Non Destructive Tests

Non - Destructive Test (NDT) are tests are carried out without destroying the weld joints. NDT plays an impossible role in reducing the chances of weld failure, both through application during fabrication and through its service.
Standard NDT methods are as follows

Ultrasonic test : 
In ultrasonic testing, a transducer connected to a diagnostic machine is passed over the object being inspected.
The transducer sends pulsed waves through the surface of the object, and receives the "sound" reflected back to the device.
Reflected ultrasound comes from an interface - such as the back wall of the object or from an imperfection.

The CRT screen on the calibrated diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance taken for the reflection to return to the transducer.
In the CRT you can see the sound wave being reflected from the bottom and from the fault, by which the location of fault can be detected.

Radiography Test : 
An X - ray machine or a radioactive source like Iridium- 192 or Cobalt - 60 is used as a source of photons. The x - ray penetrates through the object and fails on the photographic film.
The intensity of radiation emerging from the test object varies depending on the flaw.
This can be clearly seen in the negative. The dark areas in the negative of an insulated pipe are signs of corrosion.

Magnetic Field Test : 
A magnetic field is applied to the material or part under test.
The discontinuities in the surface or sub surface, will cause a leakage field.
Finely divided ferromagnetic particles applied over the surface, wil be gathered and held by the leakage field.
This magnetically held collection of particles forms an outline of the discontinuity and indicates its location, size, shape and extent.

Eddy Current Inspection : 
Eddy Current Inspection is one of several NDT methods that use the principal of "electromagnetism" as the basis of conducting examinations.
Eddy Currents are induced electrical currents that flow in a circular path.
When an alternating current is applied to a conductor, like copper wire, a magnetic field develops in and around the conductor.
This magnetic field expands as the alternating current rises to maximum, and collapses as the current is reduced to zero.
If another electrical conductor is brought brought within close proximity to this changing magnetic field, current will be induced in this second conductor.
Eddy currents are created in the last object and the the electromagnetic response is observed. A flaw inside the test object, disturb the eddy current and creates a measurable response.
Ect is used for crack detection, Material thickness measurements, coating thickness measurements.


1 comment:

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