TM 1-1500-204-23-11
core. For these reasons, the use of internal bonded
patches has been limited.
(3) Scarf or Step Bonded Joints. A scarf joint,
(or step joint) machined in the part skin reduces the
stress concentration and the adhesive shear stress at
the edge of the damage cleanup hole. In addition, the
scarf or step joint almost eliminates joint eccentricity
as the patch and part skin neutral axes are nearly
coincident. Adhesive bondline area is provided along
the scarf or step surfaces within the thickness of the
part skin. A scarf or step joint can result in joints as
strong as the original part skin. The machining of the
scarf or step joint in the part skin is time consuming,
must be done with accuracy and removes a large
quantity of sound material. A major disadvantage to
using this type of joint is the need to very accurately
layup and position replacement ply material in the
repair joint. In addition, curing of replacement ply
material can result in signiicantly reduced strength if
not cured in an autoclave. The scarf joint is preferred
over the step joint as it is more eficient from a load
transfer standpoint, and is easier to fabricate. Step
joints have been used for repair of parts fabricated
from woven materials utilizing Kevlar and glass ibers.
d. Bolted Joints. Bolted joints also make use of a
repair patch to carry loads across the damage region.
Fasteners attaching the repair patch to the damaged
structure complete the joint. Three commonly used
bolted repair joints are shown in igure 6-16. A
common misconception is that the purpose of the
fasteners is to hold the patch in place. The fasteners
do provide clamp-up between the plate and the part
surface, but their primary purpose is to allow the
load to be transferred from the original part surface
through the patch. This load is transferred through the
fasteners and patch by shear forces as the fasteners
contact the loaded structure and the plate at the edge
of the fastener hole. These forces are transferred
more eficiently with tighter fastener hole tolerances.
For fastener holes other than interference it fasteners,
a delection in the structure and plate is required for
the fasteners to contact the loaded structure and
the plate. However, interference it fasteners should
not be used in composite structures. The composite
Figure 6-16. Basic Repair Joints (Bolted)
matrix may crack or delaminate resulting in strength
degradation as the interference it fastener is pressed
(1) External Bolted Patch. The external bolted
into place. A Class II it (clearance of +0.0025/-0.000
patch is the easiest bolted repair joint to fabricate.
inch) is normally speciied for structural fasteners in
The external patch overlaps the part skin to allow
suficient fasteners to be installed for load transfer.
Like the external bonded patch, the external bolted
patch is also limited by the eccentricity due to the
offset of patch and part neutral axes. This eccentricity
results in fastener tipping, fastener failure, or bearing
failure in the composite skin/patch material.
