ULTRASONIC INSPECTION

ULTRASONIC INSPECTION

1. PRINCIPLES

   • Sound Waves
     • Human Hearing 5Hz - 15KHz
     • Mechanical vibrations
     • Ultrasonic 500KHz - 15MHz, Weld Test 2.25MHz
     • Wavelength is shortest distance between 2 particles in a wave
     • (V = λf) Velocity = Wavelength x Frequency
     • Amplitude & Power E ∝ (Af)square
     • Double Amplitude is 6 dB, 20 dB increase means 10x Amplitude
     • Attenuation
     • Tranducer
     • Piezio electric, Thickness size
   • Transducer
     • Piezoelectric : Elec to Mech
     • 'Natural' frequency : direct function of thickness
     • Probe consists of: Case, wiring, damping, electrode, wear plate.
     • EMAT : Electromag Acoustic T, have adv that direct contact not required, but low power = not widely used
     • Pulse-Echo : Transducer acts as transmit and recieve
     • Couplant eliminates air gaps

2. EQUIPMENT

   • Cathode Ray Tube
     • To maintain a stationary display, UT pulses are 30 to 1000 pulses per sec. To synchronize CRT w/pulse transmission, circuit includes a "timebase" that controls CRT sweep and pulse generator.
     • Vertical beam deflection
       • Amplifier
         • Probe
           • Pulse Generator
             • Time base
               • Oscillator
     • Horizontal beam deflection
   • A-Scan
     • Traces signal amplitude vs. time
     • Timebase sweeps CRT bem across screen in Horiz, while UT signal goes Vert
     • Base has 2 adjustments: Delay & Scale
       • Scale permits the timebase to represent any convenient distance
       • Delay allows selection of initial depth represented on screen
     • Dead zone close to transducer due to finite duration of pulse; thin materials difficult
   • B-Scan
     • X-sect view
     • Length & Depth
   • C-Scan
     • Plan view
     • Size & Shape; NOT Depth
   • P-Scan
     • Projection Image
     • 3 views; Top, End, Side
   • Water is coupling
     • Immersion Testing
     • Movable "Shoe"
   • Acoustic Emission
     • Detection of small amounts of Mech energy released from cracks or discontinuities under load
     • ADV; Triangulation, source determination, is a useful system for monitoring during welding
   • Wave modes
     • Longitudinal (compression)
       • Energy carried by particles back & forth same direction of wave
     • Transverse (Shear)
       • Particles move back & forth at right angles to direction wave is moving Only carried in solids
   • Snell's Law: Sin(Theta)/Velocity
     • Angle of incidence / Velocity in medium
   • Sensitivity
     • Ability to detect small reflectors shorter the wavelength = More sensitive
   • Resolution
     • Ability to distinguish between two closely spaced reflectors
   • Beam Spread
     • related to size of transducer and ratio of wavelength and size

3. APPLICATION

   • Features
     • ADV: High sensitivity, penetrating power, accuracy, fast response, one side access,
     • DisAdv: Unfavourable geometry, Undesirable internal structure, skill technician
   • Normal Beam Probe
     • Longitudinal Waves
     • Lamination detection
     • Not for critical butt joints
   • Angle Beam Probe
     • Normal transducers mounted on angle
   • Difficult Materials
     • Large grain size = Low frequencies (~1 MHz)
     • Austenitic SS

4. PROCEDURES & CODE REQS

   • Equipment Calibration
     • Relative intensity of UT reflections is measured using gain control (attenuator to find dB change
     • Horiz Linearity = rate which CRT trace is steady within 5%
     • Accurate location of exit (index point) determines position of discontinuities done with IIW block
     • Distance Calibration = through thickness and sweep adjustment
     • Sensitivity: Vertical deflection represents Amplitude of a reflection, and sensitivity is height adjust on screen. Flat bottom holes, Side holes, Slots are used (Transverse = 1.5mm for W59). Longitudinal = Back Wall.
     • Resolution checked by closely space reflector block
     • DAC Curve; constructed by succesive position of probe for longer distances
   • Detection, Location, Identification, Measurement
   • Discontinuities
     • Spherical (Single pore, widely spaced)
       • Same amplitude reflection from any direction
     • Cylindrical (Hollow beads, slag lines)
       • Constant amplitude for normal directions to axis of cylinder
     • Planar (Lack-of-fusion, cracks)
       • Drop in amplitude when beam is NOT NORMAL to plane of discontinuity
   • Acceptance/Rejection based on amplitude and length
     • Calibrate sensitivity 1.5mm side hole, adjust height 50-75% full screen to provide reference. 'b'
     • Attenuation factor 1dB per 12.7mm > 25.4mm. c = (sound path - 25.4) / 12.7
     • Indication rating 'd' = a - b - c
     • a & b can be +/- depending if gain /attenuation
     • Adjust gain control to bring peak of indication to ref level. 'a'