116 Lamellar Tearing

In a structural steel member subjected to tension, elongation and reduction of area in sections normal to the stress are usually much lower in the through-thickness direction than in the planar direction. This inherent directionality is of small consequence in many applications, but it does become important in design and fabrication of structures with highly restrained joints because of the possibility of lamellar tearing. This is a cracking phenomenon that starts underneath the surface of steel plates as a result of excessive through-thickness strain, usually associated with shrinkage of weld metal in highly restrained joints. The tear has a steplike appearance consisting of a series of terraces parallel to the surface. The cracking may remain completely below the surface or may emerge at the edges of plates or shapes or at weld toes.

Careful selection of weld details, filler metal, and welding procedure can restrict lamellar tearing in heavy welded constructions, particularly in joints with thick plates and heavy structural shapes. Also, when required, structural steels can be produced by special processes, generally with low sulfur content and inclusion control, to enhance through-thickness ductility.

The most widely accepted method of measuring the susceptibility of a material to lamellar tearing is the tension test on a round specimen, in which is observed the reduction in area of a section oriented perpendicular to the rolled surface. The reduction required for a given application depends on the specific details involved. The specifications to which a particular steel can be produced are subject to negotiations with steel producers.

(R. L. Brockenbrough, Chap. 1.2 in Constructional Steel Design—An International Guide, R. Bjorhovde et al., eds., Elsevier Science Publishers, Ltd., New York.)

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