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COMMENTARY



Liquid Metal Assisted Cracking (LMAC)

If steel is subject to tension, or shear stress, and if various hot liquid metals (zinc, lead) are in contact with it, and if they can enter into the surface of the steel, the liquid metal can infiltrate into the steel at a molecular level. Having molten zinc atoms between grains of the steel reduces the tensile strength to zero at that location.

If the stresses in the steel are high, a crack can form. If this crack reduces the stress, the minute crack can stop at birth, but if the crack results in an increase in stress at the tip of the crack, the size of the crack will extend. In effect, the zinc or lead can be a knife so sharp that it is one atom wide at its edge. Unlike a knife through butter, the liquid metal "knife" does not need pressure to cut: what it does need is tension in the steel; and an engine to maintain the tension in the steel by continuing to pull the faces of the cut apart, allowing the liquid metal to penetrate. When the tension disappears, the crack stops. While sufficient tension remains, the knife slices in. A crack 3/4 of the depth of a section can form in a second.

 

Factors that can lead to LMAC.

High stresses, tension or shear, in the steel.
Welds, unequal thicknesses, pattern of heating...

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A structural form that can maintain or increase the high stress level at the tip of the crack as the crack forms.
In many structures, stresses are easily relieved but with ...

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Easy starting places for the crack.
Discontinuity, oxyacetylene cut holes, punched holes, welds ...

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High yield steel and brittle steel.
Experience has shown that most cracks have occurred in...

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The risk in use

Structures built from steel members vary in their susceptibility to failure of any one of the members, and to the importance of this susceptibility. In many structures, failure of one piece would be unlikely to cause a collapse and also such a structural collapse may not be significant. In other structures failure of one piece could lead to a collapse, and present a significant danger.

 

Accumulation of risk

Risk points are totted up; the more points, the greater the risk. The greater the risk, the more need for inspection. Alternatively, the engineer could choose to design to reduce the risk, or select alternative protection.