it to receive the bolt.
This reduced its cross-sectional area.
Because the load is constant from one end of the strap to the other
and the hole cannot carry any of the load, the stress in the reduced
area is greatly increased per unit area.
In other words, besides
carrying its normal share of the load it is also carrying the load
that would have been carried by the removed metal. If the force or
load is increased until the strap breaks, the failure will occur at
or near the hole.
Figure 2-1.
Combined Forces Applied.
A member's strength under tension is determined on the basis of its
gross or total area; however, calculations involving tension must
include the net area of the member. Net area is defined as the gross
area minus that removed by drilling holes or by making other changes
in the section. Putting rivets or bolts in the holes makes little or
no difference in added strength.
The rivets or bolts will not
transfer tension force across the holes they are in.
Compression.
Compressive stress (compression) in aircraft is the
force per unit area that shortens or compresses a structural member
at any cross section.
Under compressive force, an undrilled member
is stronger than an identical member that has holes drilled through
it. However, if a plug of equivalent or stronger material is fitted
tightly into a drilled member it will transfer compressive force
across the hole, and the member can carry approximately as great a
load as if there were no hole.
Therefore, with compressive loads,
the gross or total area can be used to determine stress in a member
if all holes are tightly plugged with equivalent or stronger material.
Shear. Shear is the force per unit area that slides
adjacent pieces of material past each other. The term shear is
used because it is sideways stress of the type that is put on a
piece of paper or a sheet of metal when it is cut with a pair of
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