Creep

Definition and Classification (Why Polymers Creep)

Creep describes the slow, time-dependent change in shape of a material under continuously applied mechanical load. The question is: what happens to a seal when a contact pressure or tensile stress is applied for a long time? With plastics and elastomers this is particularly relevant, because they are viscoelastic. Viscoelastic means that the material behaves partly like a spring (elastic) and partly like a viscous fluid (viscous).

The physical background lies in the structure of polymers. Their long molecular chains can slowly rearrange, shift, or orient themselves under load. This rearrangement takes time and causes the deformation to grow further, even though the external load remains the same. As a result, creep can occur in plastics and sealing materials even at room temperature, while metals typically require significantly higher temperatures for it.

Typical Creep Behavior (Time Course)

Creep is a long-term effect. Under constant load, deformation continues to grow over minutes, hours, and days, and under sustained loading it can remain relevant over years. How fast this progression is depends strongly on temperature and on load level. Higher temperature accelerates molecular motion, while higher stress increases the “driving force” for rearrangement. Both make the deformation grow faster.

Distinction: Creep vs. Stress Relaxation vs. Compression Set

In sealing technology, what matters is which quantity stays constant in operation. In many sealing points, the seal is compressed during installation to a defined displacement — that is, brought to a constant deformation. In this case, creep as a measured value is often not the focus, but rather stress relaxation. Relaxation means: at constant deformation, force and stress drop over time. In practice, this means: the sealing force diminishes.

The compression set, by contrast, describes the residual deformation after unloading. It answers the question: how well does the seal “spring back” after a defined loading time? This is related to creep and relaxation but not the same. Therefore, the parameters are not directly interchangeable, even though they are often discussed together.

Mini-Comparison: Constant Load vs. Constant Deformation

Why Creep/Relaxation Is Critical in Sealing Points (Loss of Sealing Force and Leakage Risk)

A seal works because it generates sufficient contact pressure and thus closes leakage paths. In static sealing points — for example in a groove or between flanges — this contact pressure is usually built up by the assembly preload. When the sealing material creeps or relaxes, the effective stress in the material drops, and so does the sealing force. It becomes critical as soon as the residual contact pressure falls below a required minimum. Then the risk of micro-leakage rises, especially at long standstill periods or elevated temperatures.

In real connections, preload loss rarely results from a single cause. In addition to the time-dependent behavior of the seal, settling effects within the assembly also contribute — for example through surface conformation, roughness flattening, or minor geometry changes of the components. For practice, what counts is the system response: seal, mating surfaces, and component stiffness act together.

Practical Example: Static Seal Under Long Service Life

A static seal is compressed during installation to a defined level, so that an initial reliable sealing force is created. Over the service life, the counterforce decreases due to stress relaxation, even though the installed position remains the same. When pressure fluctuations, temperature peaks, or media-related softening are added on top, the reduced contact pressure may no longer suffice. In such cases, aging often shows up first as very small leakage before a clear loss of function becomes visible.

Influencing Factors, Testing, and Design Levers (Compact)

How pronounced creep and relaxation are depends primarily on temperature, time, and mechanical load. In sealing technology, the influence of the medium is added: swelling or extraction (leaching of components) can soften the material and accelerate the time-dependent deformation. The stiffness ratios in the system are equally decisive. A very stiff housing combined with a comparatively soft seal can cause part of the preload to be “lost” within the seal over time, because the system offers little room for compliance.

Tests and parameters should match the question at hand. For plastics, creep is frequently captured via tensile creep tests (e.g., ISO 899-1). For elastomers in sealing applications, compression stress relaxation at constant compression is particularly close to practice (e.g., ISO 3384-1). The compression set (e.g., ISO 815 or ASTM D395) complements the picture but primarily describes the residual set after unloading and does not replace relaxation data.

Selecting the Right Test Parameters

For design, the question often is: is this about deformation increase under load, or about sealing-force loss at a defined installation deformation? When operation prescribes a fixed compression, relaxation data are usually closer to reality. Creep data are particularly helpful when a load remains constant over a long time, or when components are allowed to change position through deformation. Compression-set values are well suited for material comparison but should not be used alone to predict the sealing force in the installed condition.

Short Checklist for Design

For demanding applications, it often pays to plan material- and system-related testing under realistic temperature and media conditions; in case of need, specialized consultation is sensible.