Finding Internal and Surface Defects Without Cutting the Component Open
Non-destructive testing methods detect defects — internal voids, cracks, inclusions, laminations — without damaging or destroying the component being tested. For mining and industrial components where a defect in a critical section can result in catastrophic failure in service, NDT provides the assurance that visual inspection and dimensional measurement cannot: confirmation that the internal and surface condition of the component is free from defects that would compromise its structural integrity or service life.
The appropriate NDT method depends on what kind of defect is being sought, where it is likely to be located, and what the component geometry and material allow. No single method detects all defect types in all locations. An inspection programme for a complex casting or structural forging typically applies more than one method, each chosen for the defect type and location it is most sensitive to.
Ultrasonic Testing (UT)
Ultrasonic testing introduces high-frequency sound waves into the component through a transducer in contact with the surface. The sound travels through the material and reflects from internal boundaries — defects such as voids, cracks, or inclusions return a signal to the transducer at a time delay corresponding to their depth. The technician reads the reflected signal pattern on a screen and interprets the location, estimated size, and nature of any reflectors detected.
UT is the primary method for detecting internal defects in thick-section castings and forgings. It is sensitive to planar defects (cracks, laminations) and volumetric defects (porosity, shrinkage, inclusion clusters) throughout the full section of the component, including the centre of thick sections that surface-sensitive methods cannot reach. For large structural castings — crusher mainframes, gearbox housings, heavy structural supports — UT of critical sections confirms the soundness of the material at locations where an internal defect would have the most severe structural consequence.
Acceptance criteria for UT are specified either by the client’s drawing or applicable standard (EN 12680, ASTM A609, or equivalent). Defect size limits, inspection coverage, and reporting requirements vary by standard and component criticality class. UT reports include the scanning coverage map, any recorded indications, and the accept/reject determination against the applicable standard. Multiple sets of UT equipment are maintained, and personnel qualification is to a recognised certification scheme.
Magnetic Particle Inspection (MPI)
MPI detects surface and near-surface defects in ferromagnetic materials (carbon steel, alloy steel, cast iron) by magnetising the component and applying iron particle suspension to the surface. Defects that intersect or lie close to the surface create magnetic flux leakage that attracts the particles, forming a visible indication at the defect location. MPI is sensitive to surface-breaking and shallow sub-surface cracks, laps, seams, and other linear defects that UT may not reliably detect.
For castings, MPI is applied after cleaning and before final surface treatment to detect surface-breaking shrinkage, hot tears, and cold shuts that are not visible to the naked eye but would represent failure initiation sites in service. For forgings, MPI detects forging laps and surface cracks that may result from excessive deformation or incorrect heating. For machined components, MPI after finish machining confirms that the machining process has not opened up sub-surface defects or introduced grinding cracks.
Wet fluorescent MPI — using fluorescent particles and UV illumination — is used for small or tight defect indications where the sensitivity of conventional dry or wet visible-light inspection is insufficient. For critical components or where the drawing specifies fluorescent MPI, this method is applied as standard.
Dye Penetrant Testing (PT)
Dye penetrant testing is applicable to any material — ferromagnetic or non-ferromagnetic — and detects surface-breaking defects by capillary action. A liquid penetrant is applied to the cleaned surface and allowed to enter surface-breaking defects by capillary action. Excess penetrant is removed, and a developer is applied that draws the penetrant out of defects and produces a visible indication. PT detects cracks, porosity, and other open-surface defects that penetrant can enter.
PT is used where the component material is non-ferromagnetic (austenitic stainless steel, certain non-ferrous alloys, aluminium components) and MPI is therefore not applicable, or as a supplementary method alongside MPI for surface defect detection in steel components where maximum sensitivity is required. PT is also used for weld inspection where MPI is impractical due to component geometry.
Colour contrast (red dye, white developer) and fluorescent penetrant systems are both available. Sensitivity class is selected based on the defect size requirement and the surface condition of the component.
NDT in the Production Process
NDT is applied at the stage in the production process where it provides the most useful information and where defect rejection has the lowest cost consequence. For castings, UT and MPI are typically applied after any heat treatment and surface preparation but before final machining — if a defect is found at this stage, the casting can be rejected before machining cost is added. For welded fabrications, MPI or PT after welding and before post-weld heat treatment confirms weld quality before the assembly proceeds.
For components where NDT is a specified delivery requirement, test reports — coverage maps, indication records, and accept/reject determination — are supplied with the shipment documentation. Third-party NDT witness inspection is arranged on request for components where the client’s quality system requires independent verification.
For NDT specifications or to discuss inspection requirements for a specific component, contact our engineering team. See also: Quality Assurance overview · Dimensional Inspection · Mechanical Testing.