Browsing by Author "Tiwari, Anil"

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    The development of an interpretive methodology for the application of real-time acousto-ultrasonic NDE technique for monitoring damage in ceramic composites under dynamic loads
    Tiwari, Anil (Virginia Tech, 1993)
    Research effort was directed towards developing a near real-time, acousto-ultrasonic (AU), nondestructive evaluation (NDE) tool to study the failure mechanisms of ceramic composites. Progression of damage is monitored in real-time by observing the changes in the received AU signal during the actual test. During the real-time AU test, the AU signals are generated and received by the AU transducers attached to the specimen while it is being subjected to increasing quasi-static loads or cyclic loads (10 Hz, R = 0.1). The received AU signals for 64 successive pulses were gated in the time domain (T = 40.96 µsec) and then averaged every second over ten load cycles and stored in a computer file during fatigue tests. These averaged gated signals are representative of the damage state of the specimen at that point of its fatigue life. This is also the first major attempt in the development and application of real-time AU for continuously monitoring damage accumulation during fatigue without interrupting the test. The present work has verified the capability of the AU technique to assess the damage state in silicon carbide/calcium aluminosilicate (SiC/CAS) and silicon carbide/magnesium aluminosilicate (SiC/MAS) ceramic composites. Continuous monitoring of damage initiation and progression under quasi-static ramp loading in tension to failure of unidirectional and cross-ply SiC/CAS and quasi-isotropic SiC/MAS ceramic composite specimens at room temperature was accomplished using near real-time AU parameters. The AU technique was shown to be able to detect the stress levels for the onset and saturation of matrix cracks, respectively. The critical cracking stress level is used as a design stress for brittle matrix composites operating at elevated temperatures. The AU technique has found that the critical cracking stress level is 10-15 % below the level presently obtained for design purposes from analytical models. An acousto-ultrasonic stress-strain response (AUSSR) model for unidirectional and cross-ply ceramic composites was formulated. The AUSSR model predicts the strain response to increasing stress levels using real-time AU data and classical laminated plate theory. The Weibull parameters of the AUSSR model are used to calculate the design stress for thermo-structural applications. Real-time AU together with the AUSSR model was used to study the failure mechanisms of SiC/CAS ceramic composites under static and fatigue loading. An S-N curve was generated for a cross-ply SiC/CAS ceramic composite material. The AU results are corroborated and complemented by other NDE techniques, namely, in-situ optical microscope video recordings and edge replication.
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    A feasibility study of the acousto-ultrasonic technique to assure the quality of adhesively bonded sheet metal
    Tiwari, Anil (Virginia Tech, 1990)
    This thesis contains the results of Phase-1 of a project funded by Ford Motor Company. The objective is to study the feasibility of Acousto-Ultrasonics (AU) as a nondestructive technique for assuring the quality of adhesively bonded sheet-metal used for automobiles. Other nondestructive (NDT) techniques were also applied viz., ultrasonics. radiography and thermography to supplement and verify the results of the AU technique. The AU Technique demonstrated the best results in terms of its sensitivity to the variations in the properties of the interface. Regions having kissing bonds or regions lacking adhesive were easily identified by this technique. These regions contribute to the mixed mode failure. A bond quality (BQ) model is suggested to take into account the mixed mode failure. Destructive testing results show fairly consistent correlation of BQ values with the breaking strength of the adhesive joint failing in mixed mode failure. The BQ values were calculated from the SWF (stress wave factor) values generated by the AU technique. No correlation was observed between the SWF values and the breaking strengths of the bonds failing cohesively. Cohesive failures occur at higher loads than those for mixed mode failures. These are, of course, governed by the maximum possible strength of a joint. More work needs to be done to develop a better way to analyze signals for differentiating total cohesive failure, at least for academic interest. The results strongly suggest the potential of this technique for quantitative evaluation of such types of bonding. Automation of this technique can be developed for application on the assembly line of the motor-car industry. Future work to make this technique more efficient and sensitive is suggested.