Failure: fatigue, creep, fracture

Why it matters

Parts rarely fail because a load was simply bigger than the tensile strength. Far more often they fail by a mechanism that a static strength check misses: a crack that grows a little with every load cycle (fatigue), slow stretching at high temperature (creep), or a sudden fracture that starts at a small flaw. Failure analysis engineers read broken parts like a crime scene, and the features on a fracture surface usually reveal which mechanism did it. This lesson describes those features in words so you can recognise them in photographs and real parts.

Ductile and brittle fracture

Ductile fracture happens after a lot of plastic deformation. A round tensile sample of a ductile metal necks, small voids form and join in the neck, and the final break makes the classic cup-and-cone shape: a fibrous, dull, grey centre with a shear lip around the edge. Under a microscope the surface is covered in tiny dimples. The part visibly stretches or bends first, which gives warning.

Brittle fracture happens with little or no plastic deformation, at high speed, along flat surfaces roughly perpendicular to the main tensile stress. The surface is often bright and granular (in metals, cleavage facets that glint in the light) and may show chevron or river marks that point back toward the origin of the crack. There is little or no warning.

Some steels (with BCC crystal structure) change from ductile to brittle behaviour as the temperature drops: the ductile-to-brittle transition. Several welded Liberty ships of the Second World War cracked in cold water, some breaking completely in two, partly because their steel was below its transition temperature. Impact tests such as the Charpy test measure the energy absorbed at different temperatures to find the transition. FCC metals such as aluminium and austenitic stainless steel do not show this transition.

Fracture mechanics in one equation

Brittle fracture starts at flaws: scratches, pores, weld defects, small cracks. Stress concentrates at the tip of a crack, and fracture mechanics captures this with the stress intensity factor:

K = Y · σ · √(π · a)

where σ is the applied stress, a the crack length (half the length for an internal crack), and Y a geometry factor near 1. Fracture occurs when K reaches the material's fracture toughness K_IC.

Worked example. A plate with Y = 1 carries 200 MPa and contains a 2 mm crack:

K = 1 × 200 × √(π × 0.002) = 200 × 0.0793 ≈ 15.9 MPa·√m

Typical K_IC values: glass about 0.7–0.8, alumina about 3–5, high-strength aluminium alloys about 25, and tough steels 50 or more MPa·√m. So this crack would break glass at a tiny fraction of that stress but would not grow in a tough steel. Inspection programmes (for aircraft, pipelines, pressure vessels) set inspection intervals so cracks are found long before they reach the critical size.

Fatigue

Under repeated loading, a crack can start and grow at stresses well below the yield strength. Fatigue causes a large share of mechanical failures in service: shafts, axles, springs, bolts, welded joints, aircraft skins.

A fatigue fracture surface usually shows three zones:

1. an origin, often at a surface feature such as a sharp corner, keyway, scratch, weld toe or corrosion pit;
2. a smooth, flat region with curved beach marks (also called clamshell marks) spreading out from the origin, recording pauses and changes in loading; under high magnification, finer striations mark individual cycles;
3. a rough final fracture zone where the remaining section broke in one go. A small final zone means the stress was low; a large one means it was high.

Fatigue behaviour is shown on an S-N curve: stress amplitude against the number of cycles to failure. Many steels have an endurance limit, a stress below which they appear to last indefinitely (often roughly 35–60 % of tensile strength under ideal laboratory conditions), whereas aluminium alloys have no true limit and are rated at a set number of cycles. Smooth surfaces, generous radii, compressive surface stresses (shot peening) and avoiding corrosion all improve fatigue life.

Creep

At high temperature, roughly above 0.4 of the absolute melting temperature, materials slowly and permanently stretch under a constant load. Creep curves show a short primary stage, a long steady secondary stage (whose rate designers use), and a tertiary stage of accelerating strain and voids leading to rupture. Creep limits turbine blades, boiler tubes and furnace parts, and it explains why lead pipes sag and why plastic shelves droop over time at room temperature: for polymers and lead, room temperature is already "hot". Creep fractures often show many small voids along grain boundaries and visible stretching near the break.

Common mistakes

• Assuming a part that passed a static test is safe. Fatigue can fail it at a fraction of the yield stress.
• Polishing or cleaning away evidence. Fracture surfaces must be protected for failure analysis.
• Treating handbook endurance limits as design values. Real parts need corrections for size, surface finish, environment and reliability, from design standards.

Recap

• Ductile fracture: necking, cup-and-cone, dimples, warning. Brittle: flat, shiny, chevrons, sudden.
• K = Y·σ·√(πa); fracture when K reaches K_IC.
• Fatigue: origin, beach marks, final fracture zone.
• Creep: slow strain at high homologous temperature, ending in rupture.