GB2317959A

GB2317959A - Defect detection using an indicator fluid

Info

Publication number: GB2317959A
Application number: GB9620772A
Authority: GB
Inventors: Peter David Young
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-10-04
Filing date: 1996-10-04
Publication date: 1998-04-08
Anticipated expiration: 2016-10-04
Also published as: GB9620772D0; GB2317959B

Abstract

In some types of non-destructive testing, an indicator fluid (e.g. a dye) is applied to a cast, forged or machined metal article (after washing the article) and allowed to penetrate into cracks or other flaws in the article. After excess indicator fluid has been removed, the indicator fluid still remaining in these tiny flaws can be seen and allows the flaws to be observed by an operator. The invention addresses the problem of poor drying after the initial washing stage (and the resulting blockage of recesses in the article under test, preventing penetration by the indicator fluid) by using a reduced pressure (at least a partial vacuum) to draw washing fluid away from and out of the article under test.

Description

DEFECT DETECTION USING AN INDICATOR FLUID This invention relates to defect detection using an indicator fluid, such as (for example) so-called non-destructive testing of cast, forged or machined articles by penetration of an indicator fluid.

Non-destructive testing by penetration of an indicator fluid (commonly referred to as a dye) is used to detect defects in cast, forged or machined metal articles. The technique is particularly useful for identifying flaws in metal articles for use where the article's strength or longevity is very important, which applies, for example, to engine or aircraft components.

In this technique, the article to be tested is first thoroughly washed and dried.

The indicator fluid is then applied to the article and allowed to penetrate into any surface irregularities, cracks, or holes in the article. The excess indicator fluid is then washed off or otherwise removed, leaving behind the indicator fluid still trapped in, for example, cracks in the article.

After the article has then been dried, the residual, trapped indicator fluid can then be inspected under visible light (for a simple coloured indicator fluid) or ultraviolet light (for a fluorescent indicator fluid), typically after the residual indicator fluid has been treated with a developing agent. This inspection of the trapped indicator fluid allows a technician to see the extent of any defects or irregularities in the article.

Two problems which can limit the usefulness of this process are those of (a) properly drying the article after the initial washing; and (b) obtaining a good degree of indicator fluid penetration into defects in the article. These two problems are linked to an extent, because if the article is not properly dried then washing fluid will tend to occupy the very defects into which the indicator fluid should penetrate.

One way of alleviating these problems is to use a hot washing process. The heat helps to drive off the washing fluid and, to an extent, can help the indicator fluid to penetrate by reducing its viscosity. However, even using a hot washing process the two problems described above can still be significant. Also, drying the article primarily by evaporation can be slow, increasing the cycle time of the testing process.

A further attempt at alleviating this problem has been to try using vapour degreasing with non-aqueous washing fluids, such as chlorinated organic solvents.

However, this approach introduces problems of its own, such as environmental difficulties in dealing with waste solvents.

This invention provides a method of applying an indicator fluid for defect testing of an particle, the method comprising the steps of: (i) washing the article; (ii) drying the article while subjecting the article to at least a partial vacuum; and (iii) applying the indicator fluid to the article.

The invention addresses the problems described above by using a reduced pressure (at least a partial vacuum) to draw washing fluid away from and out of the article under test. This can alleviate the problems caused by poor drying and the resulting blockage of recesses in the article under test. It can also speed up the drying time, improving the cycle time of the testing process. The technique is particularly (though not exclusively) applicable to use with aqueous washing fluids.

Although the indicator fluid could be applied under ambient pressure, or even under a positive pressure, it is preferred that the indicator fluid is applied to the article while subjecting the article to at least a partial vacuum. This can help to draw the indicator fluid into the various recesses accessible from the surface of the article.

For ease of operation, it is preferred that the vacuum is maintained between the drying step and the step of applying the indicator fluid.

Although the washing could be carried out at room temperature, in which case the applied vacuum would still help to draw washing fluid out of the article, preferably the washing fluid (step (i)) is hot - i.e. it has a temperature of at least 3540 degrees Celsius. This again increases evaporation to help the vacuum process to draw off the washing fluid.

This invention also provides apparatus for applying an indicator fluid for defect testing of an article, the apparatus comprising: a chamber for receiving the article; a supply of washing fluid in communication with the chamber under the control of a washing fluid inlet valve; a drain outlet from the chamber under the control of a drain outlet valve; a vacuum source for at least partially evacuating the chamber; and a supply of indicator fluid in communication with the chamber under the control of an indicator fluid inlet valve.

As well as the advantages described above, by performing the process in a single chamber an improved cycle time can be achieved.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, which is a schematic diagram of an apparatus for applying indicator fluid to an article for testing.

Firstly, as introduction, there are three main conventional post-washing techniques for removing excess dye penetrant from an article. For so-called waterwashable penetrants, water is used to remove the excess penetrant. Post-emulsified penetrants (rarely used) require an emulsifying agent to be applied and then rinsed off. The so-called post-remover system uses a first rinsing step, followed by an aqueous detergent rinse, followed by a final rinse to remove excess penetrant. The invention is applicable to all of these, but for simplicity it will be assumed in the drawing and accompanying description that only a single post-washing fluid is required. Naturally, other tanks (and other process steps in the list below) could be used if more than one post-washing agent was needed.

Referring now to the drawing, the apparatus comprises a chamber 100 for receiving an article 105 to be non-destructively tested by application of an indicator fluid. The chamber is sufficiently strong to withstand a vacuum down to about 50 millibars being applied. In this case, the chamber is a metal casing. A motor 110 is arranged so that the article 105 can be rotated while it is in the chamber.

A number of items are connected in fluid communication with the chamber.

Three fluid supply tanks, 120, 130 and 140, hold (respectively) a dye penetrant (e.g.

a fluorescent dye penetrant), an aqueous alkaline pre-washing solution and a postwashing solution. (Here the terms "pre-" and "post-" washing are used simply to distinguish between washing which takes place before the application of the dye penetrant and washing which takes place after application). The three tanks are connected to a fluid inlet/outlet 150 from the chamber 100, and fluid emerging from the fluid inlet/outlet 150 can be routed to any of the three tanks or to a drain outlet 160 by a pair of valves 170. Similarly, fluid from two of the three tanks can be selectively admitted to a fluid inlet pipe 180 (which enters the chamber 100 at the top of the chamber) via a series of valves 190, 210. The fluid is then driven by a pump (not shown) along the fluid inlet pipe 180. A filter element 220 is connected in series with the fluid inlet pipe 180.

The tank 120 (containing the dye penetrant) is supplied with a dip tube. Dye penetrant is sucked up the dip tube by the applied vacuum in the chamber 100 (or via a pump (not shown)) and enters the chamber 100 via the fluid inlet/outlet 150.

Excess dye can be returned to the tank 120 via the reverse route.

Thus, the fluid inlet/outlet 150 provides an outlet only from the chamber for the tanks 130 and 140, and both an inlet to and an outlet from the chamber 100 for the tank 120.

A vacuum pump 230 is also connected to the chamber. This device can at least partially evacuate the chamber (e.g. down to a pressure of 50 millibars, although less severe vacuums would also provide advantageous improvements over previous techniques) if the air inlet valve 240 is closed. A steam extractor (basically a fan) 233 is connected to the chamber by a valve 236.

The process employed with the apparatus shown in the drawing comprises a number of process steps as follows: (i) The article to be tested is placed in position within the process chamber, for example held within a suitable tray or cage structure. The chamber is then closed.

(ii) Pre-washing fluid from the tank 130 is heated to at least 35-40 "C, preferably between 40 and 80"C, and supplied via the valve 190 to the chamber 100. The article 105 is rotated in the pre-washing fluid for about one minute.

(iii) The pre-washing fluid is drained back to the tank 130 via the fluid inlet/outlet 150 and one of the valves 170.

(iv) The article 105 is rotated while steam is extracted by the steam extractor 233 (with the air inlet valve 240 open). This process may be carried on for e.g. 30 seconds.

(v) A vacuum is provided by the vacuum pump 230 down to a pressure within the chamber 100 of about 50 millibars.

(vi) The indicator fluid is supplied from the tank 120 into the chamber 100 while the vacuum at about 50 millibars is maintained. The indicator fluid is therefore sucked into pores or cracks in the article 105. The article 105 is rotated by the motor 110 while the chamber 100 fills with the indicator fluid.

(vii) Once the article 105 is submerged in the indicator fluid, the air inlet valve 240 is carefully opened to break the vacuum and allow the chamber to return to atmospheric pressure.

(viii) The indicator fluid is returned via the valve 170 to the tank 120.

(ix) A post-washing solution from the tank 140 is supplied into the chamber 100 and the article 105 rotated within the post-washing solution.

(x) The washing solution is drained from the drain outlet 160 and discarded.

(xi) A hot recyclable post-washing solution is supplied from the tank 140 to the chamber 100 and the article 105 rotated.

(xii) The recyclable post-washing solution is drained back to the tank 140.

(xiii) Finally, steam is extracted by the steam extractor 233 with the air inlet valve 240 open.

Although rotation of the article is particularly mentioned in certain of the above steps, it is preferred that the article is continuously rotated throughout the whole process.

After these steps, the article 105 can be taken out of the chamber and treated with a developer in the conventional manner.

Claims

1. A method of applying an indicator fluid for defect testing of an article, the method comprising the steps of: (i) washing the article; (ii) drying the article while subjecting the article to at least a partial vacuum; and (iii) applying the indicator fluid to the article.

2. A method according to claim 1, in which step (iii) comprises: applying the indicator fluid to the article while subjecting the article to at least a partial vacuum.

3. A method according to claim 1 or claim 2, in which step (i) comprises: washing the article using a washing fluid having a temperature of at least 3535 degrees Celsius.

4. Apparatus for applying an indicator fluid for defect testing of an article, the apparatus comprising: a chamber for receiving the article; a supply of washing fluid in communication with the chamber under the control of a washing fluid inlet valve; a drain outlet from the chamber under the control of a drain outlet valve; a vacuum source for at least partially evacuating the chamber; and a supply of indicator fluid in communication with the chamber under the control of an indicator fluid inlet valve.

5. Apparatus for applying an indicator fluid for defect testing of an article, the apparatus being substantially as hereinbefore described with reference to the accompanying drawing.

6. A method of applying an indicator fluid for defect testing of an article, the method being substantially as hereinbefore described with reference to the accompanying drawing.