Northeastern university technical writing program

Undergraduate Teaching Assistantships. The Experiential PhD. Student Research Opportunities. Research Administration Team. Funding Opportunities.

Honors in the Major. Undergraduate Advising. New Grad Students. Current Grad Students. Financial Aid and Awards. Graduate Programs Welcome Day. Learning Goals. Contact Us. Graduate Student Instructors. Research Administration Team. Funding Opportunities. Honors in the Major. Undergraduate Advising. New Grad Students. Current Grad Students. Financial Aid and Awards. Graduate Programs Welcome Day. Learning Goals. Contact Us. Graduate Student Instructors. First-Year Writing.

Advanced Writing for the Disciplines. To our best knowledge, HEA is proposed for the first time to fabricate the composite nanolattices combined with 3D laser lithography. Additionally, the mechanical properties of the obtained composite architecture were investigated via in situ scanning electron microscope SEM compression test and the presented architecture encouragingly exhibits ultrahigh compressive specific strength of 0.

This experimental observation is mainly attributed to so-called prior beta grains, which are distinct polycrystalline domains of the size of up to a few centimeters.

Tension and shear experiments are performed to characterize the plasticity and fracture response of individual prior-beta grains. Based on more than 25 experiments, the probability density functions are identified for the material parameters of a von Mises plasticity model with isotropic hardening and for the Hosford-Coulomb fracture initiation model.

Subsequently, a finite element model is built to describe the large deformation response of additively-manufacture structures. The latter are considered as composite structures where each prior beta grain is modeled as separate phase with random material properties. It is shown that the probabilistic composite model can successfully predict the structural response, including the characteristic localization of deformation within individual prior beta grains before fracture initiation.

Reeves T. Stark D. Bishop A. More recently, a high-resolution sub-micrometer variant, direct laser writing DLW via two-photon polymerization, has become available. DLW can fabricate complex 3D microstructures, such as cell scaffolding and mechanical or optical metamaterials. The use of DLW to produce highly controlled, dynamic microstructures has not yet been fully explored because methods for manipulating 3D structures in the micrometer size range are limited. Alternatively, controlled deformation through swelling can be achieved using 3D hydrogel structures, but only in aqueous conditions.

We present a practical method for manipulating soft 3D microstructures through integration with existing micro-electromechanical systems MEMS technology. These customizable electrostatic, thermal, or magnetic actuators can be externally controlled and are capable of applying and measuring forces in the nano- to milli-newton range, depending on design.

By enabling mechanical control with several-nanometer displacement precision, these hybrid DLW-MEMS structures can be used for exciting new applications like tunable 3D metamaterials or deformable cell scaffolding. Takaffoil M. Buehler Printing Nature: Hierarchical Impact Resistant Composites Talk Cancelled Abstract: Each year, millions of people are diagnosed with Traumatic Brain Injury TBI , which is caused by impact to the head or body, potentially leading to loss of consciousness, persistent headaches, memory loss, and even death.

However, despite ongoing technological innovation, contemporary helmet designs are still not energy-absorbent enough to prevent TBI. Therefore, it remains necessary to develop future superior protective systems through novel, ingenious designs. Conch shells, a type of seashell, are known for being one of the toughest biological body armors in nature. However, the complexity of the conch shell three-tier hierarchical architecture creates a barrier to emulating its cross-lamellar structure in synthetic materials.

Previous attempts at mimicking the conch shell laminate design do not capture the complexity of the material. In this work, we present a three-dimensional biomimetic conch shell prototype, which can replicate the crack arresting mechanisms embedded in the natural architecture. Specifically, a combined theoretical and experimental analysis is adopted to investigate the attributes critical to its superior impact resistance.

The overarching mechanism responsible for the impact resistance of conch shell is the generation of pathways for crack deviation, which can be generalized to the design of future protective apparatus such as helmets and body armor. Barthelat D. Multilayered structures found in nature e. Here, we propose a three-dimensionally 3D printed composites featuring a gradient of mechanical properties inspired by the microstructure of the scales of the Polypterus Senegalus, a prehistoric fish.

An ink-based manufacturing technique is developed to produce functionally graded materials in one sitting using multiple heads.

During the printing process, the different inks exhibit shear-thinning rheological behavior to facilitate the deposition of material layers and build a polymer matrix composite structure with tailored properties. Our work mainly involves the tuning a thermoset resin i. Ink mixtures made of various load fractions of mineral fibers up to 50 wt. The stiffness and strength of rectangular beams made of the various mixtures are characterized using the three-point bending method.

The measured moduli vary between 2 to Next, a sequential multimaterial printing procedure is used to generate functionally graded composites in single additive manufacturing step. A customized test to quantify the puncture resistance is designed to compare the performance of the various multimaterial structures and compare them to monolithic structures. Functionally graded materials, combined with advanced 3D printing technologies, are promising candidates for future structural applications such as in biomechanical and dental implants, personal and industrial protective equipment or semiconductor devices.

Aliheidari J. Double cantilever beam specimens of ABS were printed at different nozzle and bed temperatures, and with different layer height and layer width and then fracture-tested to measure the fracture resistance using J-integral in a finite element model. The result indicated that nozzle temperature and layer height had the most significant effects on the fracture resistance. The fracture resistance increased from The data reported here will provide insight and guidance in the design of fracture resistant printed materials for structural and functional applications.

Kochmann J. Greer Nodal Effects on the Stiffness Scaling of Lattice Architectures Abstract: Advanced techniques such as two-photon lithography and stereolithography allow for the manufacturing of complex three-dimensional structures that span a large range of relative densities, stiffnesses, and strengths.

In structural applications, lattice architectures that are manufactured through these processes require proper mechanical characterization as well as predictability of the mechanical properties.

As a consequence, beam theory is no longer applicable but instead computationally intensive continuum element models are necessary to predict the mechanical behavior of these architectures. In the current study, we present a definition for lattice nodes and perform a systematic study to isolate their effect on the stiffness scaling of lattice architectures.

Namely, we explore how modifying the nodal geometry—while keeping other parameters such as relative density and strut slenderness constant—has an effect on the stiffness of lattice architectures.

We perform uniaxial compression experiments on both rigid octahedron and non-rigid tetrakaidecahedron polymer lattice architectures, manufactured through two-photon lithography, and identify the effect of rigidity on the nodal contribution to stiffness. These computational models utilize a combination of 3D continuum elements at nodes and beam elements that accurately capture the stiffness scaling of the compression experiments, with a significant reduction in computational cost compared to typical fully-resolved continuum element models.

We discuss the possibility of applying our findings on nodal effects in a global stiffness scaling for cellular solids that can be valid beyond the limits of classical beam theory.

Abueidda M. Bakir R. Al-Rub J. Bergstrom N. Sobh I. Mathematically they are locally area minimizing surfaces with zero mean curvatures at each point on the surface. This paper investigates computationally and experimentally mechanical properties of the three TPMS-CMs mentioned above. Additive manufacturing is utilized to fabricate the polymeric cellular materials and their base material. Their properties are tested to provide inputs and serve as validation for finite element modeling.

It is shown that the architecture and specimen size of the TPMS-CMs affect the mechanical properties of these cellular materials. Moreover, the finite element results agree with the results obtained experimentally. A full representation of fracture takes into account the complex stress states of naturally occurring flaws.

In this study, the mechanics of failure for architected materials with prescribed flaws was investigated. Center-notched tension specimens were fabricated with a hollow-tube octet lattice architecture and their failure behavior under mixed-mode loading was studied by inclining a central crack with respect to the applied tensile load.

Experiments revealed that unnotched and notched lattices are sensitive to the pre-defined flaw and failed catastrophically in an elastic-brittle fashion. Post-fracture surface morphology shows that the global preferred path for failure is confined at nodal planes perpendicular to the applied load, independent of notch orientation.

Global failure occurred at the notch with cracks localized to nodal junctions. Tube wall fracture initiated failure at the crevices of the nodes and dominated crack path formation.

The failure response of an equivalent ideally-brittle continuum is compared to the experiments and is predicted to follow a similar scaling of stresses at the onset of failure. These hollow-tube octet nanolattices exhibited a high specific tensile strength, outperforming current architected materials and bulk materials with low relative densities. Fang Z. Tan M. Haberman C.

Cushing Invited: Mechanics of 3D Printed Metal Pentamode Structures Abstract: Pentamode materials PM are limiting cases of elastic solids with one of the six Kelvin moduli dominant compared to the remaining five moduli. Our interest in these materials is their ability to exhibit a one-wave property over a broad range of frequencies, which makes the PM act dynamically like an acoustic fluid. We recently demonstrated 2D PM metal structures with variable acoustic properties enabling gradient index focusing of underwater sound.

In this talk, we extend the design space and present a class of 3D metal PM structures. The range of mechanical properties that can be achieved using 3D printing will be discussed. Among the unique potentials of these PMSs is significant acoustic anisotropy.

The mechanical coupling of these structures to an exterior acoustic medium water requires a boundary between the PM and the fluid.

The pros and cons of the different fabrication techniques will be discussed, with emphasis on optimal designs using 3D printers. Work supported by ONR. Sun J. The ability to design and fabricate metamaterials that achieve desired anisotropic directions of stiffness is important for enabling a number of advanced applications. These unusual properties strongly depend upon the inherent architecture of these solids.

An example of architected materials is the phononic crystal, which consists of periodically topological structures and materials dispersions. These rationally designed structures enable the manipulation of propagating acoustic and elastic waves or phonons. The capability of tuning the wave propagation stems from the destructive interference at the interfaces of the periodic units.

As a result, Bragg-type band gaps i. Bragg-type band gaps offer a myriad of potential applications such as wave filtering, waveguiding, acoustic cloaking, thermal management and energy harvesting. The proposed lattice metamaterials are built by replacing regular straight beams with sinusoidally shaped ones, which are highly stretchable under uniaxial tension. The interplay between the multiscale ligament and cell architecture and wave propagation also enables remarkable broadband vibration-mitigation capability of the lattice metamaterials, which can be dynamically tuned by an external mechanical stimulus.

The material design strategy provides insights into the development of classes of architected metamaterials with potential applications including energy absorption, tunable acoustics, vibration control, responsive devices, soft robotics, and stretchable electronics. Bertoldi Fabrication and Mechanical Properties of Slender Nanoscale Lattice-Truss Materials Abstract: Additive manufacturing of periodic lattice-truss materials into previously inaccessible geometric configurations on some the smallest length scales achieved to date will be discussed.

Two-photon lithography is the primary technique employed to fabricate the lattice-truss structures. A combination of global post-processing methods that are subtractive in nature are then used to decrease the minimum feature size further into the nanoscale and increase the aspect ratio of the struts that comprise these cellular materials.

The details of all the manufacturing processes involved will be discussed. Furthermore, the mechanical behavior of these unique, ultra-light, nanoarchitected materials is experimentally investigated.

The behavior observed in the experiments is given context by analyzing the results of a corresponding set of numerical simulations. Graded cellular materials present new opportunities to access superior combinations of static and dynamic properties over uniform cellular materials while maintaining low density. Material extrusion additive manufacturing provides an ideal method to design, fabricate, and study the mechanical properties of graded cellular materials.

This talk will outline a novel method for fabricating honeycombs with gradients in both strut thickness and cell shape using material extrusion AM, and describe the resulting mechanical properties using compression testing, finite element analysis, and analytical modeling.

Potential areas of application will also be discussed. Based on the three-dimension lattice microstructure, we established the functionally graded structure which had high energy absorption under the in-plane compression. The length parameter is used as design parameters to achieve different effective elasticity modulus, and the functionally graded structure is based on creating gradient length parameter of the microstructure.

We compared the mechanical properties between the homogeneous structures and the graded structure on the condition of quasi static compression and the impact compression. The result shows that the functionally graded structure has high energy absorption.

The structure can find applications in structures or devices such as cars to make them lighter and safer. Raney J. Lewis K. Shea Increased Energy Absorption of Lattice Structures Through Coaxially Aligned Struts Abstract: High stiffness and high energy absorption are material properties that are typically mutually exclusive.

Nature found ways to circumvent this by growing complex, multi-material and multi-scale architectures, but conventional fabrication methods fail to reach comparable properties. More specialized processes, such as freeze-casting, are able to replicate certain features, but result in highly anisotropic materials that are unsuitable for lattice structures, which add the lightweight aspect as an additional property.

In this work, we design struts that can be fabricated through additive manufacturing with coaxially-arranged layers of similar or different materials. The layers are separated by interfacial layers that prevent cracks from propagating. Dotson S. Chizari J. These new metamaterials achieve extraordinarily large deformation motion paths with high precision due to the minimal friction that they generate.