BobinLaConner
19 September 2010, 10:03
I see several claims by manufacturers of MP Testing of Barrels and other components which is a form of Nondestructive testing or NDT. NDT is the science of inspecting materials for defects without cutting or destroying the integrity or usefulness of the material, resulting in sorting, accepting or rejecting the part based on the expected use, customer specifications or Mil-specs. Since I work for a company that specializes in nondestructive testing (NDT) of aerospace and military parts, I can shed some light on this for some of you...
"MP Testing" stands for "Magnetic Particle Testing", most of the time referred to in the NDT business as" MT" or "MPI". A lot of times you hear it called "Magnaflux", but that is only one of the original manufacturers of the equipment (kind of like saying "Polaroid"). The actual method that is used on these barrels is known as "Wet Fluorescent Magnetic Particle Testing".
MT Testing involves using a bench or portable equipment to induce a magnetic field into a metal object and then applying a carrier fluid with iron oxide particles (in this case the iron oxide particles are dyed with a fluorescent green dye). Any surface or some near surface discontinuities will redirect the magnetic lines of flux to the surface attracting the iron oxide in the shape of the crack or defect. Then inspecting with a black light will immediately show these concentration of particles as any significant cracking, lapping or other linear shaped defect. Any NDT inspector has to be certified to at least level II with sufficient training and work experience to perform the test and accept/reject parts. (Entry level = Level I, Inspector = Level II, Expert/Procedure writer = Level III).
We inspect aerospace parts, welds, engine and racing parts, industrial and misc with it every day using this method, but also use other methods of inspection as needed to supplement and "see" into other areas in parts.
Here is what we use the different NDT methods for:
MT (Magnetic Particle) Wet Fluorescent great for machined ferrous parts looking for cracking or linear surface defects. Also great for welded fabrications. Not effective for finding internal defects.
PT (Dye Penetrant) Applying a fluorescent or visible dye that seeps into surface defects and then after being cleaned off, the dye bleeds out at the defect area showing the defect outline. Great for non-ferrous (Stainless) machined or smooth surface parts that cannot be MT tested
UT (Ultrasonic Testing) -similar to a high frequency fish finder. great for finding defects within metal or composite parts. We typically inspect solid or hollow round stock looking for tiny inclusions or cracking. We inspect a lot of bar, plate and rec bar stock to certify it for manufacturers and machine shops to use. It is normal to look for a 3/64" diameter or larger flaw. This method works well on all metals but sometimes complex geometries or non-parallel surface make it challenging
RT (Radiographic Testing or X-ray) Very comprehensive test that can show great detail. Some x-ray systems are digital which can inspect small parts at high speed and generate computer based x-ray images. Nice for castings or complex geometries. Shows cracking, shrinkage or pore type defects well. It can also verify thread engagement or other internal fit issues.
ET (Eddy Current) Mostly used for finding small surface cracking in non-ferrous metals. Used on aerospace a lot for aluminum parts like airplane wheels or bolts that are inspected in service. Also good for sorting materials or heat treats
Hardness Testing - measures hardness compared to a scale that can give information on how a material will behave
There are many other specialized forms of inspection but these are the main methods used. We also use fiberoptics or borescopes for inspecting inside bores as well as inspecting inside diameter areas after MT or PT Testing (where one can't visually verify)
NDT is called out as a requirement for parts that if they failed, have the potential of life threatening or costly consequences. As you can see, there are many methods that are used and MT (MP) is just the one that works best for ferrous parts when looking for surface defects (like barrels). It is great to see that it is performed because it weeds out any defective parts that show cracks or defects and it means that company has a Quality-Focused program. Not sure, but these companies may purchase metal for their parts that has already been ultrasonic or radiography certified before they machine it. I would hope that the stainless barrels are PT tested since most types of Stainless are unable to be MT inspected.
Of course this is only a general quick summary, but I hope that clears up some of it...
"MP Testing" stands for "Magnetic Particle Testing", most of the time referred to in the NDT business as" MT" or "MPI". A lot of times you hear it called "Magnaflux", but that is only one of the original manufacturers of the equipment (kind of like saying "Polaroid"). The actual method that is used on these barrels is known as "Wet Fluorescent Magnetic Particle Testing".
MT Testing involves using a bench or portable equipment to induce a magnetic field into a metal object and then applying a carrier fluid with iron oxide particles (in this case the iron oxide particles are dyed with a fluorescent green dye). Any surface or some near surface discontinuities will redirect the magnetic lines of flux to the surface attracting the iron oxide in the shape of the crack or defect. Then inspecting with a black light will immediately show these concentration of particles as any significant cracking, lapping or other linear shaped defect. Any NDT inspector has to be certified to at least level II with sufficient training and work experience to perform the test and accept/reject parts. (Entry level = Level I, Inspector = Level II, Expert/Procedure writer = Level III).
We inspect aerospace parts, welds, engine and racing parts, industrial and misc with it every day using this method, but also use other methods of inspection as needed to supplement and "see" into other areas in parts.
Here is what we use the different NDT methods for:
MT (Magnetic Particle) Wet Fluorescent great for machined ferrous parts looking for cracking or linear surface defects. Also great for welded fabrications. Not effective for finding internal defects.
PT (Dye Penetrant) Applying a fluorescent or visible dye that seeps into surface defects and then after being cleaned off, the dye bleeds out at the defect area showing the defect outline. Great for non-ferrous (Stainless) machined or smooth surface parts that cannot be MT tested
UT (Ultrasonic Testing) -similar to a high frequency fish finder. great for finding defects within metal or composite parts. We typically inspect solid or hollow round stock looking for tiny inclusions or cracking. We inspect a lot of bar, plate and rec bar stock to certify it for manufacturers and machine shops to use. It is normal to look for a 3/64" diameter or larger flaw. This method works well on all metals but sometimes complex geometries or non-parallel surface make it challenging
RT (Radiographic Testing or X-ray) Very comprehensive test that can show great detail. Some x-ray systems are digital which can inspect small parts at high speed and generate computer based x-ray images. Nice for castings or complex geometries. Shows cracking, shrinkage or pore type defects well. It can also verify thread engagement or other internal fit issues.
ET (Eddy Current) Mostly used for finding small surface cracking in non-ferrous metals. Used on aerospace a lot for aluminum parts like airplane wheels or bolts that are inspected in service. Also good for sorting materials or heat treats
Hardness Testing - measures hardness compared to a scale that can give information on how a material will behave
There are many other specialized forms of inspection but these are the main methods used. We also use fiberoptics or borescopes for inspecting inside bores as well as inspecting inside diameter areas after MT or PT Testing (where one can't visually verify)
NDT is called out as a requirement for parts that if they failed, have the potential of life threatening or costly consequences. As you can see, there are many methods that are used and MT (MP) is just the one that works best for ferrous parts when looking for surface defects (like barrels). It is great to see that it is performed because it weeds out any defective parts that show cracks or defects and it means that company has a Quality-Focused program. Not sure, but these companies may purchase metal for their parts that has already been ultrasonic or radiography certified before they machine it. I would hope that the stainless barrels are PT tested since most types of Stainless are unable to be MT inspected.
Of course this is only a general quick summary, but I hope that clears up some of it...