Operating Manual

Chapter 1: Ultrasonic Measurement of Fasteners
Page 8 Guide to Ultrasonic Inspection of Fasteners
The BoltMike measures the time it takes for a sound wave
to travel through a fastener. The sound wave, more spe-
cifically known as an ultrasonic shock wave or longitudi-
nal wave, is created in the transducer. The wave is gen-
erated when a large electric pulse is sent to the trans-
ducer from the instrument. This pulse excites a piezo-
electric element in the transducer. The wave’s frequency
varies with the thickness of the piezoelectric element.
Frequencies most useful for measuring fasteners range
from 1 to 20 MHz.
This range of ultrasound will not travel in air. Couplant,
which is a dense liquid substance (usually glycerin or
oil) must be used to provide a pathway for the ultrasound
to travel from the transducer into the fastener.
When the ultrasonic wave encounters an abrupt change
in material density, such as at the end of the fastener,
most of the wave reflects. This reflection travels back
the length of the fastener, through the layer of couplant,
and back into the transducer. When the shock wave
enters the piezoelectric element a small electrical signal
is produced. The BoltMike detects this signal.
In I.P. mode (Initial Pulse mode is described in section
1.1.3), the BoltMike measures the elapsed time between
the sound entering the material and the returned signal.
This elapsed time is known as the wave’s time of flight.
Of course the time of flight actually represents the time
taken by the wave to travel the length of the fastener
two times. The TOF reported by the BoltMike equals half
of this value.
In M.E. mode (Multi-Echo mode is described in section
1.1.3), the BoltMike measures the elapsed time between
two consecutive returning signals. This elapsed time is
equal to the wave’s time of flight. As in I.P. mode this time
of flight actually represents the time taken by the wave
to travel the length of the fastener two times. The TOF
reported by the BoltMike equals half of this value.
The BoltMike then determines the
ultrasonic length by
first using the temperature coefficient (Cp) to correct the
TOF for any changes in temperature. The BoltMike then
multiplies the corrected TOF by the fastener’s acoustic
velocity. Acoustic velocity is represented in the BoltMike
with the variable V and is determined by the fastener’s
material type. The stress constant (K) and effective length
are then used by the BoltMike logic to determine an un-
corrected stress. As explained in Chapter 8, when the
calibration-group feature is used, the stress ratio and
offset are applied to this stress value to find a corrected
stress.
Since the actual acoustic velocity is not truly a constant,
and can vary significantly between fasteners of like ma-
terial composition, the
change in measured time of flight
(recorded before and after each fastener is tensioned)
must be used to accurately measure a fastener’s stress,
load, and elongation.
To determine the change in time of flight, the BoltMike
first records a
reference length by determining a nor-
malized time of flight for a non-tensioned fastener. A
normalized time of flight measurement of the same fas-
tener, this time while tensioned, is then recorded. The
two normalized TOF’s (which have already been cor-
rected for the effects of temperature) are then used with
the effective length, stress factor (K), and acoustic ve-
locity (V) to determine the uncorrected stress.
• The uncorrected stress is then corrected using the
stress offset and stress ratio (these values are
produced using a Cal group)
• Elongation is calculated using the corrected stress,
effective length, and the modulus of elasticity.
• Load is also determined using the corrected stress
and cross-sectional area.
1.3 Practical Limitations Of Ultrasonic
Measurement
Included in the list of fastening-system types that are
quite successfully inspected using ultrasonic techniques
are those where equal distribution of load is critical, such
as pipe flanges and head bolts where gaskets must be
compressed evenly for optimum performance.
Not all threaded fastening systems are suitable for mea-
surement by ultrasonic methods, and some systems are
better suited to either multi-echo or initial pulse mea-
surements. An understanding of ultrasonic inspection’s
practical limitations will reduce frustration and errone-
ous results.
1.3.1 Material Compatible with Ultrasonic
Inspection
Most metals are excellent conductors of ultrasound. How-
ever, certain cast irons and many plastics absorb ultra-
sound and cannot be measured with the BoltMike.
1.3.2 Significant Fastener Stretch
Since ultrasonic techniques measure a fastener’s
change in length, a significant amount of stretch is re-
quired to produce accurate measurements. Accuracy is
a significant problem in applications where the effective
length of a fastener is very short, such as a screw hold-
ing a piece of sheet metal. These applications may be
poorly suited to ultrasonic measurement because the
tensile load (and therefore tensile stress) is applied over
a very short effective length of the fastener. Because