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Creep, Brinelling, Fretting & Fretting Corrosion

MEO Class 2

times asked: 12; times asked
question: 1; question
last asked: 2026; last asked

The slides, written out

AI-drawn illustration. Simplified to explain the idea, not to scale.

Slide 1 · SMALL CAUSES

SMALL CAUSES. BIG DAMAGE. CREEP, BRINELLING, FRETTING & FRETTING CORROSION Understand the mechanisms. Protect your systems. TINY MOVEMENT. SERIOUS CONSEQUENCES.

Slide 2 · 01/04

CREEP Slow deformation under sustained stress at high temperature WHAT HAPPENS? Creep is the slow, time dependent plastic deformation of a material under sustained stress, usually at elevated temperature and below its normal yield strength. THREE STAGES 01 PRIMARY Rapid initial strain that gradually decreases. 02 SECONDARY Approximately steady minimum creep rate. This stage is particularly important for engineering design. 03 TERTIARY Creep rate accelerates until fracture. PRIMARY SECONDARY TERTIARY STRAIN TIME WHERE IN PROPULSION SYSTEMS? Steam pipes Superheater tubes Turbine components Boiler components WHY? Sustained high temperature and stress allow progressive material deformation, potentially causing dimensional changes, clearance problems and eventual rupture. KEY TAKEAWAY Temperature + Stress + Time = Creep Damage

Slide 3 · 02/04

BRINELLING Two different mechanisms – know the difference TRUE BRINELLING Permanent indentation of a bearing raceway caused by: • Excessive static loading • Heavy impact loading The indentation remains in the raceway. FALSE BRINELLING A fretting type damage mechanism occurring in rolling element bearings when they are: Stationary or nearly stationary + exposed to vibration or small oscillatory movement The rolling elements repeatedly contact approximately the same raceway positions, producing characteristic wear marks. WHERE ON BOARD? Rolling element bearings in: Standby pumps Electric motors Auxiliary machinery Other stationary bearing installations exposed to vibration PREVENTION Where appropriate, rotate idle rolling element machinery to reduce the risk of vibration induced false brinelling. EXAM POINT Brinelling ≠ False Brinelling True brinelling → Static / impact indentation False brinelling → Vibration / micro movement | |

Slide 4 · 03/04

FRETTING Tiny movement. Progressive surface damage. WHAT HAPPENS? Fretting is mechanical wear caused by small relative oscillatory movement between two metallic surfaces in close contact. MICRO SLIP Small relative oscillatory movement WEAR DEBRIS Particles are generated SURFACE DAMAGE Surface is damaged at microscopic level FURTHER WEAR Debris causes more abrasion and the cycle continues WHERE IN PROPULSION SYSTEMS? SHELL BEARINGS Slight movement between the bearing shell and its housing under alternating load. SHAFT LINERS / FITTED INTERFACES Relative movement can occur at highly stressed interfaces involving shaft liners and propeller hubs, depending on fit, geometry and cyclic loading. ENGINE STRUCTURAL INTERFACES Potential movement can occur at loaded contact surfaces such as bedplate, chock areas and where frames meet bearers. VISIBLE INDICATION Fretting of ferrous components commonly produces: Fine reddish brown or dark oxide debris WHY IT MATTERS? Surface damage creates local stress raisers Under cyclic loading, cracks may initiate Crack can propagate and cause failure KEY TAKEAWAY Small movement + repeated cycles = serious surface damage

Slide 5 · 04/04

FRETTING CORROSION Mechanical fretting + environmental oxidation THE DAMAGE CYCLE 01 MICRO MOVEMENT Small oscillatory movement occurs at the interface. 02 FILM DISRUPTION Protective oxide film is broken or removed. 03 FRESH METAL EXPOSED Bare metal reacts with the environment. 04 OXIDATION & OXIDE DEBRIS Oxide particles form at the contact area. 05 ABRASIVE ACTION Oxide debris leads to further wear and the cycle continues. 06 DAMAGE ACCUMULATION Surface damage can initiate cracks under cyclic load. WHERE IN PROPULSION SYSTEMS? Potentially at highly stressed fitted interfaces subject to cyclic micro movement. PROPELLER SHAFT / HUB INTERFACES Fretting corrosion can occur at the shaft and propeller hub interface. SHAFT LINER INTERFACES Relative movement can occur under shaft liners depending on fit, geometry and loading. OTHER LOADED INTERFACES Any tightly fitted or clamped surfaces under cyclic load can be affected. WHY IS IT SERIOUS? Surface damage acts as a stress raiser. Combined with cyclic loading in shafts, it can lead to: FRETTING DAMAGE FATIGUE CRACK INITIATION CRACK PROPAGATION FATIGUE FAILURE IMPORTANT Do not assume every fretting mark or crack can simply be ground out. Any cracking or significant fretting damage on propulsion shafting requires assessment in accordance with applicable classification, manufacturer and approved repair requirements. DESIGN RESPONSE Stress relief grooves may be provided at appropriate shaft liner and propeller hub locations to reduce local and hoop stresses associated with fitted members. STRESS RELIEF GROOVE (SHAFT LINER) STRESS RELIEF GROOVE (PROPELLER HUB) KEY TAKEAWAY Fretting + Corrosive / Oxidising Environment = Fretting Corrosion

In exams

Asked 12 times across 1 different question, most recently in 2026. Asked most in MEO Class 2 (12).

Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.

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