5.3 Service Discontinuities

5.3 Service Discontinuities

Quick Answer: Service discontinuities form after a component enters operation, under mechanical load, environment, temperature, radiation, or time-dependent degradation. Core families for Basic include fatigue, corrosion (several forms), creep, wear, overload fracture, hydrogen embrittlement in service, and stress corrosion cracking (SCC). Level III work emphasizes damage-mechanism thinking and in-service inspection (ISI) prioritization—not only naming crack types.

A part can leave the shop sound and still fail years later. Origin class tracks that timeline: if the free surface or crack grew in service, classify it as service, even when it started at a processing stress raiser (the raiser may be processing; the propagated crack is service damage).

Fatigue

Fatigue is progressive, localized permanent damage from cyclic stress below the material’s static ultimate strength.

Initiation and propagation

1. Initiation: microcracks form at stress concentrators—fillets, welds toes, corrosion pits, tool marks, inclusions at the surface, fretting contacts.
2. Propagation: the crack advances incrementally each cycle (or block of cycles).
3. Final fracture: remaining ligament fails by overload (often ductile or brittle thumbnail remainder).

Beach marks and fracture appearance

Beach marks (clamshell marks) are macroscopic arrest or load-change marks on the fatigue fracture surface. They indicate progressive growth under variable amplitude or intermittent operation. Microscopic striations (when present) are cycle-by-cycle features used in failure analysis; Basic-level recognition is that progressive, smooth, thumbnail cracks from stress raisers scream fatigue until proven otherwise.

Fatigue is service even if initiation was helped by a machining notch (processing) or an inclusion (inherent). Exam answers should still identify the active damage mode as service fatigue when growth occurred in operation.

Corrosion Types (Service Context)

Corrosion converts metal to compounds through chemical or electrochemical reaction. Forms that drive NDT planning:

Corrosion products can mask or fill cracks, reducing PT sensitivity—Level III procedures address cleaning and method limits.

Creep

Creep is time-dependent plastic deformation under sustained load at elevated temperature (roughly above ~0.4 T_m absolute for many metals, alloy-dependent). Damage progresses from microvoids on grain boundaries to aligned cavities and eventual intergranular cracking.

ISI thinking for creep-sensitive equipment (steam lines, furnace tubes, turbine components):

• Prioritize high-temperature, high-stress locations (bends, welds, reduced sections)
• Expect volumetric and surface methods depending on stage; replication/metallography often complements NDT
• Dimensional monitoring (strain) may be part of the integrity program alongside crack detection

Creep is pure service (or at least operational) degradation—not a mill inclusion class.

Wear

Wear removes material by contact: adhesive, abrasive, fretting, or erosive mechanisms. Discontinuities include:

• Loss of section and altered geometry
• Fretting cracks at clamped interfaces under micro-motion
• Embedded abrasive and surface smearing that hide cracks

NDT roles: VT and dimensional checks first; MT/PT/ET where fretting cracks are suspected; UT for remaining thickness under wear scars. Wear often interacts with fatigue at fretted surfaces.

Overload Fracture

Overload is single-event (or few-event) fracture when applied stress exceeds the load-carrying capacity of the remaining section. Features:

• Little progressive beach-mark history on the final fracture face (though overload may finish a fatigue crack)
• Gross plastic deformation in ductile materials; crystalline cleavage facets in brittle fractures
• May follow an impact, overpressure, or collision

NDT after overload events focuses on adjacent structure for secondary cracks and on confirming whether a pre-existing fatigue or manufacturing flaw reduced the section. The overload event itself is a service/operational failure mode.

Hydrogen Embrittlement in Service

Hydrogen embrittlement (HE) reduces ductility and promotes brittle cracking when atomic hydrogen is present under tensile stress. Sources in service include:

• Corrosion reactions generating hydrogen
• High-pressure hydrogen service
• Cathodic protection systems under some conditions
• Residual hydrogen from prior processes that was not fully baked out (borderline processing vs service—if cracking occurs in operation under stress, treat as service HE for ISI purposes; if cracking occurred right after plating, processing)

Cracks can be internal or surface-connected, often with little plastic deformation. Method selection depends on material and access: UT for internal cracks in susceptible alloys; MT/PT when surface-breaking; process and environmental controls remain primary prevention.

Stress Corrosion Cracking (SCC)

SCC requires a susceptible material, a specific environment, and tensile stress (applied or residual). Cracks may be intergranular or transgranular depending on the system (e.g., austenitic stainless in chlorides; brass in ammonia; high-strength steels in certain aqueous environments).

SCC is a flagship service discontinuity family on Basic exams because it forces multi-factor reasoning.

In-Service Inspection Thinking for Level III

Shop manufacturing inspection asks: “Did we make it right?” ISI asks: “How is it degrading, and where first?”

Prioritization framework

1. Damage mechanism — fatigue, corrosion, creep, SCC, HE, wear, overload risk
2. Locations of high driving force — stress concentrators, welds, supports, thermal gradients, flow disturbances, stagnant crevices
3. Detectability — surface vs embedded; insulation; coating; access
4. Consequence — leak, fracture, toxic release, structural collapse
5. History — prior repairs, known thin areas, previous indications, process upsets

Method orientation for service damage

Distinguishing old manufacturing flaws from new service growth

Level III procedures and evaluation rules must separate:

• Non-growing manufacturing remnants within acceptance limits
• Active service cracks that require repair, re-rate, or retirement

Tools include sizing trending, fracture-surface/history review, load and environment logs, and comparison to baseline inspections. A midwall lamination present at fabrication is inherent; a new crack growing from a corrosion pit after ten years is service.

Integrated Origin Triad (Exam Close)

Master this triad and Domain 4 scenario questions become systematic: read the history → assign origin class → predict morphology → select methods and inspection timing.