Flaw detections: Inspection methods for quality control

Flaw inspections represent the daily lives of Quality Assurance (QA) and Quality Control (QC) professionals, who work with flaw detectors every day in order to perform accurate inspections.

When trying to detect defects, the inspector usually focuses on finding alterations or irregularities on the sample tested.

When talking about metal part inspections, common flaw inspectors look for irregularities on the material structure.

Such irregularaties may or may not affect the usability of the product, depending on the results of the evaluation and the criteria established for product acceptability.

What types of defects can be detected by performing flaw inspections?

The most common and relevant defects that can be detected by a flaw inspections can be superficial or deep cracks, porosity on the surface and material crystallizations.

Any of these defects can be found alone or in combination on the same inspected part, which is why flaw inspections are important for quality assurance and quality control.

What are the causes of the defects found with flaw inspections?

The defects described above can have different causes.

Flaw inspections help detect those defects and evaluate them to identify the possible causes.

The most common causes include:

• Congenital causes, meaning they are inherent to the material formation.
• Casting process issues.
• Heat treatment issues.
• Hot working issues.
• Mechanical fatigue.

What are common flaw inspection methods?

Different industries have different inspection methods to ensure quality. Here you will find three of the most common non-destructive flaw inspection methods which are used.

UT Ultrasound Control

This non-destructive flaw inspection method takes advantage of the principle of using sound waves at very high frequencies, usually higher than those heard by the humans ear, to identify and verify defects while keeping the integrity of the sample.

Moreover, it is possible to obtain results by inspecting from only one side of the sample with the use of a probe. The operation consists of accurately measuring the time it takes for an ultrasound pulse to pass through the thickness of the inspected part, being reflected on the opposite surface, and finally returning to the probe.

During this process, if the ultrasound beam encounters a defect, there is an early reflection of the signal, which therefore highlights the discontinuity. The ultrasound check allows the identification of both surface and depth defects, including:

• Flaws on the surface finish which may not be visible to the human eye.
• Corrosion on the material.
• Non-metallic inclusions.
• Congenital discontinuities in the material.
• Discontinuities resulting from casting processes.
• Material cracks.
• Defects resulting from wearing or production processes.
• Defects deriving from hot working.
• Defects deriving from heat treatment.
• Fatigue failures that occur during activity.
• Welding defects.

The main applications of UT ultrasound control for flaw inspections include, but are not limited to:

• The metallurgical industry.
• Welding inspections.
• The aerospace industry.
• The shipbuilding industry.
• Sectors that require non-destructive inspections.

In addition, UT ultrasound control is a very versatile flaw inspection method, as it can be applied on many different materials, including:

• Steel
• Cast Iron
• Aluminum
• Copper
• Brass
• Zinc
• Gray Cast Iron
• Spheroidal Iron

Check out our versatile ultrasonic flaw detector models:

• Ultrasonic flaw detector – SAFD660
• Ultrasonic flaw detector – SAFD500

PT Penetrating Liquid Method

Another non-destructive flaw inspection method is the one known as liquid penetrant inspection, which takes advantage of the penetration capabilities of a highly capillary liquid.

The idea is to make a very thin liquid flow into the superficial defect, and then drawing it out with an agent called the developer. After applying the contrast varnish spray, what was initially invisible becomes visible in normal light (in the case of visible liquids) ot through the use of a Wood lamp (in the case of fluorescent liquids). Fluorescent liquids are employed, for example, for the inspection of complex samples and / or cables not accessible to natural light.

To properly apply the PT penetrating liquid method, the following steps are necessary:

1. Cleaning the surface by removing any contaminants such as grease, dust or any other kind of particles.
2. Applying the chosen penetrating liquid and waiting for capillarization into the material.
3. Removing the excess of penetrating liquid by means of water or a type of solvent.
4. Applying the developer.
5.  Inspecting the surface.
6. Cleaning with corrosion protective solutions.

The main advantages of using the PT penetrating liquid method include:

• Responsiveness to small surface defects.
• Parts with complex geometries can be easily inspected.
• Large areas and volumes of parts and different materials can be easily inspected at low costs.
• The defect is visually indicated on the surface.
• It is a portable, convenient, and inexpensive process.
• Relative size, shape, and depth of the flaw can be revealed.
• It is easy to perform this flaw inspection method, so there is no need for a special training.

Penetrating liquids have many fields of application and our products can be used to detect discontinuities on any type of non-porous material.

In regards to metallurgy, two applications are of particular interest: in the welding processes, to detect any process flaws (crater cracks, shrinkage cracks, porosities, slag cracks, marginal incisions) and when checking elements which are subjected to stress, to detect discontinuities due to repeated and prolonged punctual stress over time.

MT Magnetic Particle Detection Method

This is another non-destructive flaw inspection method, in which a magnetic field is applied to the sample tested. Any indications present produce a variation of the flow lines of the generated field that are visible by means of magnetic powders.

The magnetic detection can be performed with magnetic liquids and fluorescent magnetic liquids.

The principle of operation is that the presence of discontinuities such as cracks and other defects disrupts a magnetic field created on the subject of inspection. So, applying a liquid containing magnetic particles on the part under such conditions will make those magnetic particles  gather and accumulate in the area by the discontinuity on the material.

Due to its operation principle, this method is mainly used to detect flaws on ferromagnetic materials such as:

• Iron
• Nickel
• Cobalt
• Some alloys

There are different tools used to implement this flaw inspection method.  Some are as follows:

• Fluorescent Magnetic Spray SALP182
• Black Magnetic Spray SALP181
• Digital Gaussmeter SAGM200
• MFI magnetic field indicator SAMFI
• AC/DC magnetic detector SAYK390

There are many facilities that offer Magnetoscopic Training (MT) for operators, in accordance with UNI EN ISO 9712:2012, such as Bureau Veritas, which issues the certificate of attendance training.

This is a prerequisite for taking the exam to be certified according to the specific level.

Final considerations about flaw inspections

In this article we have highlighted what flaw inspection is, and the importance it plays in preventing the breaking of various types of pieces that we can find everywhere, around us.

All over the world, many have experienced  several cases of negligence in material checks, which have caused even fatal damage.

To make sure that this does not happen again, the work of QA and QC professionals is essential to prevent such damage by means of flaw detectors and many other inspection methods.

We provide the tools and equipment required to cover each of the flaw inspection method mentioned above. If you want to check out the flaw inspections instruments we offer, just click here.

Instead, if you’re still not sure or have any doubts about any of the flaw inspection techniques we have described here or any of our instruments, we recommend that you contact us and one of our experts will gladly assist you.