Technical Diagnostics and NDT

Olympus, a world leader in nondestructive testing technologies and pioneer in industrial phased array ultrasound, is pleased to introduce a comprehensive inspection solution for circumferential welds in pipes with a diameter as small as 21.3 mm OD. A key component of the solution is the COBRA, a new manual scanner specifically designed to

From my article "Technical diagnostics for a safer journey" for the journal Spectrum One of the most important properties of a transport system is safety. It’s said that the human factor is the main reason for many transport accidents, from aircraft to bicycle. That means there is more chance of an accident occurring because of a simple mistake of the operator (driver or a pilot), or because of the engineer who maintains or repairs the vehicle, or the other people who involved such as the air traffic controller or the policeman on roads. This article is focused on the engineering side, more precisely about identifying problems in mechanical part or a structure. What do engineers do to minimise the probability of having a failure?

WHEN they were built, no one thought they would fly for so long. But fitted with new engines and avionics, aircraft can be kept going for a very long time. The average age of the world’s airliners is more than ten years, with some passenger jets 25 years old or more. Military planes are more geriatric: the Sikorsky Black Hawk helicopter entered service 31 years ago and the Lockheed C-5 Galaxy 40 years back. Both are still going strong. Some Boeing KC-135 aerial-refuelling planes, which are based on the venerable 707, have been flying for over 50 years. Engineers reckon they could still be in the air when they are 80.  Figure 1: C-5 Galexy One thing that does ground old aircraft is the impending failure of their aluminium structure from metal fatigue. This begins in parts that are subjected to repeated strains, such as where the wings join the fuselage. Constant flexing of the structure concentrates stress, which leads to microscopic cracks. These cracks become more numerous and eve...

There are several steps in a magnetic particle inspection. Clean the surface of the ferromagnetic material Magnetize the material Spray or pour the magnetic particles   Visual inspection   Demagnetization   Cleaning Demagnetization is done to disorganize the micro atomic arrangement which produce a magnetic field. After demagnetization the material will act as a normal metal. The magnetized components can cause problems such as,

One of the most important things an inspector should do while MPI is to check for defects at least twice in separate 90 degree angles. Let’s understand this by using a small example.  I have a piece of steel, where there suppose to be a defect. Let’s assume that there is a surface crack, which is quite straight and continues parallel to one of the edges. Then we apply a magnetic field as in the picture below. We clearly see that, no change happens to the magnetic flux.   Accordingly there is no change happens to the magnetic particles we are adding. Therefore the crack will be invisible via magnetic particles method.

First, lets check out how this magnetic particles inspection carries out :). It's always easy to understand something practical by watching a youtube video. Theory can be found in the last article . Another video with a more advanced MPI This one is in Spanish, though the MPI shown very well, and the crack is well visible. MPI usually find cracks which are perpendicular to the direction of the flux, so It's necessary to search for defects twice. I'll explain more about that in a future article, but first it's important to see and understand the basic chemistry of this method.

Magnetic particle test is one of the common NDT methods. As you can see, this has to do something with magnets. For a starter, it is better to remember little bit about magnets before understanding this method. Magnet is a material produces a magnetic field. They have two poles which are known as North, and South.   Opposite poles (N+S) attracts but same poles such as N+N or S+S repels. Figure: A magnetic field, created by iron dust Magnetic field is invisible. Any ferromagnetic material or another magnet responds to a magnetic field. Ferromagnetic materials are materials which magnetized either by another magnet or an electric field.