The Penetrant Professor has always tried to give answers, to the many questions that come to Met-LChek, based on years of hands on
experience, and scientific experiments. Without a doubt the most frequently asked question is,” How small a crack will penetrant find?”
This is a great question because it challenges the volumes of NDT text books and study guides viewed as gospel, the sage opinions of level
3’s around the world and countless POD studies. In reality this is not the correct question as our following experiments will indicate. What
size void penetrant may enter is quite different from what size void indication can a human inspector resolve or see. We are mostly used to performing penetrant inspection on metals, and since these are opaque, it means that there is no way that we can actually see how deep into a crack the penetrant travels.
We have solved that problem by cracking a glass ball that is transparent, and then applying fluorescent penetrant to the crack opening on
the ball. It is then possible to actually see the penetrant travel into the crack and work its way to the bottom of it.
We know that the cracks decrease in width as they penetrate into the glass and finally end, but we do not know the internal size of these.
However, if we knew the size of the crack opening on the surface, we would understand that the fluorescent penetrant was entering cracks
that were progressively tighter as they went further into the glass. To detemine the size of the surface crack opening, we sought the expertise
This was achieved using a scanning electron microsope and the expertise of Peggy Bennett of EAG Labs, of El Segundo, CA. To our surprise,
we learned that the glass ball had to be first coated in gold to be conductive and provide an image. The first image was on a millimeter scale, and one crack was measured at 0.01 mm (0.00039”). We were pleasantly surprised with the result but Peggy didn’t stop there. The SEM was cranked up to a micrometer scale, where a crack was measured on the surface to be 0.55μm across, which is 0.0000217”.
0.55μm is an interesting length because it is right at the edge of the visible light spectrum, nearing the wave length of ultraviolet. Some other comparisons to note, this is ½ the size of the thickness of a red blood cell, 2.5 x the length of the measles virus, and 3 x the length of phage bacteria. These lengths are well beyond the limit of the naked eye.
So we knew that the opening of the crack was approximately half the wave length of visible light, but the amazing thing is that the penetrant
traveled quickly to the bottom of these cracks and could easily be seen occupying cracks which had much smaller widths. How small?
We do not know how to measure these, but we can see that the penetrant has no problem in entering them. The sight of a cracked glass
ball that has these small cracks filled with glowing fluorescent penetrant is an amazing tribute to the ability of the products to perform as they
are designed to. Now, if someone asks how small a crack the penetrant can enter, the answer is easy. It is “The size of the wave length of light and much smaller”
Magnetic Particle Innovation
During the ASNT Fall Conference in Nashville TN, numerous innovations in electronic and automated technologies were on display. It is a
rare occurrence to see significant innovation in PT and MPI materials, but this was on display by Circle Systems. They have found a way
to closely control the particle size and fluorescent brightness of their Mi-Glow® 800, resulting in a high sensitivity,bright particle with minimized
background. This means fine flaws are more readily detected by the MPI operator. The industry is well familiar with what has been considered the standard for high sensitivity fluorescent particles, but that is now oh so passe. The quantum leap Circle Systems has achieved will raise the bar for aerospace MPI. We suggest you check this out and bring your MPI process into the 21st century!