Fillets are supposed to be the solution, not the failure point. The smooth, rounded transition where repair putty meets metal exists specifically to distribute stress — yet under constant load or thermal cycling, even a well-formed fillet can develop edge creep or peeling right where it terminates, usually because the fillet was asked to do a job the underlying anchor never set it up for.

Why Fillets Still Fail Under Sustained Load

Edge creep is slow, permanent deformation at the fillet perimeter driven by continuous mechanical or thermal stress. It typically shows up not because the fillet geometry itself is wrong, but because the anchor beneath it was never mechanically locked to the substrate in the first place — a fillet applied over a lightly abraded or poorly keyed surface has nothing to distribute stress into, so the load simply concentrates at whatever point the bond is weakest, usually the outer edge where the fillet feathers to nothing. Repairs on components that see repeated start-stop thermal cycling, such as housings near a heat source, are especially prone to this because every cycle adds a small increment of permanent deformation that accumulates unnoticed until the fillet finally separates.

Building a Solid Anchor Before Filleting

A fillet has to sit on a foundation that already resists the primary tensile and shear forces on its own. That means aggressively abrading and V-grooving the immediate repair area before the putty ever goes on, so the fillet’s role is purely to distribute stress across a foundation that’s already mechanically locked into the substrate. The roughened zone should extend well beyond where the fillet’s outer edge will land — bonding to the roughest, cleanest metal available at that perimeter matters more than most repairs account for. A subtle undercut or slight inverse taper ground into the edge of the prepared area gives cure shrinkage and service loads something to pull into rather than away from, tightening the bond over time instead of loosening it.

Getting Fillet Geometry and Size Right

Counterintuitively, a bigger fillet isn’t always a stronger one. An oversized fillet introduces a larger mass of polymeric material that’s inherently more susceptible to creep under constant load — the minimum radius needed to achieve real stress distribution is the target, not the maximum radius that looks reassuring. The taper itself needs to stay consistent from base to tip; any abrupt increase in thickness partway along the fillet creates a new, unplanned stress concentration point exactly where the geometry was supposed to eliminate one. Vigorous consolidation while forming the fillet — pressing and packing rather than just smoothing — eliminates air voids and maximizes density, both of which directly improve resistance to long-term creep deformation.

Taking Load Off the Fillet Entirely

When a joint carries genuinely constant load, the most reliable fix is removing that burden from the putty altogether. Mechanical fasteners — bolts or pins — should carry the primary static and dynamic forces on any load-bearing joint, with the fillet reduced to a seal and structural shim rather than the main load path. For components running at elevated service temperature, the creep risk compounds quickly, which makes a full post-cure heating cycle per the manufacturer’s specification worth the extra step: it drives cross-link density higher and meaningfully raises the material’s resistance to deformation under sustained heat and load.

Incure’s technical team can help evaluate whether a given fillet is being asked to carry more load than its geometry supports — Email Us with a description of the joint and duty cycle for a second opinion.

Monitoring Fillets in Service

Because edge creep develops slowly, periodic inspection catches it long before a fillet actually separates. A straightedge laid across the fillet at installation, with its position marked or photographed, gives a baseline to compare against months later — any visible gap opening up under the straightedge signals creep in progress rather than a sudden failure. Scheduling that check at a fixed interval, rather than waiting for a visible problem, is the difference between planned maintenance and an unplanned outage on a critical joint. Fillets on components that see meaningful thermal swings should also be evaluated against expected differential expansion between the putty and the base metal, discussed further in this breakdown of CTE mismatch and bond-line stress, and for repairs where mechanical fastening isn’t practical, comparing putty strength against alternative bonding chemistries is worthwhile — see this guide to adhesive strength for heavy-duty repairs.

A fillet is only as good as the anchor underneath it, and getting that sequence right — anchor first, geometry second, load transfer third — is what keeps a fillet corner from becoming the weak point in an otherwise sound repair. Incure’s applications engineering group is available to review fillet designs for demanding service conditions — Contact Our Team before committing a repair to production.

Visit www.incurelab.com for more information.