Overview of Lamellar Tearing and Representative Case Studies

Overview of Lamellar Tearing and Representative Case Studies

Guest Author: Christopher Brandmeier | BAE/MAE | The Pennsylvania State University | 2014

Lamellar tearing is a weld-related failure where rolled steel ruptures in its through-thickness direction as a result of internal thermal strains.  Common in the 1960s and 1970s, lamellar tearing became a national issue following failures at the John Hancock Center in Illinois, El Paso Civic Center in Texas, Atlantic Richfield Towers in California, and Grand Coulee transmission-line towers in Colorado.  Although no lives were lost in the failures, the events were costly and litigious.  It became clear that the pace of design had outstripped the pace of materials research, and the steel industry was forced to take significant steps to develop new clean steels and connection details to mitigate lamellar tearing issues.  Even though today’s steel industry has addressed most of the factors that cause lamellar tearing, it is still important for designers to recognize the weakness of steel in its through-thickness direction and design weld connection details appropriately.

 

Susceptible and Improved Details (Image Credit: American Institute of Steel Construction

Susceptible and Improved Details (Image Credit: American Institute of Steel Construction

 

Lamellar Tearing Failure Mode

Lamellar tearing usually occurs as the direct result of welding heat.  As the weld cools, differential thermal strains develop in the steel plate along the through-dimension.  The plate ruptures along lines of brittle impurities (called inclusions), which are engrained in the steel during the deoxidation process and are elongated during rolling.  The inclusions decohere from the steel matrix, creating terraced fractures parallel to the direction of rolling.  These terraces then link together until the net area is reduced to the point where a shear fracture can occur.

Rarely do global service loads reach a magnitude that can internally fracture steel. The cause of lamellar tearing is truly small thermal strains, which create large localized stresses and tear the material apart.

 

Causing Factors

The most common factors that cause lamellar tearing are engrained inclusions formed in the rolling process, hydrogen induced cracking, and weld related susceptibilities.

Inclusions

The main factor associated with lamellar tearing susceptibility is the area percentage and length of inclusions in the parent steel.  Depending on the deoxidation process used in manufacturing, inclusions can be sulfides, silicon, carbon, phosphorus, or aluminum and range in length from 2 micrometers to 3 millimeters.  The greater the area of inclusions, the more susceptible a member becomes to internal, brittle fractures.

Hydrogen Induced Cracking

The presence of hydrogen during welding can lead to hydrogen cracking, where pockets of hydrogen form in the ruptured inclusions and add further pressure to the fractures.  Common sources of hydrogen during welding include moisture in the atmosphere, grease, rust, organic material, and even the weld electrode itself.

Welding Process

The method of welding has been shown to have more impact on lamellar tearing susceptibility than either the heat or duration of welding.  Of the various methods, gas metal arc (GMA) and submerged arc methods foster the most resistance to lamellar tearing, since these methods do not use electrodes that contain hydrogen.  Hydrogen from the welding electrode promotes hydrogen cracking and thus aggravates lamellar tearing issues.

 

Prevention Methods

In 1973, AISC released a report entitled “Commentary on Highly Restrained Welded Connections,” which outlined 13 methods for the prevention of lamellar tearing.  Some of the current industry-standard prevention methods include specifying clean-steels, preheating, buttering, edge detailing, and using vacuum packed electrodes.

Edge detailing in particular is the most common form of prevention.  Shown in Figure 2 as susceptible and improved details, the goal of modern detailing is to reduce the through-thickness dimension of steel in welded connections.  This is achieved by using the welding groove to the steel’s advantage rather than placing the full impact of shrinkage strains on the base metal.

With advances in modern material technology, careful welding techniques, and improved detailing methods, the threat of lamellar tearing in modern steel construction has been nearly eliminated.

This article provides an overview of lamellar tearing, its failure mode, common causes, and prevention methods.  Further information on lamellar tearing, including more detail into individual failure cases, can be found on the failures wiki site at: http://failures.wikispaces.com/Overview+of+Lamellar+Tearing+Failures+and+Representative+Case+Studies

 

Cover / Lead In photo: From “Commentary on Highly Restrained Welded Connections”, AISC Engineering Journal, 3rd Quarter, 1973. Reprinted with permission from AISC.

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