Browsing by Subject "Fatigue prediction"
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Item An electrical resistance-based fatigue life prediction model and its application in lithium-ion battery ultrasonic welding(2012-12) Zhao, Nanzhu; Li, Wei, doctor of mechanical engineeringUltrasonic welding is one of the leading technologies for joining multiple, thin sheets of dissimilar materials, such as copper and aluminum, for automotive lithium-ion batteries. The performance of ultrasonic welds, particularly the fatigue life, however, has not been well studied. In this work, a theoretical fatigue life model for ultrasonically welded joints was developed using continuum damage mechanics. In the model, the damage variable was defined as a function of the increase of the joint electrical resistance, resulting in an electrical resistance-based fatigue life prediction model. The fatigue model contains two constants to be determined with experimental data, depending on different fatigue loads and joint properties. As an application, the fatigue life model was validated for Al-Cu lithium-ion battery tab joints. Mechanical fatigue tests were performed under various stress loading conditions for welds made using different welding parameters. It is shown that the developed model can be used to predict the remaining life of the ultrasonically welded battery tab joints for electric and hybrid electric vehicles by monitoring the electrical resistance change. In addition, thermal and electrical fatigue tests were performed for Al-Cu battery tab welds using simulated operating conditions of electrical vehicles. These included temperature cycling between -40 and 90 °C and current cycling of 0 to 10 A. All the tests were conducted on individual weld joints. The results showed that the thermal and electrical loads imposed insignificant effect on the electrical resistance of the battery tab joints.Item Fatigue damage prediction in deepwater marine risers due to vortex-induced vibration(2011-08) Shi, Chen; Manuel, Lance; Kallivokas, Loukas F.; Kinnas, Spyros A.; Powers, Edward J.; Tognarelli, Michael A.Slender marine risers used in deepwater applications often experience vortex-induced vibration (VIV). Fatigue damage associated with VIV is of great concern to offshore engineers; however, it has proven difficult to predict this fatigue damage using existing semi-empirical tools. Similarly, approaches based on theoretical and computational fluid dynamics (CFD) generally rely on simplified assumptions on the fluid flow fields and response characteristics. To gain an understanding of VIV and associated fatigue damage, full-scale field monitoring campaigns as well as reduced-scale laboratory experiments are often carried out, wherein the riser response in the form of strains and/or accelerations is recorded using an array of a limited number of sensors distributed over the length of the riser. Simultaneously, current velocities at a proximate location are also recorded. Such measurements generally reveal complex characteristics of the dynamic response of a riser undergoing VIV, including the presence of multiple vibration harmonics, non-stationary behavior, and the existence of sustained or intermittent traveling wave patterns. Such complex features, often not accounted for in some semi-empirical and theoretical approaches, are critical to take into consideration for accurate fatigue damage estimation. In this study, several empirical methods are employed to first reconstruct the response of an instrumented riser and, then, estimate fatigue damage rates over the entire span of the riser based on a limited number of discrete measurements. The methods presented employ the measured data in different ways. One method, referred to as ``weighted waveform analysis'' relies on expressing the riser response as a summation of several weighted waveforms or riser modes; the mode shapes are ``assumed'' and time-varying weights for each mode are estimated directly from the measurements. The riser response over the entire span is reconstructed based on these assumed mode shapes and estimated modal weights. Other methods presented extract discrete mode shapes from the data directly. With the help of interpolation techniques, continuous mode shapes are formed, and the riser response is again reconstructed. Fatigue damage rates estimated based on the reconstructed strains obtained using the various empirical methods are cross-validated by comparing predictions against direct measurements available at the same locations (but not used in the analyses). Results show that the empirical methods developed here may be employed to accurately estimate fatigue damage rates associated with individual recorded segments of measurements. Finally, a procedure for prediction of long-term fatigue damage rates of an instrumented marine riser is presented that relies on combining (multiplying) the fatigue damage rates associated with short recorded segments for specific current profile types, with the relative likelihood of different incident current profiles, and integration over all current profiles. It should be noted that the empirical approaches to fatigue damage estimation presented in this study are based only on measured data; also, they explicitly account for different riser response characteristics and for site-specific current profiles developed from metocean studies. Importantly, too, such estimation procedures can easily accommodate additional data that become available in any ongoing field monitoring campaign to improve and update long-term fatigue damage prediction.