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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Now showing 1 - 8 of 8
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    Damage Assessment Using Information Entropy of Individual Acoustic Emission Waveforms during Cyclic Fatigue Loading
    (MDPI, 2017-05-30) Sauerbrunn, Christine M.; Kahirdeh, Ali; Yun, Huisung; Modarres, Mohammad
    Information entropy measured from acoustic emission (AE) waveforms is shown to be an indicator of fatigue damage in a high-strength aluminum alloy. 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. Several tension–tension fatigue experiments were performed with dogbone samples of aluminum 7075-T6, a commonly used material in aerospace structures. Unlike previous studies in which fatigue damage is measured based on visible crack growth, this work investigated fatigue damage both prior to and after crack initiation through the use of instantaneous elastic modulus degradation. Results show that one of the three entropy measurement methods appears to better assess the damage than the traditional AE features, whereas the other two entropies have unique trends that can differentiate between small and large cracks.
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    Measures of Entropy to Characterize Fatigue Damage in Metallic Materials
    (MDPI, 2019-08-17) Yu, Huisung; Modarres, Mohammad
    This paper presents the entropic damage indicators for metallic material fatigue processes obtained from three associated energy dissipation sources. Since its inception, reliability engineering has employed statistical and probabilistic models to assess the reliability and integrity of components and systems. To supplement the traditional techniques, an empirically-based approach, called physics of failure (PoF), has recently become popular. The prerequisite for a PoF analysis is an understanding of the mechanics of the failure process. Entropy, the measure of disorder and uncertainty, introduced from the second law of thermodynamics, has emerged as a fundamental and promising metric to characterize all mechanistic degradation phenomena and their interactions. Entropy has already been used as a fundamental and scale-independent metric to predict damage and failure. In this paper, three entropic-based metrics are examined and demonstrated for application to fatigue damage. We collected experimental data on energy dissipations associated with fatigue damage, in the forms of mechanical, thermal, and acoustic emission (AE) energies, and estimated and correlated the corresponding entropy generations with the observed fatigue damages in metallic materials. Three entropic theorems—thermodynamics, information, and statistical mechanics—support approaches used to estimate the entropic-based fatigue damage. Classical thermodynamic entropy provided a reasonably constant level of entropic endurance to fatigue failure. Jeffreys divergence in statistical mechanics and AE information entropy also correlated well with fatigue damage. Finally, an extension of the relationship between thermodynamic entropy and Jeffreys divergence from molecular-scale to macro-scale applications in fatigue failure resulted in an empirically-based pseudo-Boltzmann constant equivalent to the Boltzmann constant.
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    Development of a Fatigue Life Assessment Model for Pairing Fatigue Damage Prognoses with Bridge Management Systems
    (IntechOpen, 2018-12-18) Saad, Timothy; Fu, Chung C.; Zhao, Gengwen; Xu, Chaoran
    Fatigue damage is one of the primary safety concerns for steel bridges reaching the end of their design life. Currently, US federal requirements mandate regular inspection of steel bridges for fatigue cracks; however, these inspections rely on visual inspection, which is subjective to the inspector’s physically inherent limitations. Structural health monitoring (SHM) can be implemented on bridges to collect data between inspection intervals and gather supplementary information on the bridges’ response to loads. Combining SHM with finite element analyses, this paper integrates two analysis methods to assess fatigue damage in the crack initiation and crack propagation periods of fatigue life. The crack initiation period is evaluated using S-N curves, a process that is currently used by the FHWA and AASHTO to assess fatigue damage. The crack propagation period is evaluated with linear elastic fracture mechanic-based finite element models, which have been widely used to predict steady-state crack growth behavior. Ultimately, the presented approach will determine the fatigue damage prognoses of steel bridge elements and damage prognoses are integrated with current condition state classifications used in bridge management systems. A case study is presented to demonstrate how this approach can be used to assess fatigue damage on an existing steel bridge.
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    Fatigue Assessment of Highway Bridges under Traffic Loading Using Microscopic Traffic Simulation
    (IntechOpen, 2018-11-13) Zhao, Gengwen; Fu, Chung C.; Lu, Yang; Saad, Timothy
    Fatigue is a common failure mode of steel bridges induced by truck traffic. Despite the deterioration caused by environmental factors, the increasing truck traffic volume and weight pose a premier threat to steel highway bridges. Given the uncertainties of the complicated traffic loading and the complexity of the bridge structure, fatigue evaluation based on field measurements under actual traffic flow is recommended. As the quality and the quantity of the available long-term traffic monitoring data and information have been improved, methodologies have been developed to obtain more realistic vehicular live load traffic. A case study of a steel interstate highway bridge using microscopic traffic simulation is presented herein. The knowledge of actual traffic loading may reduce the uncertainty involved in the evaluation of the load-carrying capacity, estimation of the rate of deterioration, and prediction of remaining fatigue life. This chapter demonstrates a systematic approach using traffic simulation and bridge health monitoring-based fatigue assessment.
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    DEVELOPMENT OF AN OFF-LINE RAINFLOW COUNTING ALGORITHM WITH TEMPORAL PARAMETERS AND DATA REDUCTION
    (2016) Twomey, James Matthew; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Rainflow counting methods convert a complex load time history into a set of load reversals for use in fatigue damage modeling. Rainflow counting methods were originally developed to assess fatigue damage associated with mechanical cycling where creep of the material under load was not considered to be a significant contributor to failure. However, creep is a significant factor in some cyclic loading cases such as solder interconnects under temperature cycling. In this case, fatigue life models require the dwell time to account for stress relaxation and creep. This study develops a new version of the multi-parameter rainflow counting algorithm that provides a range-based dwell time estimation for use with time-dependent fatigue damage models. To show the applicability, the method is used to calculate the life of solder joints under a complex thermal cycling regime and is verified by experimental testing. An additional algorithm is developed in this study to provide data reduction in the results of the rainflow counting. This algorithm uses a damage model and a statistical test to determine which of the resultant cycles are statistically insignificant to a given confidence level. This makes the resulting data file to be smaller, and for a simplified load history to be reconstructed.
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    Damage Detection in Fiber Reinforced Concrete with Ultrasonic Pulse Velocity Testing
    (2012) Hong, Rongjin; Goulias, Dimitrios G; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In concrete, fatigue and freeze-thaw are associated with the progressive growth of internal microcracks. The Ultrasonic Pulse Velocity (UPV) technique, one of the most widely used Nondestructive Testing (NDT) methods, is promising in evaluating internal microcracks and eventually detecting damage. The primary objective of this research was to determine the effectiveness of using UPV to detect damage development in polypropylene fiber reinforced concrete under fatigue and freeze-thaw conditions. In order to realize this, i) several experiments were conducted on control samples to assess the response and limitations of UPV, and ii) fatigue and freeze-thaw samples were tested with UPV to evaluate its ability to detect crack development. In terms of modeling, three alternative models were examined and presented relating UPV with porosity and damage.
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    EVOLUTION OF THE MICROSTRUCTURE AND VISCOPLASTIC BEHAVIOR OF MICROSCALE SAC305 SOLDER JOINTS AS A FUNCTION OF MECHANICAL FATIGUE DAMAGE
    (2009) Cuddalorepatta, Gayatri; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The effect of mechanical cycling fatigue damage and isothermal aging histories on the evolution of the constitutive and fatigue responses, and microstructure of microscale SAC305 solder joints is investigated. In particular, the study examines if joint dependent behavior should be expected from as-fabricated and cycled microscale SAC305 joints that exhibit an initial non-homogenous coarse-grained Sn microstructure. In addition, the ability of traditionally used macroscale constitutive models based on continuum mechanics to represent the viscoplastic constitutive behavior of the non-homogenous as-fabricated microscale SAC305 specimens is explored. Insights into the effect of key microstructural features and dominant creep mechanisms influencing the measured viscoplastic behavior of SAC305 are provided using a multi-scale mechanistic modeling framework. Modified lap-shear microscale SAC305 specimens are characterized using the thermomechanical microscale test setup (TMM). Microscale SAC305 solder specimens show significant piece-to-piece variability in the viscoplastic constitutive properties under identical loading histories in the as-fabricated state. The mechanical response is strongly influenced by the grain microstructure across the entire joint, which is non-repeatable and comprises of very few highly anisotropic Sn grains. The statistical non-homogeneity in the microstructure and the associated variability in the mechanical properties in the microscale SAC305 test specimen are far more significant than in similar Sn37Pb specimens, and are consistent with those reported for functional microelectronics solder interconnects. In spite of the scatter, as-fabricated SAC305 specimens exhibit superior creep-resistance (and lower stress relaxation) than Sn37Pb. Macroscale creep model constants represent the non-homogeneous behavior of microscale joints in an average sense. Macroscale modeling results show that the range of scatter measured from macroscale creep model constants is within the range of scatter obtained from the stress relaxation predictions. Stress relaxation predictions are strongly sensitive to the inclusion or exclusion of primary creep models. The proposed multiscale framework effectively captures the dominant creep deformation mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as-fabricated SAC305 solder specimens. The multiscale model predictions for the effect of alloy composition on SAC solders provide good agreement with test measurements. The multiscale model can be extended to understand the effects of other parameters such as aging and manufacturing profiles, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys. Accumulated fatigue damage and isothermal aging are found to degrade the constitutive and mechanical fatigue properties of the solder. The scatter gradually decreases with an increasing state of solder damage. Compared to the elastic-plastic and creep measurements, the variability in the fatigue life of these non-homogenous solder joints under mechanical fatigue tests is negligible. Recrystallization is evident under creep and mechanical fatigue loads. Gradual homogenization of the Sn grain microstructure with damage is a possible reason for the observed evolution of scatter in the isothermal mechanical fatigue curves. The yield stress measurements suggest that SAC305 obeys a hardening rule different from that of isotropic or kinematic hardening. The measured degradation in elastic, plastic and yield properties is captured reasonably well with a continuum damage mechanics model from the literature.
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    Reliability-Based Design Of Piping: Internal Pressure, Gravity, Earthquake, and Thermal Expansion
    (2007-08-09) Avrithi, Kleio; Ayyub, Bilal M.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Although reliability theory has offered the means for reasonably accounting for the design uncertainties of structural components, limited effort has been made to estimate and control the probability of failure for mechanical components, such as piping. The ASME B&PV Code, Section III, used today for the design of safety piping in nuclear plants is based on the traditional Allowable Stress Design (ASD) method. This dissertation can be considered as a primary step towards the reliability-based design of nuclear safety piping. Design equations are developed according to the Load and Resistance Factor Design (LRFD) method. The loads addressed are the sustained weight, internal pressure, and dynamic loading (e.g., earthquake). The dissertation provides load combinations, and a database of statistical information on basic variables (strength of steel, geometry, and loads). Uncertainties associated with selected ultimate strength prediction models -burst or yielding due to internal pressure and the ultimate bending moment capacity- are quantified for piping. The procedure is based on evaluation of experimental results cited in literature. Partial load and resistance factors are computed for the load combinations and for selected values of the target reliability index, β. Moreover, design examples demonstrate the procedure of the computations. A probabilistic-based method especially for Class 2 and 3 piping is proposed by considering only cycling moment loading (e.g., thermal expansion). Conclusions of the study and provided suggestions can be used for future research.