As per PT history we may consider two periods when classifying the fluorescent penetrants used in aerospace industry.
- Those marketed before 1984, ca from 1970 to 1984.
- Those marketed from 1984 after the Amendment D of the then MIL-
I- .25135 (ASG) (*) specification
Underneath you may find explanations about:
- How a major aircraft engine manufacturer determined which marketed penetrant was the most sensitive.
- How fluorescent penetrants formula have been "adjusted" along the years.
- How their sensitivity could be increased.
1-
Among all the penetrants available on the market, which one is the most sensitive, i.e. which one allows to detect the most of the smallest discontinuities?
Ca 1980 an aircraft engine manufacturer determined the sensitivity of many fluorescent
penetrants, water-
All these penetrants were listed in the Qualified Products List (QPL) of the MIL-
The laboratory used the Nichrome panels marketed in France ca 1973 (**), now referenced
as reference blocks Type 1 in the ISO 3452-
In these times the main supplier of these Nichrome panels was not the one who has a kind of monopoly now. Some discrepancies exist with today's panels. Here is the explanation. As per the Japanese standards dealing with these panels, the ratio between the maximum width and the depth shall be at least 1/20. The then main manufacturer did not meet this requirement. We pointed out this when this manufacturer had to stop its production due to tougher environmental regulation in Japan. The only competitor, also a Japanese company, supplied panels on which discontinuities were much more difficult to detect. When cutting "old" and "new" panels the reason was obvious: the "old" panels had a ratio of 1/4. Discontinuities were then larger, had a much greater volume than the "new" panels'!
Since many years now there is only one manufacturer of these panels (ISO 3452-
This aircraft manufacturer had a complete set of panels, including the 100 µm and 5 µm ones. These two ones are no longer manufactured by the current supplier.
The 100 µm panels are of no use as colour contrast penetrants show 100% of the 50 µm ones.
We all know that post-
The 10 µm panels did not show any difference of sensitivity between the three Level 4 penetrants under test.
But the 5 µm ones were the right tool. The most sensitive system detected 80% of the discontinuities.
For hygiene and safety concerns the formula of this penetrant has been modified, and, accordingly, its reference. As far as we know the "new version" has not been tested with the same process.
Using the most sensitive penetrant seems to be a good idea to make the most of PT. Nevertheless there are some drawbacks:
- Is it useful to have a lot of indications, when maybe many of them are images of
non-
relevant discontinuities? This increases the inspector's workload, increases the number of repair/rubbish and hence increases costs and delivery time.
Nevertheless using the most sensitive system may be useful when looking for very small discontinuities on a smooth surface.
2-
The pre-
It is then that, due to a tougher competition, PT materials prices began a slow but consistent downsizing. As a consequence manufacturers optimised their formula to lower the costs while keeping a similar performance, needed to meet the specification's requirements.
Lowering costs means for instance lowering the raw materials costs without impairing the products' quality.
Two ways to achieve this:
- Choose the surfactants after their best quality/price ratio: this is the most difficult
task. A surfactant supplier may tell you that his product is the exact equivalent
of his competitor's one. This may be true for home detergents/washing products, but
by experience, we know that this untrue for most of the penetrants. When a surfactant
is no longer available replacing it is a brain-
teaser for the PT materials manufacturers, especially for the hydrophilic emulsifier. Many tests are needed, tests with large users help...and the next formula shall go through the long process of qualification! - Find the best concentration of dyes and optical brighteners and the best ratio fluorescent dyes/optical brighteners in penetrants. These ingredients are by far the most expensive ones in penetrants. Some manufacturers have dramatically lowered the content of these chemicals in their penetrants while still meeting the performance requirements. Finding the right "mix" of fluorescent dyes/optical brighteners which will deliver the highest fluorescent brightness with the lowest quantity of ingredients is one of the best kept secrets of penetrant designers.
We know that, as a general rule, the more fluorescent dyes and brighteners, the more sensitive a penetrant.
And we are close to think that some pre-
3-
Currently available penetrants meet the market requests; so PT materials manufacturers do not feel pressure to increase the sensitivity of their penetrants.
Nevertheless if some primes asked for more sensitivity, manufacturers could easily answer and show that PT is yet far from its limits.
(*) Patrick DUBOSC and Pierre CHEMIN: ''The specifications which changed the Penetrant
materials''-
(**) Patrick DUBOSC and Pierre CHEMIN: ''Penetrant testing history''-
http://www.ressuage-
ULTRA HIGH SENSITIVITY PENETRANTS
