4
en
bolt, thus causing potentially erroneous measurements. To avoid transducer placement errors, be
consistent and as methodical in your methods as possible.
Note: Do Not press while measuring elongation, as this will activate a high speed mode used
specifically with the Elcometer NDT shut-off box. The AUTO SET function (see “Auto Set” on page 29) is
only used when measuring reference lengths. If is accidentally pressed, press it again to switch the
high speed mode off.
4 ULTRASONICS - OVERVIEW
4.1 ULTRASONIC MEASUREMENT OF BOLTS
Ultrasonic measurement of bolts has proven to be the most reliable and cost effective solution when:
• Variations in friction or joint geometry prevent applied torque from controlling the clamping force
produced by the fastener with the required accuracy.
• The clamping force must be monitored over the service life of the bolt.
Ultrasonic measurement of clamping load is obtained through a predictable decrease in the sound velocity
within the body of the bolt as the tensile load is increased. By introducing a sonic pulse at one end of the
bolt and accurately measuring the time required for the echo to return from the opposite end, the ultrasonic
length is determined. As the fastener is tightened, the change in this ultrasonic length is used to calculate
and display the clamping force produced.
The physics governing this process are clearly understood, and have been employed for many years in the
fields of active sonar, or radar. Send a pulse of energy toward an object (in this case the opposite or
reflecting end of the fastener), and then measure the time between the initial pulse and the returning echo.
While the concept is comparatively simple and ultrasonic measurement can produce extremely accurate
results, the selection of the optimum transducer for the bolt being measured, and their coupling can be
difficult. Your gauge minimises these difficulties to the greatest extent possible:
• The variable width pulser system can send the maximum amount of energy to the ultrasonic
transducer, allowing the broadest possible range of transducers for a given application.
• The low noise and gain features of the receiver system allow signal detection and measurement in the
most difficult applications.
• The digital signal processor optimises the measurement process.
4.2 ULTRASONIC WAVES
Ultrasonic measurement requires the transmission of a suitable quantity of ultrasonic energy through the
length of the bolt. The relationship of the energy pulse frequency to its penetration is important in energy
transmission:
• Lower frequencies produce longer wavelengths that travel further through a given substance.
• Higher frequencies produce shorter wavelengths.
To use a familiar example: AM radio signals are broadcast at relatively low frequencies and can be received
hundreds of miles away, over the horizon. Higher quality FM radio and television signals are broadcast at
much higher frequencies, and can only be received within a comparatively short distance. The same
phenomenon exists with ultrasonic energy. A low frequency 1 MHz pulse travels much further through metal
than a 5 MHz pulse. Therefore, a lower frequency transducer is able to achieve an echo in a longer bolt, or
in a bolt made of metal with higher resistance to sound transmission (attenuation). While the lower
frequency has more penetration power, it also produces more unwanted noise. Low frequency energy tends
to spread, much like an unfocussed beam of light. When low frequency energy is introduced at the end of
a bolt, a significant portion is bounced from side to side within the cylindrical shape, producing a noisy and
distorted echo. Higher frequency pulses on the other hand tend to travel more directly down and back the
centreline of a bolt, with less noise and distortion.
To achieve a good balance between maximum frequency and noise suppression you must use a transducer
suitable for measurement of bolts. The diameter of the transducer (which is generally specified by the
diameter of the piezoelectric crystal) directly effects energy transmission; larger diameter crystals have
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