08 Material Failure and Durability

How materials fail

Materials fail when loads, environments, temperature, or time cause them to lose the strength or function required for service. Failure can be sudden, as in brittle fracture, or gradual, as in wear or corrosion. Mechanisms can interact: corrosion pits can start fatigue cracks, and wear can remove a protective coating and expose a surface to corrosion.

Fracture and crack resistance

Fracture separates a material into pieces. A crack may start at a flaw, pore, sharp corner, weld defect, or another stress concentration, then grow when local stress is high enough.

In ductile fracture, substantial plastic deformation often occurs before separation. In brittle fracture, a crack can propagate with little visible deformation, so warning may be limited. Ceramics are commonly brittle; metals and polymers may show either behavior depending on composition, temperature, loading rate, and microstructure.

Fracture resistance depends not only on strength but also on fracture toughness. Prevention includes reducing sharp notches and defects, controlling fabrication quality and residual stress, selecting adequate toughness for the service temperature, and designing to tolerate plausible flaws. Depending on the material, part geometry, and defect type, dye penetrant, magnetic-particle, ultrasonic, or radiographic testing can help find flaws.

Fatigue under repeated loading

Fatigue damage develops under repeated or fluctuating loading. A crack can initiate at a surface scratch, inclusion, hole, or other stress raiser and grow over many cycles, sometimes when peak stress is below the material’s yield strength. Eventually, the remaining section may be too small to carry the load and fractures. Fatigue cracks may show progression markings; microscopic striations can record incremental growth. Corrosion can accelerate fatigue damage, producing the combined mechanism called corrosion fatigue.

To improve fatigue life, reduce stress range and vibration, smooth or round stress concentrations, improve surface condition where appropriate, prevent corrosion, and avoid unexpected overloads. Designers use cyclic-load data and account for the actual service environment; a single static-strength value does not establish fatigue life.

Creep over time

Creep is time-dependent deformation under sustained stress. It is especially important at elevated temperatures, although polymers can creep at ordinary service temperatures. A typical creep curve has an initial period of decreasing creep rate, a longer period of near-steady rate, and a final period of accelerating deformation that may end in rupture. Temperature, stress, material structure, and exposure time all affect creep.

To limit creep, select a material qualified for the temperature and service life, reduce sustained stress or temperature where possible, and set allowable-deformation and inspection limits. Bulging, permanent dimensional change, and time-at-temperature records can provide diagnostic clues.

Wear at contacting surfaces

Wear damages or removes material at contacting surfaces in relative motion. In abrasive wear, hard particles or rough asperities cut or plow a surface. Adhesive wear involves local bonding and shearing between contacting surfaces, which can transfer or detach material. Repeated contact can also cause surface-fatigue wear, including pitting or spalling in bearings. Fretting occurs with small oscillatory motion between loaded surfaces and may combine wear, oxidation, and fatigue.

Prevention depends on the contact system. Use suitable lubrication, exclude abrasive debris, control contact loads and alignment, and select compatible surface materials or coatings. Diagnose wear by examining its pattern, the surface, and debris, and by checking lubrication, contamination, temperature, vibration, and alignment.

Corrosion and environmental degradation

Corrosion is deterioration caused by interaction with the environment. In metals, it may be relatively uniform or localized. Localized forms include pitting, crevice corrosion, galvanic corrosion between dissimilar metals, stress-corrosion cracking, and corrosion fatigue. A small pit can be dangerous because it may act as a crack starter. Polymers and composites can also degrade through ultraviolet exposure, chemical attack, moisture effects, or damage to the matrix or fiber–matrix interface.

Prevention begins with matching the material to the environment. Other measures include protective coatings, inhibitors, cathodic protection where appropriate, electrical isolation of dissimilar metals, drainage and sealing to prevent trapped moisture, and regular cleaning and inspection. Design, material selection, coatings, and maintenance work together in corrosion control.

Diagnosing failures and preventing recurrence

A sound failure investigation connects the evidence, loading history, and service environment. First, make the part safe and document its position, condition, and operating history. Preserve fracture surfaces and deposits; do not clean, cut, or rub them before documentation. Then inspect the component and, where appropriate, use nondestructive evaluation to locate damage.

Laboratory examination may include fractography, microscopy and metallography, chemical analysis, hardness or mechanical testing, and analysis of corrosion products or wear debris. Evaluation tools can include X-ray, ultrasound, penetrant inspection, and scanning electron microscopy.

A visible crack or damaged surface is evidence, not by itself a complete explanation. Distinguish the failure mode—what happened, such as fracture—from the cause—why it happened, such as cyclic overload, an unsuitable environment, a defect, or a combination. Corrective action should address the cause, for example by revising geometry, material or processing, operating limits, protection, lubrication, or inspection intervals. Verify that the change reduces the risk.

Connecting prevention and diagnosis

Fracture, fatigue, creep, wear, and corrosion are distinct but often interacting routes to material failure. Reliable prevention matches material and design to loads, temperature, time, and environment. Reliable diagnosis preserves evidence, examines the component and its operating history, and traces damage back to a cause that can be corrected.