SKH-1c-1991-09 Spiked Head

Description

Die crack propagation follows predictable physics, and SKH-1c-1991-09 of the 1991 Lincoln Memorial cent — reported on June 17, 2018 — documents one of 19 dies where these physical processes played out at the portrait-to-field boundary. Crack growth in die steel begins at the microscopic level: repeated striking creates cyclic stress that nucleates tiny fractures at points of maximum stress concentration, particularly where the die surface geometry changes abruptly. Each subsequent strike advances the crack front incrementally through a process called fatigue crack propagation, where the crack tip opens slightly under load, extends by a small increment, and then closes as the load is released — only to repeat with the next strike. Thermal cycling accelerates this process: the die surface heats during each strike and cools between strikes, creating thermal stress that supplements the mechanical striking force. At the Lincoln portrait boundary, where the die surface transitions sharply from the deep portrait recess to the flat field, both mechanical and thermal stresses concentrate along the same geometric line, making this boundary the most likely initiation site for fatigue cracks on the obverse die. The resulting spiked head is the visible evidence of this invisible material failure process. Struck at Philadelphia on copper-plated zinc planchets composed of 97.5% zinc and 2.5% copper, weighing 2.5 grams and measuring 19mm.

Attribution History

• Discovered by Zach Llewellyn
• Cross reference: none. 06/17/2018.
• Expert attribution by Cuds on Coins

External References