In adhesive joints where load application or thermal expansion builds stress in the adhesive, that stress does not remain constant indefinitely. Over time, the polymer network relaxes — chains rearrange, viscoelastic flow redistributes the stress, and the peak stress decreases. This stress relaxation is sometimes beneficial (it reduces potentially damaging stress concentrations), but in many long-term adhesive applications it causes problems: springs lose their preload, seals lose their compression, and assemblies that relied on elastic recovery from the adhesive lose their designed mechanical function.
What Stress Relaxation Means in Adhesive Joints
Stress relaxation is the counterpart to creep. In creep, constant stress produces increasing strain over time. In stress relaxation, constant strain produces decreasing stress over time. Both arise from the same underlying mechanism — viscoelastic flow of the polymer network — but they manifest in different loading conditions.
In a joint that is held at fixed deformation (constant displacement), the initial elastic stress created by that deformation decreases as polymer chains rearrange to accommodate the imposed strain. The modulus of the material effectively decreases over time at constant deformation, and the stress drops accordingly.
The rate of stress relaxation follows an Arrhenius relationship with temperature — it accelerates at elevated temperature — and it is most significant when the service temperature is within 50–80°C of the adhesive’s glass transition temperature.
Applications Where Stress Relaxation Is Problematic
Compressed Gaskets and Seals
Adhesive or sealant joints used to create pressure seals — sealing flanges, compressed window gaskets, bonded seals — are loaded in compression during assembly to achieve the sealing contact pressure. Over time, stress relaxation in the sealant reduces the contact pressure. If the contact pressure drops below the minimum needed for sealing integrity, the seal leaks.
This is particularly problematic in elevated temperature applications where relaxation rates are higher. A bonded seal that holds pressure adequately at installation and for the first year of service may develop leaks in subsequent years as stress relaxation cumulatively reduces the sealing pressure below the threshold.
Designing against seal relaxation requires either selecting sealants with very low relaxation rates at service temperature (high-crosslink density, high Tg), designing sufficient initial compression that the minimum required pressure is maintained even after maximum expected relaxation, or providing a means of periodic re-compression.
Press-Fit and Pre-Loaded Joints
Some bonded assemblies use the adhesive to maintain a preload — bearing retention, interference fit enhancement, component positioning under spring load. The adhesive is cured under a defined compressive or tensile force; after cure, the elastic recovery of the substrates is prevented by the adhesive bond. Over time, stress relaxation in the adhesive reduces the effective preload.
In bearing retention applications, adhesive retaining a press-fit bearing against a shaft or housing must maintain radial contact pressure throughout the service life. Relaxation-driven preload loss can allow bearing micro-movement that leads to fretting damage and early bearing failure.
In precision instrument assemblies, bonded elements held in position by the elastic preload of spring components rely on the adhesive to prevent the springs from driving the components to a different equilibrium position. As the adhesive relaxes, the spring force progressively moves the component to a new position.
Thermal Stress Relief — Beneficial Relaxation
Stress relaxation is not always detrimental. In bonded joints that develop significant residual stress from CTE mismatch during thermal cycling, stress relaxation at elevated temperature reduces the peak stress and prevents stress accumulation to damaging levels. The adhesive acts as a passive stress-relief mechanism — high stress drives faster relaxation until the stress is reduced to a level consistent with the material’s relaxed modulus.
This beneficial relaxation is why compliant adhesives with moderate relaxation rates are often preferred for bonded joints in thermally cycled assemblies. The adhesive relaxes thermal stress in each cycle rather than accumulating it, extending fatigue life. The tradeoff is that the same relaxation that relieves thermal stress also relaxes any beneficial preload in the joint.
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Measuring Stress Relaxation
Stress relaxation testing applies a constant strain to an adhesive specimen and measures the resulting stress as a function of time at constant temperature. Results are expressed as relaxation modulus E(t) = σ(t)/ε, where σ(t) is the time-dependent stress and ε is the constant applied strain.
Time-temperature superposition allows master curves of relaxation modulus versus time to be constructed from short-duration tests at elevated temperature. These curves predict long-term relaxation behavior at service temperature from tests of practical duration.
Compression stress relaxation for seals — a specific test where a seal or sealant specimen is compressed to a defined deformation and the sealing force (compressive load) is measured over time. This test directly measures the preload retained by the seal over time, providing design data for seal life.
Strategies for Managing Stress Relaxation
Select high-crosslink-density adhesives. The relaxation modulus at long times (the equilibrium modulus) is directly related to crosslink density. Highly crosslinked adhesives relax to a higher equilibrium modulus than lightly crosslinked adhesives — they do not relax as much in absolute terms, and they maintain more of their initial stress.
Use high-Tg adhesives for elevated temperature applications. Operating far below the adhesive’s Tg minimizes the relaxation rate. For seals and preloaded joints that must maintain their function over years, specifying an adhesive Tg well above service temperature reduces relaxation rates exponentially.
Design with relaxation allowance. For preloaded or compressed joints, design the initial preload to be sufficiently higher than the minimum required so that even after maximum expected relaxation, the remaining preload exceeds the functional minimum. The design preload = minimum required preload / (1 – expected relaxation fraction over service life).
Consider the relaxation-creep balance. For joints that experience both imposed strain (relaxation condition) and applied load (creep condition) simultaneously, the viscoelastic response involves both mechanisms. Design analyses for such joints should use full viscoelastic constitutive modeling rather than simplified elastic approaches.
Incure’s Long-Term Stress Retention Products
Incure offers adhesives and sealants with characterized stress relaxation behavior for applications requiring long-term preload or seal pressure retention. Relaxation data at multiple temperatures supports design of preloaded joints for their required service lives.
Contact Our Team to discuss stress relaxation requirements for your long-term adhesive application and identify Incure products with the preload retention characteristics your application needs.
Conclusion
Stress relaxation in long-term adhesive applications reduces the stress in constrained joints over time, causing seal pressure loss, preload reduction in press-fit and spring-loaded joints, and position drift in assemblies relying on elastic recovery to maintain component position. The relaxation rate is temperature-dependent and accelerates near the glass transition temperature. Managing stress relaxation requires high-crosslink-density, high-Tg adhesive selection, design with relaxation allowance for preloaded joints, and characterization through long-term or accelerated relaxation testing to verify that minimum required stress is maintained throughout the service life.
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