Steel-steel composite metal foam in simulated pool fire testing
Introduction
According to the US Department of Transportation, railroads are the safest means to transport hazardous materials (HAZMAT) [1]. The organizations that overlook the transportation of HAZMAT by rail are the Federal Railroad Administration (FRA), the Pipeline and Hazardous Materials Safety Administration (PHMSA), and the Department of Homeland Security (DHS) [1]. Although the number of rail HAZMAT accidents has dropped 66% since 2000, there have still been some accidents in recent years where HAZMAT transportation accidents have caused massive fatalities [1]. For example, on July 6th, 2013, a runaway train carrying 7.7 million liters of petroleum crude oil burst into flames after crashing into Lac-Mégantic, Quebec, travelling at 104 km/h [2]. The crude oil that had been spilled in the streets from the damaged rail cars led to multiple fires and explosions that destroyed most of the downtown area [2]. The Lac-Mégantic accident resulted in 47 casualties [2].
The Lac-Mégantic accident led to the development of the Department of Transportation (DOT)-117 tank car [3]. In May 2015, the DOT established the “DOT-117” specifications, which set stronger regulations on tank cars carrying HAZMAT—such as crude oil [1]. The DOT 117R set standards for the thermal protection system of tank cars [4]. According to the DOT 117R specifications, the tank car must have sufficient thermal resistance so that there will be no release of any lading within the tank car, except release through the pressure release device, when subjected to a pool fire for 100 min, and a torch fire for 30 min [5]. Furthermore, a non-jacketed tank car must have a thermal protection blanket with at least a ½ inch thick material, and the entire thermal protection system must be covered with a metal jacket of a thickness not less than 11 gauge A1011 steel or equivalent and flashed around all openings so as to be weather tight [3]. The exterior surface of a carbon steel tank and the inside surface of a carbon steel jacket must be given a protective coating as well.
The DOT-117 Specifications were deemed not strict enough in some areas, and this led to the Fixing America's Surface Transportation (FAST) Act [1]. Specifically, the FAST Act required increased thermal blanket protection for tank cars, a risk-based approach to phasing out tank cars carrying flammable liquids starting with crude oil tank cars first, followed by tank cars carrying ethanol, then followed by tank cars carrying other flammable liquids and required top fittings protection on tank car retrofits [1]. These enhancements will help to mitigate the consequences of rail accidents should they occur [1]. Therefore, there is an immediate need for novel material with superior puncture resistance, impact energy absorption capability and fire insulation properties to improve the safety of tank cars carrying HAZMAT.
Metal foams are known for their high strength to density ratio, high specific stiffness, and greatly improved energy absorption and superior thermal characteristics [[6], [7], [8]]. However, regular metal foams are not strong enough when considering huge tank car impact energies. Composite metal foam (CMF) is a unique novel material based on a combination of properties of metal matrix composites and metal foams. CMF can be made out of 100% stainless steel and be as light as aluminum while offering close to two orders of magnitude higher energy absorption compared to the parent steel material [[8], [9], [10]]. The term composite metal foam refers to a class of metal foam made with hollow metal spheres surrounded by a metallic matrix [[8], [9], [10]]. CMF has many physical and mechanical properties that make it suitable for many different applications including the structural material for tank cars carrying HAZMAT.
Due to the regularity of its structure, uniform deformation under compression, and the presence of a matrix between the pores, CMF is able to maintain a relatively uniform plateau stress over large amounts of strain when loaded under compression, which offers large strength under both quasi-static and cyclic loading compared to other metal foams [[8], [9], [10], [11], [12], [13], [14], [15], [16]]. Further studies of composite metal foams showed an increase in performance under higher loading rates [17,18], which makes CMF more suitable for protection not only against high speed impact of tank cars, but also various types of ballistic [19] and blast threats [20]. Composite metal foams made from heavy metals such as iron, tungsten, and vanadium can offer reliable radiation shielding against variety of sources from X-Ray [21], to neutron [22], and Gamma ray [23], but with the advantage of low density and great mechanical properties.
In general, metal foams have been known to make efficient and effective heat sinks due to their large surface area resulting from the presence of pores [8,24,25]. On the other hand, stainless steel composite metal foam can offer a reduction in heat transfer compared to bulk stainless steel due to the presence of air trapped within the spheres [8,24]. These pockets of air within the composite metal foam help disrupt flow of heat through the material that can be seen in bulk stainless steel samples [8,24]. As such, composite metal foams can be a safer structural material for next generation rail tank cars.
This paper will report the performance of Composite Metal Foam (CMF) panels under Simulated Pool Fire Testing conditions in accordance with the 49 Code of Federal Regulations (CFR) Part 179, Specifications for Tank Car, Appendix B Procedures through both experimental and numerical approaches.
Section snippets
Materials and processing
Three panels of 30 × 30 × 1.59 cm Steel-Steel Composite Metal Foam (S–S CMF) were manufactured using stainless steel hollow spheres embedded in a 316L stainless steel powder matrix and processed using powder metallurgy technique previously developed [[8], [9], [10], [11], [12], [13]]. Hollow steel spheres with 2 mm outer diameter were manufactured by Hollomet GmbH in Dresden, Germany using lost core technique [26,27]. 316L stainless steel powder from North American Höganäs High Alloys LLC was
Calibration results
The average initial plate temperature was measured to be 23.7 °C prior to the calibration. Calibration furnace run was conducted at 825 °C continuous temperature and the conditions specified in 2.a.6 of 49 CFR Part 179 App. B were achieved with three of nine TCs reaching 427 °C (800 °F) at 13 min ±1 min. Successful calibration results for the simulated pool-fire tests can be seen in Fig. 3. The setup used for the calibration test was duplicated for all three simulated pool-fire exposures,
Mathematical modeling
This section describes the development of a mathematical model to predict the performance of the SS-CMF samples tested according to the simulated pool fire test procedure specified in 49 CFR Part 179 Appendix B simulated pool fire test.
Uncertainty assessment
This section describes the uncertainty assessment for the measured and calculated unexposed surface temperatures in the calibration and simulated pool fire tests on the S–S CMF specimens. The assessment is based on the procedure described in ASTM E2536, Standard Guide for Assessment of Measurement Uncertainty in Fire Tests. ASTM E2536 is based on ISO/IEC Guide 98-3, also referred to as the GUM (Guide on Uncertainty of Measurements). The ASTM guide specifically describes concepts and calculation
Conclusions
Based on the experimental and modeling results as well as the uncertainty studies, the steel-steel composite metal foams tested as novel insulation system met the acceptance criteria for the simulated pool fire test in 49 CFR 179 Appendix B by a large margin and is expected to pass with near certainty if the test were to be reproduced in a different laboratory. Furthermore, the successful performance of S–S CMF in the simulated pool fire test (described in 49 CFR Part 179) can be attributed to
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Afsaneh Rabiei has 4 issued patents on composite metal foams. These patents have been released to a start-up company in which she is the founder and shareholder
Acknowledgement
This study is part of a project funded by the Department of Transportation (DOT) Pipeline and Hazardous Materials Safety Administration (PHMSA) project number DTPH5616C00001. The authors would also like to thank SWRI's members Mr. Jeremy McDonald for conducting the pool fire experiments and Ms. Alexandra Schluneker for conducting the surface emissivity on S–S CMF samples.
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