Best common metal for "passing" permanent magnet's magnetic field 4

[Kyldera]

Hi... im currently on a project where i need to design a system to inspect metal crack by using Magnetic Flux Leakage. due to portability and power factor i've decided to use permanent magnet as the system's magnetizer due to its passive properties (does not need external power). The magnet have pulling force of 60kg. This will surely stick very strongly to the metal going to be inspected. so my ideas to solve this is to attach a metal on the magnet surface. this will prevent the metal going to be inspected from sticking to the magnet but at the same time it can still "pass" the magnet's magnetic field to the metal going to be inspected.

So my question is, lets say, the metal going to be inspected is mild steel, what is the best metal to be used as the "passer" metal?

 

[IRstuff]

Unclear how that's going to help. If the magnet's field is not attenuated by this "passer" metal, then won't the outside of the passer metal "stick"?

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[prex]

Well, IRstuff, it will stick less, depending on the thickness of the non ferromagnetic material. Of course the field strength going into the inspected part will be less, so I agree with you, it's not helping very much, as one could simply use a weaker magnet.
Anyway the best material for the intended use is aluminum: light and not very expensive. Alternatives are plastics or even wood, the latter being likely not compatible with ambient conditions.

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[Kyldera]

Well does smaller volume but larger surface area does any help?? Does those spec help pass more magnetic field but still make it stick less?? The goal is, to minimize the pulling force of the magnet on the metal going to be inspected but at the same time maximizing magnetic field that flow inside the metal that are going to be inspected as the magnetic flux leakage depends on the strength of the field. The reason for doing so is because the system is designed to be a portable handheld scanner... the weight of the system combined with the pulling force will be a hindrance for scanning the metal for MFL (if there is MFL there is crack on the spot)...

 

[ndsram]

The best possible solution would be routing the magnetic flux lines. What is the shape of your magnet? Try to design a flux circuit path so that the magnetic flux line will flow through you metal and also the magnet force will be kept minimum during your inspection.

For both I would suggest you to design a conical type metal part where in you stick the metal part on the magnet pole and the smaller area would be placed on the etal you wish to inspect.

All the Best
N D Senthil Ram

 

[Kyldera]

N D Senthil Ram: well the magnet is u shaped, this is the magnet datasheet link

http://docs-europe.electrocomponents.com/webdocs/0028/0900766b80028acb.pdf...

the magnet is from eclipse model 818 power magnet. So the changes in the shape will route the magnetic flux into a more confined area and at the same time minimize magnetic force... right? So i've come up with some design with ur suggestion in mind... how bout this?

the tapezoidal part is the passer metal. the black parts is just attachment to hold the passer metal.

 

[ndsram]

Good I appreciate your effort
Yes the shape confinement will now minimize the attaction and also pass the magnetic flux
The passer metal should be low carbon steel or SS340 magnetic steel to pass magnetic flux through the passer metals
Now let me know how you test the Magnetic Flux Leakage by detecting the crack.

Based on your model the flux will narrow down on the bottom side of the passer metal and the crack which you need to detect should be between the two pole

Its better you measure the magnetic flux after attaching the passer metal on the magnet and still if you need you can confine area at the bottom portion

 

[Kyldera]

Actually its the other way around, the system can detect crack by detecting MFL. The magnetic flux that flow through the metal will emanate at the surrounding air as it came across discontinuity such as crack, dents etc. By using linear hall effect sensor, we can get detect crack by measuring the output voltage of the sensor. The sensor voltage is proportional to the magnetic field strength. so basically you get a high reading, the spot probably have cracks... ss340 is stainless steel 340? what i found on the internet they say stainless steel 430 (instead of 340) has magnetic properies...

Does low carbon steel and the ss340/430? comes in block form? (it needs to be in block form so it can be machined down to the shape) anyway the passer metal dimension is 15.9mm in width, 85.7mm in breadth and 20mm in depth.

 

[MagBen]

The idea of confined area is good, since the force is proportional to the area.
The idea to detect defects, e.g. crack, hole, dent, by magnetic flux leakage is also good.
However, unless the hole is very big, no idea how to practically detect the magnetic field changes caused by the defects. Actually, the stronger the magnet, the easier would be to mask the field difference caused by defects. Also, the low reading would indicate a crack, not the opposite.

btw, SS 4xx is magnetic, 3xx is not

 

[Kyldera]

MagBen, so what those imply is i need just the right amount magnetic field strength that can give good differences between crack and no crack reading? if that is the case how bout i increase the depth of the passer metal to reduce the magnetic field strength to a more suitable magnitude?

Is there any simulation software that is suitable to simulate permanent magnet's MFL so that i can find out what is the right depth of the passer metal to detect the crack? all i have is COMSOL Multiphysic, but i have not been able to simulate the MFL for the permanent magnet.

 

[ndsram]

Yes I regret for the typo error it is SS430. Thank you MagBen

 

[ndsram]

Dear Kyldera,
I have a suggestion here, if you can put a coil near to the crack and increase the current step by step going through the coil which will increase the flux flowing through the metal proportionately. For every step measure the change in the flux leakage. At one point you can optimize how much magnetic field is required to detect the crack with magnetic flux leakage

If you want to do with permanent magnet, you have to increase the thickness of the passer metal, say you’ve multiple passer metal thickness and you can suitable vary the thickness

 

[Kyldera]

ndsram, that is a great idea to find out how much magnetic field is required to get an optimized mfl by first using an electromagnet... but unfortunately the data sheet of the permanent magnet does not give any other information except it say that the magnet have 60kg of pulling force... so even if i have find the right amount of magnetic field, there is no way (i think) i can find out what is the thickness of the passer metal that will give the same amount of magnetic field... so i have to go with the second method u've suggested... try and error...

Unless someone can give me the formula for calculating the magnetic flux flowing inside the metal both the permanent magnet and electromagnet... i dont need to do the try and error step. By the way does electromagnet have identical properties like permanent magnet for my case? what i mean is that, does the same amount of magnetic flux flowing inside the metal will yield same result for both electromagnet and permanent magnet cases?

 

[ndsram]

Kyldera,
Please find the magnetic properties of E818 (Alcomax III)

MDL
Grade DIN Br bHc jHc BHMax
mT Gauss KA/m Oersted KA/m Oersted kJ/m3 MGO
ALCOMAX III 43/5 1260 12600 52 650 53 663 43 5,4

Yes if the magnetic field of electromagnet and the permanent magnet are identical, the flow of flux in the metal will be the same. Right now I don’t have the formula or notes in hand that would help you, however I have the material to calculate.

 

[ndsram]

Sorry for the wrond orientation of the table

MDL Grade ALCOMAX III
DIN 43/5
Br mT 1260
Gauss 12600
bHc KA/m 52
Oersted 650
jHc KA/m 53
Oersted 663
BHMax kJ/m3 43
MGO 5,4

 

[Kyldera]

tThank you... i'll try to do some calculations....

 

[MarcoKoschny]

Hello Kyldera,

Since there is no information about your kind of detection technique, may i propose to take a look at magneto-optical sensors, which are very good suited for that kind of application.

You can use either in in-plane as well as out-of-plane Bias magnetization and since MOs are visualizing flux directions and densities over up to 3 inch areas in real time you can get a very high resoltion regarding your crack inspection.

I am working on some projects regarding that my own, so if you are interested in sharing thoughts as well as you are interested in seeing some results of crack detection, just contact me.

Have a great day,
Marco