shears.
For example, if two pieces of metal are bolted or riveted
together and sufficient force is applied to opposite ends, the metal
pieces will shear, cut, the bolt.
Bending. Bending is a combination of two forces. Notice in Figure
2-1 that the bending force produces tension on the top of the strap
and compression on the bottom portion. The combined stresses produce
a shear action at the neutral axis. This occurs because these forces
act in opposite directions and are next to each other at the neutral
axis.
Shear action does not take place at the extreme upper and
lower strap surfaces.
Torsion.
Torsion in airframe repair is the force that twists a
structural member.
The stresses arising from this action are shear
ones.
They are caused by adjacent planes rotating past each other
and around a common reference axis at right angles to these planes.
As an example, assume a rod is fixed solidly at one end and is
twisted by a weight placed on a lever arm at the other.
This
produces the equivalent of two equal and opposite forces acting on
the rod at some distance from each other.
These forces create a
shearing action all along the rod, with the rod's centerline
representing the neutral axis.
GENERAL REPAIR PRACTICES
During repair of any major structural member, the structure must be
firmly supported.
The support must enable the work to be completed
without any misalignment or distortion.
If special fixtures to
support the aircraft or any of its components are not available,
temporary supports must be made.
In addition, repair materials,
layout of parts for repair, rivet replacement, and rivet-hole reuse
must be given careful selection and assessment. The paragraphs that
follow discuss repair material, layout of parts for repair, rivet
replacement, and rivet-hole reuse.
Repair Material Selection.
When repairs must be made, the prime
requirement is to duplicate the structure's original strength. To do
this, the repair material must be as strong as that used in the
original.
If it is necessary to substitute a weaker alloy than
originally used, a heavier gage metal must be used to secure
equivalent cross-sectional strength.
A lighter gage material must
never be used, even when using a stronger alloy.
If substituting a
metal becomes necessary, always consult the applicable technical
manual.
Layout of Parts for Repair. Before fitting them into the
aircraft structure, new sections made for repairs must be laid
out to the dimensions given in the applicable TM for that
aircraft. "Laid out" is used here in the sense of being
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