Materials fail in four main ways: brittle fracture (sudden, with no necking), fatigue (a crack that grows under repeated load well below the material’s strength), creep (slow stretching under constant load at high temperature) and corrosion (chemical loss of metal). Each leaves a recognisable fracture or surface, so identify the mode before replacing the part, because the same part will fail the same way again. Fatigue surprises people most, since a part can carry its load for thousands of cycles with no visible change and then break.
Part of Materials Science Principles
Failure modes are the distinct physical ways a part stops doing its job: sudden fracture, crack growth under repeated loading, slow deformation at temperature, and chemical attack.
Brittle Fracture Gives No Warning
A brittle material breaks at its tensile strength with no yielding. Tensile Strength lists cast iron, stone and concrete here and notes that internal flaws or surface notches start the crack at stresses well below what a clean specimen would suggest. The Advanced Materials article adds glass and hardened steel at low temperatures.
The fracture surface is the clue: a flat, bright, granular surface with no stretching at the edges. A ductile break instead shows a neck and a dull, fibrous surface. Hardened steel that was not tempered fails this way, and Tempering exists to prevent it. Steel with the body-centred cubic structure also turns brittle as it gets colder, which is the reason Ferritic Steel is a poorer choice than Austenitic Steel for cold service.
Fatigue Starts A Crack That Grows One Cycle At A Time
Repeated loading, even far below the yield strength, can start a tiny crack at a surface scratch, a sharp corner, a weld toe or an inclusion. Each load cycle extends the crack a little. The part shows nothing until the remaining section is too small and it snaps.
Look for a smooth, curved region with faint ripples, sometimes called beach marks, spreading from the start point, and a rougher area where the final break happened. Everyday proof: bend a piece of soft steel wire back and forth, and it fails after a modest count of bends because each bend exceeds its yield.
To reduce fatigue:
- Use generous radii at corners and shoulders, never sharp steps or square keyways.
- Finish surfaces smooth. Tool marks are notches.
- Put the surface in compression. A carburised and hardened surface resists fatigue better, as covered in Case Hardening.
- Steel has a stress below which it can be cycled indefinitely, but aluminium alloys do not, so every aluminium part has a finite life.
Creep Is Slow Stretch Under Heat And Load
At high temperature, metals deform slowly under a constant load that would never harm them cold. The Advanced Materials article names furnace components, steam pipes and turbine blades. Look for sagging, bulging and gradual wall thinning. Lead creeps even at room temperature because its melting point is low, which is why old lead pipe sags between its supports. The remedy is lower stress, a metal with a higher melting point, and inspection for growth before rupture.
Corrosion Removes The Metal Chemically
Rust Prevention says rust needs iron, water and oxygen together, that removing any one stops it, and that unprotected iron outdoors in a temperate climate loses 0.1-0.3 mm of metal per year.
Worked example: a 5 mm steel plate corroding from one face loses half its thickness, 2.5 mm, in 2.5 / 0.3 = about 8 years at the fast rate or 2.5 / 0.1 = 25 years at the slow rate. Pitting is faster than the average, because local attack concentrates the loss.
The common varieties are:
- General. Even loss from the whole surface, prevented by coatings.
- Galvanic. Two different metals in contact with moisture form a cell, and the more reactive one dissolves. This is how zinc protects steel in Galvanizing and how steel bolts in a bronze fitting eat themselves.
- Intergranular. Attack along grain boundaries, as in sensitised stainless steel (see Grain Boundaries).
- Crevice and pitting. Attack in spots where oxygen cannot reach to heal a protective film, or where chlorides (salt) break it down.
Diagnosis At A Glance
| Mark | Probable mode | First fix |
|---|---|---|
| Flat, bright, granular break | Brittle fracture | Temper, remove notches, check temperature |
| Smooth curved zone with ripples, then rough break | Fatigue | Larger radius, smoother surface, lower stress range |
| Sag, bulge, slow elongation when hot | Creep | Lower stress, cooler service, different alloy |
| Flaking, pits, thinning | Corrosion | Coating, drainage, metal pairing |
Common Mistakes
- Replacing like with like — the new part fails the same way, usually at the same place.
- Blaming the material first — a sharp corner or a pit often starts the failure.
- Looking at the break only — look at the parts either side for cracks that have not yet gone through.
A cracked load-bearing part is not a spare
A lifting hook, chain link, axle, grinding wheel or pressure vessel with a crack can fail suddenly and violently, and fragments or a falling load kill. If a crack is found, take the part out of service. Do not weld, drill a stop-hole and carry on, or “just use it lightly”.
What’s Next
- Learn next: Non-Destructive Testing — finding cracks before they go through
- Combine with: Surface Treatments — slowing corrosion and fatigue at the surface