5.3 Service Discontinuities

ASNT NDT Level III Basic Examination Exam

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1 Introduction & Exam Overview

1.1 Current Exam Facts 1.2 Level III Role and Certification Path 1.3 Eligibility, Fees, Scheduling, and Retakes 1.4 Study Strategy and Exam-Day Approach

2 Chapter 2: Materials Fundamentals for NDT

2.1 Ferrous Metals and Iron-Carbon Phases 2.2 Non-Ferrous Metals and Alloys 2.3 Mechanical Properties and Crystal Structure

3 Chapter 3: Manufacturing Processes

3.1 Casting Processes and Cast Discontinuities 3.2 Forging, Rolling, and Forming 3.3 Extrusion, Drawing, and Machining 3.4 Heat Treatment Effects

4 Chapter 4: Welding and Joining Processes

4.1 Fusion Welding Processes 4.2 Weld Metal and HAZ Discontinuities 4.3 Brazing, Soldering, and Mechanical Joining

5 Chapter 5: Discontinuity Classification

5.1 Inherent Discontinuities 5.2 Processing Discontinuities 5.3 Service Discontinuities

6 Chapter 6: Specific Discontinuity Types

6.1 Cracks and Fracture Modes 6.2 Porosity, Inclusions, and Lack of Fusion 6.3 Laminations, Seams, Laps, and Undercut 6.4 Relevant vs Non-Relevant Indications

7 Chapter 7: NDT Method Selection and Inspection Planning

7.1 Method Selection Principles 7.2 Surface vs Volumetric Methods 7.3 Inspection Planning and Flaw Expectations

8 Chapter 8: Ultrasonic and Radiographic Testing

8.1 Ultrasonic Testing Principles and Applications 8.2 UT Limitations, Calibration, and Evaluation 8.3 Radiographic Testing Principles and Applications 8.4 RT Limitations, Image Quality, and Safety Basics

9 Chapter 9: Magnetic Particle, Liquid Penetrant, and Visual Testing

9.1 Magnetic Particle Testing 9.2 Liquid Penetrant Testing 9.3 Visual Testing

10 Chapter 10: Electromagnetic, Leak, Acoustic Emission, and Infrared Testing

10.1 Electromagnetic / Eddy Current Testing 10.2 Leak Testing 10.3 Acoustic Emission Testing 10.4 Thermal / Infrared Testing

11 Chapter 11: Specialty Methods and Method Comparison

11.1 Neutron Radiography 11.2 Magnetic Flux Leakage 11.3 Method Comparison and Supplemental Examinations

12 Chapter 12: SNT-TC-1A Recommended Practice

12.1 SNT-TC-1A Overview and Written Practice 12.2 SNT-TC-1A Training and Experience 12.3 SNT-TC-1A Examinations and Recertification

13 Chapter 13: CP-189 Standard Practice

13.1 CP-189 Overview and Scope 13.2 CP-189 Mandatory Requirements 13.3 SNT-TC-1A vs CP-189 Comparison

14 Chapter 14: CP-105 and NDT Training Programs

14.1 CP-105 Purpose and Topical Outlines 14.2 Developing and Approving NDT Training 14.3 OJT, Mentoring, and Instructor Oversight

15 Chapter 15: Building and Managing Certification Programs

15.1 Building Qualification and Certification Programs 15.2 Administering Examinations and Evaluating Competency 15.3 Program Audits and Managing Certifications

16 Chapter 16: Codes, Standards, Procedures, and Quality

16.1 Interpreting Codes, Standards, and Specifications 16.2 Developing and Approving NDT Procedures 16.3 Acceptance Criteria and Quality Control 16.4 External Audits and Inspection Documentation

5.3 Service Discontinuities

Key Takeaways Explain This Topic AI

Service discontinuities form during operation under load, environment, temperature, or time—not during primary production or shop manufacturing of a sound part.

Major families: fatigue (initiation/propagation, beach marks), corrosion forms, creep, wear, overload fracture, hydrogen embrittlement in service, and stress corrosion cracking (SCC).

In-service inspection (ISI) thinking prioritizes high-stress sites, corrosive environments, thermal gradients, and prior indication history—not uniform scanning of every surface equally.

Level III roles include procedure design for ISI, method selection for expected damage mechanisms, and distinguishing manufacturing remnants from new service growth.

Last updated: July 2026

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

Initiation : microcracks form at stress concentrators—fillets, welds toes, corrosion pits, tool marks, inclusions at the surface, fretting contacts.

Propagation : the crack advances incrementally each cycle (or block of cycles).

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 cue NDT / Level III implication
Starts at surface stress raiser Surface methods (MT, PT, ET, VT) highly relevant early

Planar crack roughly perpendicular to principal tensile stress UT shear-wave / phased array for depth sizing in many geometries

Multiple initiation sites Inspect all similar geometric details, not one crack only
Weld toe / HAZ preference Focus ISI at toes, attachments, and misalignment peaks

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:

Form Brief mechanism Typical NDT focus
Uniform (general) thinning Broad surface metal loss UT thickness gauging, RT for remaining wall
Pitting Localized anodic attack VT, UT (pit depth), ET; pits initiate fatigue/SCC
Crevice corrosion Stagnant chemistry in gaps VT of joints, under deposits; hard to access
Galvanic Dissimilar metal couple Inspect joints of dissimilar couples
Erosion-corrosion Flow-assisted metal loss UT grid on elbows, tees, downstream of orifices
MIC (microbiologically influenced) Biofilms accelerate attack Localized pitting patterns; combined VT/UT
Intergranular Preferential boundary attack ET, PT of cracked surface, metallography

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 T_m 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 feature Level III implication
Often branched crack networks PT/MT for surface; UT for depth; do not assume single planar fatigue crack
May grow with little general corrosion Clean appearance does not mean “no SCC”
Residual weld stress is enough As-welded stainless systems need environmental control and focused ISI

Incubation then rapid growth Interval-based inspection may miss late-stage acceleration—risk-based programs help

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
Damage mechanism — fatigue, corrosion, creep, SCC, HE, wear, overload risk

Locations of high driving force — stress concentrators, welds, supports, thermal gradients, flow disturbances, stagnant crevices

Detectability — surface vs embedded; insulation; coating; access
Consequence — leak, fracture, toxic release, structural collapse
History — prior repairs, known thin areas, previous indications, process upsets
Method orientation for service damage

Expected service damage Often primary methods
Surface fatigue at fillets/welds VT, MT/PT, ET; UT for sizing
Internal fatigue / embedded cracks UT (including phased array where used)
Wall thinning / erosion UT thickness grids, RT profile
SCC / branched surface cracks PT/MT + UT depth; environmental review
Creep cavitation (early) Replication/metallography + targeted NDT of cracks
Hydrogen cracks UT/MT depending on location; process review
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)

Question Inherent Processing Service
When? Material production Manufacturing of the part Operation

Example Inclusion stringer, pipe, segregation Quench crack, weld LOF, grinding crack Fatigue, SCC, creep, pitting

Typical first methods UT/RT for internal; MT/PT if opened Surface methods after grind/weld/quench; RT/UT for welds Mechanism-driven mix + ISI focus

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