AMME1362 Chap.7 Fatigue, Creep, Friction and Wear
Fatigue, Creep, Friction and Wear
Time and repeated contact create failures that a single monotonic strength value cannot predict. Fatigue involves crack initiation and growth under cyclic loading; mean stress, amplitude, surface condition and environment matter. Creep is time-dependent deformation under sustained conditions and becomes important relative to melting temperature.
Friction and wear depend on contacting materials, load, motion, lubrication, debris and surface films.
What this chapter covers
- 01
Stress cycles and amplitude
- 02
S-N behaviour
- 03
Crack initiation and growth
- 04
Primary, secondary and tertiary creep
- 05
Friction and contact
- 06
Wear modes and surface evidence
Worked example · free
Resolve a stress cycle
- 1Range is 100 minus 20, which equals 80 MPa.
- 1Amplitude is half the range, which equals 40 MPa.
- 1Mean stress is half the sum, which equals 60 MPa.
- 1Stress ratio is minimum divided by maximum, or 20 divided by 100 equals 0.20.
Key terms
- Stress amplitude
- Half the difference between maximum and minimum cyclic stress.
- Mean stress
- Half the sum of maximum and minimum cyclic stress.
- Steady creep
- The stage with an approximately constant creep rate under fixed conditions.
- Wear debris
- Material removed or transferred through interacting surface processes.
- Stress ratio
- Minimum cyclic stress divided by maximum cyclic stress under a stated sign convention. It distinguishes cycles with equal amplitude but different mean levels and must match the fatigue data used for life interpretation.
- Homologous temperature
- Service temperature expressed relative to melting temperature on an absolute scale. It supports comparisons of thermally activated deformation across materials whose numerical service temperatures alone would give a misleading sense of creep severity.
- Abrasive wear
- Surface removal caused when a harder asperity or particle ploughs or cuts a softer surface. Groove direction, debris and contact history support the diagnosis, while filtration, sealing or increased supported hardness can target the mechanism.
Fatigue, Creep, Friction and Wear FAQ
Why can fatigue occur below static strength?
Repeated loading can initiate a small crack at a notch, surface defect or inclusion and grow it incrementally. The final overload occurs only after the remaining section becomes too small.
What controls whether creep matters?
Temperature relative to melting temperature, applied stress, time, atmosphere and microstructural stability all matter. A temperature that is severe for a polymer may be modest for a refractory ceramic.
Is a friction coefficient a universal material constant?
No. It describes a tribological system that includes the material pair, surface finish, normal load, speed, temperature, environment, lubrication and evolving transfer films or debris.
Exam move
Divide revision into three clocks: repeated cycles for fatigue, elapsed time under sustained conditions for creep, and accumulated contact motion for wear. For every scenario, write load, time or cycle count, temperature, environment, geometry and surface condition before choosing a mechanism. This small table reveals missing service information and stops static strength from becoming the answer to every failure.
Drill cyclic arithmetic until maximum, minimum, range, amplitude, mean and ratio can be reconstructed from any two endpoints. After each calculation, rebuild the endpoints from mean plus or minus amplitude. Compare a fully reversed cycle with a tensile-only cycle of the same amplitude and explain why mean stress can affect crack opening and life. Label whether an S-N graph uses amplitude or range before reading it.
Sketch fatigue as initiation, progressive growth and final overload. Place a notch, machining mark or corrosion pit at the origin and explain how it raises local cyclic stress. Then propose separate controls for initiation and propagation: surface finish or compressive residual stress for the first, inspection and crack-growth management for the second.
Do not describe the rough final zone as evidence that the entire life was overload. For creep, redraw primary, secondary and tertiary stages from memory. On the secondary segment, calculate a slope with strain divided by time and carry the time unit. Compare service temperatures using an absolute scale relative to melting temperature, then add the effects of stress and microstructural stability.
Practise deciding whether a design limit is allowable strain, rupture time or loss of section. Build a contact-mechanism matrix for adhesive, abrasive and surface-fatigue wear. Record the likely surface evidence, the role of hardness or lubrication, and one intervention. Keep friction separate: it measures tangential resistance under stated conditions, while wear measures material change.
A low coefficient does not guarantee a low wear rate. Add coupled cases such as corrosion fatigue, oxidation-assisted wear and thermal fatigue. Identify the first local feature that accelerates the second process instead of listing both names. Finish with evidence-based diagnoses.
For each failed component, state the proposed origin, propagation path, final event and supporting observation, followed by an alternative explanation and a measurement that could reject it. The last step protects against overconfidence from one photograph. Review by changing one service variable at a time and predicting which stage of the history responds.
