Fatigue

Definition and Context (What Does Fatigue Mean for Seals?)

Fatigue is a progressive damage process caused by repeated loading. In sealing technology, this means: a seal is not overloaded once, but deformed again and again over many load cycles. As a result, micro-damage can build up, from which cracks initiate. These cracks grow with further cycles until the sealing function fades or a material fracture occurs.

For elastomer seals (elastomers are rubber-like, highly stretchable plastics), fatigue often shows up first as fine cracks at heavily loaded zones. These are usually edges, lip regions, or zones with local overstrain. Over time, this develops into a dominant crack that propagates along the stress field. In many test approaches, “failure” is therefore defined as crack growth up to separation or up to loss of function.

What Is “Cyclic Loading” at the Sealing Point?

A cycle is a complete repetition of a load. In real sealing points, a cycle arises, for example, from a stroke out and back, a pressure rise and drop, or from periodic vibration. Particularly in hydraulics and pneumatics, this cyclic loading is typical, because motion, pressure changes, and contact deformation interact.

Many small cycles can be critical even if the individual loading appears moderate. The reason is the accumulation of micro-damage: small crack nuclei are repeatedly “nudged” until they begin to grow stably.

Mechanisms: Crack Initiation and Crack Growth (Why Fatigue Occurs)

Fatigue life can often be split into two parts. First, a crack initiates (crack initiation). Then it grows further per load cycle (crack growth) until it reaches a critical size. For seals, this distinction is helpful, because design and material choice can influence both phases.

Crack initiation often starts at weak points. These can be notches (geometric stress raisers), pores, inclusions, or sharp edges in the installation space. With elastomers, an additional factor is that they are viscoelastic: they show both elastic and time-dependent behavior. Therefore, fatigue processes in elastomers are often described energetically — that is, through a driving quantity that “drives” the crack growth. Without going deeper into theory, the classification suffices here: the higher the locally introduced strain energy per cycle, the sooner a crack grows.

Some elastomers show strain-induced crystallization. This means that local crystalline regions can form under tension, which slow down crack growth at the crack tip. This effect is material-dependent and does not replace good design, yet it can improve fatigue strength.

Which Loading Types Act in the Sealing Contact?

In a sealing point, “tension” is rarely the only load. A typical case is a superposition of tensile, compressive, and shear loading. Shear arises primarily from relative motion — that is, when the seal slides over the mating surface. Pressure changes raise contact pressure and therefore local stresses, especially at lip edges and transitions.

The most important amplifiers are notch effects and local overstrain. A small geometry change can significantly raise local strain, even though the global loading remains the same. As a result, damage shifts from “uniform aging” toward targeted crack growth at hotspots.

Operating and Design Factors That Accelerate Fatigue (Practical Focus)

In practice, fatigue is rarely determined by a single parameter. A combination of motion, pressure changes, friction, and temperature frequently accelerates the process. Friction and hysteresis losses (internal damping losses under repeated deformation) lead to self-heating. Rising temperature can soften elastomers, reduce strength, and thereby promote crack growth.

The medium also plays a role. Swelling or extraction of constituents can change properties, which then worsens crack resistance. Speed and strain rate likewise influence viscoelastic behavior, and thus the local stress distribution and heat generation.

From a design perspective, installation-space details are decisive: sharp edges, unfavorable gaps, or high local contact pressures. In addition, set behavior (time-dependent permanent deformation under load) can shift contact conditions. As a result, local loading rises at certain points, even though the system as a whole appears unchanged.

Typical Application Example (Rod Seal in a Cylinder)

For a rod seal in a hydraulic cylinder, a cycle is often a complete stroke out and back. With each stroke, the seal is deformed at the mating surface and slides under contact pressure. When pressure peaks are added, local stresses rise particularly at the sealing lip or at geometry transitions.

A small initial crack can arise at this point, for example through a notch or local overstrain. The crack then grows further with each stroke. Eventually, the sealing edge loses its effective contact geometry, leakage occurs, or material breaks out.

Identification, Testing, and Distinction (Fatigue vs. Aging vs. Wear)

Fatigue is often recognized through crack-dominated damage patterns. What matters is the question of where the crack starts and how it propagates. Fatigue cracks frequently initiate at mechanically heavily loaded zones and grow with the load sequence. Depending on the stress state, individual main cracks or network cracks can occur, although a dominant crack is usually decisive for loss of function.

In tests, fatigue is often described through “cycles to crack”. For this purpose, strain, load shape, and frequency are defined so that results become comparable. Controlling self-heating is important, because temperature would otherwise overlay the assessment. Transferability to real sealing points remains limited, because combined loads, friction, media, and geometry effects act there simultaneously.

For practice, a clear distinction from other mechanisms helps:

• Aging is more strongly time- and environment-driven — for example, by temperature, oxygen, or media. It changes material properties and can indirectly accelerate fatigue.
• Ozone cracks are a separate mechanism. They are triggered by ozone in the environment and often show cracks transverse to the tensile direction, particularly under static strain.
• Wear shows up as material removal or abrasion. It is contact- and friction-driven and must be distinguished from crack-based fatigue failure, even though both can occur together.

Test and Comparison Variables: “Cycles to Crack” and Operating Conditions

“Cycles to crack” is a practice-near parameter, because it directly answers how long a material or design endures under repeated loading. For the informational value to remain intact, operating conditions must be controlled: strain level, frequency, specimen temperature, and avoidance of strong heating from internal damping.

Nevertheless, the rule remains: a sealing point is more complex than a test specimen. Therefore, test results should be used as a comparison and as a trend. For design work, the actual contact geometry, the pressure curve, and the tribology (friction and lubrication) are additionally decisive.

A short closing note: with recurring crack damage, a specialized analysis of material, installation space, and operating profile is often sensible, because fatigue is almost always driven by several factors at once.