Flaw detectors use well-established and completely non-destructive ultrasonic technology to pass sound waves through metals, composites, plastics, and ceramics to detect hidden flaws such as cracks, voids and softness which can lead to failure. Sound waves act in highly predictable ways and produce distinctive echo patterns that can be displayed and recorded by portable instruments making them quite useful as an inspection tool.
As an ultrasonic device, flaw detectors use a transducer to both create vibrations and receive the echo that returns. As those vibrations pass through a medium they do so in a predicable direction and at speeds and velocities specific to the medium. When a boundary, such as a new medium or a flaw, is reached, the vibrations will echo back to the transducer, again, in a predictable manner. The returning signal is converted into a waveform pattern that can be analyzed for inconsistencies.
Ultrasonic flaw detection is a comparative technique. Using appropriate reference standards along with a knowledge of sound wave propagation and generally accepted test procedures, a trained operator can identify specific characteristics, including flaws, from the waveform pattern of a test material.
Flaw detectors consist of the instrument and a transducer. Different types of transducers are available for different measuring techniques and applications. There are, generally, two types of measuring techniques:
Straight Beam Testing: Straight beam testing involves placing the transducer perpendicular to the material being tested. It is used to find cracks and delaminations parallel to the surface of the test piece as well as voids and porosity. Straight beam testing can done with four types of transducers, each designed for a specific application:
- Contact Transducers are used in direct contact with the test piece.
- Delay line transducers incorporate a short plastic waveguide or delay line between the active element and the test piece which improves near surface resolution as well as protects the transducer in high temperature applications.
- Dual element transducers utilize separate transmitter and receiver elements in a single assembly which improves resolution detecting porosity or when working with rough surfaces or coarse grained materials.
- Immersion transducers transmit sound energy into the test piece through a water column or water bath which is useful in automated scanning applications as well as in situations where a sharply focused beam is needed to improve flaw resolution.
Angle Beam Testing: Angle beam testing addresses the main problem with flaw detection: flaws perpendicular to the surface of a test piece are usually invisible from straight beams due to their orientation with respect to the sound beam. Angle beam transducers, which utilize plastic wedges to introduce sound energy into the test piece at a selected angle, solve this problem. They are especially useful in weld inspection.
Calibration
Like most test and measurement equipment, flaw detectors need to be periodically calibrated to maintain accuracy. In the case of NDT equipment, calibrations are performed using precision test blocks that simulate conditions and serve as reference standards. Different types of test blocks simulate different conditions. Test blocks can include steps of different thicknesses for use with thickness gauges, holes and other “flaws” for use with flaw detectors, or predetermined hardness for hardness detectors.
Things to Consider When Selecting a Flaw Detector:
- What parameter (thickness, flaw detection, etc.) is being tested for?
- Does the equipment require specialized training?
- What accessories (transducers, couplant, calibration blocks) are required?
- Are there professional standards which must be adhered to?
If you have any questions regarding flaw detectors please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
