Development of Method for Measuring Susceptibility of Steel Plate to Lamellar Tearing

Abstract

1. Introduction During the construction of steel structures problems sometimes arise due to a cracking phenomenon that has been termed lamellar tearing". A case in point is the construction of drilling rigs and other offshore platforms where problems of this kind is known to occur quite frequently. Lamellar tearing is a type of weld defect that may occur in a steel plate when it is subjected to appreciable strains in the short transverse direction (z-direction) due to shrinkage of the weld deposit. Since it is essential that the strains are predominantly in the z-direction, the defect is most often found in welded joints where the fusion boundary is roughly parallel to the surface of the plate. Typical examples of such welds are shown schematically in Fig. 1. The appearance of lamellar tears occurred in illustrated in Figs. 2 and 3. It is well established now that the main variables in lamellar tearing are:The mechanical properties in the z-direction of the plate,The design of the structure, in particular the joints, andThe welding parameters. It is the intent of the present paper to deal only with the first and most important of these variables, namely the material properties. The main factor in determining the z-direction properties of a steel plate is the non-metallic inclusions. Their effect, which is mainly to reduce the ductility in this direction, is caused by the inclusions being elongated and flattened in planes parallel to the plane of the plate, a feature which also gives rise to the stepped appearance of lamellar tears. A typical fracture surface of an opened lamellar tear is shown in Fig. 4. Once lamellar tearing has occurred in a construction it is difficult to get rid of since removal of the crack by grinding or gouging and rewelding often results in new lamellar tears forming further into the material There pairwork is therefore complicated, time consuming and often very expensive. For these reasons there is a need for a testing method that will provide quantitative information about the susceptibility of steel plate to lamellar tearing. It has been the aim of the present work to develop such a method. 2. Test for assessing susceptibility to lamellar tearing. In the literature a number of methods are described that have been suggested as a means for testing the susceptibility of steel plate to lamellar tearing. See for instance references (1), (2), (3), (4), (5), (6) and (7). Some of these methods are purely mechanical tests, others are weld-cracking tests that rely on welding strains to trigger off possible cracking, while others, again, are combinations of these methods. In principle, however, all the methods have as a common objective the provision of information about the mechanical properties in the z-direction of the material being tested.