Non Destructive Testing
Non Destructive Testing (NDT) is a range of tests that can be performed on many
materials to determine if there are any faults in the piece. As the test piece is not
dismantled or altered in any way, it makes it a very valuable technique to learn and use.
What NDT's Are There?
Dye Penetrant Inspection (DPI)
Magnetic Particle Inspection (MPI)
Ultrasonic Testing (UT)
Radiographic Testing (RT)
Eddy Current Testing (ECT)
Which One Is Best?
There is no "best" one, it depends on what you are testing as each method had pros and cons.
Which Is The Best Way To Detect Turbo Cracks?
DPI is probably the best option in this case as MPI can be a bit tricky.
UT, RT and ECT uses equipment costing thousand of pounds.
What Is Dye Penetrant Inspection?
This process uses osmosis to draw a coloured dye into any cracks. The excess dye is
wiped clear and a developing spray is applied. This draws any of the dye back out of
the crack and clearly displays it on the surface.
I gained an NDT qualification in 2009 and very quickly saw the opportunity for
crack testing on the smart turbo manifold. DPT is the cheapest NDT process.
Where Can I Buy A DPI Kit?
DPI kits can be bought from most places that deal with and sell welding supplies.
There are loads of online companies including eBay sellers who can help you.
Performing The Dye Penetrant Test
You do not necessarily have to remove the turbo from the car but it would
help to remove a few things from around it to make access easier.
These are the 3 sprays you get with the dye penetrant testing kit.
Cleaner, penetrant dye and contrast developer.
Liberally spray the test area with the cleaner and use a lint free cloth to clean the area
as best as you can. Spray the entire area again and allow the cleaner to evaporate.
This clears out any cracks for the dye process.
Shake the can of dye for a minute and spray the test area.
Leave the dye there for 15 minutes to allow it to soak into any cracks.
It is now time to remove the excess dye. Spray the cleaner onto a lint free
cloth and wipe the excess dye off. There are 3 important things you must do:
1- Only wipe the dye off with a rag that has cleaner on it, a dry rag can pull dye out of the cracks.
2- Always wipe in 1 direction to remove the dye, it doesn't matter which way but going
in different directions can clear the dye out of the cracks and introduce dirt into any fault.
3- NEVER spray the cleaner directly onto the test area for this stage in the process.
It will force all of the dye out of any cracks.
With the excess dye removed, leave it for 5 minutes for any excess cleaner to clear.
Shake the can of developer for a minute, hold it approximately 10 inches away
from the test piece and apply a light coat with several continuous passes.
Exactly the same way as using a standard spray paint can.
Leave for approximately 5 minutes for any trapped dye to be drawn to the surface.
On castings such as the turbo, you will get speckling over the entire piece.
However, what we are looking for is a distinct line as circled in below.
This denotes the presence of a sub surface crack. Over time, more dye will
be drawn out depending on the depth of the crack. If the initial line becomes
blotchy and wide, it is an indication that the crack runs quite deep.
Here is a close up of the fault that shows a sub surface crack appearing from the wastegate.
Once you have completed your test you should remove the developer as the heat from an operational turbo will create a lot of smoke until the developer has burnt off.
Take the can of cleaner and spray the test area at close range, this
easily removes the developer so all you need to do is wipe it over.
The last picture shows a close up shot of the area that was found to be cracked.
It is not visibly obvious that this area has a fault even now you know that there is.
This process can be used on anything that could crack, turbo manifold, suspension
springs, con rods, cylinder head, valve stems, crank shaft, pistons etc. It is a handy
process to know and could save you a lot of time and money in the future if you are
rebuilding an engine or testing for fatigue on worn parts.
Health & Safety & COSHH
When using the 3 sprays from the DPT kit it is essential you wear goggles,
work in a well ventilated area and ideally you should wear gloves.
NEVER spray any of the 3 sprays onto a hot turbo or exhaust.