An improved joint is described for reducing the peak shear stress in adhesive bonds when adhesives are used to bond two materials which are in a lapped relationship and which differ in value of modulus of elasticity. An insert placed between the adhesive and one of the two materials acts to cushion the discontinuity of material stiffness thereby reducing the peak shear stress in the adhesive bond

Sainsbury-Carter, J. B.

English

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United States Patent Office 3,809,601Patented May 7, 1974
3,809,601
BONDED JOINT AND METHOD
James. C. Fletcher, Administrator of the National Aero-
nautics and Space Administration, with respect to an
invention of John B. Sainsbury-Carter, Newtown, Conn.
Filed Sept. 18,1972, Ser. No. 290,021
Int Cl. B32b 3/02, 3/10
U.S. Cl. 161—116 18 Claims
ABSTRACT OF THE DISCLOSURE
An improved joint for reducing the peak shear stress in
adhesive bonds: when adhesives are used to- bond two ma-
terials which are in a lapped relationship and which dif-
fer in value of modulus of elasticity. An insert placed
between the adhesive and one of the two materials acts
to cushion the discontinuity of material stiffness thereby
reducing the peak shear stress in the adhesive bond.
ORIGIN OF THE INVENTION
The invention described herein was made in- the per-
formance of work under a NASA contract and is subject
to the provisions of Section 305 of the National Aeronau-
tics and Space Act of 1958, Public Law 85-568: (72 Stat.
435;42U.S.C. 2457).
BACKGROUND OF THE INVENTION
The invention relates generally to adhesive bonding and
more specifically involves the reduction of the peak shear
stress in an adhesive joint when an adhesive is used to
bond two materials which are in a lapped relationship and
which have diverse stiffness values or moduli of elasticity.
Previously, whenever an adhesive was used: to bpnd
two-materials of diverse stiffness, the adhesive bond would
be subjected to a high peak shear stress located at. the
lap joint end of the stiffer material. Peak shear stress
above acceptable limits causes shortened bond life,, weak'
bonding and delamination of the adhesive bonds. One
prior art solution to this problem has involved tapering
the stiffer material of the bond joint at the end nearest
the lap joint. Tapering this end reduces the discontinuity
of stiffness values between the two bonded materials
thereby reducing the peak shear stress in. the adhesive
bond. Limitations of the prior art concern the minimum
thickness that can be achieved in the tapered material. For
example, when fibrous composite materials are bonded',
the minimum thickness taper is often inadequate to reduce
the peak shear stress in. the adhesive bond to an acceptable
level. Therefore, there is a definite need in the art for an
improved adhesive bonded joint for connecting materials
having diverse stiffness characteristics.
Accordingly, it is an object of the present invention to-
provide an improved joint for reducing the peak shear
stress in an- adhesive bond when two materials having-
diverse stiffness values or moduli of elasticity are bonded:
by an adhesive.
Another object of the present invention is a method for
reducing the peak shear, stress in an adhesive-bond when
two materials having diverse stiffness values or moduli
of elasticity are bonded by an adhesive.
BRIEF DESCRIPTION OF THE DISCLOSURE
The foregoing and other objects are attainable .in the
present invention by 'providing an undercut surface in the
end face of one of the materials to be joined by the ad-
hesive bond. An insert is then positioned to fit snugly
within this undercut surface with a portion .thereof ex-
tending beyond the -material having the undercut" surface
therein. At least one surface of the insert engages an
adhesive layer.
When an adhesive bonds two materials having diverse
stiffness values or moduli of elasticity, a high peak shear
stress is induced in the adhesive bond.
Use: of this insert provides a graduating- effect between
5 the two bonded materials and thereby reduces the sharp
discontinuity of stiffness between- the two bonded ma-
terials which; in turn,, reduce the peak shear stress in the
adhesive bond to an acceptable level.
. A. more complete appreciation of the present invention
10 and many more of the attendant advantages thereof will
be. readily appreciated as the same becomes better under-
stood by reference to the following detailed description
when considered in connection with the accompanying
drawing wherein:. '
15 FIG. 1 shows a cross sectional view of the novel bonded
joint of the present invention.
FIGS. 2, 3, 4,, and 5 show, various insert modification
forms useful in. the present invention.
DETAILED' DESCRIPTION OF THE INVENTION
20
Referring now to the embodiment of the invention se-
lected for illustration in FIG. 1, the relationship of the
various elements comprising the adhesive bond utilized in
the present invention will now be described.
25 Two materials 10 and 12 having diverse stiffness values
or moduli of elasticity are bonded by. an adhesive layer
14 therebetween. Insert 16 formed of a material having
a stiffness value which does not exceed that of either ma-
terials 10 or 12 is positioned between material 10 and ad-
30, hesive layer 14. One side 17 of'insert 16 fits snugly into
preformed angular undercut 18 formed in material 10. at
the lap joint 20. The base 19 of insert. 16 is in parallel
engagement with adhesive layer 14 and the other side 21
of insert 16 extends beyond material 10 such that, to-
35 gether with the tapered surface 22 of material 10, a planar
surface is formed'.
In a specific example of the invention, material 10 is
formed of essentially 50% boron and 50% epoxy by vol-
ume; material' 12 is 7075-T6 aluminum alloy; adhesive
40' layer 14 is AF l'26-2, a trade name epoxy adhesive manu-
factured'by Minnesota Minnihg and Manufacturing Com-
pany of St. Paul; Minn.; and' insert 16 is formed of es-
sentially 60% fiber glass and 40% epoxy by volume.
In this specific example, the boron/epoxy had the high-
4i> e'st- modulus of elasticity, the aluminum had' the next high-
est modulus of elasticity and the fiber gl'ass/epoxy has
the lowest modulus of'elasticity.
The composition of insert 16 is not considered critical
in any embodiment of the present invention so long as the
50
 modulus of elasticity thereof does not equal' or exceed
that of the stiffest material of the lap joint: In the opera-
tion of this invention, insert 16: provides a cushion or a
soft spot between the two bonded'material's 10 and 12 and
in- effect' acts to spread bond- deformation over a larger
55
 area and thus reduces- the peak shear stress- in the adr
hesive bond.
Referring-now-to FIGS; 2, 3, 4, and 5 various modifica-
tions that cant be taken for the- insert are illustrated and
distinguished, respectively, by reference numerals 26, 36,
60 46, 56 wherein 26. is an- insert with essentially a stepped
triangular crossrsection; 36- is an- insert having a- circular
cross-section;; 46 is an insert with- a diamond cross-section,
and 56 is an insert having a semi-circular cross-section.
It is to be understood that the forms of the invention
65 shown and described herein are to be taken as preferred
embodiments only, and that the invention may be prac-
ticed otherwise than as specifically described; For exam-
ple, although various apparent modifications of the insert
form have been illustrated the invention is not so limited
70
 and other features of the invention will be readily ap-
parent to those skilled in the art without departing from
the spirit and scope of the appended claims.
3,809,601
What is claimed as new and desired to be secured by
Letters Patent of the United States is:
1. An improved joint for reducing the peak shear stress
in an adhesive bond when an adhesive is used to join two
materials in lapped relationship and which materials have
 g
diverse stiffness values or moduli of elasticity, comprising:
first and second materials having diverse stiffness prop-
erties, an adhesive layer for bonding the first and
second materials, in an overlapping relationship,
an insert formed of a third material, 10
an undercut surface formed in the end face of one of
the first and second materials at the lap joint formed
therebetween,
said undercut corresponding in shape to and adapted to
receive a portion of said insert therein, 15
said insert when so positioned in said undercut having
a portion thereof in parallel engagement with the ad-
hesive layer, and another portion thereof extending
beyond the material having the undercut surface
therein, . . 20
said insert thereby providing a cushioning effect be-
tween the two bonded materials and thereby reducing
the peak shear stress induced in the adhesive.
2. An improved joint as in claim 1 wherein said under-
cut surface is angular with respect to the adhesive layer 25
and said insert is provided with essentially a triangular
cross-sectional area.
3. An improved joint as in claim 2 wherein one side of
said insert is adapted to snugly engage said angular under-
cut surface. 30
4. An improved joint as in claim 3 wherein an angular
tapered face is formed on the said material having the
undercut therein and the other side of said triangular in-
sert forms a continuous surface with said angular tapered
face when said triangular insert is positioned within said 35
undercut surface.
5. An improved joint for reducing the peak shear stress
in an adhesive bond when an adhesive is used to join two
materials in a lapped relationship and which materials
have stiffness values'or moduli of elasticity, comprising:
. first and second materials having diverse stiffness prop-
erties,
an adhesive layer for bonding said first and second
materials in an overlapping relationship,
 45
an insert formed of a third material that has a stiffness
value which does not exceed that of said first and
second materials,
an undercut surface formed in the face of one of said
first and second materials at the lap joint formed 59
therebetween,
said undercut corresponding in shape to and adapted to
receive a portion of said insert therein,
said insert when so positioned in said undercut being in'
parallel engagement with the adhesive layer and 55
thereby providing a cushioning effect between the two
bonded materials and reducing the peak shear stress
induced in the adhesive.
6. An improved joint as in claim 5 wherein said under-
cut surface is formed in the face of the material having 60
the highest stiffness value used in the bond.
• 7. An improved joint as in claim 5 wherein the cross-
sectional area of said insert is triangular in form.
8. An improved joint as in claim 5 wherein the cross-
sectional area of said insert is semicircular in form. 65
9. An unproved joint as in claim 5 wherein the cross-
sectional area of said insert is polygonal in form.
40
10. An improved joint as in claim 5 wherein the cross-
sectional area of said insert is circular in form.
' " 11. An improved joint as in claim 5 wherein at least
one surface area of said insert is of stepped configuration.
12. An improved joint as in claim 5 wherein the
bonded material having the highest modulus of elasticity
is boron/epoxy.
13. An improved joint as in claim 5 wherein the-
bonded material having the next highest modulus of elas-
ticity is aluminum.
14. An improved joint as in claim 5 wherein the
bonded material having the lowest modulus of elasticity
is fiber glass/epoxy.
15. A method for reducing the peak shear stress in an
adhesive joint used to bond two materials having diverse
stiffness values or moduli of elasticity, comprising the
steps of:
providing first and second materials to be bonded,
providing an adhesive layer therebetween,
providing an insert made from a material whose modu-
lus of elasticity value does not equal or exceed the
higher modulus of elasticity value of the two bonded
materials,
making an undercut surface in the stiffer of the two
materials in the joint at the end thereof where the
peak shear stress in the adhesive bond occurs, and
positioning said insert in said undercut surface such
that a portion thereof is in parallel engagement with
the adhesive and such that another portion thereof
extends beyond the end of said material having the
undercut surface formed therein.
16. Securing said insert into position permanently by
means of the adhesive bonding material so that said in-
sert provides a cushioning effect between the two bonded
materials and thereby reduces the peak shear stress in-
duced in the adhesive bond.
17. The method of claim 15 wherein the material hav-
ing the highest modulus of elasticity is essentially 50%
boron and 50% epoxy by volume, the material having
the next highest modulus of elasticity is 7075-T6 am--
minum and the material having the lowest modulus of
elasticity is essentially 60% fiber glass and 40% epoxy
by volume.
18. The method of claim 15 wherein adhesive layer
is an epoxy adhesive.
References Cited
UNITED STATES PATENTS
2,751,321 6/1956 Sans 161—254
3,231,450 1/1966 Riel 156—309
3,545,795 12/1970 Hertel 161—38
3,723,214 3/1973 Meraz, Jr. 161—213
OTHER REFERENCES
Economy, High Performance Composites, Chemical
Engineering Progress, October 1969 (vol. 65, No. 10),
pp. 46-49.
WILLIAM J. VAN BALEN, Primary Examiner
S. S. SILVERMAN, Assistant Examiner
1. X.R.
156—250. 38, 39, 118, 145, 149


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