Non Destructive Evaluation

NDE Methods

Visual-Optical

Liquid Penetrant Inspection (LPI)

Magnetic Particle Inspection (MPI)

Ultrasonic Testing (UT)

Radiographic Testing (RT)

Eddy Current Testing (ET)

Acoustic Emission Testing (AET)

Thermal Imaging

Visual-Optical Inspection

Borescope: (i) Rigid (ii) Flexibile

(a) Rigid and fixed – Series of achromatic relay lenses in an optical tube with an incandescent lamp for illumination: Tube dia – 4-70 mm. Mag – 3-50X

(b) With rotating shaft having light guide bundle for orbital scan

Flexible fiberscope : Dia 1.4 – 13mm. Consists of light guide bundle (optic fibers of 30 m dia) and image guide bundle with interchangeable viewing heads. May also consists of a CCD image sensor at the distal tip.

Drive shaft Bearing Casting

Shaw bladeWeldment

Crack originating at fastener hole

Fastener hole MPI indications

Cold lap

Incomplete fusion

Lack of penetration

RT Image Weld defects

Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into the base material without bonding.

Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

Incomplete fusion is a condition where the weld filler metal does not properly fuse with the base metal. Appearance on radiograph: usually appears as a dark line or lines oriented in the direction of the weld seam along the weld preparation or joining area.

Porosity

Internal or root under cut

Offset or mismatch

Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

Offset or mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image shows a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by the failure of the weld metal to fuse with the land area.

Gas porosity Sand inclusion Shrinkage cavity

RT Image Casting defects

Gamma ray radiography

Digital Radiography

Flat panel detectors (FPD)

Phosphor plate, barium fluorohalide activated by europium. Latent image is excited by laser, photomultiplier converts the light out put to an analog image which is converted to a digital one by A/D converter.

Direct conversion of X-ray to electrical signal - a two-dimensional array of thin film transistor (TFT) with an X-ray converter. Amorphous Si (TFT) coated with a image-conversion layer (CsI)

The light from phosphor falls on photodiode which creates charges proportional to light intensity. They get deposited on signal storage capacitors.

Row of TFTs (pixels) – Gate line – Stores the chargeColumns – Data or Drain line – Transfers the chargeTFTs in ‘Off’ mode during exposure. A positive bias after exposure opens the gates (Transfers the charge to Data lines D1 ----- Dn) gate-by-gate.

Computed Tomography

2-D and 3-D cross-sectional images of an object

Images are taken at different angles by rotating the sample as if the sample is sliced at different planes and imaged. Tomography - Tomos – slice graphein – to write

When the density data of these slices are stretched and put together (reconstructed) they create a 3D image of the object or the defects

Radiation safety

Exposure vaults

Exposure cabinets

Safety control -

Engineered control – Shielding, door interlocks, alarms, warning lights. Rope and signal portable radiography

Administrative control –

Postings, procedure, dosimetry and training

Ultrasonic testing

Electromagnetic Acoustic Transducer (EMAT)

F = J x B

C- scan

Immersion testing

Calibration

Data Display

Impedance plane on Oscilloscope screen

Analog meter

Surface probe – ‘Pancake’ type OD or Encircling Coils ID or Bobbin Probes

Reference standard

Should be manufactured from the same material type, alloy, material thickness, and chemical composition that will be found on the component to be inspected. Sizes and tolerances of flaws introduced in the standards are usually regulated by inspection specifications.

Common eddy current reference standards include:Conductivity standards. Flat plate discontinuity standards. Flat plate metal thinning standards (step or tapered wedges). Tube discontinuity standards. Tube metal thinning standards. Hole (with and without fastener) discontinuity standards.

Narrow notches produced with electron discharge machining (EDM) and saw cuts are commonly used to represent cracks, and drilled holes are often used to simulate corrosion pitting.

3 or 4 Al plates fastened together in a lap join fashion, 2nd or 3rd layer contains EDM notches or naturally/artificially induced cracks

Thermal Imaging

wavelength of thermal radiation extends from 0.1 microns to several hundred microns.

Heat transfer

Conduction – solid

Convection – liq, gas

Radiation – no medium

Infrared (IR) radiation has a wavelength that is longer than visible light - greater than 700 nanometers. As the wavelength of the radiation shortens, it reaches the point where it is short enough to enter the visible spectrum and can be detected with the human eye.

An infrared camera has the ability to detect and display infrared energy. The basis for infrared imaging technology is that any object whose temperature is above 0 K radiates infrared energy.

Emissivity

Measure of a surface’s ability to radiate infrared energy

ratio of thermal energy emitted by a surface to the energy emitted by a perfect blackbody at the same temperature.

Equipment

Detector and display system

Detector – Thermal and Quantum

Thermal – Heat sensitive coating that melts at certain temp or change color with temp. Thermoelectric or pyroelectric devices like thermocouple, thermistors or bolometers

ThermopileBolometer

IR incoming photons of a certain wavelength absorbed by the sensor and creates electron-hole pair which can be detected as electrical current. Thus radiation energy is directly converted to a signal.

But signal output is very small and hence suffers from noise. Since noise is partly proportional to temperature, detector is operated at cryogenic temp.

Photoconductive and photovoltaic

Photoconductive – generates electrons, holes or electron-hole pairsIndium antimonide (InSb), quantum well infrared photodetector (QWIP, GaAs/AlGaAs multilayer), mercury cadmium telluride (mercad, MCT), lead sulfide (PbS), and lead selenide (PbSe).

Photovoltaic - require an internal potential barrier (p-n junction or Schottky barriers) with a built-in electric field in order to separate photo-generated electron-hole pairs.indium antimonide (InSb), mercury cadmium telluride (MCT), platinum silicide (PtSi), and silicon Schottky barriers

ImagingScanning a detector (or group of detectors) –

single element detector scanning along each line in the frame (serial scanning)- requires very high scan speedseries of elements are commonly scanned as a block, along each line

The frame movement can be provided by frame scanning optics (using mirrors) or in the case of line scan type imagers, by the movement of the imager itself

Focal plane array (FPA) - group of sensor elements organized into a rectangular grid. The entire scene is focused on the array, each element cell then provides an output dependent upon the infrared radiation falling upon it.

The advantage of FPAs is that no moving mechanical parts are needed and that the detector sensitivity and speed can both be slower. The drawback is that the detector array is more complicated to fabricate and manufacturing costs are higher.

A special lens focuses the infrared light emitted by all of the objects in view.

The focused light is scanned by FPA detectors. The array of detector lies in the focal plane of the lens.

A detailed temperature pattern called a thermogram, is created based on the radiation energy falling on each sensor. This information is obtained from several thousand points in the field of view of the detector array.

The thermogram created by the detector elements is translated into electric impulses.

The impulses are sent to a signal-processing unit, a circuit board with a dedicated chip that translates the information from the elements into data for the display.

The signal-processing unit sends the information to the display, where it appears as various colors depending on the intensity of the infrared emission. The combination of all the impulses from all of the elements creates the image.

Thermal imaging devices

Un-cooled - This is the most common type of thermal-imaging device. The infrared-detector elements are contained in a unit that operates at room temperature. natural or artificial pyroelectric materials, usually in the form of a thin film, out of gallium nitride (GaN), (CsNO3), polyvinyl fluorides or micrbolometers. This type of system is completely quiet, activates immediately and has the battery built right in.

Cryogenically cooled - these systems have the elements sealed inside a container that cools them to sub zero temperature. The advantage of such a system is the incredible resolution and sensitivity that result from cooling the elements. Cryogenically-cooled systems can detect a temperature difference as small as 0.1 C from a distance of more than 1,000 ft (300 m).

Image interpretation

White areas indicate hot spots and black areas indicate cooler regions

Modern equipments are capable resolving the temperature in terms of different colors

Application to NDT

Thermal image technique will detect defects or anomalies in the underlying material as changes in the surface temperature. Owing to different thermal coefficients between defects and the surrounding material, heat will flow in different way and with different rate, and the result will be a different temperature distribution in correspondence of defects.

Passive and active method

Passive - No external heating. Heat is generated due mechanical load (stress). cracks tips or stress concentration zones are areas where heat is accumulated and dissipated and they appear as hot spots.

Active – part to be tested is heated externally to get radiation from the surface.

Single sided – sample is heated by an external heat source and then sensor is scanned to record the radiation

Double sided – The heat source and the sensor are placed on opposite sides and operated simultaneously

Composite sandwich structure

Brazed joint of dissimilar metals

Brazed joint of cooling tube to a heat sink

Thermal image X-ray image

Civil structures

Bright patches indicate missing and poorly positioned insulation in the walls of a new industrial unit.

Survey of concrete roof slab in full sun. Dark (cool) patches indicate presence of moisture trapped in upper insulation layers

Can be used for inspecting parts in service e.g. Mechanical and Electrical Systems – Simply pointing an infrared camera at a component and looking for areas of uneven heating and localized hot spots.

Electrical – loose connection, failed transformer

Mechanical – improper bushing and bearing lubrication, over loaded motor or pumps, coupling misalignment

Electronics – temperature of the junction is a critical factor for the life of semiconductor device

Corrosion damage:

material thinning of relatively thin structures e.g. aircraft fuselage

Surface being inspected needs to be heated – out put obtained from different parts will give an idea about the damage. Heating is done xenon flash lamps.

Heat will be conducted away from the surface faster from thicker region. From the temp profile a thickness map can be generated

Corrosion damage and disbonding in the inside surface of an aircraft skin

Gas turbine engines liners

SiC-SiCf liner with EBC

Defect (delamination )

delamination/dedonding of the EBC layers after 13,937 hrs of engine operation

2 mm2/s 17 mm2/s

Correlation of cross-sectional photomicrographs with one-sided thermal image showing locations where EBC debonding and pre-spall occur

Flaw Detection

Sound material, a good weld, or a solid bond will see heat dissipate rapidly through the material, whereas a defect will retain the heat for longer.

Vibrothermography image of a 7075 Al plate

Vibrothermography – exciting the sample by bursts of high energy, low frequency acoustic waves. this generates frictional heat at the faces of a crack and hence the crack can be detected by thermal imaging

Process monitoring

no water for cooling. There are no stripes from improper headers.

New headers were installed to quench slab, however, the header nozzles were not properly installed causing hot and cold stripes.

Steel slab

Product quality

On the tyre testbed: infrared camera detects rolling resistance and heat development of tyre

Audi cars

Monitoring heat distribution in the catalytic converter

Thermal imaging of the engine testbed to find out temp distribution and potential failure sites

Comparison of NDT methods

Selection of NDT methods