ABM NDT Ltd.

Ultrasonic testing is a non-destructive testing method where pulses of ultrasound are introduced into the sample under test and by analysing the resulting echo or echoes an experienced operator can make a judgement of the quality of the part.

Some of the benefits of ultrasound are:

• Often cheaper than radiography, especially for large parts or high volumes.
• Can be advantageous over radiography on heavy sectioned parts or dense materials.
• In many instances a satisfactory ultrasonic examination can take place with access to only one side of the part.
• Under some circumstances ultrasonic examination can take place whilst the item under test is still in use, for example, testing welds or wall thicknesses on pipework.
• Modern ultrasonic equipment is lightweight, highly portable and powered by batteries and unlike radiography, it poses no risk of ionising radiation to either the operator or people nearby.
• Accurate wall thickness measurements can easily be taken, often even under unfavourable conditions and with only one side accessible.

Some of the problems with using ultrasound are:

• The accuracy and correct interpretation of results is highly dependent on operator skill, judgement and experience. This risk can be greatly reduced by making sure detailed procedures and instructions are in place before testing.
• Often no permanent record of ultrasonic data is produced, so analysis at a later stage is not possible. Modern techniques are seeking to address this shortcoming but currently have limited applications, with pipe welds being the most common use.

Typical uses of the ultrasonic testing method are:

• Testing of steel castings, looking for shrinkage voids, porosity, cracks, cold laps etc.
• Wall thickness checking of castings to ensure that any cores have not moved during the casting process.
• Testing of forged and rolled bar stock, looking for axial looseness, forging bursts, surface cracks, etc.
• Testing of rolled steel plate, looking for laminations and inclusions.
• Wall thickness checking of tanks, vessels and pipes.
• Testing of many types of welds for lack of fusion, lack of root penetration, cracks, porosity, etc.
• Checking of composite materials for thickness or incomplete bonding.

Penetrant testing, dye penetrant testing, or sometimes shortened to dye pen is a method that can be used to find surface-breaking defects on nearly all non-porous materials.

A bright dye is applied on the surface of the object and then left for a specified period. During this time dye will be drawn into any surface-breaking defects due to a phenomenon known as capillary action. After the ‘dwell’ time has elapsed the dye is removed from the surface of the part, with care being taken not to over-clean the part. After the part has been suitably prepared a special developer is applied, often a chalky suspension which is sprayed on in a thin coating, leaving a absorbent residue on the part.

The part is now left to ‘develop’. After a specified period, the part is inspected and areas where any residual dye has been absorbed by the developer become readily visible revealing flaws in the parts.

Some of the benefits of penetrant testing are:

• A quick and low-cost method for finding surface-breaking defects.
• Highly portable, to conduct a basic penetrant test only three cans of aerosol consumables are required. Excellent for use in remote locations., Unlike MPI, penetrant testing can be used on any non-porous material, including stainless steel, magnesium, aluminum, glass, and glazed ceramics.
• Indications found by penetrant testing are by and large fairly easy for the operator to detect and correctly interpret.

Some of the problems associated with using penetrant testing are.

• The penetrant method is only able to find defects that break the surface of the item under test.
• Parts with coatings, galvanizing, or platings are unsuitable for penetrant testing.
• Processing large parts can be time-consuming and labour-intensive.
• Post-test residual dye can corrode certain metals and alloys and cause other unwanted effects, so parts must be thoroughly cleaned after testing.
• Correct interpretation of indications is down to operator skill and experience. As always, this risk can be greatly reduced by making sure high-quality procedures and instructions are in place before testing.

Typical uses of penetrant testing are:

• Detection of hot tears, cold shuts, gas, porosity, and inclusions in castings, especially in non-ferrous castings or with castings with geometry that makes MPI inspection difficult.
• Finding defective areas of weld in fabrications made from stainless steel, aluminum, and other non-ferrous materials.
• Looking for tears, cold shuts, or porosity in white metal-bearing linings.
• Finding cracks and laps in austenitic stainless steel forgings.
• Checking for surface defects on composite welds. For example, lack of fusion defects in Inconel weld overlay sections on stainless steel parts.