What is Liquid Penetrant Testing (PT)?

Liquid Penetrant Testing is a non-destructive technique capable of detecting various types of discontinuities, as long as they are 'surfacing', and is probably the oldest method of inspection, along with simple visual one. The principle on which it is based involves:

• the application, on the surface under inspection, of a coloured liquid (red or fluorescent) capable of penetrating into the discontinuities;
• the removal of the liquid’s excess from the surface, once the necessary time has elapsed (known as 'penetration time');
• the application of a white powder – commonly known as 'developer' – capable of absorbing the liquid penetrated into the discontinuities, while at the same time causing it to emerge on the surface to obtain clear and defined indications, easily visible to the naked eye.

In the different manufacturing processes (rolling, casting, forging, welding, etc.), a large number of discontinuities can be generated, partly located inside and partly emerging on the surface of the parts under examination. The former, such as non-metallic inclusions, porosity, lack of fusion, etc., cannot be detected with liquid penetrants, unless subsequent cutting, grinding or machine tool operations bring them to the surface. The latter, on the other hand, can be detected as soon as they are formed: folds, grinding cracks, fatigue and crater cracks, recalcifications, etc. can be easily detected.

Liquid Penetrant Testing can be carried out on all ferrous and non-ferrous materials; exceptions are extremely porous materials (such as unglazed ceramics, certain types of sintered materials, etc.) and soft rubbers, whose high elasticity tends to close up any cuts not allowing the necessary penetration. The penetrant liquid is obviously the fundamental product of this type of inspection: it generally consists of a liquid (water or oil – petroleum derivatives) into which a pigment and other chemical substances are introduced to give it the desired characteristics. A penetrant liquid applied to a surface does not act by gravity, but slowly seeps into discontinuities driven by capillarity, which also causes it to act from the bottom upwards.

Regarding the colors, penetrants are divided into three types:

• fluorescent (type I)
• color contrast (type II)
• Dual-purpose (type III)

Fluorescent penetrants take their name from the type of pigment they contain: they emit bright yellow-green rays of visible light when irradiated with ultraviolet light with a specific wavelength. The inspection must be performed in a darkened area, with a UV lamp as the only source of illumination. Discontinuities are highly visible and easy to interpret.

Color contrast penetrants get their name from the characteristic of contrasting strongly under normal lighting conditions (daylight or electric light) with the background to which they are applied. They generally have a deep, bright red color in contrast to the white of the background and do not require a darkened environment.

Dual-purpose penetrants contain colored pigments that are visible in white light and can also be highlighted in ultraviolet light if required, resulting in fluorescent indications.

For more than 65 years, CGM CIGIEMME S.p.A. has been offering complete solutions for liquid penetrant testing: from simple equipment to automatic systems, from accessories to fluorescent or dye consumables.