Microstructure-based Constitutive Models for Residual Mechanical Behavior of Aluminum Alloys after Fire Exposure
Summers, Patrick Timothy
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Aluminum alloys are increasingly being used in a broad spectrum of applications such as lightweight structures, light rail, bridge decks, marine crafts, and off-shore platforms. The post-fire (residual) integrity of aluminum structures is of particular concern as a severe degradation in mechanical properties may occur without catastrophic failure, even for short duration, low intensity fires. The lack of research characterizing residual mechanical behavior results in an unquantified mechanical state of the structure, potentially requiring excessively conservative repair. This research aims to develop an in-depth understanding of the mechanisms governing the residual aluminum alloys so as to establish a knowledge-base to assist intelligent structural repair. In this work, the residual mechanical behavior after fire exposure of marine-grade aluminum alloys AA5083-H116 and AA6061-T651 is characterized by extensive mechanical testing. Metallography was performed to identify the as-received and post-fire microstructural state. This extensive characterization was utilized to develop constitutive models for the residual elasto-plastic mechanical behavior of the alloys. The constitutive models were developed as a series of sub-models to predict (i) microstructural evolution, (ii) residual yield strength, and (iii) strain hardening after fire exposure. The AA5083-H116 constitutive model was developed considering the microstructural processes of recovery and recrystallization. The residual yield strength was calculated considering solid solution, subgrain, and grain strengthening. A recovery model was used to predict subgrain growth and a recrystallization model was used to predict grain nucleation and growth, as well as subgrain annihilation. Strain hardening was predicted using the Kocks-Mecking-Estrin law modified to account for the additional dislocation storage and dynamic recovery of subgrains. The AA6061-T651 constitutive model was developed considering precipitate nucleation, growth, and dissolution. A Kampmann-Wagner numerical model was used to predict precipitate size distribution evolution during elevated temperature exposure. The residual yield strength was calculated using solid solution and precipitate strengthening, considering both shearable and non-shearable precipitates. A modified KME law was used to predict residual strain hardening considering the additional effects of the precipitate-dislocation interactions, focusing on the efficient of dislocation (Orowan) loop storage and recovery about the precipitates. In both cases, the constitutive models were bench-marked against experimental data.
- Doctoral Dissertations