Magnetic Particle Crack Detection
Cracks in cast iron or steel components, which can be magnetised, can be found using magnetic particle inspection. The component in question is temporarily magnetised using a permanent or electro magnet and then small iron oxide particles in dry powder form or in a solution in water or kerosene are then sprayed or brushed on. Prior to this the component must be thoroughly cleaned and free from oil grease and dirt
Any crack in the component disrupts the magnetic field and attracts the iron particles which shows up the crack. To assist with the detection the particles may be dyed in bright colours or the component painted with a white paint.
However the most sensitive method is to use particles treated with a fluorescent dye and then view the results using an ultra violet lamp under low light conditions.
It may be necessary to rotate the magnetic field through 90° to allow the crack to show up. This is because the lines of magnetic force must cross the crack at an angle for it to show up.
Afterwards the component should be demagnetised especially if it is a bearing journal or thrust collar This is because of the danger of wear particles being attracted to the surface after the inspection. Components that are to undergo weld repair after a crack is detected (e.g. cylinder head) must also be demagnetised as residual magnetic fields can interfere with welding repairs.
CRACKS IN A THRUST COLLAR
Dye Penetrant Crack Detection
This is the most common method used to detect cracks in components on board ship. The penetrant, which is the same as a penetrating oil used to unseize a rusted nut and bolt except it contains a dye will find its way into the smallest of cracks, even those invisible to the naked eye. Some penetrants use a fluorescent dye which is then used in conjunction with an ultraviolet light which makes the cracks glow green when ordinary lighting is reduced, others use a developer which makes the dye stand out as a red line. This type usually comes in three aerosols. The first is a cleaner which is sprayed on. After the component is allowed to dry, the penetrating dye is sprayed on and after 5 minutes the excess coating the surface is wiped off. The developer is sprayed on which will highlight any crack present.
ON THE THRUST COLLAR ABOVE SEVERAL SMALL CRACKS ARE VISIBLE
CLOSE UP OF CRACKS
Ultrasonic Crack Detection
The disadvantage of magnetic particle and dye penetrant crack detection is that they will only reveal surface cracks. Ultrasonic testing can also be used to find internal flaws. A transducer generates high frequency ultrasonic energy at up to 25MHz that passes through the component under test. If there is a crack, some of the sound is reflected back to the transducer which is is transformed into an electrical signal by a pulser/receiver and displayed on a screen.