3.1 What NDT Can and Cannot Detect

ASNT NDT Level II written exams — the 50-question general paper and the 40-question specific paper — reward candidates who know what a method can actually see. Interpretation, reporting, and Applications/Techniques items all start from capability and limitation, not from brand names of equipment.

Why capability comes first

A Level II who treats every particle buildup as a rejectable defect, or who claims radiography "sees everything inside the weld," fails both the exam and the job. Nondestructive testing (NDT) is a family of methods that examine a material, weldment, or component for discontinuities, dimensions, or properties without impairing intended use. The part stays in the production sequence or remains a candidate for continued service.

Destructive testing is the opposite: the specimen is intentionally damaged or consumed. Tensile tests, Charpy V-notch impact tests, nick-break and fillet-weld break tests, macroetch, and burst tests all destroy or alter the coupon. They give quantitative mechanical properties. They do not map the location of a crack in the actual production part you must ship or return to service.

NDT answers: is there an indication here, and what are its type, size, and location under this procedure? NDT does not, by itself, prove fitness-for-service (FFS). Fitness-for-service is an engineering evaluation (for example API 579 / ASME FFS-1) that uses NDT results plus loads, material properties, and remaining-life models. If a specific-exam stem says a Level II "accepted the vessel for continued service based only on a clean magnetic particle test," that is overreach. Report what the method found. Let the acceptance standard decide rejectability. Let engineering decide remaining life.

The five written methods in this ASNT NDT Level II program are Magnetic Particle Testing (MT), Liquid Penetrant Testing (PT), Radiographic Testing (RT), Ultrasonic Testing (UT), and Visual Testing (VT). Eddy current testing (ET) is a common industrial method and a useful contrast — it can find surface and near-surface flaws in electrically conductive materials without a ferromagnetic requirement — but ET is not one of the five methods currently offered on this program's written exams. Do not study ET as an ASNT Level II method here.

Surface-breaking, near-surface, volumetric, and planar

Classify the target before you pick a method. The specific exam loves this decision.

What each method can reveal — and its hard limits

Visual Testing (VT) finds only what can be resolved with the available lighting, access, contrast, optical aids, and the inspector's vision. A Level II can document undercut, incomplete fusion at a visible root, arc strikes, misalignment, overlap, and surface porosity. VT cannot see a tight subsurface crack, a lamination in plate mid-wall, or a root crack hidden by backing or slag. Industry procedures written to ASME Section V, Article 9 and workmanship codes such as AWS D1.1 treat VT as a controlled method with lighting and distance rules, not as a casual glance.

Liquid Penetrant Testing (PT) finds surface-breaking discontinuities that have a clean, open path to the surface. Capillary action draws penetrant into the opening; a developer draws it back out as a visible or fluorescent bleed-out. PT works on ferromagnetic metals, nonmagnetic metals, and many nonmetals when the procedure allows. PT cannot find subsurface porosity, a mid-wall lamination, or a crack sealed by paint, scale, oil, or machining smear. ASTM E1417 and ASME Section V, Article 6 assume the surface is prepared so openings are not closed.

Magnetic Particle Testing (MT) finds surface and some near-surface discontinuities in ferromagnetic materials by leakage-field collection of magnetic particles. It is excellent on fatigue cracks at weld toes and on seams in bar stock. MT cannot be used as a primary method on austenitic stainless steel, aluminum, copper alloys, titanium, or plastics — those materials will not support the flux required for a leakage field. A crack that runs parallel to the magnetic field produces little leakage. ASTM E709 (guide), ASTM E1444 (practice), and ASME Section V, Article 7 are the usual industry references.

Radiographic Testing (RT) images differences in radiation absorption through the thickness. Gas porosity, slag, and many volumetric voids show well because they change the amount of material along the beam. Tight planar cracks and incomplete fusion are a classic RT miss: if the crack plane is nearly perpendicular to the beam, the beam crosses only the microscopic opening and density barely changes. Even when the beam is aligned along the crack plane (the favorable orientation), a tightly closed fatigue crack or a fused-but-unbonded interface may still be invisible. RT also needs two-side access for a conventional source-and-film (or source-and-detector) setup, plus radiation safety controls. ASTM E94 and ASME Section V, Article 2 govern technique and image quality.

Ultrasonic Testing (UT) uses high-frequency sound. A reflector returns an echo when acoustic impedance changes and the beam is oriented to hit it. Angle-beam shear wave is the workhorse for weld fusion-line and mid-wall planar flaws. Straight beam is the workhorse for laminations and remaining thickness. UT needs a couplant and an accessible scanning surface. It struggles with coarse-grained austenitic welds (scatter and beam skew), cladding interfaces, and complex geometry that lifts the probe or sends the beam into a corner trap. ASTM E164 and ASME Section V, Article 4 (welds) and Article 5 (other UT) are the common references.

Discontinuity, indication, defect, relevant, and nonrelevant

These words are not interchangeable. ASNT study materials and the method standards use them with care, and the written exam will punish loose language.

• A discontinuity is an interruption in the typical structure of a material — a void, crack, seam, inclusion, or lack of homogeneity. It is a physical condition. It is not automatically a reject.
• An indication is the response or evidence from an NDT method that requires interpretation — a particle buildup, a penetrant bleed-out, a film density change, an A-scan echo, or a visual mark.
• A defect is a discontinuity that does not meet the specified acceptance criteria. Defect is an accept/reject decision against a code, specification, or procedure. It is not a synonym for crack.
• A relevant indication is caused by a discontinuity that requires evaluation against the acceptance standard.
• A nonrelevant indication is caused by a known geometry or permitted condition — a keyway, a thread root, a change in section, magnetic writing, or a press-fit edge.
• A false indication is not caused by a discontinuity at all — lint on a fluorescent PT surface, incomplete penetrant removal, a light leak, or electrical noise on an instrument.

Realistic exam scenarios

A procedure calls for fluorescent PT on a titanium aerospace fitting. The Level II finds no bleed-out after correct dwell and developer times. That result means no open-to-surface discontinuity large enough to produce a relevant indication under that procedure. It does not prove the fitting is free of internal shrinkage.

A carbon-steel nozzle-to-shell weld is examined with yoke alternating current (AC) MT. A sharp linear particle buildup appears at the toe, transverse to the weld axis. That is a classic relevant surface indication. The same yoke on a Type 304 stainless nozzle will not produce a valid flux-leakage test, because 304 is not ferromagnetic.

An RT shot of a double-V-groove weld shows scattered rounded indications in the weld metal. Those are consistent with porosity (volumetric). The same radiograph can miss a tight incomplete-fusion plane along a bevel that presents almost no gap to the beam.

A UT 70° shear-wave scan from both sides of a 1-inch (25 mm) butt weld finds a mid-wall reflector at the original land. That planar reflector is exactly the condition RT often under-calls and UT is chosen to find.

What NDT does not do

NDT does not measure yield strength, toughness, or remaining life by itself. A clean examination means no reportable indication under this procedure, not that the component will not fail. Residual stress, dissolved hydrogen, and early creep can exist with little or no conventional MT, PT, RT, UT, or VT response until a crack or void forms. Thickness UT can trend wall loss; it does not by itself recertify a vessel.

When you write a report, record the method, procedure, technique, area examined, and the indication's type, size, and location. Let the acceptance standard decide defect. Let engineering decide remaining life.