Comparison of Methods
Each of the main methods of Non Destructive Testing has it's own uses and advantages but it should be remembered that it is usually possible to cross check an indication by the use of one or more other methods.
This means that if some doubt exist as to the nature of a indication it should be possible to select another method or technique to assist with evaluation.
Such cross-checking can often save a good item, or at least satisfy the operator of the accuracy of the first method.
Method Measures or detects Advantages Disadvantages
Surface and near surface cracks and seams. Alloy content, Conductivity and Permeability. Heat treatment variations. Wall thickness. Crack depth.
Clean and Quick, no need to remove surface coatings. High speed, low cost. Automation possible. Permanent record capability. No couplant or probe contact required.
Conductive materials only. Shallow depth of penetration. Trained operator skills required. Need reference standards. Particular probes required for particular job.
Surface and slightly sub-surface defects, cracks, seams, porosity and inclusions. Permeability variations. Extremely sensitive for locating small tight cracks.
It indicates near surface indications, inclusions.
Relative fast and low cost. May be portable.
Alignment of magnetic field is critical. Demagnitisation of parts required after test. Parts must be cleaned before and after inspection. Indications can be masked by surface coatings.
Defects open to the surface parts; - cracks, porosity, seams, laps etc.
Low cost, portable, indication may be further examined visually. Results are easily interpreted.
Surface films, such as coatings, scale and smeared metal may prevent detection of defects. Parts must be cleaned before and after inspection. Defects must be open to the surface.
Internal defects and variations, cracks, lack of fusion, porosity, inclusions, delaminations, lack of bond, texturing. Thickness or velocity.
Most sensitive to cracks. Test results known immediately. Automation and permanent record capability. Portable. high penetration capability.
Couplant required. Small thin, complex parts may ne difficult to check. Reference standards required. Trained operator required for manual inspection. Special probes.