Fillet corners—the smooth, rounded transition applied where the repair putty meets the metal—are intended to distribute stress. However, under constant load or thermal cycling, even these areas can fail due to edge creep (slow deformation) or peeling, especially where the fillet ends. This failure mode often results from a lack of mechanical anchor combined with the material’s tendency to deform over time under load.

1. Enhancing the Anchor Before Filleting

A fillet must be built on a rock-solid foundation that resists the primary tensile and shear forces.

• Deep Base Keying: Before applying the putty to form the fillet, ensure the immediate area is aggressively abraded and V-grooved. The fillet should act only as a stress distributor for a foundation that is already mechanically locked into the substrate.
• Extend Abrasion: Aggressively feather the roughened area out significantly beyond where the fillet will end. The fillet’s perimeter must bond to the roughest, cleanest metal surface possible.
• Undercutting: For defects at a seam or corner, use a grinder to create a subtle undercut (a slight inverse taper) at the edge of the prepared area. When the putty is applied and cured, any minor shrinkage or load pulls the material into the undercut, tightening the bond instead of facilitating peeling.

2. Optimizing Fillet Geometry and Application

The shape and thickness of the fillet are crucial in determining its resistance to creep and peeling.

• Avoid Overly Large Fillets: While fillets distribute stress, an overly thick fillet introduces a large mass of polymeric material that is highly susceptible to creep under constant load. Use the minimum necessary radius to achieve the desired stress distribution, keeping the fillet size as small and tight as possible.
• Consistent Taper: Ensure the fillet transitions smoothly and consistently. Avoid creating a “foot” or abrupt increase in thickness at the fillet’s perimeter, as this spot becomes a new stress concentration point for peeling. The fillet should gradually taper down to a fine, feathered edge.
• Consolidation: Vigorously press and pack the putty when forming the fillet, especially near the substrate. This consolidation eliminates air voids and ensures maximum density, which increases the material’s resistance to long-term creep deformation under load.

3. Mitigating Long-Term Load (Creep Control)

If the fillet is under constant load, steps must be taken to transfer that load to the metal.

• Mechanical Load Transfer: For joints bearing load, utilize mechanical fasteners (bolts, pins) to carry the primary static and dynamic forces. The putty, shaped into a fillet, should function purely as a seal and structural shim, not as the primary load-bearing material.
• Low-Temperature Cure: If the component operates at elevated temperatures, the risk of creep increases dramatically. Ensure the putty is fully post-cured according to the manufacturer’s specifications to maximize the material’s internal cross-linking, which significantly improves its long-term resistance to deformation under heat and load.