THE NEED FOR NDT
Visual observations, even with magnification, cannot locate all small, below the surface defects in cast, forged metals or in weld -metals such as found in pressure vessels, boilers and nuclear components. Proper NDT will, however revel, all such indications, without damage to the components.
THE TYPES OF NDT
- Magnetic particle.
Magnetic Particle Test
MP inspection test is the most commonly use test method for forgings. It utilizes magnetism to attract and hold very fine magnetic particles right on the part itself. If a defect present, it interrupts the magnetic field and is clearly shown by the pattern made by the particles.
The part is magnetized in suitable direction by DC line voltages transformed to low voltage (4 to 18 volts) high amperage AC, half wave current, or three phase full wave current.
- All electric circuits should be installed and grounded according to National Electrical Code or any other appropriate code.
- Local exhaust should be provided to control the dust particles used for testing.
- If local exhaust is not feasible, operator should wear respirator protective equipment.
- Eye protection should be worn by the operator to guard against the irritating effects of dust particles and arcing.
- Personal protective equipment should be worn to protect against the possible skin irritation from the powder.
Penetrant Inspection (DP)
DP tests are used for detecting cracks, pores and similar indications which are open to the surface in metals or other solid material. First the part to be inspected is cleaned A penetrant is then applied to the surface and within a few minutes is drawn into defects by capillary action. The penetrant is removed from the surface but will remain in the surface opening until removed by the developer.
- Most penetrants are organic compounds that may cause dermatitis.
- Skin contact should be avoided with good personal hygiene practices strictly followed.
- Smoking, smoking materials, food or drinks should not be stored or used in the test area.
- Exposed skin should be washed before smoking, eating or drinking.
Ultrasonic Tests (UT)
Ultrasonic waves (above the audible range of 20,000 Hz are created by an electronic generator which supplies high frequency voltage to a piezoelectric crystal. Three basic ultrasonic methods have been developed. The reflection method the through transmission method and the resonance frequency method.
- Equipment should be disconnected from power supply and
- The condensers discharged whenever a cathode ray tube must be adjusted or removed.
Two types of electromagnetic tests are currently used: magneto inductive and eddy current.
The magneto inductive method uses variations in the permeability of magnetic materials to create variations in a pickup coil or probe.
T he eddy current utilizes alternating current in a coil or probe to induce eddy current in the part being tested. Defects and variation in properties or geometry causes changes in the strength and distribution in the eddy current. Readout is presented on a cathode ray tube, on a meter, by a suitable or by visible alarm, or by a combination of these methods.
- In some reader frequency testing installations, operators have been burnt internally, when they passed between object and the testing device. Special regulations should be formulated and enforced.
- Barriers should be setup to prevent operators and others from entering such areas.
- Recommendations of the manufacturer of the equipment should be followed explicitly.
Radiography uses X rays and gamma rays. X rays are unidirectional and their wave lengths can be varied within certain limits, to suit the conditions Gamma radiography differs from he X ray radiography, in that the gamma rays are multidirectional and their wave lengths, being characteristic of the source, cannot be regulated.
S AFETY PRECAUTIONS
- All sources of ionizing radiations are potentiall y dangerous They should be stored and handled in accordance with the instr tions of the supplier of he source.
- The exposure to the radiation by the operators should be regularly monitored, in accordance with the supplier s instructions.
- Only trained and certified operator should be allowed to do the work.
- The surrounding area should be effectively cordoned off and ensured that no one enters this area.
- Where-ever possible, the work should be done in a suitable enclosed room, built in accordance with the supplier of the source.
- All operators should be medically examined at frequency suggested by the supplier of the source.
- After the work is over, the source should be stored in accordance with the instruction of the supplier of the source.