P.T. Summers

Post-Fire Mechanical Properties and Hardness of 5083 and 6082 Aluminum Alloys

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.Copyright

Mechanical Properties of 5000 Series Aluminum Alloys Following Fire Exposure

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.

Microstructure‐Based Constitutive Model for Yield Strength and Strain Hardening of 5XXX‐Series Aluminum Alloys after Non‐Isothermal Fire Exposure

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.

Sensitivity Analysis of a Thermo-Structural Model for Materials in Fire

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.