Materials testing (Non-destructive)

Question 1

What is non‑destructive testing (NDT)?

Answer 1

Methods that check a material/component for defects or measure properties without causing damage that would prevent its use.

Question 2

Why use NDT in manufacturing? Give two reasons.

Answer 2

To detect defects before parts reach customers and to verify quality without scrapping good parts.

Question 3

Name six common NDT methods.

Answer 3

Visual testing (VT), Dye penetrant (PT), Magnetic particle (MT), Ultrasonic (UT), Radiographic (RT), Eddy current (ET).

Question 4

Is hardness testing NDT?

Answer 4

Usually no; it leaves a permanent indent (minimally destructive). NDT should leave the part fit for use.

Question 5

Visual testing (VT): what is it?

Answer 5

Looking for visible defects such as cracks, dents, corrosion, poor weld bead; often with magnifiers, mirrors, borescopes, and good lighting.

Question 6

VT: two advantages.

Answer 6

Low cost and quick to apply to many parts.

Question 7

VT: one limitation.

Answer 7

Only finds surface‑visible problems; relies on inspector skill and lighting.

Question 8

Dye penetrant testing (PT): what does it detect?

Answer 8

Surface‑breaking defects (cracks, porosity) on non‑porous materials (metals, some plastics/ceramics).

Question 9

PT: basic steps in order.

Answer 9

Clean → apply penetrant → dwell → remove excess → apply developer → inspect (often under UV for fluorescent systems).

Question 10

PT: one advantage and one limitation.

Answer 10

Adv: very sensitive to fine surface cracks; Lim: cannot find subsurface defects and needs clean, non‑porous surfaces.

Question 11

Magnetic particle testing (MT): what materials?

Answer 11

Ferromagnetic materials only (e.g., carbon steels, some irons).

Question 12

MT: what does it detect?

Answer 12

Surface and near‑surface cracks and discontinuities that disturb the magnetic field, attracting iron particles.

Question 13

MT: one advantage and one limitation.

Answer 13

Adv: quick and good for fine surface/near‑surface cracks; Lim: only works on ferromagnetic materials and needs magnetization/cleanup.

Question 14

Ultrasonic testing (UT): simple definition.

Answer 14

High‑frequency sound waves are sent into a part; echoes from flaws or the back wall are measured to find defects or thickness.

Question 15

UT: two uses in a school workshop context.

Answer 15

Thickness gauging of metal plate/tube and detecting internal flaws in simple blocks/weld coupons (if equipment available).

Question 16

UT: one advantage and one limitation.

Answer 16

Adv: finds subsurface defects and gives depth info; Lim: needs trained operator, good surface contact, and calibration blocks.

Question 17

Simple calc (UT thickness): sound speed in steel ≈ 5900 m/s; round trip time 3.4 μs. What is thickness?

Answer 17

Thickness = (velocity × time)/2 = 5900×3.4e‑6/2 ≈ 0.01003 m ≈ 10 mm.

Question 18

Radiographic testing (RT): basic idea.

Answer 18

X‑rays or gamma rays pass through the part to a detector/film; differences in absorption show internal defects like voids or inclusions.

Question 19

RT: school safety note.

Answer 19

Ionising radiation requires strict controls and trained personnel—generally demonstration‑only, not student‑operated.

Question 20

RT: one advantage and one limitation.

Answer 20

Adv: permanent image of internal structure; Lim: safety/cost and can miss very tight planar cracks depending on orientation.

Question 21

Eddy current testing (ET): what materials?

Answer 21

Conductive materials (metals).

Question 22

ET: what can it detect?

Answer 22

Surface and near‑surface cracks, coating thickness, and material mix/heat‑treat differences via changes in coil impedance.

Question 23

ET: one advantage and one limitation.

Answer 23

Adv: no couplant and very fast scanning; Lim: shallow penetration and needs reference standards and skilled setup.

Question 24

Leak testing: name two methods.

Answer 24

Pressure decay (measure pressure drop over time) and bubble testing (pressurise under water and look for bubbles).

Question 25

Acoustic emission (AE): simple idea.

Answer 25

Sensors listen for high‑frequency sound bursts from crack growth or leaks during loading; used for monitoring structures/tanks.

Question 26

Infrared thermography (IRT): what does it show?

Answer 26

Surface temperature patterns; defects can create hot/cold spots due to differences in heat flow.

Question 27

NDT selection: which method best finds tight surface cracks on non‑magnetic aluminium?

Answer 27

Dye penetrant (PT) or eddy current (ET) depending on access; PT for open cracks, ET for rapid scanning.

Question 28

NDT selection: which method measures wall thickness of a steel tube from the outside?

Answer 28

Ultrasonic thickness gauging (UT).

Question 29

NDT selection: which method sees internal porosity in a casting with an image?

Answer 29

Radiography (RT).

Question 30

Surface prep—why important for PT/ET/UT?

Answer 30

Clean, smooth surfaces improve sensitivity and signal quality; dirt or paint can hide defects or block signals.

Question 31

Calibration/standards—why needed?

Answer 31

To ensure results are reliable and comparable; reference blocks/standards set sensitivity and verify equipment.

Question 32

Documentation—what should NDT reports include?

Answer 32

Method, equipment/settings, calibration, inspector, results/indications, acceptance criteria, and part identification.

Question 33

Acceptance criteria—who defines pass/fail?

Answer 33

Engineering drawings/standards or customer specifications define allowable defect sizes/locations.

Question 34

Give two examples of NDT you can realistically demo in a school workshop.

Answer 34

Dye penetrant testing with aerosol kits and visual inspection with magnifiers and lighting.

Question 35

Give one example of NDT that usually requires a specialist contractor.

Answer 35

Industrial radiography (RT) due to radiation safety rules and licensing.

Question 36

Safety PPE: name two items for PT/MT work.

Answer 36

Gloves and eye protection; ensure ventilation when using aerosols/solvents.

Question 37

Environmental note: what to do with used penetrant/developer wipes?

Answer 37

Dispose as per local hazardous waste rules; do not pour chemicals down sinks.

Question 38

Limitations: why might PT miss a crack?

Answer 38

Crack not open to the surface, poor cleaning, insufficient dwell time, or developer not applied correctly.

Question 39

Limitations: why might UT miss a defect?

Answer 39

Poor coupling, wrong probe angle/frequency, rough surface, or defect oriented to reflect sound away from the probe.

Question 40

Limitations: what does MT not work on?

Answer 40

Non‑ferromagnetic materials like aluminium, copper, and most stainless steels (austenitic).

Question 41

NDT and Lean (TIMWOOD): how does NDT cut waste?

Answer 41

Early detection prevents Defects reaching assembly, avoids Overprocessing/rework, and reduces Waiting/Transport from returns.

Question 42

Simple calc (pressure decay): a sealed part at 2.0 bar drops to 1.98 bar in 10 min. Is that a leak?

Answer 42

It indicates a possible leak or temperature change; repeat with temperature controlled and compare to acceptance limits.

Question 43

PT fluorescent vs colour‑contrast—difference?

Answer 43

Fluorescent penetrant is viewed under UV for high sensitivity; colour‑contrast shows red indications on white developer in normal light.

Question 44

ET lift‑off effect—what is it?

Answer 44

Changes in probe‑to‑surface gap alter the signal; keep contact consistent or compensate during calibration.