skip to main content


Title: A pyramidal lattice frame: Pathways to inversion
This paper considers the load–deflection behavior of a pyramid-like, shallow lattice structure. It consists of four beams that join at a central apex and when subject to a lateral load, it exhibits a propensity to snap-through: a classical buckling phenomenon. Whether this structural inversion occurs, and the routes by which it happens, depends sensitively on geometry. Given the often sudden nature of the instability, the behavior is also examined within a dynamics context. The outcome of numerical simulations are favorably compared with experimental data extracted from the testing of three-dimensional (3D)-printed specimens. The key contributions of this paper are that despite the continuous nature of the physical system, its behavior (transient and equilibria) can be adequately described using a discrete model, and the paper also illustrates the utility of 3D-printing in an accessible research context.  more » « less
Award ID(s):
1926672
NSF-PAR ID:
10233560
Author(s) / Creator(s):
;
Date Published:
Journal Name:
International journal of structural stability and dynamics
Volume:
21
Issue:
2
ISSN:
0219-4554
Page Range / eLocation ID:
2150020
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Background: Internships for college students can enhance their grades, skills, and employment prospects, but finding and completing an internship sometimes requires considerable resources. Consequently, before postsecondary institutions consider mandating this high-impact practice, more evidence is needed regarding the various obstacles students face as they seek an internship. Focus of Study: The purpose of this study was to document the prevalence and nature of obstacles to securing a college internship and how these factors interact in the lives of particular students. Field theory is used to highlight the ways that structural inequalities and forms of capital serve to facilitate or constrain access to an internship experience. Population: The participants in this study included students attending five postsecondary institutions—three comprehensive universities, one historically Black college and university (HBCU), and one technical college in the U.S. states of Maryland, South Carolina, and Wisconsin. Research Design: This concurrent mixed-methods study included the collection of survey (n = 1,549) and focus group and interview (n = 100) data from students who self-selected into the study. Given that this is a descriptive study, the aim was to document student experiences with obstacles to internships using varied sources of data. Data Collection and Analysis: Data were collected via an online survey (with a 26% response rate) and in-person focus groups or interviews at each campus. Data were analyzed using inductive thematic analysis, social network analysis, and logistic regression techniques and interpreted in ways that highlight the situated and critical role of capital and structure in shaping opportunity and behavior. Findings: Among the 1,060 (69%) survey respondents who reported not having had an internship, 638 indicated that they had in fact wanted to pursue an internship but could not because of the need to work, a heavy course load, insufficient positions, and inadequate pay. The role of financial, social, and cultural capital also impacted students differentially depending on their majors, socioeconomic status, race, and geographic location, highlighting how context and enduring systemic forces—and not solely the possession of capital(s)—intersect to shape students’ abilities to pursue an internship. Conclusion: Internships are not universally accessible to all college students and instead favor students who have access to financial, social, and cultural capital while also being positioned in particular majors, geographic locations, and institutions. Before actively promoting internships for their students, colleges and universities should secure funding to support student pay and relocation costs, identify alternative forms of experiential learning for working students, and engage employers in creating more in-person and online positions for students across the disciplines. 
    more » « less
  2. This paper experimentally investigates the mechanical behavior of inactive and low-pressure fluidic artificial muscle (FAM) actuators under applied axial load. In most cases, the active characteristics of an actuator are of interest because they provide valuable information about its force-strain relationship. However, a system of actuators requires attention to the interaction between individual units. One such configuration is a bundle of McKibben artificial muscle actuators arranged in parallel and used for load-adaptive variable recruitment. This bio-inspired actuator bundle sequentially increases the number of actuators activated depending on the load required, which is analogous to how motor units are recruited in a mammalian muscle tissue. While using the minimum number of actuators allows the bundle to operate efficiently, the resistive force of inactive elements acts against total bundle contraction due to their inherent stiffness. In addition, when the bundle transitions between recruitment levels, motor units for a given recruitment level may be gradually pressurized; these low-pressure motor units can also cause resistive forces. Experiments were conducted to characterize the complex interaction between the bladder and braided mesh that cause the resistive force and deflection of inactive and low-pressure elements. Based on observations made from experiments, the paper proposes the initial criteria for developing a model of the resistive forces of a McKibben actuator, both individually, and within the context of a variable recruitment bundle. 
    more » « less
  3. null (Ed.)
    Classical mechanics courses are taught to most engineering disciplinary undergraduate students. Due to the recent advancements of multiscale analysis and practice, necessary reforms need to be investigated and explored for classical mechanics courses to address the materials’ mechanics behaviors across multiple length scales. This enhanced understanding is needed for engineering students to consider materials more broadly. This paper presents a recent effort for the development of a multiscale materials and mechanics experimentation (M3E) module that can be potentially implemented in undergraduate mechanics courses, including Statics, Dynamics, Strength of Materials, and Design of Mechanical (Machine) Components. The developed education module introduces the concepts of multiscale materials behavior and microstructures in the form of micro and macro-scales. At the micro-scale, both 3D printed aluminum and cold-rolled aluminum samples were characterized using scanning electron microscope. Microstructures, including grains, grain boundaries, dislocation, precipitates, and micro-voids, were demonstrated to students. At the macro-scale, experiments following ASTM standards were conducted and full strain fields carried by all the samples were analyzed using digital image correlation method. The experimental data were organized and presented to the students in the developed M3E module. The implementation of the developed module in undergraduate mechanics classes allows students to not only visualize materials behavior under various load conditions, but also understand the reasons behind classical mechanics properties. To assess the effectiveness of the developed M3E education module, an evaluation question was developed. Students are required to classify key mechanics, materials, and processing concepts at both micro and macroscales. More than 40 fundamental concepts and keywords are included in the tests. The study outcomes and effectiveness of the M3E education module will be reported in this paper. 
    more » « less
  4. There is a need for better 3-D model representations of cerebrovasculature particularly on the order of arterioles. Such a model would have many applications and could be a useful tool for those conducting studies involving the brain and its function. The load bearing effects of the vasculature can be better studied with such a model, such as in the case of large strains. In addition, by having a continuous hollow structure, studies involving flow properties can be conducted at a whole scale rather than in a segmented view. Such studies are critical to the advancement of knowledge about the brain and its mechanics which can lead to advancements in preventative and curative care, as well as preventative safety measures. The model developed in this paper could serve as a tool in such studies. A fractal L-system is used to define the branching nature of the model. As such a growing tree structure is developed and characterized by its bifurcation at the end of a vessel segment. The index of bifurcation, α, is a parameter that controls the behavior of the two generated daughter vessels. The model presented here grows from a single parent branch into a bifurcation each of which then bifurcates as many times as specified. The length and diameter of the two daughter vessels will be a function of the respective parent’s length and diameter as well as a value α. The branching angle of the two daughter vessels will be entirely controlled by α. The hollow continuous nature of the model allows for it to be used as a representation of the arteriole structures in the brain. There is also use for such a model in other areas of the body, however, this study will focus on the representation of the cerebrovasculature. The end result is a branching tree model generated in Abaqus which is continuous, hollow and capable of extensive generation with uses in modeling complex cerebrovascular mechanics. 
    more » « less
  5. Scanning probes reveal complex, inhomogeneous patterns on the surface of many condensed matter systems. In some cases, the patterns form self-similar, fractal geometric clusters. In this paper, we advance the theory of criticality as it pertains to those geometric clusters (defined as connected sets of nearest-neighbor aligned spins) in the context of Ising models. We show how data from surface probes can be used to distinguish whether electronic patterns observed at the surface of a material are confined to the surface, or whether the patterns originate in the bulk. Whereas thermodynamic critical exponents are derived from the behavior of Fortuin–Kasteleyn (FK) clusters, critical exponents can be similarly defined for geometric clusters. We find that these geometric critical exponents are not only distinct numerically from the thermodynamic and uncorrelated percolation exponents, but that they separately satisfy scaling relations at the critical fixed points discussed in the text. We furthermore find that the two-dimensional (2D) cross-sections of geometric clusters in the three-dimensional (3D) Ising model display critical scaling behavior at the bulk phase transition temperature. In particular, we show that when considered on a 2D slice of a 3D system, the pair connectivity function familiar from percolation theory displays more robust critical behavior than the spin-spin correlation function, and we calculate the corresponding critical exponent. We discuss the implications of these two distinct length scales in Ising models. We also calculate the pair connectivity exponent in the clean 2D case. These results extend the theory of geometric criticality in the clean Ising universality classes, and facilitate the broad application of geometric cluster analysis techniques to maximize the information that can be extracted from scanning image probe data in condensed matter systems. 
    more » « less