Introduction

Non-destructive testing inspection usually implies a process for finding defects in such a manner that product integrity and surface texture remain unchanged. The application of NDT process is involved in various field of engineering from mass productive system to maintenance of machine. In recent years, the word “defective” and “defect” have often been interpreted as meaning unfit for service, and their use is declining in favour of terms such as flaw, imperfection, discontinuity and non-conformance. [1] Generally, NDT is normally carried out in one of the following circumstances:

1. As a control check and an aid to the monitoring of quality during manufacture. In this case NDT is used to check the maintenance of required quality levels during manufacture. The presence of manufacturing imperfections and defects may be detected with regard to assessment of their characteristics and severity.

2. During in service inspection. In the case NDT methods are used in order to detect any physical deterioration in a component or structure that might occur under service conditions. NDT methods are most usually employed to detect cracking, or crack propagation from existing defects. Such cracking is commonly associated with some form of fatigue.

3. Critical defect assessment. This use of NDT normally involves an in-depth examination of known defects in a structure in order to provide detailed information for “fitness for purpose” assessments and fracture mechanics calculations.

Advantages of NDT:

1. May reveal failure mechanism.2. Can be done on entire production, defective parts can be removed instantaneously.3. May be performed on parts in service.4. Little or no specimen preparation is required.5. Different tests can be applied and repeated to the same item simultaneously.6. Little or no specimen preparation is required.7. Equipment is often portable for use in field.

Methods of NDT testing

The methods generally used for non-destructive testing are as follows:

Visual Inspection.[2]

Hardness Testing. Liquid-Penetrant Inspection. Magnetic-Particle Inspection. Eddy current Inspection. Electromagnetic Sorting on Testing. Radiographic Inspection. Inspection by Neutron Radiography. Ultrasonic Inspection. Inspection by Optical Holography. Acoustic-Emission Inspection. Microwave Inspection. Leak testing. Rapid Identification of Metals and Alloys. Thermal Inspection. Spark testing of Ferrous Alloys. Chemical Spot Testing.

The most widely used NDT methods in engineering industry are in bold characters. In this article we will define the main methods employed and their steps to achieve results.

Visual Inspection

This method is widely used in automotive industry where vehicles undergo this inspection as final stage before being marketed. The inspection is carried out in a good illumination chamber, basically this chamber consists of white fluorescent light built and arranged in a manner to facilitate visual inspection. Vehicles, with some kind of defects, are resent in the production line for proper repair. This method is done for only surface analysis.

Liquid-Penetrant Inspection

Penetrant testing aims to increase the visible contrast between the defects and the background by treating the whole component with a searching liquid of high penetrating power (i.e. low velocity). [2] In this method, fluids are applied to the surfaces of the part and allowed to penetrate into surface cracks, seams, and pores. The condition for usage of this method is the lower limit of the crack must be as such to allow Penetrant to fully enter and the upper limit must be sufficiently large for cleaning stage. The Penetrant can seep into cracks as small as 0.1µm in width. [3] Searching liquid is such that it must be clearly visible to inspectors. To enhance the vision, Penetrants used are of fluorescent type with ultraviolet light detection and visible type using dyes (usually red in colour) which appear as bright outlines on the surface.

Sequence of operations in penetrant testing

Fig 1

1. Component preparationAnything that would prevent or absorb penetrant must be removed, for example grease, dust, scale, rust, water or coating. Liquid or vapour degreasing in organic solvent is effective in removing oil and grease, while acid pickling will remove scale and rust. Then the component is made to dry in a dryer.

2. Application of penetrantFor the treatment of surface with penetrant, is done by either dipping into searching liquid or spraying. Then the component is left to soak and dry. The drying time is usually 20-30min depending on the type of manufacturing process. []

3. Removal of excess penetrant Normally penetrant are removed by water but if the penetrant is of post-emulsifiale, it is treated in an emulsifying stage before wash. It is important that excess penetrant is completely removed, since any residue will give false indication of the presence of flaw.

4. DryingComponent passed into a dryer.

5. DevelopingA developing agent is applied, to allow the penetrant to seep back to the surface and to spread to the edges of openings, thus magnifying the size of defects.About 20mins are usually allowed for development of the surface.

6. InspectionFor fluorescent penetrant, this stage takes action in a UV chamber. A good intensity of light is needed for proper inspection for other developer.

This method can be used to detect a variety of surface defects. The equipment is simple and easy to use, can be portable, and is less costly to operate than that of other methods. However, this method can only detect defects that are open to the surface.

Practical example: An example came from a seminar last year 2013 from CEB; liquid penetrant technique is practiced in hydraulic blades and hot gas removal fan. Hydraulic blades are susceptible to high speed debris that collides on propulsion and eventually give rise to small surface cracks. To maintain the speed and consequently the shaft speed constant (as a result power), a proper maintenance schedule for this blade is done at CEB. Also same procedure is respected for the hot gas removal fan.

Magnetic-Particle inspection

This method utilises small ferro-magnetic particles to reveal the defect from the component. Magnetic-particle testing detects cracks and flaws from surface and sub-surface of component. The line of flux produced by the component is used to find defects. The defect under the surface produces an uneven form of the flux line as seen from the figure below. But note that the deformation in flux lines must be such that it crosses the surface limits for detection otherwise nothing will be detected.

Fig 2

Note: The component must be strictly cleaned and demagnetised prior to testing.

Sequence of operation:

Magnetisation

The component under test can be magnetised either by connecting wires like poles for passage of current or wrapping a coil around the component with current passing into the coil. The former method give rise to magnetic field in circular motion and the latter give rise to magnetic field in longitudinal form. Both method of magnetisation can be used to find defect on the longitudinal or lateral plane.

Applying magnetic particles on the surface of the component

Magnetic particle (e.g. Fe3O4) is mixed in a carrier medium usually kerosene and the component is dipped into it. The magnetic particles are usually of fluorescent type which enables detection of flaws by UV light. Then inspection is done.

Demagnetisation

Once a component has been magnetised it tends to retain some magnetic field that have adverse effect if the component is machined or assembled into machine. Machine this component results into swarf collection resulting into tool damage and during assembly it can upset other nearby components. Demagnetising is done by moving the magnetising coil away from the specimen or by gradually reducing the current flowing through it.

Application

The principal industrial uses of magnetic particle inspection are final inspection, receiving inspection, in-process inspection and quality control, maintenance and overhaul in the transportation industries, plants and machinery maintenance and inspection of large components.

Eddy-current inspection

This method is based on electromagnetic induction. The component is placed in an electric coil in which alternating current flows at frequencies from 60 Hz to 6 MHz depending on component size. Eddy current is produced by this alternative magnetic field. Any crack or flaws distort the direction of eddy current. A searching coil carrying ac is moved over the surface of the component under test and any change in the electrical properties of the coil is assumed to indicate the presence of flaws. Defect can be detected under a depth limited to 13mm. [] in the non-ferrous wrought metal industries, eddy-current testing is the most widely used non-destructive testing technique.

Note: This technique is still in a research phase in a way that it can also identify properties and composition of the metal.

Figure 3 shows the changes in eddy-current flow caused by a defect in a workpiece

Radiographic inspection

This method consists of two types of analysis:

I. X-Ray analysisII. Gamma Ray analysis

Radiography inspection is used to detect internal flaws such as cracks and porosity. For X-ray, the principle involved is difference in density; the metal surrounding the defect is denser and, hence, shows up as lighter than the flaws on an X-ray film. The film is placed at the back of the component and after radiation it is examined in a light intensity. Light dense region is examined carefully and if rejected if non-compliant. X-ray radiography can be used to examine engineering structures, components and devices.

Gamma Ray inspection is used to inspect thick sections otherwise it would require the use of very large X-ray sets. A radioactive isotope is exposed to a position of the work. The work may be large pipes or weld that has to be inspected for crack that may propagate to produce leakage. All this depend on the structure and accessibility of the pipe. All the procedures are designed as per standards for safety of people and environment. Like X-ray, film is examined to detect any flaws in the weld.

Figure 4 shows the positioning of gamma film prior to clearance of pipe

Ultrasonic Inspection

Ultrasonic is the general subject of sound whose frequency is above the upper pitch limit of the human ear. Ultrasound is used to test engineering materials for cracks and defects, and to measure material thickness. The ultrasonic wave sent to the workpiece is reflected back to the piezo with little distortion and if defects present, part of the wave is reflected back according to its position. Ultrasonic waves are generated by a transducer. The transducer can both be used for transmission and reception. A coupling medium (gel) is used to prevent any hard attenuation when transmitted from transducer to the surface of the specimen.

There are 2 ways for Flaw detection:

I. Pulse-reflection methodII. Transmission method

In pulse reflection method, waves are transmitted into the component under test and the time which elapses between the transmission signal and the return of the reflected echo from the back wall of the component. If any defect is present on half way of the ultrasonic wave, the wave is partly reflected to the transducer while the other wave struck the back wall. The defect echo gives the inspector the position of the defect with respect to the back wall. Note that there is only one transducer and consequently make is easier and more rapid compared to the other detection.

Figure 5 illustrates the reflection method

In transmission method, a continuous beam is used. The ultrasonic beam passes from the transmitter into the workpiece and then to the receiver.

Two peaks appear on the cathode ray oscilloscope; one shows a peak with roughly no attenuation and the other one shows an attenuated peak.

Figure 6 illustrates the transmission method of ultrasonic flaw detection

Application:

Steel plate and flat bar from 5mm to 30mm thick can be checked for laminations, seams, rolling cracks and inclusion. This method is widely used to check for casting defects and welding inspection. Ultrasonic inspection is used for quality control and material inspection in all major industries, this includes electrical and electronic-component manufacturing; production of steel, aluminium and titanium; and fabrication of structures such as air frames, pressure vessels, ships, bridges, motor vehicles, machinery and jet engines.[1]