Browsing by Author "Summers, Patrick T."
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- Microstructure-based Constitutive Models for Residual Mechanical Behavior of Aluminum Alloys after Fire ExposureSummers, Patrick T. (Virginia Tech, 2014-04-24)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.
- Overview of Aluminum Alloy Mechanical Properties During and After FiresSummers, Patrick T.; Chen, Yanyun; Rippe, Christian; Allen, Ben; Mouritz, Adrian; Case, Scott W.; Lattimer, Brian Y. (Springer, 2015)Aluminum alloys are increasingly being used in a broad spectrum of load-bearing applications such as lightweight structures, light rail, bridge decks, marine crafts, and off-shore platforms. A major concern in the design of land-based and marine aluminum structures is fire safety, at least in part due to mechanical property reduction at temperatures significantly lower than that for steel. A substantial concern also exists regarding the integrity and stability of an aluminum structure following a fire; however, little research has been reported on this topic. This paper provides a broad overview of the mechanical behavior of aluminum alloys both during and following fire. The two aluminum alloys discussed in this work, 5083-H116 and 6061-T651, were selected due to their prevalence as lightweight structural alloys and their differing strengthening mechanisms (5083 – strain hardened, 6061 – precipitation hardened). The high temperature quasi-static mechanical and creep behavior are discussed. A creep model is presented to predict the secondary and tertiary creep strains followed by creep rupture. The residual mechanical behavior following fire (with and without applied stress) is elucidated in terms of the governing kinetically-dependent microstructural mechanisms. A review is provided on modeling techniques for residual mechanical behavior following fire including empirical relations, physically-based constitutive models, and finite element implementations. The principal objective is to provide a comprehensive description of select aluminum alloys, 5083-H116 and 6061-T651, to aid design and analysis of aluminum structures during and after fire.
- Predicting Compression Failure of Fiber-reinforced Polymer Laminates during FireSummers, Patrick T. (Virginia Tech, 2010-04-30)A thermo-structural model was developed to predict the failure of compressively loaded fiber-reinforced polymer (FRP) laminates during fire. The thermal model was developed as a one-dimensional heat and mass transfer model to predict the thermal response of a decomposing material. The thermal properties were defined as functions of temperature and material decomposition state. The thermal response was used to calculate mechanical properties. The structural model was developed with thermally induced bending caused by one-sided heating. The structural model predicts out-of-plane deflections and compressive failure of laminates in fire conditions. Laminate failure was determined using a local failure criterion comparing the maximum combined compressive stress with the compressive strength. Intermediate-scale one-sided heating tests were performed on compressively loaded FRP laminates. The tests were designed to investigate the effect of varying the applied stress, applied heat, and laminate dimensions on the structural response. Three failure modes were observed in testing: kinking, localized kinking, and forced-response deflection, and were dependent on the applied stress level and independent of applied heating. The times-to-failure of the laminates followed an inverse relationship with the applied stress and heating levels. The test results were used to develop a relationship which relates a non-dimensionalized applied stress with a non-dimensionalized slenderness ratio. This relationship relates the applied stress, slenderness ratio, and temperature of the laminate at failure and can be used to determine failure in design of FRP laminate structures. The intermediate-scale tests were also used to validate the thermo-structural model with good agreement.