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 SPECTRAL METHODS FOR MODELING AND ESTIMATING VIBRATION FATIGUE DAMAGE IN ELECTRONIC INTERCONNECTS(2023) Welch, Jacob Adam; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The purpose of this thesis is to explore the accuracy of fatigue damage estimation in printedwiring assembly (PWA) interconnects, using purely frequency-domain (also known as spectral) information such as the power spectral density (PSD) of the input excitation. The test case used in this study is the estimation of fatigue damage accumulation rate in the critical interconnects of low profile quad flat-pack (LQFP) components on a PWA, under broad-band random vibration excitation. this study examines whether the fatigue predictions made with this frequency-domain approach are consistent with those obtained from a direct time-domain approach. The frequency-domain response modeling is achieved using a two-stage global-local modeling process using a finite element model (in ABAQUS©), where the dominant modal participation factors for the dynamic response is obtained using a dynamic global simplified dynamic finite element model consisting of shell elements to represent the entire PWA. The PSD of the input excitation is applied as a boundary condition and the PSD of the PWA strain response is recorded at the base of critical components. The corresponding PSD for the dynamic strain response at critical interconnects is estimated with strain-transfer functions (STFs) for each dominant mode, obtained from detailed 3D quasi-static nonlinear local models of the component, adjacent PWB, and the interconnects. The global-local STF provides a relationship between the level of equivalent strain in the critical interconnects and the flexural strain at the adjacent surface of the PWB. The STF for each of the dominant vibration modes is obtained by imposing the corresponding mode-shape predicted by the dynamic global model on the PWB, in the quasistatic local model, using multi-point constraint equations. The PSD of the equivalent strain in the critical interconnect is then estimated via linear modal superposition. A deterministic estimate of the cyclic fatigue damage accumulation rate in the critical interconnect is then conducted with the Basquin high cycle fatigue (HCF) model and linear damage superposition approach, by using three different spectral approaches for representing the strain severity with estimated probability density functions (PDFs). The three approaches include: (i) Raleigh method; (ii) Dirlik method and (iii) Range distribution function created with the Rainflow cycle counting method. Methods (ii) and (iii) are derived from a pseudo time-history created with an inverse Fourier transform. These frequency-domain results are compared to corresponding fatigue damage estimates from a multi-modal time-domain analysis method, to assess the consistency of the two approaches.Item GRAIN-SCALE ANISOTROPIC STUDY OF TENSILE VS. SHEAR MECHANICAL CONSTITUTIVE AND FATIGUE BEHAVIOR IN OLIGOCRYSTALLINE SAC305 SOLDER JOINTS(2021) Deshpande, Abhishek Nitin; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solder joints in microelectronic assemblies experience a multiaxial combination of cyclic extensional and shear loads due to combinations of thermal expansion mismatch and flexure of printed circuit assemblies (PCAs) during thermal cycling or during vibrational loading of constrained PCAs. Although, a significant amount of research has been conducted to study cyclic fatigue failures of solder joints under pure-shear loading, most of the current literature on cyclic tensile loading of solders is on long dog-boned monolithic solder coupons. Unfortunately, such coupon specimens do not capture the critical interactions between key micro-scale morphological features (such as grain orientation, grain boundaries, intermetallic compounds [IMCs] and substrates) that are believed to play important roles in the fatigue of functional solder joints under life-cycle loading. Therefore, Part I of this study uses a combination of experiments and finite element analysis to investigate the differences in mechanisms of cyclic fatigue damage in Sn-3.0Ag-0.5Cu (SAC305) few-grained (oligocrystalline) microscale solder joints under shear, tensile and multiaxial loading modes at room temperature. Cyclic fatigue durability test results indicate that tensile loads are more detrimental compared to shear loads. Tensile vs. shear loading modes are found to cause distinctly different combinations of interfacial damage vs. internal damage in the bulk of the solder (transgranular and intergranular damage), which correlates with the differences observed in the resulting fatigue durability. The test results also confirm that this type of multimodal fatigue damage cannot be modeled with the traditional approach of a power-law dependence on the cyclic amplitude of equivalent deviatoric strain. Instead, multiaxial fatigue damage results are seen to be affected not only by the cyclic equivalent strain amplitudes, but also by the severity of the stress-triaxiality, as hypothesized in models such as Chaboche model.Estimating the true deviatoric strains and triaxiality ratios at the failure sites is not a trivial task in typical oligocrystalline SAC305 solder joints, because the strong anisotropy of the individual grains - and the interactions of such grains with surrounding grains as well as with the interfacial boundaries - make the strain field unique in each joint. Thus, the current approach of modeling solder joints as homogenous isotropic structures, are clearly inadequate because they fail to capture the true grain-scale stress fields at the failure sites. The joint-to-joint variation in the grain morphology leads to variability in fatigue damage accumulation rates under cyclic loading. Part II of this study thus focuses on grain-scale study of the fatigue results presented in Part I, by: (a) characterizing multi-scale anisotropic elastic-plastic properties of SAC305 single crystals, using a hybrid combination of experiments and finite element simulation, (b) applying a grain-scale parametric study to explain the variability seen in Part I, in the bimodal fatigue failures under multiaxial cyclic loading. The anisotropic elastic-plastic properties in Part IIa were determined by conducting monotonic tensile and shear tests on SAC305 single crystal specimens. The anisotropic elastic behavior is modeled using anisotropic elastic stiffness constants for SAC305, whereas anisotropic plasticity is modeled using Hill’s potential in conjunction with a Holloman-type power-law plastic constitutive model. Microstructurally motivated scaling factors are empirically developed, to assess the effect of dendritic and eutectic microstructural features on single-crystal stress-strain properties. This facilitates extrapolation of constitutive properties across different cooling rates and different isothermal aging protocols. Additional empirical scaling factors are also developed to account for the influence of characteristic grain sizes and grain aspect ratios (relative to principal loading directions). The parametric study in Part IIb, was conducted using the anisotropic properties of Part IIa, to quantify the effect of grain anisotropy on variability in cyclic mechanical fatigue curves of SAC305 solder. This study demonstrates an efficient computational approach for determining variability in mechanical response and fatigue behavior of Sn-rich solder joints, thereby reducing the time and costs associated with physical testing.Item Design and Characterization of Additively Manufactured Lightweight Metal Structures with Equivalent Compliance and Fatigue Resistance(2021) Santos, Luis S; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Additive Manufacturing (AM) has been a disruptive manufacturing technology allowing for control of geometric features and material distributions, potentially starting at the atomistic level, to realize structures with lighter weights. However, it is still begin used primarily as a rapid prototyping tool due to challenges arising from various issues that need to be addressed before commercial parts can be deployed. Three of those issues are: (1) characterization of mechanical properties that may vary spatially, (2) identification of novel defects in the parts, and (3) new design approaches that account for the unique capabilities of AM processes and their impact on fatigue resistance.This dissertation addresses these three issues by developing a cyclical indentation technique to characterize the fatigue properties of geometric features only capable with AM. The method produces the degradation of the material stiffness as the number of cyclic loads increases and is capable of generating an entire S-N curve with a single test at sub-millimeter scales. Geometric features are then analyzed by running a thermal and mechanical simulation of a Direct Metal Laser Sintering (DMLS) printing process. The new simulation can account for buckling of features with high aspect ratios, such as low percentage infills or high levels of unit cell porosity, and predicts distortions with less than 5% error. This computational approach is useful for analyzing parts before printing and informs designers about regions in the part that may need modification to prevent buckling. Finally, the experimental and computational techniques are combined to design structures with macroscale topological features and microscale unit cell features that are fatigue resistant.Item PARAMETRIC DESIGN AND EXPERIMENTAL VALIDATION OF CONJUGATE STRESS SENSORS FOR STRUCTURAL HEALTH MONITORING(2021) Kordell, Jonathan; Dasgupta, Abhijit; Yu, Miao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, conjugate stress (CS) sensing is advanced through a parametric evaluation of a surface-mounted design and through experimental validation in monotonic and cyclic tensile tests. The CS sensing concept uses a pair of sensors of significantly different mechanical stiffness for direct query of the instantaneous local stress-strain relationship in the host structure, thus offering measurement of important health indicators such as stiffness (modulus), yield strength, strain hardening, and cyclic hysteresis. In this study, surface-mounted CS sensor designs are parametrically evaluated with finite element modeling, with respect to the sensors’ location, thickness, and modulus and the external loading state. An analytic pin-force model is developed to infer the host structure’s stress-strain state, based on the strain outputs of the CS sensor-pair. Two CS sensor designs are fabricated – one employs resistive foil strain gauges and the second employs fiber optic sensors – and paired with the pin-force model for experimental demonstration of the measurement of: (i) stress-strain history of three different isotropic metal bars (aluminum, copper, and steel) as they experience monotonic tensile loads well into plasticity and (ii) stress-strain hysteresis of a steel bar as it is subject to cyclic tensile fatigue. In the cyclic tests, two machine learning algorithms – anomaly detection and neural net classification – are used in conjunction with the estimated host stiffness from the CS sensor and pin force model to predict the onset of damage in the steel beams.Item THERMODYNAMIC AND INFORMATION ENTROPY-BASED PREDICTION AND DETECTION OF FATIGUE FAILURES IN METALLIC AND COMPOSITE MATERIALS USING ACOUSTIC EMISSION AND DIGITAL IMAGE CORRELATION(2021) Karimian, Seyed Fouad; Modarres, Mohammad; Bruck, Hugh A.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Although assumed to be identical, manufactured components always present some variability in their performance while in service. This variability can be seen in their degradation path and time to failure as they are tested under identical conditions. In engineering structures and some components, fatigue is among the most common degradation mechanisms and has been under extensive study over the past century. A common characteristic of the fatigue life models is to rely on some observable or measurable markers of damage, such as crack length or modulus reduction. However, these markers become more pronounced and detectable toward the end of the component or structure’s life. Therefore, more advanced techniques would be needed to better account for a structure's fatigue degradation. Several methods based on non-destructive testing techniques have developed over the past decades to decrease the uncertainty in fatigue degradation assessments. These methods seek to exploit the data collected by sensors during the operational life of a structure or component. Hence, the assessment of the health state can be constantly updated based on the operational conditions that allow for condition-based monitoring and maintenance. However, these methods are mostly context-dependent and limited to specific experimental conditions. Therefore, a method to effectively characterize and measure fatigue damage evolution at multiple length scales based on the fundamental concept of entropy is studied in this dissertation. The two entropic-based indices used are: Thermodynamic entropy, and, Information entropy.The objectives of this dissertation are to develop new methods for fatigue damage detection and failure prediction in metallic and FRP laminated composite materials by using AE and DIC techniques and converting them to information and thermodynamic entropy gains caused by fatigue damage. 1. Develop and experimentally validate fatigue damage detection, failure prediction, and prognosis approaches based on the information entropy of AE signal waveforms in both metallic and FRP laminated composite materials. 2. Develop and experimentally validate fatigue damage detection, failure prediction, and prognosis approaches based on thermodynamic entropy using the DIC technique in both metallic and FRP laminated composite materials. 3. Develop a framework for RUL estimation of metallic and FRP laminated composite structures based on the two entropic measures.Item 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.Item Evaluation of Information Entropy from Acoustic Emission Waveforms as a Fatigue Damage Metric for Al7075-T6(2016) Sauerbrunn, Christine Marie; Modarres, Mohammad; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Information entropy measured from acoustic emission (AE) waveforms is shown to be an indicator of fatigue damage in a high-strength aluminum alloy. Several tension-tension fatigue experiments were performed with dogbone samples of aluminum alloy, Al7075-T6, a commonly used material in aerospace structures. Unlike previous studies in which fatigue damage is simply measured based on visible crack growth, this work investigated fatigue damage prior to crack initiation through the use of instantaneous elastic modulus degradation. Three methods of measuring the AE information entropy, regarded as a direct measure of microstructural disorder, are proposed and compared with traditional damage-related AE features. Results show that one of the three entropy measurement methods appears to better assess damage than the traditional AE features, while the other two entropies have unique trends that can differentiate between small and large cracks.Item NUMERICAL SIMULATION STUDY OF VIBRATION MITIGATION EFFECTIVENESS OF TUNED MASS DAMPERS FOR TRAFFIC SIGNAL MAST ARM STRUCTURES(2016) Zhu, Yifan; Zhang, Yunfeng; Fu, Chung C; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fatigue damage in the connections of single mast arm signal support structures is one of the primary safety concerns because collapse could result from fatigue induced cracking. This type of cantilever signal support structures typically has very light damping and excessively large wind-induced vibration have been observed. Major changes related to fatigue design were made in the 2001 AASHTO LRFD Specification for Structural Supports for Highway Signs, Luminaries, and Traffic Signals and supplemental damping devices have been shown to be promising in reducing the vibration response and thus fatigue load demand on mast arm signal support structures. The primary objective of this study is to investigate the effectiveness and optimal use of one type of damping devices termed tuned mass damper (TMD) in vibration response mitigation. Three prototype single mast arm signal support structures with 50-ft, 60-ft, and 70-ft respectively are selected for this numerical simulation study. In order to validate the finite element models for subsequent simulation study, analytical modeling of static deflection response of mast arm of the signal support structures was performed and found to be close to the numerical simulation results from beam element based finite element model. A 3-DOF dynamic model was then built using analytically derived stiffness matrix for modal analysis and time history analysis. The free vibration response and forced (harmonic) vibration response of the mast arm structures from the finite element model are observed to be in good agreement with the finite element analysis results. Furthermore, experimental test result from recent free vibration test of a full-scale 50-ft mast arm specimen in the lab is used to verify the prototype structure’s fundamental frequency and viscous damping ratio. After validating the finite element models, a series of parametric study were conducted to examine the trend and determine optimal use of tuned mass damper on the prototype single mast arm signal support structures by varying the following parameters: mass, frequency, viscous damping ratio, and location of TMD. The numerical simulation study results reveal that two parameters that influence most the vibration mitigation effectiveness of TMD on the single mast arm signal pole structures are the TMD frequency and its viscous damping ratio.Item Truck Loading Simulation for the Fatigue Assessment of Steel Highway Bridges(2015) Zhao, Gengwen; Fu, Chung C.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fatigue is a common failure mode for steel bridges. About 80-90% of failures in steel structures are related to fatigue and fracture. Despite the deterioration caused by environmental factors, the increasing traffic volume and weight pose a premier threat of steel highway bridges. The total number of truck passages in the 75-year life of a highway bridge could exceed 100 million. With the aging of existing steel highway bridges and the accumulated damage under truck loading, the fatigue assessment for continuing service has become important for decisions making on the structure maintenance, component replacement, and other major retrofits. This research seeks to develop a framework for the fatigue assessment of steel highway bridges based on simulated truck loading. The I-270 Bridge over Middlebrook Road was numerically studied with the proposed methodology. With the help of the available long-term monitoring traffic data and information, truck loading was obtained through the probability-based full velocity difference model. Then, the three-dimensional finite element (FE) global and local bridge models were studied subjected to the simulated truck loading. Meanwhile, the preliminary field test and the long-term monitoring test were also been conducted. The FE models were calibrated by the collected field measurements through monitoring systems, and the simulated numerical structural responses were validated. Lastly, Miner's rule and the rainflow counting algorithm were used in the analysis of simulated numerical structural responses to estimating the fatigue life. Thus, the proposed methodology could be used to realistically simulate the fatigue behavior of steel highway bridges under current or future truck loading, to direct the experimental designs and instrumentation plans before performing experiments on laboratory or on site, and to better understand the fatigue mechanism and prevent the fatigue damage of steel highway bridges.Item Fatigue Properties of Graphene Interconnects on Flexible Substrates(2014) Paradee, Gary; Christou, Aris; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis represents the first determination of the fatigue behavior of Graphene as interconnect material electronic components on flexible substrates. The potential application of this interconnect material is for displays on flexible substrates where fatigue resistance is required due to the stress placed on the interconnect during mechanical bending. As the display is cyclically deformed (fatigued) during normal operation, cracks in the interconnect layer initiate and propagate leading to the lineout failure condition. The major contribution of this work is to show that Graphene is a superior interconnect material to the present state of the art Indium Tin Oxide (ITO) due to its electrical, optical and mechanical properties. The experimental approach in this thesis is based on Graphene samples which were fabricated on Silicon Nitrite (Si3N4)/Polyethylene Naphthalate (PEN) substrates. For comparison, both patterned and uniform ITO films ITO films on Si3N4/PEN were fabricated. The results of the in-depth characterization of Graphene are reported and based on Atomic Force Microscopy (AFM), Raman Spectroscopy and Scanning Electron Microscopy (SEM) are reported. The fatigue characteristics of ITO were determined at stress amplitudes ranging from 2000 MPa to 400 MPa up to 5000 cycles. The fatigue characteristics of Graphene were determined at stress amplitudes ranging from 80 GPa to 40 GPa up to 5000 cycles. The fatigue S-N curves were determined and showed that Graphene's endurance limit is 40 GPa. Beyond the endurance limit, there is no observable high cycle or low cycle fatigue indication for Graphene on a flexible substrate such as PEN. The microstructural analysis by SEM and AFM did not reveal normal fatigue crack growth and propagation. This thesis presents the first comprehensive behavior of Graphene in a bending fatigue stress environment present in numerous flexible electronic applications. The design and stress environments for safe operation has been defined.