SummaryThe structural response of welded aluminium in fire is computationally and experimentally analysed. A finite element (FE) model is developed to compute the deformation and failure of gas metal arc welded (GMAW) aluminium plate under combined loading and one‐sided unsteady‐state heating representative of fire. The FE model predicts the deformation of the weld, heat‐affected zone and parent plate based on the combined effects of elastic softening, plastic softening and creep. The effects of residual stresses in the weld and thermal expansion on the deformation response are also analysed. The numerical accuracy of the model is rigorously evaluated using a large amount of deformation and failure stress data obtained from fire structural tests performed with welded AA5083–AA5083, AA5083–AA6061 and AA6061–AA6061 plates. Good agreement is found between results computed with the FE model and experimental testing. The results reveal that GMAW welds do not reduce the structural performance of aluminium in fire unless the maximum temperature remains below the recrystallisation temperature. Copyright © 2014 John Wiley & Sons, Ltd.

An experimental study was performed comparing changes in microstructure and mechanical properties of six different 5000 series alloys following a simulated fire exposure. To simulate the fire exposure, specimens were subjected to a constant heating rate of 25 °C /min (up to 500 °C) and then water quenched. Quasi-static tensile tests were conducted to quantify yield strength. Additionally, grain evolution was examined by optical microscopy for each alloy. The 5000 series alloys with different tempers resulted in residual strengths between 85 and 157 MPa following the fire exposure. Most alloys exhibited recovery between 100 °C to 280 °C followed by recrystallization between 300 °C to 340 °C. However, the 5456-H116 alloy, which has the highest magnesium content, maintained 60% of room temperature yield strength. This alloy underwent recovery but did not have a clear recrystallization, as apparent in both the micrographs and mechanical testing.

An integrated infrared thermography and 3-D digital image correlation (TDIC) technique has been developed which allows for simultaneous measurement of spatial and temporal distributions of temperatures and displacements. For this, a novel technique was developed to calibrate the IR thermal cameras with a stereo-vision digital image correlation (DIC) system using the standard pin-hole stereo calibration model. This method fuses thermal and displacement information and compensates for the difference in camera resolutions. Several high temperature black and white paints were evaluated to determine their characteristics including the temperature-dependent emissivity of each paint, the mixed emissivity of both paints in the speckle pattern, and optical thickness. The advantages of evaluating linked full-field temperatures and strain measurements through the TDIC technique are demonstrated through measurements obtained on an E-glass/vinyl ester/balsa wood sandwich composite subjected to simultaneous one-sided heating and compressive loading.

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.

Aluminum alloys are increasingly being used in a broad spectrum of load-bearing applications such as light rail and marine crafts. The structural performance of such aluminum structures during and after a fire is a major concern. Post-fire evaluation of structural integrity and assessment for structural member replacement requires an understanding of the residual (post-fire) mechanical state of the material. In this work, a model is developed to predict the residual constitutive behavior of AA5083-H116 at room temperature following fire exposure. This model comprises several sub-models to predict (i) microstructural evolution, (ii) residual yield strength, and (iii) residual strain hardening behavior. Time-temperature dependent kinetics models were implemented to predict microstructural evolution, i.e., recovery and recrystallization, during a non-isothermal fire exposure. The residual yield strength is predicted as a function of the subgrain (recovery) and grain (recrystallization) evolution based on kinetic modeling. The residual strain hardening behavior is predicted using the Kocks–Mecking–Estrin law modified to account for the additional dislocation storage and dynamic recovery of subgrains. Constitutive model predictions of residual yield strength and strain hardening show good agreement with experimental data residual yield strength and strain hardening data.

One of the major concerns regarding the use of lightweight materials in ship construction is the response of those materials to fire scenarios, including the residual structural performance after a fire event. This paper presents a study on creep damage evolution in 5083 marine-grade aluminum alloy and its impact on residual mechanical behavior. Tests conducted at 400°C and pre-selected tensile stress levels were interrupted at target amplitudes of accumulated engineering creep strains to investigate the stress-induced damage using ex-situ characterization. Two-dimensional optical and electron microscopy and three-dimensional X-ray tomography were utilized on samples extracted from these test specimens to characterize the external and internal creep damage. The stress-induced damage is primarily manifested as cavitation and dynamic microstructural evolution. Cavitation morphology, orientation and grain structure evolution were investigated on three perpendicular sample surfaces. A 3D examination of the damage state provided consistent damage information to that obtained from the 2D analysis. The post-fire mechanical properties were also evaluated and linked to the microstructural change. The competing processes of cavitation and grain structure evolution were investigated to develop an understanding of the stress-induced damage associated with high temperature creep.

Aluminum alloys are increasingly being used in a broad spectrum of load-bearing applications such as light rail and marine crafts. Post-fire evaluation of structural integrity and assessment of the need for structural member replacement requires an understanding of the residual (post-fire) mechanical behavior. In this work, models are presented to predict the residual (post-fire) constitutive behavior, including yield strength and strain hardening, at ambient conditions following fire exposure. This model consists of a series of sub-models for (i) microstructural evolution, (ii) residual yield strength, and (iii) residual strain hardening behavior. Kinetics-based (time-temperature dependent) models were implemented to predict microstructural evolution during fire, i.e., recovery and recrystallization for 5xxx-series Al alloys.. The residual yield strength is predicted using individual strengthening contributions and which are function of the microstructural material state. The residual strain hardening behavior is predicted using the Kocks-Mecking-Estrin law modified to account for the additional dislocation storage and dynamic recovery from subgrains. The constitutive model for residual mechanical behavior was bench-marked against AA5083-H116 specimens exposed to conditions resembling those in fire. The residual yield strength and strain hardening models show good agreement with experimental data.

Aluminum alloys are increasingly being used in lightweight transportation applications such as naval vessels and light passenger rail. The aluminum alloys considered include 5xxx-series (Al-Mg) and 6xxx- series (Al-Mg-Si) alloys due to their mechanical strength, corrosion resistance, and weldability. A major concern in the use of aluminum alloys in lightweight structural applications is fire exposure. Aluminum mechanical properties are significantly reduced at 300°C. After fire exposure, structural damage will vary due to the local thermal history which is governed by the fire size and proximity and other environmental factors. It is paramount to understand the nature of this structural damage in terms of residual (post-fire) constitutive behavior so as to allow for informed damage assessment. AA5083-H116 and AA6061-T651 residual constitutive behavior was characterized using quasi-static tension tests. The alloys were exposed to elevated temperatures at controlled heating rates using an induction heater to simulate the varying conditions in a fire environment. The thermal history dependence of residual constitutive behavior was elucidated in terms of the microstructural strengthening mechanisms. The primary strengthening mechanisms of AA5083 and AA6061 evolve at elevated temperatures due to recrystallization and precipitate coarsening, respectively. Strengthening mechanism evolution has been shown to be kinetically dependent which explains the thermal history dependence of residual constitutive behavior. Structural damage from a wide range of fire scenarios may be understood using these underlying strengthening mechanisms which govern residual constitutive behavior.

Aluminum alloys are increasingly being used in a broad spectrum of load-bearing applications such as light rail and marine crafts. Post-fire evaluation of structural integrity and assessment of the need for structural member replacement requires understanding of the residual (post-fire) mechanical behavior. Aluminum alloys are strengthened by either strain hardening (cold/hot rolling) or precipitation hardening (heat treatment). Prevalent structural alloys strengthened by each method were investigated in this research: AA5083-H116 (strain hardened) and AA6061-T651 (precipitation hardened). An experimental study was conducted to quantify the residual mechanical behavior of aluminum alloys following a fire exposure. Heating of the aluminum alloys results in evolution of material microstructure, which governs mechanical behavior. Microstructural evolution can be predicted as a kinetically-driven process. As a result, the effects of exposure temperature and heating rate on the residual mechanical properties were quantified. Monotonic, uniaxial tension tests were performed to measure the residual stress strain relations, which were used to quantify the residual Young's modulus, yield strength, ultimate strength, and work hardening behavior. AA6061-T651 was determined to have a more significant decrease in strength after fire exposure as compared to AA5083-H116. The heating rate affected the temperatures at which property degradation initiated as well as the residual property magnitude at a given temperature. This is most prevalent in temperature regions with significant microstructural changes, such as recrystallization and precipitate growth. The knowledge elucidated in this study was used to develop empirical evolution models to estimate residual yield strength after linear (ramp) heating and isothermal exposure. Utilizing these models, residual yield strength evolution after realistic fire exposure, which includes combinations of linear and isothermal heating, may be estimated and understood. (C) 2014 Elsevier Ltd. All rights reserved.

Aluminum alloys are being increasingly used in lightweight transportation applications such as naval vessels and passenger rail. The primary aluminum alloys considered are Al-Mg (5xxx) and Al-Mg-Si (6xxx) due to their mechanical strength, corrosion resistance, and weldability. A major concern in the use of aluminum alloys for lightweight structural applications is fire exposure. Aluminum mechanical properties begin to significantly degrade at temperatures above 300°C. After fire exposure, structural integrity will be governed by the residual, post-fire strength of the aluminum. However, scarce data is available regarding the post-fire mechanical response. The post-fire mechanical properties were characterized for several aluminum alloys: 5083-H116, 6082-T651 plate, and 6082-T6 extrusion. The alloys were exposed to elevated temperatures in a furnace to simulate a fire environment. Tension tests were performed to determine the mechanical response of the alloys. Vickers hardness measurements were also performed on specimens exposed for varying durations and temperatures to quantify the time and temperature-dependent behavior. The observed behaviors were explained in relation to the microstructural strengthening mechanisms for each alloy. Correlations were developed between the mechanical properties and Vickers hardness indentations.

A thermo-structural model was developed and validated to predict the failure of compressively loaded fiber-reinforced polymer (FRP) laminates during one-sided heating from a fire. The model consists of a one-dimensional pyrolysis model to predict the thermal response of a decomposing material and an integral structural model based on the bending equation. The thermo-structural model predicts temperatures, out-of-plane deflections, and compressive failure of laminates exposed to fire conditions. Model results were validated with intermediate-scale compression load failure tests with a one-sided heat flux exposure. Through a sensitivity analysis of the model predictions to input parameters, the residual elastic modulus was determined to be of utmost importance to both prediction of out-of-plane deflection and time-to-failure.

A thermo-structural model was previously developed and validated for predicting the failure of compressively loaded fiber-reinforced polymer (FRP) laminates by one-sided heating in fire. The model consists of a one-dimensional pyrolysis model to predict the temperature and decomposition response. An integrated structural model uses the thermal predictions to predict thermally-induced bending caused by one-sided heating. Failure is predicted based on a localized failure criterion using the compressive strength of the material. The analysis was performed by slightly perturbing the thermal and mechanical properties to determine their effect on predictions of the out-of-plane deflection and time-to-failure. The predicted out-of-plane deflections were affected by several properties, including the in-plane thermal expansion and residual elastic modulus. The residual elastic modulus also had a significant effect on time-to-failure predictions. This demonstrates the sensitivity of the model to these parameters in predicting both the time-to-failure and deflection behavior of the laminate.

A new thermo-structural model was developed and validated to predict the failure of compressively loaded fiber-reinforced polymer (FRP) laminates during one-sided heating from a fire. The model consists of the best thermal and structural models in the literature integrated into a single predictive model. This includes a one-dimensional pyrolysis model to predict the thermal response of a decomposing material. Using the thermal response to calculate the mechanical properties, an integral structural model was developed considering thermally induced bending caused by one-sided heating. The thermo-structural model predicts out-of-plane deflections and compressive failure of laminates in fire conditions. This paper also provides an improved failure model for FRP laminates exposed to fire, a first validation study on the modeling approach using intermediate-scale compression load failure tests with a one-side heat flux exposure, and a first sensitivity study of the input parameter effects on the structural response of FRP laminates. Through the sensitivity study, the out-of-plane deflection predictions exhibited little sensitivity to the thermal inputs. However, the time-to-failure predictions were significantly affected by the virgin conductivity and specific heat capacity. The structural inputs exhibited a significant impact on the out-of-plane deflection predictions. The in-plane thermal expansion, residual elastic modulus above the glass transition temperature, and vertical temperature profile significantly affected the magnitude of the out-of-plane deflection; however, only the in-plane thermal expansion and residual elastic modulus affected the failure direction. The time-to-failure prediction was only significantly affected by the residual elastic modulus. A better agreement between the predicted and observed times-to-failure was achieved by reducing the residual elastic modulus.

Intermediate-scale, one-sided heating tests were performed on compressively loaded E-glass vinyl ester composite laminates. The tests were designed to investigate the effect of varying the applied stress, applied heat flux, and laminate dimensions on structural response. Three failure modes were observed in testing: large-scale buckling, localized kinking, and forced-response deflection. The failure modes were dependent on applied stress and independent of applied heating. The times-to-failure of the laminates exhibited an inverse relationship with the applied stress and heating levels. The use of a single temperature was incapable of quantifying laminate failure due to variations in temperature at failure for a given stress level. A dimensionless relationship was developed as a function of temperature for the applied stress and slenderness ratio. This relationship compares the applied stress, slenderness ratio, and laminate temperature at failure and may be used in design of composite laminate structures to determine failure.

Fiber-reinforced plastic (FRP) composite materials are being considered for structural applications on the topside of naval and commercial surface ships [1]. FRP composite materials are being considered for these applications in part because of the weight savings, lower construction costs, and improved life cycle costs. Though the FRP composite materials are designed to have the required structural response for strength under normal operating conditions, the structural performance of the composite degrades rapidly at elevated temperatures easily achievable during fires [2, 3]. The structural properties of FRP composite materials have been measured to dramatically decrease at temperatures around the glass transition temperature of resin [1]. For inexpensive polyester and vinyl ester resins typically used in ship construction, the glass transition temperature is typically around 120°C. Maintaining temperatures below the glass transition temperature, thereby protecting the structural integrity of the composite design, would require 100mm of insulation on each face of the laminate [4].