Operating Manual

Chapter 1: Ultrasonic Measurement of Fasteners
Guide to Ultrasonic Inspection of Fasteners Page 5
1.1.9 Stress Factor (K)
The velocity at which a longitudinal wave moves through
an object is affected by stress. When a fastener is
stretched there are two influences on its ultrasonic length
(as determined by multiplying the sound wave’s time of
flight by the constant value of acoustic velocity). First,
the length of material through which the sound must travel
increases. Also, the fastener’s actual acoustic velocity
decreases as stress increases. In other words, even
when the stretching effect on the fastener’s physical
length is ignored, tensile stress leads to an increase in
the fastener’s ultrasonic length. In the BoltMike, a mate-
rial constant known as the
Stress Factor (K) compen-
sates for the effect stress has on the fastener’s actual
acoustic velocity.
A great deal of confusion surrounds this effect. Con-
sider the example shown in Figure 1-5 as you read the
following description. In Figure 1-5A, no load is applied
to the fastener when the reference ultrasonic length
(UL1) is recorded. In Figure 1-5B, a load is applied and
a new ultrasonic length (UL2) is recorded. Note that
Figure 1-5A and B also identify the physical length when
unloaded (Physical Length 1) and loaded (Physical
Length 2). The actual physical elongation of the fastener
equals Physical Length 1 – Physical Length 2. The dif-
ference between the ultrasonic lengths (UL1 and UL2)
is about three times the actual physical elongation of
the fastener.
FIGURE 1-5—Applied tensile stress affects the ultrasonic (measured) length of a fastener in two ways. First, it
stretches the fastener, thus increasing the actual length. Second, tensile stress reduces the fastener’s acoustic
velocity, further increasing its ultrasonic length. In the BoltMike, the material constant K (stress factor) is used to
compensate for the effect of tensile stress on acoustic velocity.