Clamp-zone tearing begins at or near the sheet boundary. It may propagate into the part or remain hidden in the trim web while still changing material distribution.
An underheated edge can crack because it cannot accommodate load. An overheated edge loses strength and thins between grip points. Sharp clamp features, excessive pressure, burrs, notched cut edges, misalignment, and high pull-in force can initiate the tear.
Corrective action
The fracture appearance and position relative to pins or serrations should be examined. Clamp surfaces must be smooth where they should support and sharp only where a designed grip requires it. Cut-sheet edges should be free of notches that can grow under load.
Perimeter heating and clamping must be developed together. More pressure is not a reliable fix when the edge is thermally weak or mechanically damaged.