Operating Manual

Chapter 1: Ultrasonic Measurement of Fasteners
Page 6 Guide to Ultrasonic Inspection of Fasteners
It is important to note that in order to change the acous-
tic velocity, stress must be applied in the same direction
traveled by the ultrasonic shock wave. Thus shear and
torsional stress have no effect on the acoustic velocity
when measured along the fastener’s length.
1.1.10 Temperature Coefficient (Cp)
The temperature of a fastener affects its physical length.
As the temperature of a fastener increases, its physical
length increases. In addition, as a fastener’s tempera-
ture increases the amount of time it takes for sound to
travel through the fastener also increases. In other words,
when a fastener is subjected to increased temperature,
its acoustic velocity decreases and, therefore, its ultra-
sonic length increases. In fact, temperature’s affect on
ultrasonic length is even greater than its affect on physi-
cal length. The thermal expansion of the fastener and
the ultrasonic velocity change with temperature are two
separate effects. However, for the purpose of the
BoltMike they are compensated for with a single com-
bined factor known as the
Temperature Coefficient (Cp) .
The Bolt Mike relies on a temperature compensation
system to normalize the measured time of flight (TOF)
and thus correct for temperature-caused changes in its
physical and ultrasonic length. The compensation sys-
tem normalizes the TOF to the value expected at 72
degrees Fahrenheit (22 degrees C) before attempting
to calculate the fastener’s stress, load, and elongation.
This compensation greatly improves accuracy when the
temperature has changed during tightening.
1.1.11 Calibration-Group Correction Factors —
Stress Ratio and Offset
The accuracy of the BoltMike’s stress, load, and elon-
gation calculations depends on many factors. Two ma-
jor influences on the accuracy of these calculations are
the material-property constants inputted and the
fastener’s geometric characteristics.
While the material-property constants (including elastic-
ity, acoustic velocity, and stress factor) are considered
to be standard values, actual material properties vary
widely. This variation is even found among fasteners
produced in the same manufacturer’s lot. The BoltMike’s
accuracy depends partly on the difference between the
fastener’s actual material properties and those proper-
ties represented by the standard material constants.
Similarly, variations in fastening systems’ physical char-
acteristics affect the accuracy of load and elongation
calculations.
When BoltMike III users desire to calculate load, elonga-
tion, stress, or TOF (time of flight) values with a higher
degree of accuracy, they generally choose to create
calibration groups. During the process of creating a cali-
bration group, the BoltMike uses inputted values of ac-
tual tensile load, as well as its own measured load data,
to calculate two correction factors: Stress Ratio and
Stress Offset. These correction factors are used to con-
vert the BoltMike’s raw stress value into a corrected stress,
as shown in Chapter 8 of this guide.
The BoltMike uses one of two methods to determine these
correction factors. The first method, called a regression
correlation, uses a linear regression technique to deter-
mine the stress factor and offset. (Figure 1-6) The stress
factor is actually the slope of a line that represents the
relationship between actual and calculated load. The
stress offset represents the Y intercept of the actual
verses calculated load line. This value can be thought
of as the level to which actual load can increase before
the BoltMike can measure an observable load.
The second method used to determine correction fac-
tors is known as vector correlation. With this approach
the BoltMike calculates only a stress ratio. The value of
the stress offset is set to zero. (Figure 1-6)
When creating a calibration group, the user must de-
cide which correction method to use. This decision should
be based on the application. If accuracy over a wide
range of loads (including low-level loads) is desirable,
the vector correction is usually preferred. If the highest
level of accuracy at a single target load is desired, the
regression method is best.
Why are two methods required? Often the relationship
between actual and measured stress is non-linear,
especially at the low end of the curve (as shown in
Figure 1-6). This can be caused by a skin effect. When
a small amount of load is applied to a fastener, most of
the stress is in the surface layers, not evenly distributed
across the cross-section. Since the longitudinal wave
travels predominantly down the center of the fastener,
less of the actual stress is observed.
1.1.12 Fastener Geometry
Several geometrical characteristics of fasteners affect
the ultrasonic measurement of load, stress, and elonga-
tion. While these characteristics are described in great
detail in Chapter 6 and the Appendix, Figure 1-7 briefly
illustrates them.