❌ Influenced by environmental factors such as lighting, contrast, glare, viewing angle.
Subjective to person inspecting
Needs direct access to the component
Clean surface required

Lab prep of areas of interest, slight over-etching required. Can use field microscope but usually replicas are made

❌ Time consuming and requires experienced personel

Uses ferrous particle spray and an electromagnet. Part to be inspected is magnetised and ferrous particles are attracted to magnetic flux leakage fields so cluster over discontinuity

Reveals surface flaws when coloured or fluorescent dye leaks from flaw. Relies on ability of low viscocity liquid to be drawn into flaw through capillary action. After time period (dwell) excess dye is removed and absorbant developer is used to draw penetrant from the flaw to reveal where.

Piezoelectric transducer produces ultrasonic waves and transmitted inside teh component using a couplant. Defects will cause variations in time and amplitude of reflection. Frequencies of 0.1 to 50MHz

Features inspected:
Cracks, inclusions, delaminations, laminations, voids, lack of fusion, blow holes, thickness, corrosion, microstructural changes

Snells Law:
When ultrasonic wave passes through two materials at oblique angle, and materials have different indices of refraction, both reflected and refracted waves are produced

Crack tip diffraction is used to estimate length of a crack:

Ultrasonic waves do not originate from single point, instead from all over surface of piezoelectric element

Lower frequency than conventional ultrasonics (20-200kHz), used to inspect large sections in one set up.

Applications:
Pipes, Rails, wire cables in bridges, windturbine towers, sheet piling in harbour walls

Piezoelectric transducers mounted on structures surface monitor release of stored energy when structure elastically deforms

❌ Noise from friction, loose parts, impacting dust or rain

Uses magnets and coils to produce interrogating ultrasonic waves.

✅ Non-contact so ideal for high temp applications
Dry inspection, less sensitive to surface condition, easier to generate certain wave types

❌ Only applicable for conductive or ferromagnetic materials.
Weaker signal than piezoelectric, Larger size transducers, low efficiency and signal to noise ratio

Responsible for US waves in non-ferromagnetic materials. Works by placing wire near surface of electrically conducting component and eddy current produced at same frequency of current in wire. Eddy currents will experience lorentz forces causing ions to oscillate and produce US waves

Magnetic dipoles are alligned along magnetic domains below curie temperature. Magnetic field is applied to component, net strain in direction of field.