IIM Hyderabad 15th July 2016 Overview of NDT Methods K Kapoor DGM(QA), NFC, Hyderabad
Interruption in Normal Physical Structure - Keyways, Grooves, Holes present by design Discontinuity Flaw Defect Discontinuity with undesirable connotation - Slag, Porosity, Lamination Flaw which makes component unfit for service - Cracks, Lack of Fusion in Weld
Types of Flaws: Planar Flaws (2D) Lack of Fusion, Cracks Volumetric Flaws (3D) Porosity, Inclusions
Important Flaw Characteristics in materials Location Surface Flaw Sub-surface Flaw Surface flaw more severe as compared to sub-surface flaw because of higher stress intensity associated with it Code provides guidelines on classification of flaw in to surface or subsurface, if it lies just below the surface of the component
Important Flaw Characteristics in materials Shape Flaws with sharp tip like crack more severe than flaws with smooth surface like porosity Small root radius leads to higher stress intensity
Important Flaw Characteristics in materials Proximity If two flaws are very close, they influence the stress intensity associated with each other Two flaws shall be separated by the length of the longest flaws, or else they shall be considered together as a singe flaw including the sound region in between
Important Flaw Characteristics in materials Nature Weld Flaw - Operator: Porosity, lack of fusion, slag inclusion, undercut - Metallurgical origin: Cold crack, Hot crack, laminar tearing Planar or Volumetric Flaw Planar flaw more severe than volumetric flaw
Classification of NDT Methods Non-Destructive Testing Surface Examination Volumetric Examination Performance Test Visual Radiography Acoustic Emission Liquid Penetrant Ultrasonic Testing Leak Test Magnetic Particle Eddy Current
NDT Techniques for Welds Conventional X- ray and Gama-ray Radiography Ultrasonic testing Eddy Current testing Magnetic Particle Dye Penatrant Advanced Infra-red Thermography Acoustic Emission Micro-focal radiography Real time radiography Time of Flight Diffraction (UT) Phased array ultrasonic testing X-ray residual stress measurement
Dye Penetrant Inspection Surface breaking defects only detected Penetrant applied to the component and drawn into the defects by capillary action Applicable to all non- porous and non absorbing materials. Penetrants are available in many different types Water washable contrast Solvent removable contrast Water washable fluorescent Solvent removable fluorescent Post-emulsifiable fluorescent
Dye Penetrant Inspection
Dye Penetrant Inspection
Dye Penetrant Inspection Step 1. Pre-Cleaning Cleaning preparation is very important on this method. Usually solvent removal is been used
Dye Penetrant Inspection Step 2. Apply penetrant After the application of the penetrant the penetrant is normally left on the components surface for approximately 15 minutes (dwell time). The penetrant enters any defects that may be present by capillary action
Dye Penetrant Inspection Step 3. Clean off penetrant After sufficient penetration time (dwell time) has been given,excess removal penetrant stage take place. A damped lint free tissue with solvent is used to clean the excess penetrant.
Dye Penetrant Inspection Step 3. Apply developer After the excess penetrant is been removed, a thin layer of developer is applied.a penetrant drawn out by reversed capillary action.
Dye Penetrant Inspection Step 4. Inspection / development time Inspection should take place immediately after the developer has been applied.any defects present will show as a bleed out during development time.
Dye Penetrant Inspection Step 5. Post-Cleaning After the inspection has been performed post cleaning is required to prevent corrosion.
Dye Penetrant Inspection Fluorescent Penetrant Bleed out viewed under white light Bleed out viewed under a UV-A light source Colour contrast Penetrant
Magnetic Particle Inspection
Magnetic Particle Inspection Surface and slight sub-surface detection Relies on magnetization of component being tested Ferro-magnetic materials only can be tested A magnetic field is introduced into a specimen being tested Methods of applying a magnetic field, yolk, permanent magnet, prods and flexible cables. Fine particles of iron powder are applied to the test area Any defect which interrupts the magnetic field, will create a leakage field, which attracts the particles Any defect will show up as either a dark indication or in the case of fluorescent particles under UV-A light a green/yellow indication
Magnetic Particle Inspection Electromagnet (yolk) DC or AC Collection of ink particles due to leakage field Prods DC or AC Crack like indicati on Crack like indicati on
Magnetic Particle Inspection A crack like indication
Magnetic Particle Inspection Alternatively to contrast inks, fluorescent inks may be used for greater sensitivity. These inks require a UV-A light source and a darkened viewing area to inspect the component
Magnetic Particle Inspection Typical sequence of operations to inspect a weld Clean area to be tested Apply contrast paint Apply magnetisism to the component Apply ferro-magnetic ink to the component during magnetising Interpret the test area Post clean and de-magnatise if required
Magnetic Particle Inspection Advantages Simple to use Inexpensive Rapid results Little surface preparation required More sensitive than visual inspection Disadvantages Surface or slight subsurface detection only Magnetic materials only No indication of defects depths Detection is required in two directions
Radiographic Inspection The principles of radiography X or Gamma radiation is imposed upon a test object Radiation is transmitted to varying degrees dependant upon the density of the material through which it is travelling Thinner areas and materials of a less density show as darker areas on the radiograph Thicker areas and materials of a greater density show as lighter areas on a radiograph Applicable to metals,non-metals and composites
Industrial Radiography X - Rays Electrically generated Gamma Rays Generated by the decay of unstable atoms
Industrial Radiography X - Rays Electrically generated
Industrial Radiography Gamma Rays Generated by the decay of unstable atoms
Radiographic Inspection Source Radiation beam Radiographic film Image quality indicator Test specimen
Radiographic Inspection Source Radiation beam Image quality indicator Test specimen Radiographic film with latent image after exposure
Radiographic Sensitivity 7FE12 Step / Hole type IQI Wire type IQI
Image Quality Indicators Step/Hole Type IQI Wire Type IQI
Radiographic Techniques Single Wall Single Image (SWSI) - film inside, source outside Single Wall Single Image (SWSI) panoramic - film outside, source inside (internal exposure) Double Wall Single Image (DWSI) - film outside, source outside (external exposure) Double Wall Double Image (DWDI) - film outside, source outside (elliptical exposure)
Single wall single image SWSI Film Film IQI s should be placed source side
Single wall single image SWSI panoramic Film IQI s are placed on the film side Source inside film outside (single exposure)
Double wall single image DWSI Film IQI s are placed on the film side Source outside film outside (multiple exposure) This technique is intended for pipe diameters over 100mm
Double wall single image DWSI Identification Unique identification IQI placing Pitch marks indicating readable film length EN W10 A B ID MR11 Radiograph
Double wall double image DWDI elliptical exposure Film IQI s are placed on the source or film side Source outside film outside (multiple exposure) A minimum of two exposures This technique is intended for pipe diameters less than 100mm
Double wall double image DWDI Identification 4 Unique identification 3 EN W10 IQI placing Pitch marks indicating readable film length 1 2 ID MR12 Shot A Radiograph
Radiographic Inspection Advantages Disadvantages Expensive equipment Permanent record Bulky equipment ( x-ray ) Little surface preparation Harmful radiation Defect identification Detection on defect No material type depending on orientation limitation Slow results Required license to operate
Basic Principles of Ultrasonic Testing To understand and appreciate the capability and limitation of UT
Ultrasonic Inspection Sub-surface detection This detection method uses high frequency sound waves, typically above 2MHz to pass through a material A probe is used which contains a piezo electric crystal to transmit and receive ultrasonic pulses and display the signals on a cathode ray tube or digital display The actual display relates to the time taken for the ultrasonic pulses to travel the distance to the interface and back An interface could be the back of a plate material or a defect For ultrasound to enter a material a couplant must be introduced between the probe and specimen
Ultrasonic Inspection Ultrasonic testing is a good technique for the detection of plate laminations and thickness surveys Laminations detected using normal beam probes
Ultrasonic Inspection defect echo initial pulse Back wall echo Material Thk defect 0 Compression Probe 10 20 30 40 CRT Display 50
Ultrasonic Inspection Pulse echo signals A scan Display Compression probe UT Set, Digital Thickness checking the material
Ultrasonic Inspection Ultrasonic testing requires high operator for defect identification Most weld defects detected using angle probes
Ultrasonic Inspection UT Set A Scan Display Angle Probe
Ultrasonic Inspection initial pulse defect echo Surface distance defect sound path 0 Angle Probe 10 20 30 40 CRT Display 50
Ultrasonic Inspection Advantages Disadvantages Trained and skilled Rapid results operator required Sub-surface detection Requires high operator Safe skill Can detect planar defect Good surface finish Capable of measuring the required depth of defects Difficulty on detecting May be battery powered volumetric defect Portable Couplant may contaminate No permanent record
Principles and Application of Eddy Current Testing Basic Principle Parameters influencing ECT Equipment for ECT Probe (types, construction and design) Supply, display and measuring unit Reference Standards General Applications Tube testing (Welded and Seamless) Thickness measurement (bare surface and coating) Surface Crack Detection Conductivity measurement (sorting, heat-treatment etc)
Illustration material
Basic Applications of ECT Crack detection Material thickness measurements Coating thickness measurements Conductivity measurements for: o Material identification o Heat damage detection o Case depth determination o Heat treatment monitoring
Advantages Sensitive to small cracks and other defects Detects surface and near surface defects Inspection gives immediate results Equipment is very portable Method can be used for much more than flaw detection Minimum part preparation is required Test probe does not need to contact the part Inspects complex shapes and sizes of conductive materials
Limitations Only conductive materials can be inspected Surface must be accessible to the probe Skill and training required is more extensive than other techniques Surface finish and and roughness may interfere Reference standards needed for setup Depth of penetration is limited Flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are undetectable
Parameters Influencing ECT Material Parameters Conductivity Permeability Magnetic Coupling (sensor and test object) Depth of penetration (frequency)
Conductivity ( ) Variables affecting : Chemical composition Impurity content Heat-treatment Lattice distortion and lattice defects (Cold work) Hardness Temperature Discontinuities
Permeability Material s Flux Density (B) Coil s Mag. Force (H) Non-magnetic Material Material s Flux Density (B) Coil s Mag. Force (H) Magnetic Material Example Cu, SS 304 Steels, Ni-base alloys Low DOP for eddy currents High DOP for eddy currents
Depth of penetration
Depth of penetration Material Conductivity Resistivity Permeability DOP (in inch) at frequencies (% IACS) ( -cm) 1KC 4KC 16KC 64KC Cu 100 1.7 1.0 0.082 0.041 0.021 0.010 Al (6061-T6) 42 4.1 1.0 0.126 0.063 0.32 0.016 Mg 37 4.6 1.0 0.134 0.067 0.34 0.017 Pb 7.8 22 1.0 0.292 0.146 0.073 0.037 Zr 3.4 50 1.0 0.446 0.223 0.112 0.056 304 SS 2.5 70 1.02 0.516 0.258 0.129 0.065 Alloy Steel 2.9 60 750 0.019 0.01 0.0048 0.0024 Cast Iron 10.7 16 175 0.018 0.009 0.0044 0.0022
Phase Lag (with depth) Phase lag is a parameter of the eddy current signal that makes it possible to obtain information about the depth of a defect within a material In radians In degrees Where: q=phase Lag (Rad or Degrees) x=distance Below Surface (in or mm) d=standard Depth of Penetration (in or mm)
Probes Mode of operation Configuration Applications Absolute Bobbin(ID), Encircling(OD), Surface Flaw detection, conductivity measurements and thickness measurements. Differential Bobbin(ID), Encircling(OD), Surface Flaw detection Reflection Probes Flaw detection and thickness measurement Bobbin(ID) (remote field: driver and sensor) Encircling (OD) Surface Bobbin (ID)
Reference Standards 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
Basic Applications Crack detection (tubes, components) Material thickness measurements Coating thickness measurements Conductivity measurements for: o Material identification o Heat damage detection o Case depth determination o Heat treatment monitoring
Application 1: Tube Testing: Analysis of Signals
Tube Testing: Analysis of Signals
Tube Testing: Examples Typical flaws which show up, include: Pin-holes Cross cracks lamination Seam cracks Porosity Hook cracks Lack of fusion Edge damage Open seams
Application 2:Thickness measurement-principle 2 1 3 4
Thickness measurement-example Thickness of thin metal sheet and foil, and of metallic coatings on metallic and nonmetallic substrate Cross-sectional dimensions of cylindrical tubes and rods Thickness of nonmetallic coatings on metallic substrates
Application 3: Coating Thickness Measurement of Thin Conductive Layers It is also possible to measure the thickness of a thin layer of metal on a metallic substrate, provided the two metals have widely differing electrical conductivities. A frequency must be selected such that there is complete eddy current penetration of the layer, but not of the substrate itself. The method has been used successfully for measuring thickness of very thin protective coatings of ferromagnetic metals (i.e. chromium and nickel) on non-ferromagnetic metal bases.
Thickness Measurements of Non-conducting Coatings on Conductive Materials o Principle: The thickness of nonmetallic coatings on metal substrates can be determined using effect of liftoff on impedance. The coating serves as a spacer between the probe and the conductive surface. o Applications: thickness measurement of paint and plastic coatings. o Accuracy: Thickness between 0.5 and 25 µm can be measured to an accuracy between -10%/+ 4%.
Application 4: Surface Crack Detection Excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength Important Considerations A knowledge of probable defect type, position, and orientation. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw. Selection of a reasonable probe drive frequency. For surface flaws, use high freq. for maximum resolution and high sensitivity. For subsurface flaws, use lower frequencies to get the required depth of penetration and this results in less sensitivity. For ferromagnetic or highly conductive materials use of an even lower frequency to arrive at some level of penetration. Reference specimens of similar material as component and with features that represent the defect or condition being inspected
Calibration for crack detection