Delamination, the peeling or separation of the cured putty from the metal substrate at the bonded interface, is a common failure mode in repairs subjected to repeated heating and cooling (thermal cycling). This failure is primarily due to the mismatch in the Coefficient of Thermal Expansion (CTE) between the putty and the metal, which introduces continuous, cyclic shear stress at the bond line.

Here are genuine solutions focused on reducing this stress and maximizing the bond’s resistance to fatigue.

1. Maximizing Bond Strength Through Preparation

A bond that is stronger than the internal stresses induced by thermal cycling is less likely to delaminate.

• Ultimate Surface Preparation: This is non-negotiable. As established, any contaminant acts as a bond breaker.The surface must be perfectly clean, degreased, and aggressively abraded to a bright metal finish (40-80 grit roughness). The mechanical bond created by the roughness is crucial for resisting the shear forces generated by expansion and contraction.
• Deep Mechanical Keying: Focus on creating a robust mechanical lock. For defects, V-grooving or chamferingthe edges forces the putty to penetrate deeply and anchor itself into the metal substrate. When the metal and putty try to move at different rates, the mechanical interlock resists the direct peeling or shearing of the bond line.
• Feathering Edges: Delamination often begins at the sharp edge of the repair where stress concentrates. Always feather or taper the putty application outwards to a very thin edge. This gradual transition distributes the cyclic stress over a larger surface area, preventing stress from peaking at the perimeter.

2. Managing the Internal Stress (Geometry & Transition)

The way the putty is applied can reduce the stress generated by the CTE mismatch.

• Minimize Putty Mass: Where possible, use the minimum effective thickness of putty. The thicker the putty, the greater the volume undergoing differential movement, leading to higher internal stress. A thinner bond line has less material mass to pull or push against the substrate.
• Radius Edges and Fillets: Never leave sharp 90∘ angles where the putty meets the metal. Use a fillet (a rounded internal corner) in the putty to transition smoothly from the repair to the substrate. Sharp angles act as stress concentrators, accelerating fatigue failure and delamination.

3. Controlling the Thermal Environment (Cure and Service)

The way the putty is cured and first introduced to heat determines its long-term stability under thermal cycling.

• Controlled Post-Cure Conditioning: The final step for high-temperature applications is post-curing. Fully cure the putty at the manufacturer’s specified ambient temperature first. Then, gradually expose the repaired part to increasing temperature (e.g., in an oven or via a controlled engine run). This controlled heating process fully cross-links the polymer and relieves residual stress, hardening the bond and significantly improving its resistance to thermal fatigue.
• Gradual Service Introduction: Avoid immediately exposing the freshly cured repair to wide, rapid temperature swings. Allowing the component to go through several mild thermal cycles (e.g., using a component intermittently) before full service operation helps the bond “settle” and further stress-relieve itself naturally,preparing it for the rigors of repeated thermal cycling.