the interaction of nitrogen with edge dislocations was found to give rise to both an athermal (long-range) and a thermal (short-range) component of the frictional stress. A detailed analysis of the interactions between the nitrogen atoms and the edge dislocations suggests that the athermal component of the frictional stress observed in the present work is most likely responsible for the experimentally observed athermal flow stress in Fe---Ni---Cr polycrystalline austenite. On the other hand, with a possible exception of very low temperature, the contribution of the thermal frictional stress associated with the interaction of edge dislocations with nitrogen to the strength of Fe---Ni---Cr polycrystalline austenite is small.     

Coarsening resistance of M2C carbides in secondary hardening steels: Part II. Alloy design aided by a thermochemical database

In high Co-Ni steels containing the strong carbide-forming elements Mo, Cr, and W, secondary hardening is accomplished by the precipitation of fine-scale M₂C alloy carbides. Thermodynamic stability and coarsening resistance of these carbides depend on the alloy content of these elements. A model for the M₂C coarsening kinetics in multicomponent alloys has been used to identify the optimum alloying addition for maximum coarsening resistance and as a basis for selection of four experimental alloy steels. Necessary information pertaining to the equilibrium in these steels was obtained using the Thermo-Calc software and database developed at the Royal Institute of Technology, Stockholm, Sweden.     

Ballistic-performance optimization of a hybrid carbon-nanotube/E-glass reinforced poly-vinyl-ester-epoxy-matrix composite armor

The material model for a multi-walled carbon nanotube (MWCNT) reinforced poly-vinyl-ester-epoxy matrix composite material (carbon nanotube reinforced composite mats, in the following) developed in our recent work (M. Grujicic et al. submitted), has been used in the present work within a transient non-linear dynamics analysis to carry out design optimization of a hybrid polymer-matrix composite armor for the ballistic performance with respect to the impact by a fragment simulating projectile (FSP). The armor is constructed from E-glass continuous-fiber poly-vinyl-ester-epoxy matrix composite laminas interlaced with the carbon nanotube reinforced composite mats. Different designs of the hybrid armor are obtained by varying the location and the thickness of the carbon nanotube reinforced composite mats. The results obtained indicate that at a fixed thickness of the armor, both the position and the thickness of the carbon nanotube reinforced composite mats affect the ballistic performance of the armor. Specifically, it is found that the best performance of the armor is obtained when thicker carbon nanotube reinforced composite mats are placed near the front armor face, the face which is struck by the projectile. The results obtained are rationalized using an analysis of the elastic wave reflection and transmission behavior at the lamina/met and laminate/air interfaces.     

Atomistic simulation of ∑3 (111) grain boundary fracture in tungsten containing various impurities

The effect of various impurities and micro-alloying additions (B, N, C, O, Al, Si, S and P) on the intrinsic resistance of the ∑3 (111) grain boundary in tungsten has been investigated using the molecular dynamics simulation. The atomic interactions have been accounted for through the use of Finnis-Sinclair interatomic potentials. The fracture resistance of the grain boundary has been characterized by computing, in each case, the ideal work of grain boundary separation, the mode I stress intensity factor and the Eshelby's F₁ conservation integral at the onset of crack propagation. The results obtained suggest that pure tungsten is relatively resistant to grain boundary decohesion and that this resistance is further enhanced by the presence of B, C and N. Elements such as O, Al and Si however, have a relatively minor effect on the cohesion strength of the ∑3 (111) grain boundary. In sharp contrast, S and P greatly reduce this strength making tungsten quite brittle. These findings have been correlated with the effect of the impurity atoms on material evolution at the crack tip.     

A computational analysis of the percolation threshold and the electrical conductivity of carbon nanotubes filled polymeric materials

Percolation of individual single walled carbon nanotubes (SWCNTs) and of SWCNT bundles dispersed in a non-interacting polymeric matrix has been analyzed computationally using an analytical model and a numerical simulation method. While the analytical model used is strictly valid only in the limit of an infinite length-to-diameter aspect ratio of the dispersed phase, good agreement is found between its predictions and the ones obtained using a computationally-intensive numerical method for the aspect ratios as small as 350. Since the aspect ratio of the individual SWCNTs is on the order of 1,000–10,000, this finding suggests that the analytical model can be used to study SWCNT percolation phenomena.An electrical network model is also applied to the percolating and near-percolating SWCNT clusters in order to compute the dc electrical conductivity of a CP2 polyimide + SWCNT composite material. A reasonably good agreement is obtained between the computational and the experimental results with respect to both the magnitude of the electrical conductivity and to its behavior in the vicinity of the percolation threshold.     

Factors Influencing the Self‐Efficacy Beliefs of First‐Year Engineering Students

A survey incorporating qualitative measures of student self‐efficacy beliefs was administered to 1,387 first‐year engineering students enrolled in ENGR 106, Engineering Problem‐Solving and Computer Tools, at Purdue University. The survey was designed to identify factors related to students' self‐efficacy beliefs, their beliefs about their capabilities to perform the tasks necessary to achieve a desired outcome. Open‐ended questions prompted students to list factors affecting their confidence in their ability to succeed in the course. Students were then asked to rank these factors based on the degree to which their self‐efficacy beliefs were influenced. Gender trends emerged in student responses to factors that affect confidence in success. These trends are discussed in light of the categories identified by efficacy theorists as sources of self‐efficacy beliefs. The results presented here provide a useful look at the first‐year engineering experiences that influence students' efficacy beliefs, an important consideration in explaining student achievement, persistence, and interest.     

Structural-Response Analysis,Fatigue-Life Prediction,and Material Selection for 1 MW Horizontal-Axis Wind-Turbine Blades

The problem of mechanical design, performance prediction (e.g., flap-wise/edge-wise bending stiffness, fatigue-controlled life, the extent of bending-to-torsion coupling), and material selection for a prototypical 1 MW horizontal-axis wind turbine (HAWT) blade is investigated using various computer-aided engineering tools. For example, a computer program was developed which can automatically generate both a geometrical model and a full finite-element input deck for a given single HAWT-blade with a given airfoil shape, size, and the type and position of the interior load-bearing longitudinal beam/shear-webs. In addition, composite-material laminate lay-up can be specified and varied in order to obtain a best combination of the blade aerodynamic efficiency and longevity. A simple procedure for HAWT-blade material selection is also developed which attempts to identify the optimal material candidates for a given set of functional requirements, longevity and low weight.