Mechanical Engineering

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Energy Absorbing Cellular Structures for Crashworthiness Applications
(2024) Murray, Colleen; Wereley, Norman; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Energy absorbing materials are utilized in many applications. Aircraft, automobiles, and helmets all use energy absorbing materials to ensure the safety of the individual during an impact event. The seats in aircraft are made from a material that can minimize the force that is transferred from the impact to the occupant. In a similar manner, the material in the front of an automobile is designed to absorb the energy from an impact event and redistribute it in a manner that minimizesthe amount of energy experienced by the main cabin. Helmets perform in the same way: by taking the impact and distributing the load to protect the wearer. The materials used in these applications were tailored to meet the needs of the application, particularly the density and strength of the material. Using cellular structures allow for more control of the design for energy absorbing applications, particularly when looking to increase the performance of the material. There are three options for increasing the energy absorption in materials for crashworthiness applications: decrease the force with a constant mean crush stress, increase the mean crush stress with a constant force, or decrease the force while increasing the mean crush stress. In a force- displacement diagram, the area under the curve is the amount of energy that a material can absorb during an impact. By decreasing that initial force, the initial peak force will begin to equilibrate with the mean crush, resulting in a higher energy absorption. The structures that have been relied on throughout history for these applications are cellular structures. Cellular structures are described as any structure that is made of one phase composed of either air or fluid. As Lakes describes in his work, foams, honeycombs, and lattices are categorized as such; the voids allow the materials to reach physical limits beyond their previous. With the improvements of technology, it is important to re-asses these structures to determine whether they too can be manufactured and remain as effective in their original crashworthiness applications as before. Throughout this work, different methods of additive manufacturing are used to create honeycomb structures specifically for energy absorption applications. Each of these studies focuses on a different attribute that additive manufacturing can help improve in energy absorption materials. In this dissertation, four case studies involving the out-of-plane compression of additively manufactured honeycomb will be discussed. The first chapter will center on the applications of visco-elastic theromplastic polyurethane (TPU) as a potential material of choice for energy absorption materials. TPU is a material that has the ability to achieve significant deformation and return to its original shape within a matter of minutes. This material is of interest due to the need to re-use helmet liners and other safety mechanisms before buying a new one. This work also focuses on the impact that adding buckling initiators will have to the structure in terms of energy absorption during quasi-static conditions. The next chapter is centered on the applications of these TPU honeycomb undergoing dynamic testing. Crashworthiness materials experience impact velocities bordering on 10- 15 m/s (22- 35 mph). These tests differ from the previous due to the velocity no longer being constant. As the impactor falls, the velocity changes, while the quasi-static tests were completed under a constant velocity. This set of dynamic tests is most representative of long term applications, however the performance of these materials change drastically as discussed. In some applications, a visco-elastic plastic is not going to be able to absorb the energy from the impact. In these situations, a stiffer material would be necessary. To provide an alternative for these applications, acrylonitrile butadiene styrene (ABS) was studied since it is a commonly used plastic when additively manufacturing. Once again, honeycomb were manufactured and tested under out of plane, uni-axial quasi static compression. The samples were studied to determine the effects of buckling initiator location as well as the effect of the inscribed diameter. For this, samples were manufactured with an internal diameter of 10, 15, or 20 mm. The buckling initiators were located either 1/2, 3/4, or at the top of the samples to determine the design which enables the best energy absorption. The final study recognizes that traditional honeycomb has been manufactured using metals like aluminum and steel. By moving towards an additively manufactured honeycomb, this work has been focusing on polymeric honeycomb instead. The metallic additive manufacturing methods require drastic safety precautions be taken. A safer alternative is proposed in this last study: combining stereolithography and electroplating. Here, an isotropic material can be the core of the structure, with a thin layer (about 150 μm) of metal creating the ductile layer. These samples demonstrate a ductile failure as opposed to their plastic only counterparts who experience a brittle failure. The energy absorption performance is then characterized as a function of buckling initiator height as well.
MECHANICS AND THERMAL TRANSPORT MODELING IN NANOCELLULOSE AND CELLULOSE-BASED MATERIALS
(2023) RAY, UPAMANYU; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Cellulose, the abundantly available natural biopolymer, has the potential to be a next generation wonder material. The motivation behind this thesis stems from the efforts to obtain mechanical properties of two novel cellulose-based materials, which were fabricated using top-down (densified engineered wood) and bottoms-up (graphite-cellulose composite) approaches. It was observed that the mechanical properties of both the engineered wood (strength~596 MPa; toughness ~3.9 MJ/m3) and cellulose-graphite composite (strength~715 MPa; toughness ~27.7 MJ/m3) surpassed the equivalent features of other conventional structural materials (e.g., stainless steel, Al alloys etc.). However, these appealing properties are still considerably inferior to individual cellulose fibrils whose diameters are in the order of nanometers. A significant research effort needs to be initiated to effectively transfer the mechanical properties of the hierarchical cellulose fibers from the atomistic level to the continuum. To achieve that, a detailed understanding of the interplay of cellulose molecular chains that affects the properties of the bulk cellulosic material, is needed. Modeling investigations can shed light on such underlying mechanisms that ultimately dictate multiple properties (e.g., mechanics, thermal transport) of these cellulosic materials. To that end, this thesis (1) applies molecular dynamics simulations to decipher why microfibers made of aligned nanocellulose and carbon nanotubes possess excellent mechanical strength, along with understanding the role of water in fully recovering elastic wood under compression; (2) delineates an atomistically informed multi-scale, scalable, coarse grained (CG) modeling scheme to study the effect of cellulose fibers under different representative loads (shearing and opening), and to demonstrate a qualitative guideline for cellulose nanopaper design by understanding its failure mechanism; (3) utilizes the developed multi-scale CG scheme to illustrate the reason why a hybrid biodegradable straw, experimentally fabricated using both nano- and micro-fibers, exhibits higher mechanical strength than individual straws that were built using only nano or microfibers; (4) investigates the individual role of nanocellulose and boron nitride nanotubes in increasing the mechanical properties (tensile strength, stiffness) of the derived nanocellulose/boron-nitride nanotube hybrid material; (5) employs reverse molecular dynamics approach to explore how the boron nitride nanotube based fillers can improve thermal conductivity (k) of a nanocellulose derived material. In addition, this thesis also intends to educate the readers on two perspectives. The common link connecting them is the method of engineering intermolecular bonds. The first discussion presents a few novel mechanical design strategies to fabricate high-performance, cellulose-based functional materials. All these strategies are categorized under a few broad themes (interface engineering, topology engineering, structural engineering etc.). Another discussion has been included by branching out to other materials that, like nanocellulose, can also be tuned by intermolecular bonds engineering to achieve unique applications. Avenues for future work have been suggested which, hopefully, can act as a knowledge base for future researchers and help them formulate their own research ideas. This thesis extends the fundamental knowledge of nanocellulose-based polymer sciences and aims to facilitate the design of sustainable and programmable nanomaterials.
Rupture Mechanisms Of Porcine And Human Ascending Aortic Tissue Under Dynamic Translational Shear Deformation
(2021) Harwerth, Jason W.; Haslach, Henry W; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Tissue Engineered Vascular Grafts (TEVGs) may be grown in living pigs to further their development towards use in humans to repair damaged aorta. To explore whether porcine grown TEVGs are good models for human grown TEVGs, normal human and porcine aortic tissues are loaded in shear deformation to compare the differences in the dissection response of these viscoelastic tissues. Shear is strongly related to aortic dissection. Translational constant rate and sinusoidal shear deformation tests characterize dynamic mechanical properties of aortic tissue. Knowledge of the tissue microstructure helps determine the effect of interstitial fluid-solid interaction on the shear response of the specimens. Transient and quasi-periodic response characteristics provide baseline material properties of normal porcine aortic tissue to compare its dissection resistance with TEVG porcine aortic tissue. The results show that normal porcine aortic tissue is sufficiently similar to human aortic tissue to justify the continued development of porcine grown TEVG models.
Coupled Oscillator Arrays: Dynamics and Influence of Noise
(2021) Alofi, Abdulrahman Mohammed; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Coupled oscillator arrays can be used to model several natural systems and engineering systems including mechanical systems. In this dissertation work, the influence of noise on the dynamics of coupled mono-stable oscillators arrays is investigated by using numerical and experimental methods. This work is an extension of recent efforts, including those at the University of Maryland, on the use of noise to alter a nonlinear system's response. A chain of coupled oscillators is of interest for this work. This dissertation research is guided by the following questions: i) how can noise be used to create or quench spatial energy localization in a system of coupled, nonlinear oscillators? and ii) how can noise be used to move the energy localization from one oscillator to another? The coupled oscillator systems of interest were harmonically excited and found experimentally and numerically to have a multi-stability region (MR) in the respective frequency response curves. Relative to this region, it has been found that the influence of noise depends highly on the excitation frequency location in the MR. Near either end of the MR, the oscillator responses were found to be sensitive to noise addition in the input and it was observed that the change in system dynamics through movement amongst the stable branches in the deterministic system could be anticipated from the corresponding frequency response curves. The system response is found to be robust to the influence of noise as the excitation frequency is shifted toward the middle of the MR. Also, the effects of noise on different response modes of the coupled oscillators arrays were investigated. A method for predicting the behavior is based on so-called basins of attractions of high dimensional systems. Through the findings of this work, many unique noise influenced phenomena are found, including spatial movement of an energy localization to a neighboring oscillator, response movement gradually up the energy branches, and generation of energy cascades from a localized mode.
Anisotropic Multi-scale Modeling for Steady-state Creep Behavior of Oligocrystalline SnAgCu (SAC) Solder Joints
(2021) Jiang, Qian; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Heterogeneous integration is leading to unprecedented miniaturization of solder joints. The overall size of solder interconnections in current-generation microelectronics assemblies has length-scales that are comparable to that of the intrinsic heterogeneities of the solder microstructure. In particular, there are only a few highly anisotropic grains in each joint. This makes the mechanical response of each joint quite unique. Rigorous mechanistic approaches are needed for quantitative understanding of the response of such joints, based on the variability of the microstructural morphology. The discrete grain morphology of as-solidified oligocrystalline SAC (SnAgCu) solder joints is explicitly modeled in terms of multiple length scales (four tiers of length scales are used in the description here). At the highest length-scale in the joint (Tier 3), there are few highly anisotropic viscoplastic grains in each functional solder joint, connected by visoplastic grain boundaries. At the next lower tier (Tier 2), the primary heterogeneity within each grain is due to individual dendrites of pro-eutectic β-Sn. Additional microscale intermetallic compounds of Cu6Sn5 rods are located inside individual grains. Packed between the dendrite lobes is a eutectic Ag-Sn alloy, The next lower length-scale (Tier 1), deals with the microstructure of the Ag-Sn eutectic phase, consisting of nanoscale Ag3Sn IMC particles dispersed in a β-Sn matrix. The characteristic length scale and spacing of the IMC particles in this eutectic mix are important features of Tier 1. Tier 0 refers to the body-centered tetragonal (BCT) anisotropic β-Sn crystal structure, including the dominant slip systems for this crystal system. The objective of this work is to provide the mechanistic framework to quantify the mechanical viscoplastic response of such solder joints. The anisotropic mechanical behavior of each solder grain is modeled with a multiscale crystal viscoplasticity (CV) approach, based on anisotropic dislocation mechanics and typical microstructural features of SAC crystals. Model constants are calibrated with single crystal data from the literature and from experiments. This calibrated CV model is used as single-crystal digital twin, for virtual tests to determine the model constants for a continuum-scale compact anisotropic creep model for SAC single crystals, based on Hill’s anisotropic potential and an associated creep flow-rule. The additional contribution from grain boundary sliding, for polycrystalline structures, is investigated by the use of a grain-boundary phase, and the properties of the grain boundary phase are parametrically calibrated by comparing the model results with creep test results of joint-scale few-grained solder specimens. This methodology enables user-friendly computationally efficient finite element simulations of multi-grain solder joints in microelectronic assemblies and facilitates parametric sensitivity studies of different grain configurations. This proposed grain-scale modeling approach is explicitly sensitive to microstructural features such as the morphology of: (i) the IMC reinforcements in the eutectic phase; (ii) dendrites; and (iii) grains. Thus, this model is suited for studying the effect of microstructural tailoring and microstructural evolution. The developed multiscale modeling methodology will also empower designers to numerically explore the worst-case and best-case microstructural configurations (and corresponding stochastic variabilities in solder joint performance and in design margins) for creep deformation under monotonic loading, for creep-fatigue under thermal cycling as well as for creep properties under isothermal aging conditions.
ACTIVE CONTROL OF NON-RECIPROCAL ACOUSTIC METAMATERIAL WITH A DYNAMIC CONTROLLER
(2019) Raval, Suraj; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Reciprocity is one of the fundamental properties in the field of wave propagation. In acoustics, this property helps in various practical applications. But, breaking this reciprocity also has useful applications. As a result of which, many researchers have tried to break the reciprocity in acoustics, which is comparatively difficult, unlike in fields such as electro-magnetics. Majority of these proposed methods to break the reciprocity are hard-wired systems, which work for a very limited frequency range. Thus, we have introduced a non-reciprocal metamaterial having boundary control with the help of piezoelectric sensors and actuators. A theoretical model is introduced to induce the nonreciprocal behavior, and it is backed up by providing experimental evidence. Our setup consists of a cylindrical cell made up of acrylic, filled with water, having four piezo sensors/actuators, two on each end. The idea is to excite the piezo cell through an actuator on one side, collect the resulting signal from the piezo sensor on the other side, and perform appropriate mathematical operations on this signal to produce a feedback/control signal via a specially designed dynamic control action. This control signal affects the propagation of pressure waves through the water medium inside the cell as it introduces a virtual gyroscopic effect of controlled magnitude and direction. Thus, this is how non-reciprocity is introduced and controlled into the metamaterial cell. The obtained theoretical and experimental results demonstrate the effectiveness of the dynamic controller in breaking the acoustic reciprocity. Extension of this work to multi-cell metamaterial configuration is natural extension to be pursued.
Entropic Approaches for Assessment of Metal Fatigue Damage
(2019) Yun, Huisung; Modarres, Mohammad; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Prognostics and Health Management (PHM), a promising technique assessing individual life of engineering systems, requires metrics that indicate the current level of degradation and aging. However, traditional methods of fatigue life estimation have a restriction to apply to PHM due to scale dependency of measurements. An alternative to the conventional fatigue assessment is the entropic approach, initially de-rived from the second law of thermodynamics. The entropic approach is scale-independent and able to monitor degradation and aging from the early periods of life. The entropic endurance indicates a certain level of damage that a component can tolerate before failure. Not only the thermodynamic theory but also information and statistical mechanics laws introducing entropy apply to the various modes of energy dissipations. This dissertation introduces the extension of the entropic approaches as the representation of damage by empirically examining the theoretical basis of three en-tropic theorems. Metallic coupons were fatigue tested to confirm the applicability of three entropic measures: irreversible thermodynamic entropy, information (Shannon) entropy, and Jeffreys divergence, by measuring variables used to compute energy dissipations during fatigue. In addition to the entropic approaches to damage, short-term loading process (STLP) is designed to minimize the difficulties associated with acoustic emission background noise when used to measure information entropy of the generated signals. Without damaging the material, high-frequency/low-amplitude loading is expected to generate acoustic signals through quiet background noise excitation loading to infer the current damage status. The results of this research help identifying multiple damage measurement methods and will broaden understanding and selecting practical applications, and reduce the prognosis uncertainty in PHM applications.
DEFORMATION MECHANICS OF SOFT MATTER UNDER EXTERNAL STIMULI
(2019) Cheng, Jian; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Artificial soft matters are a class of materials which can be easily deformed by external stress, typical examples include foams, colloids, elastomers, and hydrogels. Due to their unprecedented and unique properties, such as large deformability, high resemblance to biological systems, versatile response to multi-physical stimuli, and biological compatibility, soft matters have found applications in fields like soft actuators and robots, soft sensors, bio-mimicking material systems, micro-fluidic system control, biomedical engineering, etc. In these applications, the large deformability of soft matters has taken an enabling role. The deformation theory of polymeric soft matters can date back to 1940s in the early infancy of the statistical mechanics sketch of rubbery materials, with a fast growth in the most recent decade concurring the latest progress in soft matters. However, the mechanical modeling of soft matter leaves many open questions. This doctorate research is devoted to advance the understanding of the deformation mechanics of soft matter, specifically, from the following aspects: (1) how the chemo-mechanical interaction between the solvent molecules and the polymeric network invokes anomalous behaviors of a thin-walled hydrogel structure under internal pressure, in contrast to its polymer counterpart; (2) the application of the dielectric elastomer as sensing medium in soft sensor technology; (3) the development of a novel light-responsive hydrogel material system with the application in bio-mimicking shape transform; (4) and enriching the existing theory to facilitate the mechanistic understanding of the deformational behaviors of a type of fiber-reinforce anisotropic hydrogels. For that, this dissertation (1) reveals the delayed burst of hydrogel thin-shell structures as a new failure mechanism, which is dissimilar from the instantaneous burst of a rubber shell: at a subcritical applied pressure the burst occurs with a delay in time; (2) presents a facile design of capacitive tactile force sensor using a dielectric elastomer subjected to a modest voltage and a pre-stretch; (3) develops a theoretical framework to simulate the light-responsive deformation of the proposed hybrid hydrogel system; and (4) from the perspective of micromechanics, constructs a constitutive model suitable for the microfiber-reinforced anisotropic hydrogel, with large deformation, mass transportation, and the origin of anisotropy are intrinsically captured.
COMPARISON OF HIGH STRAIN RATE PROPERTIES OF ADDITIVELY MANUFACTURED AND WROUGHT INCONEL 625 VIA KOLSKY BAR TESTING
(2019) Morin, Jason; Fourney, William; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Additive manufacturing is becoming an important part of modern manufacturing technology. Before additively manufactured parts gain widespread adoption, the material properties of the additively manufactured material itself must be accurately quantified. Stress strain curves must be produced over a wide variety of test conditions so that accurate modeling of material behavior can be done. Materials that may undergo dynamic loading must therefore be tested under dynamic conditions. In this study the tensile and high strain rate compressive material properties of additively manufactured Inconel 625 are compared to conventionally formed wrought material. The results of testing showed that there is a clear difference in material properties between wrought and additively manufactured Inconel 625 in tension and compression. The additively manufactured tensile samples showed anisotropy between print directions of approximately ±10%. The printed samples had a 35% higher yield strength, a similar ultimate strength, and 20-40% the elongation when compared to wrought. There was also a significant difference in properties between the additive and wrought materials during the compressive tests. The additive material showed little anisotropy and had a 30% higher yield stress than wrought. Additionally, the additive material had a higher strain hardening rate than the wrought samples. No significant strain rate effects were noted.
NONLINEAR RESPONSE OF ELECTRONIC ASSEMBLIES UNDER MULTIAXIAL VIBRATION EXCITATION
(2018) Massa, Samuel; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Electronic packages are exposed to complex life-cycle environments, and in many cases that environment involves exposure to multiaxial vibration which can dangerously affect the integrity of the electronic package’s functionality due to nonlinear amplification of the multiaxial response, in comparison to the corresponding uniaxial responses. This has particular implications in vibration durability testing of electronic assemblies, since conventional tests in industry are often run sequentially as set of uniaxial tests along orthogonal axes. This is in part because multiaxial vibration tests can be expensive and complex when the response becomes significantly nonlinear. The severity of the nonlinear response is known to depend both on the multiaxial excitation parameters and on the component architecture. Prior studies have investigated the nonlinear effects of varying the loading parameters through modeling and testing, while this study focuses on quantifying the effects of component geometry. The approach is based on a combination of multiaxial vibration testing and modeling to conduct a parametric study with components of different geometries. The findings of this study will provide important guidance when developing guidelines about when multiaxial response is important, instead of sequential uniaxial testing along orthogonal axes.

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