Operating Manual
Chapter 1: Ultrasonic Measurement of Fasteners
Page 4 Guide to Ultrasonic Inspection of Fasteners
1.1.6 Stress
Stress occurs when load is applied to a fastener. When
a tensile load (like the one shown in Figure 1-3) is ap-
plied to a fastener, the tensile stress is equal to the ten-
sile load divided by the fastener’s average cross-sec-
tional area (see the Appendix for average cross-sec-
tional areas). The BoltMike calculates tensile stress in
units of pounds per square inch (psi) or mega Pascal
(MPa). This calculation is performed using the change
in ultrasonic length, the effective length, acoustic veloc-
ity (described in section 1.1.1), the material’s stress fac-
tor (a property that is described below), and stress com-
pensation parameters known as Stress Ratio and Stress
Offset. These are instrument correction parameters that
are described in section 1.1.11.
1.1.7 Elongation
As a tensile load is applied, a fastener stretches in the
same way a spring would. The amount of stretch, known
as
elongation
, is proportional to the tensile load as long
as the load is within the fastener’s working range (which
means at loads that are less than the fastener’s yield
strength – a term we’ll describe shortly). Using the effec-
tive length, the material’s modulus of elasticity, and the
calculated value for corrected stress the BoltMike calcu-
lates elongation. (Figure 1-3)
1.1.8 Modulus of Elasticity (Eo)
When a fastener is loaded with a tensile force, its length
increases. As long as the loading does not approach
the fastener’s
yield strength
(defined as the loading point
beyond which any change in material shape is not com-
pletely reversible), the relationship between the tensile
stress and elongation is linear. By this we mean that if
the stress level increases by a factor of two, the amount
of elongation also increases by a factor of two. For load
levels in the fastener’s elastic region (meaning that the
loads are less than the yield strength of the fastener),
the relationship between stress and elongation is de-
scribed by a material constant known as the
modulus of
elasticity
. The variable Eo in the BoltMike represents the
modulus of elasticity. The concepts of tensile stress, elon-
gation, modulus of elasticity, and yield strength are illus-
trated in Figure 1-4.
FIGURE 1-4—This graph shows the relationship between tensile stress and elongation in a fastener. The material’s
modulus of elasticity equals the slope of the straight portion of this curve (this area is known as the material’s elastic
region). The point at the top of the curve, where it is no longer linear, represents the material’s yield strength. Note
that the graph actually plots stress verses strain. Strain is simply the amount of elongation, divided by the original
length of the stressed section.