Non Destructive Testing of Welding

There are Numerous Non-Destructive tests used to evaluate the base metal to be joined as well as completed welds. However these all NDT shares several common elements, these essential elements are summarized below:

o A Source of Probing energy or Medium

o A Discontinuity must cause change or alteration of probing energy

o A means of detecting this change

o A means of indicating this change

o A means of observing or recording this indication so that an interpretation can made.

Over the years Numerous Non-Destructive Testing Methods have been developed, each one has associated with its various advantage & Limitations.

Followings are the Noted NDT Methods
o Penetrant Test (PT)

o Magnetic Particle Test (MT)

o Radiographic Test (RT)
o Ultrasonic Test (UT)
o Eddy Current Test (ET)   1. Penetrant Testing (PT)

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface

penetrant is removed and a developer applied.This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

Detection of Defect using Black-light

Table for Dwell time

2. Magnetic Testing (MT)

Magnetic particle inspection is a nondestructive testing method used for defect detection. MPI is a fast and relatively easy to apply and part surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.  

      MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement from an inspectability standpoint is that the component being inspected must be made of a ferromagnetic material such iron, nickel, cobalt, or some of their alloys. Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.
     The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness-for-use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines


Electromagnetic Yoke Detail Diagram

Electromagnetic Yoke Application



Application of Dry Powder

The Magnetic Field Intensity Measure



Defect Detection in Weld Using MPI (Dry Powder)



Before and after Inspection MPI Detection

3. Radiographic Testing
Covered in detail in my older post



4. Ultrasonic Testing (UT)
Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more. To illustrate the general inspection principle, a typical pulse/echo inspection configuration as illustrated below will be used.

 

A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulse. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

cross-section of the Probe



Beam spread occurs because the vibrating particle of the material (through which the wave is traveling) do not always transfer all of their energy in the direction of wave propagation. Recall that waves propagate through that transfer of energy from one particle to another in the medium. If the particles are not directly aligned in the direction of wave propagation, some of the energy will get transferred off at an angle. (Picture what happens when one ball hits another second ball slightly off center). In the near field constructive and destructive wave interference fill the sound field with fluctuation. At the start of the far field, however, the beam strength is always greatest at the center of the beam and diminishes as it spreads outward.