The Relationship Between Microstructure and Mechanical Properties of Late 19th/Early 20th Century Wrought Iron Using the
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THE RELATIONSHIP BETWEEN MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LATE 19TH/EARLY 20TH CENTURY WROUGHT IRON USING THE GENERALIZED METHOD OF CELLS MODEL Jennifer J. Hooper1, Lori Graham2, Tim Foecke3, and Timothy P. Weihs1 1 Department of Materials Science and Engineering 2 Department of Civil Engineering The Johns Hopkins University, Baltimore, MD 21218 3 National Institute of Standards and Technology, Gaithersburg, MD ABSTRACT The discovery of the RMS Titanic has led to a number of scientific studies, one of which addresses the role that the structural materials played in the sinking of the ship. Chemical, microstructural, and mechanical analysis of the hull steel suggests that it was state-of-the-art for 1912 with adequate fracture toughness for the application. However, the quality of the wrought iron rivets may have been an important factor in the opening of the steel plates during flooding. Preliminary studies of Titanic wrought iron rivets revealed an orthotropic, inhomogeneous composite material composed of glassy iron silicate (slag) particles embedded in a ferrite matrix. To date, very little is understood about the properties of wrought iron from that period. Therefore, in order to assess the quality of the Titanic material, contemporary wrought iron was obtained from additional late 19th/early 20th century buildings, bridges, and ships for comparison. Image analysis completed on the Titanic wrought iron microstructure showed a high slag content that is very coarse and unevenly distributed. To investigate how microstructure impacts the mechanical properties, and hence the quality of late 19th/early 20th century wrought iron, a detailed analysis of the relationship between the microstructural features and the mechanical behavior was completed. Here we present the first step in that process: the use of the Generalized Method of Cells (GMC) to predict the mechanical response of composites with variable microstructural properties.1 The GMC tool is used to generate the effective inelastic behavior of the composite from the individual constituent properties. INTRODUCTION Wrought iron can be defined as commercially pure iron (less than 0.02wt% C) containing approximately 1-4wt% slag (iron silicate) inclusions. For centuries, wrought iron was used for applications requiring resistance to fatigue-failure and corrosion, as well as the ability to be easily formed and machined.2 Large-scale manufacturing of wrought iron began in the late 18th century for use on bridges, ships' hulls and parts of machinery. The high demand for material was met using the puddling process, a technique requiring constant agitation of a molten pool of iron and slag. Subsequently, the product was extruded or rolled to form shapes for a variety of applications. Despite its widespread use as a structural material in the 19th century, most wrought iron was replaced by steel by the mid-20th century. 3 Published results of tensile tests on mid-20th century rolled wrought iron reveal a yield strength that is 1.5 times higher in the longitudina
