A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item LENGTH-SCALE DEPENDENCE OF VISCOPLASTIC PROPERTIES OF SILVER SINTER REVEALED BY INDENTATION TESTING AND MODELING(2024) Leslie, David; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This doctoral dissertation research focuses on using a combination of indentation testing and modeling to characterize the creep behavior of heterogeneous silver sinter at different temperatures, using multiple indenter sizes to interrogate length-scale effects. The measured steady-state creep deformation is characterized with three different modeling approaches, that rely on: (i) conventional deviatoric creep potential; (ii) pressure-sensitive Drucker-Prager creep potential; and (iii) length-scale dependent deviatoric creep potential. The creep flow rule for all three cases is Norton’s power-law creep. The materials in this study are from a family of sintered silver materials used for interconnects and die-attach in high-temperature electronics and for conductor traces in printed electronics. The dissertation focuses on identifying and quantifying the length-scale dependence presented by sintered materials due to their non-homogeneous morphology. Testing consists of constant-force indentations using spherical indenters of two different radii at three different temperatures: 25°C, 75°C, and 125°C. The indentation results were first analyzed using two different post-processing methods: an empirical approach with closed-form models (CFM) and a computational FEA approach based on classical continuum mechanics. Differences found between the CFM and numerical (FEA) analyses, while significant at room temperature, reduce with temperature. Both models reveal that indenters of different radii cause significantly different viscoplastic behavior. This dependence on tip radius increases with temperature The research was extended to examine two second-order influences of the metallic agglomerated phase and the discontinuous compliant phase of the microstructure of sintered silver on its viscoplastic behavior: (i) dependence on hydrostatic stress; (ii) dependence on microstructural length-scale. The aim of incorporating the pressure-sensitive modeling was to investigate what effect the intrinsic compressive hydrostatic stress in indentation tests might have on the measured viscoplastic properties. Results from using the Drucker-Prager creep model further confirmed the increasing dependence on length-scale with temperature. The length-scale dependence seen in all the above results is investigated and quantified with the help of a simplified strain-gradient viscoplastic model. This modeling approach is motivated by the conventional mechanism-based strain gradient (CMSG) model that is widely used in plasticity theory to quantify length-scale effects. The characteristic length-scale metric in this problem is presented by the agglomerate size distribution in the sintered material and is quantified in this study with ‘watershed analysis’ of cross-sectional features observed via electron microscopy. This discrete length scale is believed to cause the variations in the observed creep response when queried with indenters of different radii, because of the different strain gradients produced by the two different indenters. The length-scale dependence is incorporated in a strain-gradient viscoplastic constitutive model suitable for finite element modeling of deformation fields containing strong strain-gradients (e.g. in the die attach layer in microelectronics chip assembly). Finally, a procedure is proposed, to incorporate the scale-dependence in the empirical closed-form approach, currently available in the literature, for extracting viscoplastic properties from indentation tests. This approach provides corrected model constants for the strain-gradient viscoplastic model, using simple closed-form equations instead of expensive finite element modeling.Item MECHANICAL CHARACTERIZATION OF NORMAL AND CANCEROUS BREAST TISSUE SPECIMENS USING ATOMIC FORCE MICROSCOPY(2014) Roy, Rajarshi; Desai, Jaydev P.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Breast cancer is one of the most common malignancies among women worldwide. Conventional breast cancer diagnostic methods involve needle-core biopsy procedures, followed by careful histopathological inspection of the tissue specimen by a pathologist to identify the presence of cancerous lesions. However, such inspections are primarily qualitative and depend on the subjective impressions of observers. The goal of this research is to develop approaches for obtaining quantitative mechanical signatures that can accurately characterize malignancy in pathological breast tissue. The hypothesis of this research is that by using contact-mode Atomic Force Microscopy (AFM), it is possible to obtain differentiable measures of stiffness of normal and cancerous tissue specimens. This dissertation summarizes research carried out in addressing key experimental and computational challenges in performing mechanical characterization on breast tissue. Firstly, breast tissue specimens studied were 600 um in diameter, about six times larger than the range of travel of conventional AFM X-Y stages used for imaging applications. To scan tissue properties across large ranges, a semi automated image-guided positioning system was developed that can be used to perform AFM probe-tissue alignment across distances greater than 100 um at multiple magnifications. Initial tissue characterization results indicate that epithelial tissue in cancer specimens display increased deformability compared to epithelial tissue in normal specimens. Additionally, it was also observed that the tissue response depends on the patient from whom the specimens were acquired. Another key challenge addressed in this dissertation is accurate data analysis of raw AFM data for characterization purposes. Two sources of uncertainty typically influence data analysis of AFM force curves: the AFM probe's spring constant and the contact point of an AFM force curve. An error-in-variable based Bayesian Changepoint algorithm was developed to quantify estimation errors in the tissue's elastic properties due to these two error sources. Next, a parametric finite element modeling based approach was proposed in order to account for spatial heterogeneity in the tissue response. By using an exponential hyperelastic material model, it was shown that it is possible to obtain more accurate material properties of tissue specimens as opposed to existing analytical contact models. The experimental and computational strategies proposed in this dissertation could have a significant impact on high-throughput quantitative studies of biomaterials, which could elucidate various disease mechanisms that are phenotyped by their mechanical signatures.Item The Effect of Phase Constitution and Morphology on Room Temperature Deformation Behavior of Binary Titanium Alloys(2011) Wyatt, Zane W.; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Currently, titanium alloys are used in a variety of applications, including defense, aerospace, biomedicine, and even common consumer products such as bicycles and golf clubs. In many applications such as the landing gear of aircraft and geothermal energy production, titanium components may be subjected to stresses for extended periods of time. It has long been known that single-phase α (HCP), single-phase β (BCC), and two-phase α + β Ti alloys can creep at low temperatures (<0.25Tm). For this reason, creep is an important factor to consider when designing titanium alloys for various applications. The first part of this investigation is concerned with single-phase α-Ti alloys. It was found that the twin size (lamellar thickness) decreases with an increase in strain rate. This behavior is unexpected based on the classical understanding of instantaneous twinning. This investigation was able to for the first time demonstrate a time-dependent twinning phenomenon during high strain rate tensile deformation. The second part of this investigation is concerned with experimentally and theoretically studying low-temperature creep deformation behavior of two-phase α + β Ti alloys. Deformation mechanisms were seen in two-phase α + β Ti alloys that are not present during creep of the respective single-phase alloys with compositions equivalent to the individual phases. To investigate the possible interphase interaction stresses, 3D anisotropic Finite element modeling (FEM) was used. These simulations revealed that due to the Burgers orientation relationship between the two phases, deformation such as slip or twinning in the α phase can create very high additional shear stresses on different slip systems in the β phase. This work also revealed that the interfacial stresses that develop between the two phases during elastic deformation will often be much greater than the applied stress. These results were used to help explain the additional deformation mechanisms seen in two-phase alloys that are not seen in the respective single-phase alloys during creep. This work was supported by the National Science Foundation under Grant Number DMR-0906994.Item Traveling Wave Thermoacoustic-Piezoelectric Energy Harvester: Theory and Experiment(2011) Roshwalb, Andrew Zvi; Baz, Amr; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents a theoretical and experimental investigation of a piezoelec- tric energy harvester coupled to a traveling wave thermoacoustic engine (TWTAE). By simplifying the engine using a lumped-parameter model, the performance pa- rameters such as pressure oscillation frequency and amplitude, regenerator hot end temperature, and piezoelectric output voltage are predicted. Also, an axisymmetric finite element model of the piezoelectric energy harvester is developed, resulting in a two-part reduced-order model of the electromechanical impedance of the harvester. The predictions of the finite element model are compared with those of ANSYS finite element analysis and validated experimentally. The two-part model is utilized in a numerical analysis of the TWTAE using DeltaEC (Design Environment for Low- Amplitude ThermoAcoustic Energy Conversion). Results from pressure transducers and the piezoelectric disc attached to a physical realization of the TWTAE are com- pared with theoretical predictions of the lumped-parameter models and DeltaEC analysis. The developed theoretical techniques and experimental validation provide invaluable tools for effective design of the thermoacoustic-piezoelectric harvester.