Jun Li

Jun Li
University of Massachusetts Dartmouth | UMD · Department of Mechanical Engineering

PhD

About

45
Publications
13,074
Reads
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688
Citations
Education
August 2007 - August 2012
University of Illinois, Urbana-Champaign
Field of study
  • Mechanical Engineering

Publications

Publications (45)
Article
Full-text available
This paper builds on the recently begun extension of continuum thermomechanics to fractal porous media that are specified by a mass (or spatial) fractal dimension D, a surface fractal dimension d and a resolution length scale R. The focus is on pre-fractal media (i.e. those with lower and upper cut-offs) through a theory based on a dimensional regu...
Article
Full-text available
Elastic–plastic transitions were investigated in three-dimensional (3D) macroscopically homogeneous materials, with microscale randomness in constitutive properties, subjected to monotonically increasing, macroscopically uniform loadings. The materials are cubic-shaped domains (of up to 100 × 100 × 100 grains), each grain being cubic-shaped, homoge...
Article
Full-text available
A theoretical experimentally based multi-scale model of the elastic response of cortical bone is presented. It portrays the hierarchical structure of bone as a composite with interpenetrating biopolymers (collagen and non-collagenous proteins) and minerals (hydroxyapatite), together with void spaces (porosity). The model involves a bottom-up approa...
Article
Full-text available
This paper presents a constitutive thermoviscoelastic model for thin films of linear low-density polyethylene subject to strains up to yielding. The model is based on the free volume theory of nonlinear thermoviscoelasticity, extended to orthotropic membranes. An ingredient of the present approach is that the experimentally inaccessible out-of-plan...
Article
A combination of computational and experimental investigation is performed to study additively manufactured (AM) polymers for enhanced fracture properties. Single edge notch tension specimens made of acrylonitrile-butadienestyrene (ABS) materials through fused deposition modeling with various build/raster orientations are studied, namely, horizonta...
Presentation
Advanced Manufacturing - is sponsored by the Manufacturing Engineering Division of the ASME. The Track contains a collection of Topics in the manufacturing ranging from nanomanufacturing, to fastening and joining, to material removal and forming, as well as additive manufacturing. The topics are organized by leading researchers in the field. The To...
Article
Motivated by the abundance of fractals in the natural world, this paper further develops continuum-type models for product-like fractals. The theory is based on a version of the non-integer dimensional space approach, in which global balance laws written for fractal media are expressed in terms of conventional (integer-order) integrals. Key relatio...
Article
Full-text available
The underlying complicated spatiotemporal thermo-mechanical processes in additive manufacturing (AM) technology pose challenges in predicting and optimizing the as-built part quality for production use. Physics-based simulations are being developed to provide reliable predictions such as part distortions, residual stresses/strains, microstructure c...
Presentation
Additive manufacturing (AM), or 3D printing, has been transiting from demonstrative prototypes to functional products that are impacting a wide variety of sectors, from biomedical, electronic, and automotive to aerospace industries. However, the high thermal gradients, non-negligible part distortions, and residual stresses could adversely affect th...
Article
This article advances continuum-type mechanics of porous media having a generally anisotropic, product-like fractal geometry. Relying on a fractal derivative, the approach leads to global balance laws in terms of fractal integrals based on product measures and, then, converting them to integer-order integrals in conventional (Euclidean) space. Prop...
Presentation
Additively manufactured functional parts with high precision are broadly adopted by the multi-layer manufacturing strategy. The spatially different thermal cycles result in residual stresses within the additively manufactured which leads to the part distortion that could impact product precision and properties. The inherited multi-physics system ca...
Article
Full-text available
The mechanical property of 3D-printed components often exhibits anisotropic behaviors and a strong dependence on printing orientations and process parameters. In this study, computational models based on microstructures of 3D-printed ABS polymers are developed using micromechanics of a representative volume element (RVE) to investigate the orthotro...
Article
Full-text available
The mechanical properties and fracturing mechanism of shale containing beddings are critically important in shale gas exploitation and wellbore stability. To investigate the effects of shale bedding on crack behavior and fracturing mechanism, scanning electron microscope (SEM) with a loading system was employed to carry out three-point bending test...
Article
Full-text available
This paper proposes a novel data-driven approach for predicting and optimizing the additive manufacturing process parameters. The integrated scheme consists of three popular algorithms: (1) group method for data handling (GMDH) as the engine of neural networks, (2) autoregressive integrated moving average (ARIMA) for characterizing spatial collinea...
Article
Full-text available
The use of robust multiresponse constrained optimization techniques in which multiple-objective responses are involved is becoming a crucial part in additive manufacturing (AM) processes. Common and popular techniques, in most cases, rely on the assumption of independent responses. In practice, however, many of the desired quality characteristics c...
Article
The fracture of additively manufactured polymer materials with various layer orientations is studied using the extended finite element method (XFEM) in an anisotropic cohesive zone model (CZM). The single edge notched bending (SENB) specimens made of acrylonitrile-butadiene-styrene (ABS) materials through fused filament fabrications with various cr...
Chapter
This chapter is a survey of continuum-type mechanics of porous media having a generally anisotropic fractal geometry. The approach relies on expressing the global balance laws in terms of fractional integrals based on product measures and, then, converting them to integer-order integrals in conventional (Euclidean) space. Via localization, this all...
Conference Paper
Full-text available
Fused Deposition Modeling (FDM) is a 3D printing process where thermoplastic materials are deposited layer by layer through an extrusion nozzle. One of the main advantages of an FDM process is that any complex shape geometry can be printed directly from a CAD model. However, the process also shows disadvantages such as air-gap, porosity, weak inter...
Conference Paper
Additive manufacturing (or 3D printing) is being increasingly used in a wide range of areas including civil, aerospace and biomedical engineering where it offers significant advantages over conventional methods for model prototyping. However, the reduced fracture resistance typically observed in 3D printed materials limits its application to functi...
Conference Paper
XFEM combined with different isotropic and anisotropic damage criteria is able to model distinct fracture behaviors in 3D printing materials with various layer directions: brittle, ductile or kinked fractures.
Chapter
This chapter reviews the modeling of fractal materials by homogenized continuum mechanics using calculus in non-integer dimensional spaces. The approach relies on expressing the global balance laws in terms of fractional integrals and, then, converting them to integer-order integrals in conventional (Euclidean) space. Via localization, this allows...
Article
Full-text available
Composition-structure-property relations of bone provide fundamental understanding of bone quality. The objective of this paper was to investigate age dependent changes in the composition, structure and mechanical properties of porcine femoral cortical bone at mid-diaphysis region from six age groups (1, 3.5, 6, 12, 30, 48 months). This study was m...
Article
The elastic moduli of trabecular bone were modeled using an analytical multiscale approach. Trabecular bone was represented as a porous nanocomposite material with a hierarchical structure spanning from the collagen-mineral level to the trabecular architecture level. In parallel, compression testing was done on bovine femoral trabecular bone sample...
Conference Paper
Full-text available
This paper presents a constitutive model capable of predicting the thermoviscoelastic behavior of the balloon thin film StratoFilm subject to large strains up to yielding. The model is based on the free volume theory of nonlinear thermoviscoelasticity and extended to orthotropic membranes. An ingredient of the present approach is that the experimen...
Conference Paper
Full-text available
This paper presents a constitutive model capable of predicting the thermoviscoelastic behavior of the balloon thin film StratoFilm subject to large strains up to yielding. The model is based on the free volume theory of nonlinear thermoviscoelasticity and extended to orthotropic membranes. An ingredient of the present approach is that the experimen...
Article
Full-text available
This paper studies fractal patterns forming at elastic-plastic transitions in soil-and rock-like materials. Taking either friction or cohesion as nonfractal vector random fields with weak noise-to-signal ratios, it is found that the evolving set of plastic grains (i.e., a shear-band system) is always a monotonically growing fractal under increasing...
Article
This paper presents an overview of modeling fractal media by continuum mechanics using the method of dimensional regularization. The basis of this method is to express the balance laws for fractal media in terms of fractional integrals and, then, convert them to integer-order integrals in conventional (Euclidean) space. Following an account of this...
Article
Full-text available
In two recent papers [Phys. Rev. E 85, 025302(R) (2012) and Phys. Rev. E 85, 056314 (2012)], the authors proposed fractal continuum hydrodynamics and its application to model fluid flows in fractally permeable reservoirs. While in general providing a certain advancement of continuum mechanics modeling of fractal media to fluid flows, some results a...
Conference Paper
Full-text available
The failure of balloons made of Linear Low Density PolyEthylene (LLDPE) is investigated. The chosen film is 38 µm thick StratoFilm 420, currently used for the NASA Super-Pressure balloons [1]. The visco-elastic behaviour of the film has been extensively studied and is already accounted for in the balloon design [2, 3, 5]. The next step in the devel...
Article
Full-text available
Cortical and trabecular bones were modeled as nanocomposite materials with hierarchical structures spanning from collagen-mineral level to cortical and trabecular bone levels. In order to verify theoretical models, compression testing was done on cortical and trabecular bovine femur bone samples and the experimental data were compared with the theo...
Conference Paper
Full-text available
Trabecular bone is a porous nanocomposite material with a hierarchical structure. In this study, a multi-scale modeling approach, addressing scales spanning from the nanometer (collagen-mineral) to mesoscale (trabecular bone) levels, was developed to determine the elastic moduli of trabecular bone. Then, the predicted modeling results were compared...
Article
Full-text available
Over the past few decades, various conjectures were advanced that Saturn's rings are Cantor-like sets, although no convincing fractal analysis of actual images has ever appeared. We focus on the images sent by the Cassini spacecraft mission: slide #42 "Mapping Clumps in Saturn's Rings" and slide #54 "Scattered Sunshine". Using the box-counting meth...
Article
Full-text available
In this paper the amount and morphology of cortical and trabecular bone porosities were estimated using optical microscopy and micro-computed tomography technique. The hierarchical structure of porosity at different structural scales spanning from a single lacuna (sub-microscale) to trabecular or cortical bone levels (mesoscale) was characterized a...
Article
a b s t r a c t This paper builds on the recently begun extension of continuum thermomechanics to frac-tal media which are specified by a fractional mass scaling law of the resolution length scale R. The focus is on pre-fractal media (i.e., those with lower and upper cut-offs) through a technique based on a dimensional regularization, in which the...
Article
Full-text available
Fractal patterns are observed in computer simulations of elastic-plastic transitions in linear, locally isotropic thermoelastic-hardening plastic heterogeneous materials. The models involve 2D aggregates of homogeneous grains with weak random fluctuations in thermal expansion coefficients, equivalent to modeling the effects of random residual strai...
Article
Full-text available
This paper builds on the recently begun extension of continuum thermomechanics to fractal porous media that are specified by a mass (or spatial) fractal dimension D, a surface fractal dimension d and a resolution length scale R. The focus is on pre-fractal media (i.e. those with lower and upper cut-offs) through a theory based on a dimensional regu...
Article
Full-text available
In this review, recent advances on the measurement and modeling of elastic properties of cortical and trabecular bone are presented. Bone is a multifunctional material which among its other functions serves as a support for other tissues in the body. As a structural material it is stiff, strong, tough, lightweight and is adaptable. Its excellent me...
Chapter
Full-text available
This paper builds on the recently begun extension of continuum thermomechanics to fractal porous media which are specified by a mass (or spatial) fractal dimension D, a surface fractal dimension d, and a resolution length scale R. The focus is on pre-fractal media (i.e., those with lower and upper cut-offs) through a theory based on a dimensional r...
Article
Full-text available
Plastic grains are found to form fractal patterns in elastic-hardening plastic materials in two dimensions, made of locally isotropic grains with random fluctuations in plastic limits or elastic/plastic moduli. The spatial assignment of randomness follows a strict-white-noise random field on a square lattice aggregate of square-shaped grains, where...
Article
Full-text available
Continuing in the vein of a recently developed generalization of continuum thermomechanics, in this paper we extend fracture mechanics and beam mechanics to materials described by fractional integrals involving D, d and R. By introducing a product measure instead of a Riesz measure, so as to ensure that the mechanical approach to continuum mechanic...
Article
Full-text available
Fractal patterns are observed in computational mechanics of elastic-plastic transitions in two models of linear elastic/perfectly-plastic random heterogeneous materials: (1) a composite made of locally isotropic grains with weak random fluctuations in elastic moduli and/or yield limits; and (2) a polycrystal made of randomly oriented anisotropic gr...

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Projects

Projects (3)
Project
Nodaway, additive manufacturing (AM) technologies are becoming a transubstantiate for the conventional manufacturing methodologies that can deliberate a flexible and efficient production system. AM uses the computer-aided-design (CAD) software to deliver the desired output by depositing material, layer upon layer, through thermo-mechanical cycles. This process implies a multi-physics system; on the other hand, due to AM process capabilities, the process can build a full-scale functional part with a complex geometrical shape within short cycle-time. Thus, AM machine suppliers do not guarantee the best optimal settings for the AM process parameters. Accordingly, a multi-objective optimization problem (MOOP) is formulated constrained with space mapping problem. The computational complexity of the emerged MOOP prevents a broader adoption of additively manufactured functional parts. This work proposes a realistic and efficient data-driven approach for predicting, optimizing and simulating the additive manufacturing process parameters using surrogate modeling. The proposed themes consist of three popular methods: (1) Group Method for Data Handling (GMDH) variants as the engine of prediction modeling technique, the modified polynomial neural network can characterize the spatial collinearity for the input arguments along with dependency relationship among the multiple outputs. (2) Evolutionary algorithms (EA) to find the optimal operational settings and extract the Pareto-frontier space from the approximated design space. (3) Reduce-order modeling techniques to provide a numerical simulation for the AM process, which could expand the exploration of the design space. The numerical results from different data-driven techniques correlate well with the Abaqus FE simulation and the lab experimental results. This approach provides a feasible and realistic approach for the AM process modeling and optimization.
Project
The developed anisotropic damage models for 3D printed polymer materials based on layer orientation has potential applications on the study of orthotropic fracture behavior of bone as well as 3D printed bone substitutes.
Project
Unlike the fractional derivative, the Hausdorff derivative, one kind of fractal derivatives (also called the non-local fractional derivative), introduced by Chen (2006), is a local derivative instead of a global operation. Thus, the computing costs of the Hausdorff fractal derivative are far less than the global fractional derivative, while performing almost equally well in modeling a variety of complex problems. In particular, the Hausdorff fractal derivative diffusion equation characterizes the stretched Gaussian process in space and fractal exponental decay, also known as the stretched relaxation, in time. In contrast, the fractional derivative diffusion equation underlies the Levy statistics in space and the ML function power law decay in time. By extending the concept of fundamental solution of integer-order differential operators to fractal by Chen et al (2016), the fractal differential operators are defined and employed to describe various mechanical behaviors of fractal materials. Fractal calculus operator significantly extends the application scope of the classical calculus modeling approach under the framework of continuum mechanics to fractal materials. It is noted that there exist quite a few different definitions of fractal derivative, among which, to the best understanding and knowledge, the Hausdorff derivative is mathematically the most simple and numerically the easiest to implement with clear physical significance and the most real-world applications.