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Penetrant Testing: Penetration time vs Kinematic viscosity

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Written by Administrator
Friday, 12 July 2013 13:00

August 2013

In this paper, we use the term “penetration time” listed in the ISO 12706:2009 standard instead of the term “dwell time” listed in the Section F of the ASTM E1316 –11b standard.

We often hear and read that at low temperature, when the temperature decreases, the penetrant becoming more viscous, the penetration time shall be increased and conversely, at high temperature, the penetration time can be reduced because the penetrant is less viscous.

The penetration time is specified in many standards/codes/specifications.

In theory, due to the capillary pressure, the penetration time may be short (one to two minutes). However, the discontinuities may contain something other than air and especially residual pollutants traces that were not removed before penetrant testing. This is why longer penetration times are generally stated: this contact time between the penetrant and pollutants may allow at least for a partial dissolution of pollutants and for the going up in the penetrant layer of some of the pollutants.

Moreover, in tiny discontinuities, the air pressure rises very quickly, when the penetrant enters, which may block it ... and decrease sensitivity.

For over three decades, we have emphasized, in training courses, the fact that the most important parameter determining the ability of the penetrant to the discontinuities is the surface tension, and not the kinematic viscosity. The ISO 3452-6:2008 remembers it.

 

1- Penetrants for use at room temperature(1)

Let us consider the case of penetrants used at ambient temperatures between 4 and 52 °C (40 and 125 °F), as stated in the ASTM E1417/E1417M-11ε1 or between 10 and 50 °C (50 and 122 °F), as stated in the ISO 3452-1:2013 standard.

The sensitivity test method for penetrant systems listed in the ISO 3452-2:2008 standard and the SAE-AMS 2644E specification states to dip test panels into the penetrant and, then, to let them drain for 5 minutes. Therefore, this means a rather short penetration time, a little longer than 5 minutes.

Types 1 and 2 penetrants are therefore submitted to the same penetration time, whatever their kinematic viscosities.

Let us consider the case of Types 1 and 2, Method A penetrants listed in the Qualified Products Database (QPD) of the SAE-AMS 2644E specification.

A quick search shows that their kinematic viscosities range from 2.05 to 27 mm²/s at 40 °C (104 °F).

(2) Note that in the legal SI system, the unit of measurement of the kinematic viscosity is mm²/s and NOT cSt (cSt is the obsolete unit of measurement of the kinematic viscosity).

We found out the kinematic viscosity of 2.05 mm²/s for a Type 2, Method A penetrant of sensitivity Level 2 according the ISO 3452-2 :2008 standard (Note: there is no sensitivity level classification for Type 2 penetrant systems in the SAE-AMS 2644E specification) and the 27 mm²/s figure for a Level 4, Type 1, Method A penetrant.

This could be seen as a good proof that the "viscous" penetrants do not require a longer penetration time than the "fluid" penetrants, and that their detection sensitivity does not depend on their kinematic viscosity, at least at room temperature.

2- Low temperature penetrants(1)

What does happen at lower temperature? The main problem of penetrant testing at low temperature is ... the presence of condensation water (dew, ice). This is a SIGNIFICANT barrier for the penetrant to go into the discontinuity. There is a very thick fluorescent penetrant, almost jellified, specifically designed for...low temperature penetrant testing, because it is able to get rid of some of the water already entered.

The ASTM E1417/E1417M-11ε1 states a minimum dwell time of 10 minutes. This minimum is increased to 20 minutes at temperatures between 4 and 10 °C.

It is not surprising that this stipulation, multiplying the penetration time by 2, is included in specifications, inspection procedures and PT suppliers’technical data sheets, for temperatures between 0 and 10 °C (40 and 50 °F). At lower temperatures, down to -30 °C (-22 °F), the penetration time may reach 40 minutes.

3- High temperature penetrants(1)

At high temperature, the penetration and development times are, sometimes, significantly shortened, usually for fear of penetrant degradation, due to temperature. It is also true that at 150 °C (302 °F), or higher, up to 200 °C (392 °F), the penetrant which entered the discontinuities tends to bleed out quickly, greatly expanding indications.

The penetration time may vary from 30 seconds [180 °C (356 °F)] to 3 minutes [140 ° C (284 °F)] approximately.

4- Thixotropic penetrants

Thixotropic and classic products have a very important difference: the former's behaviour does not obey to the Newton's law. There is no relationship between the shear rate and the shearing forces. In other words, a thixotropic material has a viscosity, which is related to the shear time and decreases along the shear time.

Thixotropy is a physical phenomenon, which may be reversible: if the product, liquefied thanks to the mechanical action, is let without any further action, it comes back to its initial gel-like consistency(3).

Therefore, at rest, these penetrants and, where applicable, their emulsifiers, are very viscous; after stirring, they are very fluid.
The penetration time of thixotropic penetrants is not different than that of non-thixotropic penetrants.

The capillary pressure due to the discontinuity tightness exerts a mechanical action such that the penetrant goes perfectly well in the discontinuity. Quite often, after application of a developer, the indications appear with a better contrast than with a "normal" penetrant ... because the jellified penetrant tends NOT TO SPREAD into the developer: the indications are thinner, and as the same amount of penetrant bleeds out of the discontinuity, the dye is more concentrated, hence, more seeable indications.


References

(1) Pierre CHEMIN and Patrick DUBOSC, Penetrant testing industrial uses, November 2010:
on our Website.

(2) Patrick Dubosc and Pierre Chemin, MT/PT units: follow the rules stop the mess, Materials Evaluation, Vol. 68, No. 5, 2010, ©American Society for Nondestructive Testing.
This paper was reproduced with the permission of Materials Evaluation, ©American Society for Nondestructive Testing in our DPCNewsletter N° 026 –July 2010: on our Website.

(3) Pierre CHEMIN and Patrick DUBOSC, Special products for PT, DPCNewsletter N° 017, October 2009: on our Website.


Normative references

• ISO 12706:2009, Non-destructive testing - Penetrant testing – Vocabulary, International Organization for Standardization, Geneva, Switzerland, 2009.

• ASTM E1316 –11b, Standard Terminology for Nondestructive Examinations, Section F: Liquid Penetrant Testing (PT) Terms, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2011.

• ISO 3452-1:2013, Non-destructive testing -- Penetrant testing -- Part 1: General principles, International Organization for Standardization, Geneva, Switzerland, 2013.

• ISO 3452-2:2006, Non-destructive testing -- Penetrant testing -- Part 2: Testing of penetrant materials, International Organization for Standardization, Geneva, Switzerland, 2006.

• ISO 3452-6:2008, Non-destructive testing -- Penetrant testing -- Part 6: Penetrant testing at temperatures lower than 10 degrees C, International Organization for Standardization,
Geneva, Switzerland, 2008.

• ASTM E1417/E1417M-11ε1, Standard Practice for Liquid Penetrant Testing, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959, USA, 2011.

• SAE-AMS 2644E, Inspection Material, Penetrant, Society of Automotive Engineers (SAE), 400 Commonwealth Drive, Warrendale, Pennsylvania 15096, USA, 2006.

Last Updated ( Friday, 12 July 2013 15:48 )