Albert C. To

Albert C. To
University of Pittsburgh | Pitt · Mechanical Engineering and Materials Science

Ph.D.

About

115
Publications
31,895
Reads
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2,880
Citations
Additional affiliations
September 2008 - present
University of Pittsburgh
December 2005 - June 2008
Northwestern University
August 1999 - December 2005
University of California, Berkeley

Publications

Publications (115)
Article
Full-text available
This paper presents an efficient and compact MATLAB code for three-dimensional stress-based sensitivity analysis. The 146 lines code includes the finite element analysis and p-norm stress sensitivity analysis based on the adjoint method. The 3D sensitivity analysis for p-norm global stress measure is derived and explained in detail accompanied by c...
Article
The current work proposes a finite element method (FEM) to accelerate scanwise thermal process simulation of the laser powder bed fusion (L-PBF) process with computational fluid dynamics (CFD) resolution near the melt pool. Termed the CFD imposed FEM (CIFEM), the transient thermal fields from a high-fidelity CFD simulation and inferred by deep lear...
Article
This work proposes to combine matrix-free finite element modeling (FEM), adaptive remeshing, and graphical processing unit (GPU) computing to enable, for the first time, scanwise process simulation of the Laser Powder Bed Fusion (L-PBF) process with temperature-dependent thermophysical properties at the part scale. Compared to the conventional FEM...
Article
This paper proposes a subtractive geometry projection topology optimization approach to simulate the multi-axis machining process and remove material from the initial design domain. Each geometric primitive is described by a plane extrusion operation based on a 2D closed cubic spline curve controlled by boundary points. The reference plane for the...
Article
Effective post-heat treatment is critical to achieve desired microstructure for high performance of additive manufacturing (AM) components. In this work, the timing influence of homogenization and its tuning mechanism on microstructure-property relationships of 17-4PH steels fabricated by laser powder bed fusion have been studied systematically. A...
Article
Sintering of binder jet 3D printed (BJ3DP) parts results in significant nonlinear distortion with typical shrinkage value of 5–20%, which makes design for BJ3DP and post-machining difficult. In this work, a computational modeling framework with calibration and validation procedure is developed to simulate distortion during sintering of BJ3DP parts...
Article
Reduction of the thermal distortion caused by the fabrication process of metal additive manufacturing is an important issue. Considering the problem of forming an object having lattices inside it to reduce the thermal distortion, we perform a lattice density distribution optimization and its experimental verification. In the optimization, assuming...
Article
Full-text available
Numerical simulations have recently shown their potential as a robust, cheap and reliable tool for predicting the quality of components produced by metal additive manufacturing (MAM) processes. Despite the advantages of the MAM processes over conventional manufacturing methods, there is still a lack of thorough understanding on how different defect...
Article
As a metal additive manufacturing (AM) process, laser powder bed fusion (L-PBF) has been widely used to produce parts with complex geometries. The large thermal gradient caused by the fast, intense, and repeated laser scanning induces significant residual deformation and stress to the as-built parts, which increase manufacturing difficulty and geom...
Article
Full-text available
This research proposes a novel topology optimization method using neural style transfer to simultaneously optimize both structural performance for a given loading condition and geometric similarity for a reference design. For the neural style transfer, the convolutional layers of a pre-trained neural network extract and quantify characteristic feat...
Preprint
This paper proposes a novel density-based method for structural design considering restrictions of multi-axis machining processes. A new mathematical formulation based on Heaviside function is presented to transform the design field into a geometry which can be manufactured by multi-axis machining process. The formulation is developed for 5-axis ma...
Article
Laser powder bed fusion (L-PBF) has emerged as one of the mainstream additive manufacturing approaches for fabricating metal parts with complex geometries and intricate internal structures. However, large deformation associated with rapid heating and cooling can lead to build failure and requires post-processing which may increase manufacturing cos...
Preprint
This paper presents an efficient and compact MATLAB code for three-dimensional stress-based sensitivity analysis. The 146 lines code includes the finite element analysis and p-norm stress sensitivity analysis based on the adjoint method. The 3D sensitivity analysis for p-norm global stress measure is derived and explained in detail accompanied by c...
Article
Full-text available
Phononic crystals (PnCs) have seen increasing popularity due to band gap property for sound wave propagation. As a natural bridge, topology optimization has been applied to the design of PnCs. However, thus far most of the existent works on topological design of PnCs have been focused on single micro-scale topology optimization of a periodical unit...
Article
Mechanical simulations are usually implemented via inherent strain method in a sequential layer-by-layer manner to predict residual deformation of large metal builds given the bottom-up fabrication nature of the laser powder bed fusion (L-PBF) process. However, it is very time-consuming since too many layers need to be simulated in a large componen...
Preprint
Full-text available
This paper proposes a new parametric level set method for topology optimization based on Deep Neural Network (DNN). In this method, the fully connected deep neural network is incorporated into the conventional level set methods to construct an effective approach for structural topology optimization. The implicit function of level set is described b...
Article
This paper proposes a density‐based boundary evolving algorithm for continuum‐based topology optimization. The boundary of voids in the design domain is described by RBF (radial basis function) function controlled by RBF knots in polar coordinate, where the voids are projected onto a fixed grid using Heaviside function. For merging overlapped multi...
Article
Full-text available
Epitaxial columnar grain growth is a prevalent microstructural feature in the additive manufacturing (AM) of metal components such as Inconel, with cubic unit cell crystal lattice structure (face centered cubic (FCC) or body centered cubic (BCC)). These columnar grains evolve from the partly molten grains in the substrate or the solidified metal. T...
Article
Residual stress induced cracking at the solid-lattice support interface is often observed in laser powder bed fusion (LPBF) additive manufactured metals. Therefore, it is crucial to predict possible cracking before printing a part especially when it is large and complex. Previously, a method has been proposed to predict interfacial cracking during...
Article
Laser powder bed fusion (L-PBF) has been the most popular metal additive manufacturing (AM) process thus far. However, residual deformation of the metal builds has been a significant issue. Laser scanning strategies adopted in the laser-assisted fabricating process have proved to have important influence on the residual stress and deformation. As t...
Article
In laser powder bed fusion (L-PBF) additive manufacturing, the mechanical performance, microstructure and defects of fabricated parts are closely associated with the melt pool morphology, e.g., its dimension and shape through the building process. Past studies have largely focused on how the process parameters such as laser power and scan speed aff...
Article
This paper presents an efficient GPU-based part-scale thermal process simulator for laser powder bed fusion (L-PBF) additive manufacturing (AM). To take full advantage of modern GPU computing, a matrix-free preconditioned conjugate gradient (PCG) finite element algorithm with voxel mesh is proposed to solve the transient heat transfer problem invol...
Article
Full-text available
Wire arc additive manufacturing (WAAM) is an energy-efficient manufacturing technique used for near-net-shape production of functional industrial components. However, heat accumulation during deposition and the associated mechanical and metallurgical changes result in complex residual stress profiles across the cross section of the fabricated compo...
Article
Advances in self-terminating etching processes have brought dissolvable supports to selective laser-melted stainless-steel alloys. Preliminary data showed that the amount of support material removed could be larger than the amount of material removed from the bulk material. This article details a small study aimed at understanding this phenomenon....
Preprint
This paper proposes a projection-based implicit modeling method (PIMM) for functionally graded lattice optimization, which does not require any homogenization techniques. In this method, a parametric projection function is proposed to link the implicit function of functionally graded lattice with the finite element background mesh. To reduce the nu...
Article
Full-text available
This paper proposed a new topology optimization method based on geometry deep learning. The density distribution in design domain is described by deep neural networks. Compared to traditional density-based method, using geometry deep learning method to describe the density distribution function can guarantee the smoothness of the boundary and effec...
Article
A new density field representation technique called the Bézier skeleton explicit density (BSED) representation scheme for topology optimization of stretchable metamaterials under finite deformation is proposed for the first time. The proposed approach overcomes a key deficiency in existing density-based optimization methods that typically yield des...
Article
This paper presents a process-microstructure finite element modeling framework for predicting the evolution of volumetric phase fractions and microhardness during laser directed energy deposition (DED) additive manufacturing of Ti6Al4V. Based on recent experimental observations, the present microstructure evolution model is formulated to combine th...
Article
A new topology optimization scheme called the projection‐based ground structure method (P‐GSM) is proposed for linear and nonlinear topology optimization designs. For linear design, compared to traditional GSM which are limited to designing slender members, the P‐GSM can effectively resolve this limitation and generate functionally graded lattice s...
Article
Full-text available
It is challenging to predict the residual deformation in the part-scale by performing detailed process simulation for the large part. In this work, the modified inherent strain theory is proposed to enable efficient yet accurate prediction of the residual deformation of large components produced by the Direct Metal Laser Sintering process. The prop...
Preprint
A new density field representation technique called the Bezier skeleton explicit density (BSED) representation scheme for topology optimization of stretchable metamaterials under finite deformation is proposed for the first time. The proposed approach overcomes a key deficiency in existing density-based optimization methods that typically yield des...
Article
Conventional topology optimization presentations generally highlight the numerical and optimization details established on the specially-customized discrete geometric modeling system, which is incompatible with the existing CAD/CAE systems. Therefore, tedious pre- and post-processing are required to improve the editability and manufacturabilitym, w...
Article
Full-text available
Highly stretchable material is widely used in the engineering field ranging from soft robots to stretchable electronics. Some highly stretchable 3D-architected mechanical metamaterials have been developed recently. However, failure of material is still the most critical design constraint when stretchability of the structure is considered, and exist...
Article
Full-text available
This paper presents a manufacturing cost constrained topology optimization algorithm considering the laser powder bed additive manufacturing process. Topology optimization for additive manufacturing was recently extensively studied, and many related topics have been addressed. However, metal additive manufacturing is an expensive process, and the h...
Conference Paper
Full-text available
One of the challenges of working with Additive Manufactured (AM) metal parts involves checking accuracy and reliability before production. Techniques used Computed Tomography (CT) scans, 3D image processing, and Finite Element (FE) simulation help detect problems prior to costly faults. A workflow has been developed by Synopsys, ANSYS, North Star I...
Article
Full-text available
Additive manufacturing (AM) eliminates many of the geometric restrictions in conventional manufacturing, and hence complex geometry, such as lattice structures, can be produced with little additional cost. AM designs based on lattice structuring have become increasingly popular as it possesses tunable properties and can be designed to be self-suppo...
Article
Full-text available
Manufacturing-oriented topology optimization has been extensively studied the past two decades, in particular for the conventional manufacturing methods, for example, machining and injection molding or casting. Both design and manufacturing engineers have benefited from these efforts because of the close-to-optimal and friendly-to-manufacture desig...
Article
Laser engineering net shaping (LENS) is one of the representative processes of directed energy deposition (DED) in which a moving heat source having high-intensity melts and fuses metal powders together to print parts. The complex and nonuniform thermal gradients during the laser heating and cooling cycles in the LENS process directly affect the mi...
Chapter
In recent years, carbon nanotube (CNT) has been one of the most prominent nanomaterial ever to be investigated due to its extraordinary mechanical, thermal, and electrical properties. Recently, the application of CNTs has been extended to CNT networks in which the CNTs are joined together in a two- or three-dimensional space. In this regard, CNT ne...
Article
The concurrent atomistic/continuum coupling method Multiresolution Molecular Mechanics (MMM) has been presented for statics Yang et al. (2013) and dynamics Biyikli et al. (2014), its convergence and error structure has been analyzed Yang et al. (2014), and a unified and consistent framework for general finite element shape functions has been introd...
Article
Full-text available
This paper presents the effects of lithium doping on the hydrogen storage capability of heat welded random carbon nanotube (CNT) network structures having different cross-link densities at room temperature. Cluster based and atom based doping strategies are taken into consideration in the current simulations. Moreover, different doping ratios are u...
Article
A typical feature of additive manufactured photopolymers is that their mechanical behavior depends on the printing direction. Hence, this work aims at developing a material model to predict the inelastic deformation and failure of glassy polymers with such an effect. To achieve this goal, a transversely isotropic hyperelastic-viscoplastic model is...
Article
Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced b...
Article
The point group symmetry of materials is closely related to their physical properties and quite important for material modeling. However, superlattice materials have more complex symmetry conditions than crystals due to their multilevel structural feature. Thus, a theoretical framework is proposed to characterize and determine the point group symme...
Article
Cellular structures are promising candidates for additive manufacturing (AM) due to their lower material and energy consumption. In this work, an efficient method is proposed for optimizing the topology of variable-density cellular structures to be fabricated by certain AM process. The method gains accuracy by relating the cellular structure's micr...
Article
Full-text available
The objective of this study is to investigate hydrogen storage capability of heat welded random carbon nanotube (CNT) network structures. To achieve this objective, different three-dimensional random CNT network structures are generated by using a stochastic algorithm and molecular dynamic simulations. The interaction of CNT networks with hydrogen...
Article
We present a general mathematical framework for the newly proposed energy-based concurrent atomistic/continuum method Multiresolution Molecular Mechanics (MMM) (Yang et al., 2013). The main features of the generalized framework are: (1) Consistency with the atomistic framework by directly employing the interatomic potential to calculate force and e...
Article
Full-text available
A new topology optimization method called the Proportional Topology Optimization (PTO) is presented. As a non-gradient method, PTO is simple to understand, easy to implement, and is also efficient and accurate at the same time. It is implemented into two MATLAB programs to solve the stress constrained and minimum compliance problems. Descriptions o...
Article
Direct heat welding of commonly found single-walled carbon nanotubes (SWCNTs) (i.e. diameters >1 nm) requires a temperature of >4000 K which is too high to be practical. This work reports a novel welding method of SWCNTs at experimentally-relevant temperature by means of nanobuds to facilitate C-C bonded junction formation between the nanotubes. Th...
Article
This study reports the investigation on C60 bombardment on the carbon nanotube (CNT) by molecular dynamic (MD) simulations. We found that carbon nanotube nanobuds or nanotube-fullerene hybrid nanostructures can be formed by C60 bombardment. Different from the nanobuds in earlier studies, more structural patterns are found in the bombardment formed...
Article
A new static concurrent atomistic/continuum coupling method under the Multiresolution Molecular Mechanics (MMM) framework has been presented in the first part of this paper (Yang et al., 2013 [28]). The present paper presents the dynamic coupling method under the MMM framework called the Multiresolution Molecular Dynamics (MMD) method. First, the d...
Article
Full-text available
The compressive response of nanoporous (np) Au with different porosities and ultrathin ligaments of widths 0.5-16 nm is investigated through molecular dynamics (MD) simulations. From the results of the uniaxial compressive loading, it was found that these materials behave in a ductile manner and possess characteristic high yield strength, suggestin...
Article
This work employs a multiscale modeling approach to probe the thermal conductivity in sintered randomly oriented carbon nanotube (CNT) networks, in which a fraction of close contacts between nanotubes has welded together to become CNT junctions. In the multiscale approach, a macroscopic analytical model is proposed to predict the overall thermal co...
Article
Non-equilibrium molecular dynamics (NEMD) simulations are employed to investigate the longitudinal thermal conductivity of non-orthogonal extended X-junction (EX-junction) of single-walled carbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, two distinct jumps in the temperature profile around the EX-junction are observed, which...
Article
Most composites exhibit a damping figure of merit, a crucial index of a material's dynamic behavior, lower than the value predicted by the Hashin-Shtrikman bound. This work found that the biomimetic hierarchical staggered composites inspired by bone structure can have a damping figure of merit tens of times higher than the Hashin-Shtrikman composit...
Article
A new normalization criterion has recently been proposed for constructing the localization function in Hardy's atomistic-to-continuum thermomechanical theory. The modification involves changing the normalization integral into a summation over the discrete volumes occupied by the atoms. The resulting thermomechanical quantities such as stress and he...
Article
Full-text available
This work proposes a modification to Hardy's atomistic-to-continuum thermomechanical theory, so that it can more accurately conserve mass, momentum, and energy for non-equilibrium thermomechanical processes. The modification proposed is a new normalization rule for the localization function employed in the theory. The improved accuracy of the modif...
Article
An ensemble averaging approach was investigated for its accuracy and convergence against time averaging in computing continuum quantities such as stress, heat flux and temperature from atomistic scale quantities. For this purpose, ensemble averaging and time averaging were applied to evaluate Hardy's thermomechanical expressions (Hardy 1982 J. Chem...
Article
Carbon nanotubes (CNTs) are one of the presents of nanotechnology being investigated due to their extraordinary mechanical, thermal and electrical properties. Carbon nanotube networks feed the idea that CNTs can be used as the building blocks of new advanced materials utilizing the superior characteristics of CNTs. In this way, nanoscale features o...
Article
This paper presents a new concurrent atomistic-continuum coupling method called the multiresolution molecular statics (MMS). By introducing a novel energy sampling framework, MMS aims at accurately and efficiently approximating the atomic energy of the system at different resolutions without the cumbersome interfacial treatment in existing methods....
Article
Broadband wave filtering effect is observed in a phononic crystal with hierarchical structure inspired by biocomposites. Through a multilayered model with hierarchical structure, it is demonstrated that the overall bandwidth covered by closely adjacent bandgaps for this structure is orders of magnitude broader in frequency than that of conventional...
Chapter
The many-realization approach is reviewed for determining the thermal and mechanical state of individual atoms in nickel crystal under shock impact with different shock velocities, modeled using nonequilibrium molecular dynamics simulations. It has been revealed that the atoms under shock impact can deviate from equilibrium and the duration of the...
Article
We report a fast computational method for generating atomistic models of complex-shaped carbon graphitic nanostructures from their computer-aided design (CAD) representation. The novelty of the method is the adaptation of a fast finite element (FE) triangular surface mesh generation algorithm for complex geometric surfaces.
Article
The formation and structural stability of defect-free non-orthogonal X-junctions, composed of ultrathin SWCNTs (UTCNTs) with varying angles, through heat treatment is investigated by molecular dynamics simulation. It is found that non-orthogonal X-junctions of UTCNTs are prone to transform into parallel junctions (P-junctions), and further coalesce...
Article
This paper presents a new concurrent atomistic-continuum method called the atom collocation method (ACM). By adopting the framework of continuum collocation method, ACM aims at overcoming the current difficulties in interfacial mismatch, adaptive analysis, and parallel implementation of existing atomistic-continuum methods. The proposed ACM is trul...
Article
Though X-junctions of single-walled carbon nanotubes (SWCNTs) have been intensively studied, studies concerning non-orthogonal X-junctions are still very rare. In this paper, the heat welding of defect-free non-orthogonal X-junctions with different crossed angles are investigated by molecular dynamics simulations. The difference between the heat we...
Article
The mechanical behavior of seven different carbon nanotube (CNT) X-junctions with a varying number of bonds was investigated through molecular dynamics simulations. The X-junctions are composed of two (6,0) single-walled carbon nanotubes (SWNTs) created via vibration-assisted heat welding. The junctions, containing anywhere between one and seven bo...