Evaluation and Repair of Wrought Iron and - Purdue e-Pubs ...

122The reasons for the differences in fracture toughness relates to the fibrousstructure of historic wrought iron. As mentioned in chapter two, the structure of wroughtiron consists of elongated strands of iron and iron silicate, also known as slag, thattypically run along the length of an eyebar. The charpy impact strengths found fromtesting for a crack running perpendicular to the length of the eyebar, or against the grainstructure of the wrought iron, was found to be between 10 ft lbs and 40 ft lbs at roomtemperature. The charpy impact strength for a crack running parallel to the grain of aneyebar, however, was never found to exceed 3 ft lbs for all temperatures.Since the charpy impact strength was so low for a crack running parallel to thedirection of rolling, it is essential to inspect existing eyebars to ensure no cracking isrunning along this direction. It is also very important to ensure that any repair does notcreate stress concentrations that might propagate a crack in this direction. Fortunately,cracking is not very common in this direction since it is parallel to the primary memberstresses. Figure 5.8 is a diagram showing the various directions and locations wherepotential crack growth might occur.The impact strength for cracks growing perpendicular to the grain structure of thewrought iron varied considerably and were mainly below the AASHTO Zone 2 minimumrequirements for the charpy v-notch strength for fracture critical members of A36 steel.Hence, low fracture toughness values indicate that it is essential to have thoroughinspection procedures to identify any existing fatigue cracking. This can be done bymany methods such as visual inspection, dye penetrant examination, magnetic particle orultrasonic examination which are outlined in AASHTO’s Manual for ConditionEvaluation of Bridges.