Joint Design for
Structural Adhesives
Joint design Specific stress resistance is a major consideration.
Joints should be specifically designed for use with structural adhesives. This is largely a matter of common sense
and experience. Two basic factors should be the design guidelines. First, structural joints should be designed so
that all of the bonded area equally shares the load. Second, the joint configuration should be designed so that
basic stress is primarily in shear of tensile, with cleavage and peel minimized or eliminated. Finally, appropriate
joint thickness should be accounted for in the design (typically, 0.1-0.5mm depending on joint design and
stresses, temperature fluctuations during use, etc).
The following structural joints and their advantages/disadvantages illustrate some typical design alternatives. They
are not, of course, the limit of possible adhesive bonded joints.
Lap joints
are most common. They are most practical and applicable
in bonding thin materials.
The simple lap joint is off-set. This can result in cleavage and peel
stress under load when using thin materials.
The joggle lap joint gives more uniform stress distribution than
either the simple or tapered lap joint. The joint can be formed by
simple metal forming operations. The curing pressure is easily
applied.
A tapered single lap joint is more efficient than a simple lap joint.
The tapered edge allows bending of the joint edge under stress.
The double butt lap joint gives more uniform stress distribution in
the load-bearing area than the above joints. However, this type of
joint requires machining which is not always feasible with thinner
gauge metals.
Double scarf lap joints have better resistance to bending forces
than double butt joints. However, this type of joint also requires
machining.
Angle joints
give rise to either peel or cleavage stress depending on the
gauge metal. Typical approaches to the reduction of cleavage
are illustrated.
Butt joints
A straight butt joint has poor resistance to cleavage. The
following recessed butt joints are recommended: landed scarf
tongue and groove, conventional tongue and groove, and scarf
tongue and groove. Landed scarf tongue and groove joints act as
stops which can control adhesive line thickness. Tongue and
groove are self- aligning during assembly and act as a reservoir
for void filling type adhesives.
Technical Bulletin November 2017
LANDED SCARF
TONGUE AND GROOVE
SCARF
TONGUE AND GROOVE
CONVENTIONAL
TONGUE AND GROOVE
TYPICAL
ADHESIVE BONDED
BUTT JOINT
GOOD
GOOD GOOD GOOD
GOOD
GOOD
GOOD
GOOD
GOOD
FAIR
GOOD
GOOD
Technical Bulletin
Joint Design for Structural Adhesives
Cylindrical Joints
The T-joint and overlap slip joint are typical for bonding cylindrical
parts such as tubing, bushings, and shafts. With adhesive bonding,
all available contact area contributes to carrying the load.
Adhesives also provide a joint with better appearance and
eliminate distortion caused by high welding temperatures.
Corner joints sheet metal
Corner joints can be assembled with adhesives by using simple
supplementary attachments. This permits joining and sealing in a
single operation. Typical designs are right angle butt joints, slip
joints and right angle support joints. These joints increase the
structure s rigidity. Void filling adhesives are most frequently used.
Use of a heat curing adhesive depends on the heat resistance of
the materials being bonded.
With this technique, thin gauge metals or sandwich panels can be
easily formed into boxes, cases, housings, vehicle bodies, metal
boat hulls, etc.
Corner joints Rigid members
Corner joints, as in storm doors or decorative frames, can be
adhesive bonded. End lap joints are the simplest design type
although they require machining. Adhesives requiring pressure
during cure may be utilized in such designs. Mortise and tenon
joints are excellent from a design standpoint but also require
machining. Mitered joint with spline is best if members are hollow
extrusions. In this case, a void filling adhesive is recommended.
Stiffener Joints
Deflection and flutter of thin metal sheets can be minimized with
adhesive bonded stiffeners. When such assemblies are flexed,
peel stresses are exerted on the adhesive. If the flanges on the
stiffening section can bend with the sheet, minimum peel stress
on the bond will result. Increasing sheet gauge or decreasing the
gauge of the stiffener flange will give equivalent results. Stiffening
members such as T-sections, hat sections and corrugated backing
can be used depending on desired rigidity.
T JOINT
End attachment or fitting
bonded to pipe.
Cylindrical joint
for bonding bushing
to sheet metal. Note:
sheet metal is formed to
provide increased resist-
Bonded shaft assembly. ance to cleavage forces.
RIGHT ANGLE
SUPPORT
SLIP JOINT
RIGHT ANGLE BUTT
END LAP MITERED JOINT
WITH SPLINE
MORTISE AND TENON
T SECTION HAT SECTION
CORRUGATED
BACKING
Technical Bulletin
Joint Design for Structural Adhesives
Technical Information
The technical information, recommendations, and other statements contained in this
document are based upon tests or experience that 3M believes are reliable, but the
accuracy or completeness of such information is not guaranteed.
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purpose and suitable for user s method of application.
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Industrial Adhesives and Tapes Division
3M Center, Building 225-3S-06
St. Paul, MN 55144-1000
800-362-3550 877-369-2923 (Fax)
www.3M.com/structuraladhesives
3M is a trademark of 3M
Company.
3M 2017 78-6900-
9939-1