When an adhesive joint carries a sustained load, the adhesive slowly deforms over time even at stress levels well below its instantaneous failure load. This time-dependent deformation is creep. For most adhesive joints, creep is negligible at room temperature — the polymer network is glassy and chain mobility is very low. But at elevated temperatures, as the adhesive approaches its glass transition temperature, creep rates increase dramatically and can become the dominant factor limiting joint performance. High-temperature applications that require dimensional stability or sustained load-bearing capacity must account for creep in their adhesive design.
The Physical Basis of Creep in Adhesives
Creep in polymer materials occurs because polymer chains are not locked in place — they have some freedom to rearrange their configuration in response to stress, even in the solid state. Under an applied stress, the polymer network gradually adopts a new configuration that partially accommodates the stress, resulting in macroscopic deformation. This rearrangement happens slowly because it requires chain segments to overcome activation energy barriers as they move past their neighbors.
Temperature dramatically accelerates creep because thermal energy helps chains overcome activation barriers. The Arrhenius relationship for creep rate means that a 10–15°C increase in temperature can double the creep rate. At temperatures near the glass transition temperature, where chain mobility increases by orders of magnitude, creep rates become very high — the adhesive deforms substantially under loads it would barely creep under at room temperature.
For high-temperature adhesive applications, the critical parameter is not just the adhesive’s instantaneous strength at temperature but its creep behavior — how much it deforms under sustained load at service temperature over the intended service life.
How Creep Manifests in Bonded Joints
Bondline Dimension Change
Under sustained compressive or tensile load, the adhesive bondline thickens or thins over time at elevated temperature. Compressive load causes the adhesive to cold-flow outward, thinning the bondline and causing squeeze-out at the joint edges over time. Tensile load causes the bondline to elongate, increasing its thickness. Either change alters the joint’s mechanical performance and, in precision assemblies, changes component positions.
Component Misalignment
In assemblies where the bonded joint maintains a precise geometric relationship — optical systems, sensor mounts, precision instruments — creep deformation shifts the component position over time. The rate of shift depends on the creep rate at service temperature and the applied load. For joints near Tg, this shift can be significant over months or years of service.
Creep misalignment is particularly insidious because it is gradual and may not be immediately apparent. A system that performs correctly when assembled degrades slowly as creep accumulates, making it difficult to distinguish from other drift mechanisms.
Creep Rupture
Under high sustained loads at elevated temperature, creep can proceed to failure — the adhesive deforms until it separates, even though the applied stress is well below the instantaneous failure load. Creep rupture sets a maximum sustained load limit at each temperature: the creep rupture stress, which is typically 20–50% of the instantaneous failure stress at temperatures near Tg.
Creep rupture is a limiting failure mode for structural adhesive joints that carry sustained loads (dead weight, spring preload, pressure) in high-temperature service. Design for sustained load at temperature must use creep rupture allowables, not instantaneous strength values.
Email Us to discuss creep performance requirements for your high-temperature adhesive application.
Measuring and Characterizing Creep
Creep compliance — the fundamental material property characterizing creep, defined as the time-dependent strain per unit stress. Creep compliance is measured by applying a constant stress to an adhesive specimen and measuring the resulting strain as a function of time at a constant temperature. The compliance increases with time as the material creeps.
Creep compliance master curves using time-temperature superposition allow creep behavior at one temperature to be predicted from shorter-duration tests at higher temperatures. The principle is that time and temperature are equivalent in terms of polymer chain mobility: what takes years of creep at room temperature may be reproduced in hours at elevated temperature. Master curves compiled from tests at multiple temperatures predict long-term creep from accessible test durations.
Sustained load test to creep rupture — specimens loaded at specified fractions of their short-term strength and held at temperature until failure. The creep rupture time is plotted against stress to construct a stress-rupture curve for the adhesive at each test temperature. This curve defines the maximum stress for a given required service life.
Factors That Control Creep Rate in High-Temperature Adhesives
Distance from Tg. Creep rate is low far below Tg and increases dramatically near Tg. Selecting adhesives with Tg well above service temperature provides the most reliable low-creep performance. For every 10°C of additional margin between service temperature and Tg, creep rate decreases roughly one order of magnitude.
Crosslink density. Higher crosslink density restricts chain mobility and reduces creep rate. Highly crosslinked aromatic epoxies, bismaleimides, and polyimides have lower creep rates at equivalent temperature relative to Tg than more lightly crosslinked systems.
Filler content. Inorganic fillers mechanically restrict polymer chain mobility at the filler-polymer interface. High filler loading reduces creep compliance proportionally to the filler volume fraction, and anisotropic fillers (fibers, platelets) with aspect ratio provide additional creep restriction through geometric constraint.
Physical aging. Physical aging (free volume reduction) decreases chain mobility and reduces creep rate, but also increases brittleness. The creep reduction from physical aging at long service times may be offset by embrittlement that reduces creep rupture resistance.
Design Strategies for Creep-Limited Applications
Use Tg margins as a design parameter. For sustained-load applications, the margin between service temperature and Tg should be set based on the creep rate at the service conditions, not just on instantaneous strength. Larger margins reduce creep rate exponentially.
Select high-Tg formulations. Adhesives formulated for high Tg — bismaleimide, high-temperature epoxy, polyimide — have inherently lower creep rates at the elevated temperatures where lower-Tg adhesives show significant creep.
Limit sustained load levels. Design sustained loads to be below the creep rupture limit for the required service life, not just below the instantaneous failure load. This typically requires reducing design loads to 20–40% of instantaneous strength for elevated temperature sustained load applications.
Provide mechanical stops. In precision assemblies where creep displacement must be limited, mechanical stops adjacent to the adhesive bond prevent deformation beyond a defined limit even if the adhesive creeps.
Incure’s Creep Performance Data
Incure provides creep compliance and sustained load data at elevated temperatures for high-temperature adhesive products, supporting design of load-bearing assemblies in thermally demanding service conditions.
Contact Our Team to discuss creep requirements for your high-temperature adhesive application and identify Incure products with the creep resistance needed for your service conditions.
Conclusion
Creep deformation in high-temperature adhesives is a time-dependent deformation mechanism that accelerates dramatically near the glass transition temperature. It causes bondline dimension change, component misalignment in precision assemblies, and creep rupture under sustained loads. Measuring and characterizing creep through compliance measurements and sustained load testing provides the data needed for design. Preventing excessive creep requires Tg margins above service temperature, high-crosslink-density adhesives, and sustained load levels set by creep rupture allowables rather than instantaneous strength values.
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