Fretting Wear

An example of fretting can be observed in the connection between an aircraft’s wing and fuselage. In this application, slight oscillatory movements occur between the mating surfaces due to aerodynamic forces during flight. This micro-motion leads to repeated contact and friction, causing the removal of material from the surfaces in contact. The resulting damage, known as fretting wear, manifests as pitting, grooving, and the formation of oxide debris, which can weaken the structural integrity of the connection.

Preventive Measures:

• Use of Lubricants: Applying appropriate lubricants to reduce friction and wear.
• Surface Treatments: Employing surface coatings or treatments to increase hardness and resistance to fretting.
• Design Modifications: Improving joint design to minimize relative motion and increase contact stability.

Fretting wear and fatigue wear are two distinct types of wear that can occur in mechanical components. Here’s a comparison of the two:

Fretting Wear:

• Mechanism: Fretting wear occurs due to small amplitude oscillatory motion between two contacting surfaces. This repeated micro-motion causes the surfaces to stick and slip, leading to the removal of material.
• Characteristics: Results in surface damage such as pitting, grooving, and the formation of debris. Often accompanied by oxidation, creating a reddish-brown appearance known as fretting corrosion.
• Common Locations: Found in joints and connections, such as splined shafts, bolted joints, and bearings, where slight movements occur under load.
• Prevention: Use of lubricants to reduce friction, surface treatments to increase hardness, and design modifications to minimize relative motion.

Fatigue Wear:

• Mechanism: Fatigue wear results from cyclic loading and unloading, leading to the initiation and propagation of cracks within the material. These cracks grow over time and cause material to break away.
• Characteristics: Manifests as surface cracking, spalling, and material flaking. Fatigue wear typically occurs after a high number of load cycles.
• Common Locations: Common in components subjected to repeated stress, such as gears, springs, and rotating shafts.
• Prevention: Improve material properties to enhance fatigue resistance, optimize component design to reduce stress concentrations, and implement proper load management.

Key Differences:

• Motion Type: Fretting wear is associated with small oscillatory motions, while fatigue wear involves cyclic loading.
• Failure Mechanism: Fretting wear results from surface interactions, whereas fatigue wear originates from internal material failures due to repeated stress.

The formula for fretting wear is typically based on the Archard wear equation, which can be adapted to describe fretting conditions. The basic form of the Archard wear equation is:

𝑉=𝐾⋅𝐹⋅𝑠 / 𝐻​

Where:

• 𝑉 = Volume of material worn away
• 𝐾 = Wear coefficient (dimensionless)
• 𝐹 = Normal load (force) applied to the contact surfaces
• 𝑠 = Sliding distance or relative motion between surfaces
• 𝐻 = Hardness of the softer material in contact

In the context of fretting wear, the sliding distance 𝑠 is often very small due to the limited oscillatory motion, but the repeated cycles can still lead to significant material removal over time.

Factors Affecting Fretting Wear:

• Material Properties: Hardness and toughness of the materials in contact.
• Surface Roughness: Smoother surfaces generally reduce fretting wear.
• Load and Frequency: Higher loads and frequencies of oscillation increase wear rates.

By understanding and controlling these factors, it is possible to reduce the extent of fretting wear and prolong the lifespan of components.

Metal coatings are used to impart a variety of properties to components that aren’t otherwise present in the base metal itself. These properties can include improved wear, corrosion, or chemical resistance, they can be applied for aesthetic reasons, to help to reduce friction, fretting, and galling, or for a variety of other reasons. Coatings can be a cost-effective way of meeting performance requirements without having to resort to the use of an exotic or expensive base metal.