16:13 Mar-11-2010 aldo Engineering Italy, Joined Jan 2009 39
The lenght of one piece that has been magnetized by direct flow current (hend to hend) has some influence on intensity magnetic field produced? This is a question on quiz MT
I want to know the your suggestion...
Tanks and best regards
When inducing a current directly, like when the part is clamped in the head shot, the length has no effect on the field induced. The current runs between the head and tail stock creating a series circuit inducing the circular magnetic field into the part where the current, in theory, should be the same throughout the length of the part. The strength of the magnetic field induced is the variable depending on the current level, diameter, geometry and permeability of the part under test. That's why the general formulas for determining current level only take the diameter or cross sectional area into account when determining the required current. On a side note, these formulas are only general theory and have been found to be excessive, which is where the use of QQI's and Hall Effect Meters come into play. Hope this helps.
Hi and tanks for your replay...But in attached there is small dimostration on Biot-sevart law for one finte straight wire where there is a magnetic field that is function of the distance of point (R) and the lenght of wire (L)...The minimun is at ends of wire and the maximun is in middle lenght of the wire...Is correct my interpretation?
Tanks and excuse me if i insist on this topic..
What Amy meant by her "Side Note" was that if we only use our calculation to find out what is the magnetic field required; we end up with the part being overmagnetised. When I was teaching MT, I always told my students the calculations where a starting point and whatever your answer was for the magnetising force, always start with 200 - 300 Amperes less and increase your filed strength until you obtain a proper indication from your Pie Gauge or QQI. Because one of the factor that we do not take in consideration when we calculate the magnetising force is permeability (we assume it is 1). But permeability will change according to physical dimention of the part, heat treatment, alloying and so on. So the best thing is to proof your inspection by using your Pie Gauge or QQI.
I'm not really good with this kind of equations, but when it comes to magnetisation, your part in not at the end of a circuit but rather is place in a circuit completing a loop allowing the current to flow through. This is why the circular magnetic field will have the same value through the whole length of the part.
tanks and micheal for your perfect explanation...but why when i do the ketos ring is necessary to use a bar that has a lenght of 400mm o more? is there correlation between lenght of bar and the quality the test on ketos ring?
I do this question because in a recently NADCAP audit the auditor has done a NCR because the ispector has used a conductor for the ketos ring of 300mm and not 400mm o more...
I thought then, on basis of this NCR, there is some correlation between magnetic field and lenght of bar or piece...
This is a good question for which i do not have an answer. Maybe someone with more knowledge in the MT field could answer this one. But, I would suspect that it may have to be for the same reason our calculations always come out with more magnetising force than we really need.
Don't forget many of these standards and practices where written 30 - 40 years ago and very often the tendancy is not to fix what ain't broken. With the progress in UT and ET many regards MT and PT as techniques of the past.
As an ASNT Level III in MT, I would say length of the bar should not affect the field strength. I have also wondered why there is a minimum length for the bar when using a ketos ring. My only guess is that if the ring is too close to the copper contact pads on the head and tail stocks, the current in that location could interfere with the magnetic field in the ketos ring. This is only an idea. Mabee it is just so auditors have something more to look for.
Aldo, per other inspector's posts, it is a fact that the length of the conductor has no direct effect on the intensity of the magnetic field and for a given diameter, the circular magnetic field is the same across the length of the conductor. Most Level I-II courses cover this aspect of magnetic fields.
Regarding a Nadcap audit where the auditor gigs you for a short conductor, a good question to ask any auditor is "where is that requirement?" It is not in Nadcap checklist AC7114-2 Rev. D or in AC7114-2S Rev. D. Nor is it in ASTM E1444-05. E1444-05 states in the "nonmandatory" appendix X1 in paragraph X1.2.1, "Place a non-ferromagnetic conductor with a diameter between 1 and 1.25 in. [25.4 and 31.75 mm] through the center of the ring." It does not specify the length of the ring. I trust this is helpful.
ASTM E07.03 (Magnetic Particle/Liquid Penetrant) Subcommittee Chairman
The length of a copper central conductor would have some impact on the amount of current passing through it with a specific setting of a specific machine's current control. The required numer of hole indications is given for particlar current readings. Therefore, adjusting the machines's current control makes the impact of central conductor length "invisible" and not really relevant for the KIetos ring test.