NDE_UltraSonicTesting

Introduction

Ultrasonic testing is widely used by industry for quality control and equipment integrity studies. Major uses include flaw detection and wall thickness measurements. When using ultrasonic techniques, it is possible to detect flaws and determine their shape, size and location. Ultrasonic testing uses high frequency acoustic waves generated by piezoelectric transducers. Ultrasonic waves propagate effectively through most structural materials, but are dissipated or reflected by inhomogeneities or discontinuities.

Principle

When a piezoelectric crystal is driven by high voltage electrical pulses the crystal “rings” at its resonant frequency and produces short bursts of high frequency vibrations. These “sound wave trains” generated by the ultrasonic transducer or “search unit” are transmitted into the material being tested. When the search unit is in direct contact with the test material, the technique is known as contact testing.

If flaws or discontinuities are present, an acoustic mismatch occurs and some or all of the ultrasonic energy is reflected back to the search unit. The piezoelectric crystal in the search unit converts the reflected sound wave or “echo” back into electrical pulses. The pulses' amplitude are related to flaw characteristics. Travel time or time of flight, through the material are proportional to the distance of the flaw from the entrant surface. Ultrasonic pulses are also reflected from the back surface of the material and this signal represents the total distance travelled. The pulse received from the back surface can also represent the width, length, or thickness of the material depending upon the orientation. Ultrasonic flaw and ultrasonic thickness testing indications are frequently displayed on a cathode ray tube screen.

Test methods

Ultrasonic testing is performed using one of the following procedures:

  1. Normal incident pulse echo testing: The ultrasonic energy is coupled to the component being inspected through a couplet usually oil or grease or glycerin that transmits the ultrasound between the face of the transducer and the surface of the component. When ultrasonic energy travels through a test sample and strikes a discontinuity, a portion of the energy will be reflected back and the remaining portion propagates in the material in the forward direction. Also a refracted beam from the material is available for interpretation. Ultrasonic energy that has been reflected and returned to the probe is the source of the latest defect indications shown on the instrument screen. The sound energy that travelled completely through the test piece will be reflected at the end giving a large back-echo indication.
  2. Angle beam pulse echo testing: Angle beam transducers provide access to areas that are inaccessible to normal beam probes. Angle beam contact units have the piezoelectric crystal units mounted on a plastic wedge so that ultrasonic beam enters the test material at a predetermined angle to the entrant surface. Angle beam testing must be done when test part geometry precludes straight beam testing. Tests are done using 45°, 60° or 70° angle beam contact probes. Angle beam inspection is accomplished by using shear waves inclined at a specific angle.

Flaw detection equipment

Many types of pulse echo ultrasonic transmitters are available in the market. In a flaw detector, there are three essential working units. There is also a power supply unit that provides direct current at appropriate voltages to the three working units.

The three essential units are as follows: pulse transmitter, receiver amplifier and cathode ray oscilloscope (cathode ray tube plus a saw tooth generator). The pulse transmitter delivers a fast rising, short, high voltage spike. The spike shock excites the piezoelectric transducer, causing it to vibrate at its own resonant frequency. In the receiver amplifier, the echo signals are amplified and filtered to some extent. After amplification at the ultrasonic carrier frequency, the signals are rectified and further amplified before being fed to the deflection plates of the cathode ray tube.

Applications
  1. Weld testing is the largest application of ultrasonic testing.
  2. Testing of semi-finished products in steel and non-ferrous metal industries is done using ultrasonic waves.
  3. Another important application of ultrasonic testing is the finding of dangerous incipient defects, such as fatigue cracks or corrosion in components in service. In power plants, boiler components must be tested regularly for fatigue cracks in welds and for corrosion in certain areas.

Advantages
  1. Testing can be carried out from only one accessible surface unlike in radiography where accessibility for the two opposite sides of the specimen is required.
  2. A large section thickness can be tested at one time.
  3. Results are immediate and on the spot decisions can be made.
  4. Cost involved is cheaper compared to other test methods.

Limitations
  1. The test method is highly operator dependent. Hence, an extremely skilled operator is needed for data interpretation.
  2. Coarser structures, like cast iron and stainless steel, scatter ultrasonic waves very significantly. Hence penetration of ultrasonic waves is drastically reduced in these materials.

Some variables that complicate test analyses are:

  1. Beam spread which is caused by finite transducer size.
  2. Beam attenuation - caused by scattering or dampening.
  3. Geometry - false signals caused by irregular surfaces, fillets or corners.
  4. Beam non-uniformity - caused by near field effects or physical properties.
  5. Defect impedance - dependent on defect size, shape and surface condition.
  6. Noise problems - caused by external RF interference, dirt, grease, air bubbles, or internal signal to noise ratio characteristics of the equipment.