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Techniques - Coggle Diagram
Techniques
Visual techniques
Vision
:check: Easy, cheap, quick
:red_cross: Influenced by environmental factors such as lighting, contrast, glare, viewing angle.
Subjective to person inspecting
Needs direct access to the component
Clean surface required
Field Metallography and replicas
Lab prep of areas of interest, slight over-etching required. Can use field microscope but usually replicas are made
:red_cross: Time consuming and requires experienced personel
Magnetic Particle Inspection
Uses ferrous particle spray and an electromagnet. Part to be inspected is magnetised and ferrous particles are attracted to magnetic flux leakage fields so cluster over discontinuity
:red_cross: Only applicable on ferrous materials
Needs good lighting :bulb:
Needs surface prep sometimes
:check: Applicable for surface and near-surface defects
Dye Penetrant inspection
Reveals surface flaws when coloured or fluorescent dye leaks from flaw. Relies on ability of low viscocity liquid to be drawn into flaw through capillary action. After time period (dwell) excess dye is removed and absorbant developer is used to draw penetrant from the flaw to reveal where.
:check: High sensitivity to small cracks
Few material limitations
Large areas and volumes inspected quickly at low cost
Complex geometries
Indications produced directly onto part
Aerosol spray cans means portable
Inexpensive
:red_cross: Only surface breaking defects
Only non-porous surfaces
Pre-cleaning is essential
Metal smearing must be removed
Inspector requires direct access to surface
Surface roughness effects sensitivity
Multiple operations must be performed
Post cleaning
Chemical handling and correct disposal
Ultrasound based techniques
Ultrasonic testing
Piezoelectric transducer produces ultrasonic waves and transmitted inside teh component using a couplant. Defects will cause variations in time and amplitude of reflection. Frequencies of 0.1 to 50MHz
Features inspected:
Cracks, inclusions, delaminations, laminations, voids, lack of fusion, blow holes, thickness, corrosion, microstructural changes
Snells Law:
When ultrasonic wave passes through two materials at oblique angle, and materials have different indices of refraction, both reflected and refracted waves are produced
Crack tip diffraction is used to estimate length of a crack:
Ultrasonic waves do not originate from single point, instead from all over surface of piezoelectric element
Ultrasonic guided waves
Lower frequency than conventional ultrasonics (20-200kHz), used to inspect large sections in one set up.
:red_cross: Complex due to large number of wave nodes
Applications:
Pipes, Rails, wire cables in bridges, windturbine towers, sheet piling in harbour walls
Acoustic Emission
Piezoelectric transducers mounted on structures surface monitor release of stored energy when structure elastically deforms
:red_cross: Noise from friction, loose parts, impacting dust or rain
Electromagnetic Acoustic Transducers
Uses magnets and coils to produce interrogating ultrasonic waves.
:check: Non-contact so ideal for high temp applications
Dry inspection, less sensitive to surface condition, easier to generate certain wave types
:red_cross: Only applicable for conductive or ferromagnetic materials.
Weaker signal than piezoelectric, Larger size transducers, low efficiency and signal to noise ratio
Lorenz force
Responsible for US waves in non-ferromagnetic materials. Works by placing wire near surface of electrically conducting component and eddy current produced at same frequency of current in wire. Eddy currents will experience lorentz forces causing ions to oscillate and produce US waves
Magnetostriction
Magnetic dipoles are alligned along magnetic domains below curie temperature. Magnetic field is applied to component, net strain in direction of field.
Measuring residual stresses
Can be either tensile, compressive or shear
Measured using synchrotron
How to measure
Destructive:
Slitting, contour, inherent strain, block removal, Sach's boring
Semi-destructive:
Centre-hole drilling
Ring coring
Deep-hole drilling
Electronic speckle pattern interferometry
Non-destructive methods:
XRD
Neutron diffraction
Synchrotron x-ray diffraction
Barkausen noise
Ultrasonics
Info in notes
Electromagnetic Techniques
Magnetic flux leakage
Metal is saturated with magnetic flux until it cannot hold any more. Remaining flux leaks out and is detected by hall effect sensor
:red_cross: Three sensors are needed to measure axial, radial and circumferential parts as MFL is vector
Probe distance from test area needs to be constant
Limited defect sizing capacity
No sudden changes in speed
Used to detect pitting, circumferential cracks, wall losses
:check: No cleaning required
Lead differential coil is employed to detect pitting,
Eddy Current testing
Alternating current applied to conductor, forms magnetic field around which expands and collapses with alternating current. Current is induced in a second conductor
Uses:
Crack detection, thickness measurements, conductivity measurements
Impedance
Total opposition that circuit presents to alternating current. Vector addition must be used to calculate impedance
Alternating Current Potential Drop
Used for quantification of depth of known surface breaking cracks
Introducing an alternating uniform current into test area, current will be undisturbed if no defects present. If defect present then current flows into the crack
Radiographic Techniques
Radiography
Uses gamma ray or x-ray sources, based off thickness variation
Uses higher energy, shorter wavelength
Film selection:
Dependant on composition, shape, size and sometimes weight and location. Also dependent on type of radiation
Angle of defects is also very important
:check: Not limited by material type or density
Can inspect assembled components
Minimum surface preparation
Sensitive to thickness, corrosion, voids, cracks and density
Both surface and sub surface defects
Permanent record
:red_cross: Many safety precautions
Many hours of training
Requires access to both sides of the component
Requires film exposure which is timeconsuming and dangerous
Orientation
Flaw depth is impossible without additional angles
Expensive initial equipment
Thermographic Techniques
Infrared thermography
Measures emissivity of material
