German firebox sidesheets were suceptible to cracking around the staybolt holes. This is thermal fatigue cracking. Two fundamental conditions exist for fatigue cracks to occur.
1. The tensile stresses are cycled above and below the yield strength of the material.
2. Cracks appear perpendicual to the greatest tensile stress.
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To understand the orientation of cracking in the German side sheets it is necessary to understand the physics of metal expansion. If a plate of metal is heated uniformly, it expands uniformly in all directions from the geometric center. If the plate is restrained from expanding, compression stress builds up uniformly in the plate. As heat increases the force exerted increases. Once the yield strength is passed the atoms take a permanent set in the expanded condition.
As noted, this cracking required side sheet replacement at 100,000 km or about 60K miles.
From the paper "Tross New Knowledge Locomotive Rear Boiler" from Matt Janssen's website:
As the side sheet cools, it pulls and shrinks in the opposite direction to the expansion. As shown by the Tross drawing, the cracks appear vertical in the center of the side sheet and rotate to an angular direction in the lower corners. Note the cracks appear in the lower half of the side sheet. Tross in his papers, identifies that the staybolts in the upper half of the side sheets absorb the expansion and cause a permanent S-curve set in the staybolts. Mudring bulging and crown sheet sagging contribute to this condition.
The lower side sheet cracking and the upper side sheet staybolt bending is caused by the thermal expansion from the geometric center of the side sheet. The sheet is realatively restrained in the horizontal direction but not in the vertical direction. The thin mudrings do not provide significant resistance to downward expansion and the U profile mud troughs even less.
North American practice with the solid mudrings prevents downward and horizontal expansion of the side sheet where it is attached. For practical purposes, the side sheet expands as if the geometrical center of expansion starts at the center of the mudring attachment.
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