21.2 Common Traps by Method

A trap on the ASNT NDT Level II papers is not a trick question. It is a physically plausible wrong choice. The 50-item general tests one method's principles, equipment, and evaluation. The 40-item specific tests codes, applications, and techniques for General Industry or Pressure Equipment. Both reward the candidate who can name which physical condition each option would actually produce. This section is the last pass through the five methods ASNT currently offers at NDT Level II — magnetic particle testing (MT), liquid penetrant testing (PT), radiographic testing (RT), ultrasonic testing (UT), and visual testing (VT) — written as the mistakes that cost items after the content chapters are already behind you.

If a choice would violate orientation, current type, capillary fill, the inverse-square law, wave mode, or demonstrated lighting, it is wrong even when it sounds like something a hurried technician would do on Friday afternoon.

MT traps: orientation, current type, residual versus continuous

Field versus crack orientation. Magnetic particle testing finds discontinuities that distort flux and leak. Leakage — and therefore a particle indication — is strongest when the discontinuity cuts across the field (the long axis of the crack is perpendicular to the flux). A crack parallel to the field produces little or no leakage. The trap is magnetizing a bar or pipe longitudinally and expecting a longitudinal seam, lap, or stringer to light up. That discontinuity runs with the flux. Use a circular field (head shot, central conductor, or prods placed so current is parallel to the crack) for longitudinal discontinuities, and a longitudinal field (coil shot, or a yoke whose legs span a transverse crack) for discontinuities that cut the long axis. A 90° second shot exists because one field direction cannot cover both orientations. If the stem shows a longitudinal crack and a coil field along the same axis, the physics says weak or no leakage, not "the coil is always stronger."

AC versus DC depth. Alternating current is forced to the surface by the skin effect. It is the right current family for surface-breaking fatigue cracks, especially with a yoke on in-service carbon steel. Direct current, half-wave DC, and full-wave DC produce fields that reach deeper, which is why they are chosen when the procedure wants near-surface or subsurface sensitivity in a suitable product form. The trap is claiming AC "sees deeper because it is stronger," or claiming DC cannot find a surface crack. AC is the surface tool. DC families add depth. Neither current type removes the orientation rule. A deep slag stringer inspected only with AC can be a miss even when the field is perfectly across it.

Residual versus continuous. Continuous magnetization is applied while the current or yoke field is on and particles are applied. Residual magnetization inspects in the field that remains after the magnetizing current is removed. Residual works only when the material has enough retentivity to hold a useful field — typically harder, higher-carbon, or heat-treated ferromagnetic alloys. Many mild steels and some weld heat-affected zones will not hold a residual field worth inspecting. The trap is using residual on a low-retentivity part because it is faster, or claiming continuous is forbidden. Continuous is the default when retentivity is unknown or low. Residual is a technique for high-retentivity material, not a universal time-saver.

PT traps: overwash, dry penetrant, Type I lighting

Overwash. After dwell, excess penetrant must leave the surface and stay in the opening. A coarse water wash, a long rinse, a nozzle held too close, high pressure, or an emulsifier left on too long strips dye from a shallow or tight crack. The part looks clean. The trap is calling that part acceptable. Control wash time, pressure, temperature, and emulsifier contact to the procedure (ASTM E1417 / ASME Section V, Article 6 families of controls, as the invoked document states). If the stem describes a hard spray and a vanished indication, the failure is removal, not "no crack." Water-washable Method A and over-emulsified Method B or D parts are the usual victims. Solvent wipe that is too wet and too aggressive is the Method C version of the same physics: you cleaned the reservoir out of the opening.

Dry penetrant. Penetrant is a liquid. Capillary fill requires a wetting liquid, not a dust. Dry powder is a developer form, not a penetrant type. The trap is dusting a dry "penetrant" on a part and expecting bleed-out, or confusing dry developer with a dry inspection method that skips the liquid. If no liquid entered the discontinuity, there is no reservoir for the developer to blot. A dry developer applied after a correct liquid penetrant process is legitimate. A dry powder used as the penetrant is not PT.

Type I lighting. Under ASTM E1417 / AMS 2644 language used throughout this guide, Type I is fluorescent dye inspected under ultraviolet-A in a darkened booth or shroud. Type II is visible dye inspected in white light. The trap is inspecting a Type I part in a bright bay as if it were red dye, or using a white-light meter reading as proof that a fluorescent indication should have been visible. Type I needs specified UV-A irradiance at the surface and controlled visible-light background. A legal white-light number does not make a Type I indication glow. Conversely, a Type II red-dye line does not become more sensitive because you borrowed the fluorescent booth. Type is dye family and viewing light, not wash method.

RT traps: inverse square, artifacts, tight cracks

Inverse-square mistakes. Intensity follows I₁/I₂ = (D₂/D₁)². Doubling source-to-film distance quarters intensity and quadruples exposure time for the same density. Halving distance quadruples intensity and cuts time to one-fourth. The common trap is treating intensity as linear with distance ("twice as far, twice the time") or inverting the ratio and dividing when you should multiply. Write the squared ratio every time. Example: 3.0 min at 20 in becomes 3.0 × (40/20)² = 12 min at 40 in, not 6 min. Time scales with the square of the distance ratio. Geometric unsharpness is a different formula (Ug = f × t / d); do not "correct" Ug with the inverse-square law or correct exposure with the Ug equation.

Film artifacts versus defects. A dark or light mark on film is not automatically a discontinuity in the part. Static marks, pressure marks, crimps, processor streaks, screen scratches, and light leaks are process artifacts. A defect in the part must be consistent with manufacturing or in-service damage at that location and must survive a check of film handling. The trap is calling a roller mark a crack, or dismissing a true incomplete-fusion line as "just an artifact" because it is straight. Use geometry, image-quality indicator (IQI) sensitivity, density, and whether the mark tracks the part or the film path. If the mark moves when you reshoot after cleaning the cassette, it was not in the weld.

Tight cracks. A tight crack images when it produces enough subject contrast and is sufficiently aligned with the beam. A crack that is nearly parallel to the film and poorly aligned with the central ray can disappear. High geometric unsharpness and high kilovoltage (low subject contrast) make tight cracks worse. The trap is claiming radiography always finds every crack, or blaming the isotope when the geometry was wrong. Volumetric methods are not automatically better than surface methods for a tight, surface-breaking fatigue crack that PT or MT would have shown.

UT traps: skip versus half-skip, mode conversion, wrong velocity

Skip versus half-skip. In shear-wave weld inspection, a half-skip (first leg, ½ V) is the path from the probe to the opposite surface. A full skip (1 V) is the path that reflects and returns to the inspection surface one skip distance away. The skip distance is the surface distance for that full V at the refracted angle. The trap is calling the first-leg opposite-surface node a full skip, or parking the probe at one skip when the procedure wants a half-skip examination of the root. Draw the V. Root geometry on the far face is a half-skip problem until a bounce puts the beam back on the near face.

Mode conversion. When a longitudinal wave hits an interface at an angle, part of the energy can convert to shear (and the reverse). Past the first critical angle, the longitudinal wave in the second medium disappears and shear remains — that is why angle-beam weld inspection is usually a shear inspection in steel. The trap is calibrating or sizing as if every echo is the intended mode. A wedge designed for 45° shear in carbon steel will not produce the same angle or mode in a different-velocity material. Unexpected signals after a geometry change, a backwall, or a counterbore are often converted modes, not extra cracks. If the stem gives an incident angle and two velocities, compute the refracted mode before you name the reflector.

Calibrating on the wrong velocity. Range, thickness, and skip distance all use velocity. Carbon-steel longitudinal velocity is not aluminum, not every stainless product form, and not shear in the same steel. The trap is locking an International Institute of Welding (IIW) or IIW-type block calibrated for carbon-steel shear and then reading thickness or skip on a part whose velocity was never verified. Distance equals time multiplied by velocity. If the stem gives a velocity, use that velocity. A 10% velocity error is a 10% thickness and skip error, which is how a root indication gets plotted in the wrong half of the weld.

VT traps: lighting, ripple versus crack, remote resolution

Lighting too low. Visual testing is a contrast problem. ASME Section V, Article 9 and many procedures use a familiar 100 fc / 1000 lx class white-light number for direct VT — attribute the number to the code or procedure, not to a secret ASNT cut. Light that is too low hides undercut, porosity, and crack mouths. The trap is inspecting a dark tank with a dying flashlight and calling the surface acceptable because nothing was seen. Inverse-square applies to illuminance too: doubling the lamp distance quarters the light on the work. Angle of incidence, glare, and shadows can make a legal lux reading useless. Measure at the surface you are actually grading.

Calling ripple a crack. Weld ripple, overlap, undercut, grinding marks, and oxide lines can look linear. A crack is a fracture with a sharp mouth and usually no intentional process pattern. The trap is rejecting every linear shadow as a crack, or accepting a crack because "welds always look like that." Use lighting angle, magnification, and the acceptance standard named in the procedure. Classify relevant / nonrelevant / false before you size. Ripple is a process texture. A crack crosses ripple.

Remote resolution not demonstrated. Borescopes, cameras, and remote VT systems must demonstrate the required resolution — a wire, a chart, or a character height the procedure names — before you call the inspection valid. The trap is treating a blurry video as a completed VT because the camera powered on. If resolution was not shown at the working distance, the remote exam is not demonstrated. Zoom that makes a smear larger is not the same as resolving the required line pair.

The specific-exam trap that crosses every method

The 40-item specific (General Industry or Pressure Equipment) will print codes, applications, and techniques. If the stem prints a table, that table is the given. Do not replace it with the shop traveler you memorized, a different ASME edition, or a number from a general-exam memory aid. The exam is testing whether you can apply the document in front of you. Memory is for formulas and definitions when no table is given. Memory is a liability when the stem already did the lookup. This is the same discipline as section 21.1's 3.0-minute specific budget: the extra time is for reading the printed row, not for arguing with it.

Cross-method trap table

Scenarios that match exam stems

MT yoke on a longitudinal seam. A technician applies a yoke so the field runs along a pipe axis and reports no indication on a known longitudinal seam. The field was parallel to the crack. Rotate the field so flux crosses the seam.

PT water-washable overwash. A Method A part is blasted with a close, high-pressure rinse. Developer shows nothing. The Level II who says "accept" walked into the overwash trap. Recheck removal parameters before you trust a blank surface.

RT distance doubling. Exposure was 2.5 min at 30 in. The source is moved to 60 in and the candidate doubles the time to 5.0 min. Intensity fell by four, not by two. The correct scale is 2.5 × (60/30)² = 10 min.

UT skip on a 1-inch plate. A candidate places the probe at a full skip to look at the root on the first leg. The root on the opposite face is a half-skip problem. Draw the V.

VT remote camera. A tank camera powers on and the technician reports "no cracks visible" without showing the procedure's resolution target at working distance. The remote exam was not demonstrated.

Specific table. The stem's thickness band allows 1/8 in reinforcement. The candidate's shop allows 1/16 in. The scored answer follows the printed band.

Carry one sentence per method into the booth: MT — field across the crack, AC on the surface, continuous unless retentivity is known; PT — do not wash the crack empty, penetrant is liquid, Type I needs UV-A; RT — square the distance ratio, artifacts are not defects, tight cracks need alignment; UT — draw the V, name the mode, use the given velocity; VT — measure the light, do not call ripple a crack, demonstrate remote resolution; specific — read the printed table.