This lecture describes the factors governing the strength of adhesive joints in order to appreciate these factors for the design of adhesively bonded joints, i.e. geometry of joint, stiffness and strength of the adjoining parts, stress distribution in the adhesive layer as well as the effects of humidity and ageing. General background in production engineering and material science, some knowledge of mechanics and polymer science is assumed.

1. TALAT Lecture 4702
Factors Influencing the Strength of
Adhesive Joints
13 pages, 13 figures
Basic Level
prepared by
Lutz Dorn, Technische Universität, Berlin
Objectives:
− to describe the factors governing the strength of adhesive joints in order to appreciate
these factors for the design of adhesively bonded joints, i.e. geometry of joint,
stiffness and strength of the adjoining parts, stress distribution in the adhesive layer
as well as the effects of humidity and ageing
Prerequisites:
− general background in production engineering and material science
− background in mechanics and polymer science
Date of Issue: 1994
 EAA - European Aluminium Association

2. 4702 Factors Influencing the Strength of Adhesive
Joints
Table of Contents
4702 Factors Influencing the Strength of Adhesive Joints ........................2
4702.01 Basic Factors Governing Strength of Joints.......................................... 3
Interdependence of material and design factors.......................................................3
Loading factors ........................................................................................................3
Summary of influencing parameters ........................................................................4
4702.02 Stress Distributions in Lap Joints .......................................................... 5
Load distribution in adhesive sheet joints................................................................5
Stress distribution in a brittle and an elastic-plastic adhesive layer.........................5
Distribution of stress during peeling........................................................................6
4702.03 Effects of Geometric Parameters............................................................. 7
Influence of overlapping on adhesive joints ............................................................7
Correlation of overlap length and joining part elongation.......................................7
4702.04 Effects of Stiffness and Strength of the Joining Parts .......................... 8
Correlation between adhesive strength and joining part thickness ..........................8
Correlation between adhesive strength and strength of joint parts ..........................9
4702.05 Effects of Ageing under Stress and Humidity ....................................... 9
Behaviour of aluminium alloy 6060 - T6 under stressing........................................9
4702.06 Fatigue Behaviour of Adhesive Joints................................................... 10
Deformation behaviour of adhesive layers under repeated stress ..........................10
Correlation between fatigue strength under repeated stress and a number of cycles
for different strengths of joining parts ...................................................................11
4702.07 Literature/References ............................................................................. 12
4702.08 List of Figures............................................................................................ 13
TALAT 4702 2

3. 4702.01 Basic Factors Governing Strength of Joints
• Interdependence of material and design factors
• Loading factors
• Summary of influencing parameters
Interdependence of material and design factors
The peculiar behaviour of the strength of adhesively joint metals is a result of the fact
that the joint system is not homogeneous but consists instead of a composite system in
which the resulting properties are a combination of the individual properties of the parts
to be joint, the adhesive layer and the interface layers (Figure 4702.01.01).
Strength of Adhesive Metal Joints
Adhesive Layer Surface
Adhesive Joining Materials
Adhesive Joint
Strength of Adhesive Joint
Geometric Design Optimally Constructed Joint Stressing
Source: Habenicht
alu
Factors Influencing the Strength of Adhesive Joints 4702.01.01
Training in Aluminium Application Technologies
The specific properties of the adhesive joint are a result of the strengths obtained due to
the geometrical and material design.
The following statement can serve as a basis for assessing the behaviour of adhesive
joints in metals and consequently in dimensioning and designing such joints.
Loading factors
The overall performance of an adhesive metal joint is characterised by the measure in
which it is able to withstand loads without any appreciable change in its original
strength values (Figure 4702.01.02)
TALAT 4702 3

4. S t r e s s in g T y p e
Environmental Stress:
Mechanical Stress: (Aging)
Shear
Tension
Tensile-Shear Temperature
Complex Stress: Physical
(Reaction)
Bending (Peeling) (Both Types)
Compression Climate gen.
Torsion Corrosive Chemical
Climate
Time-Dependent
Stress
Long-Time
Short-Time Static
Reversed
Impact, High-Rate Impat Repeated
Static Static
Dynamic
Source: Habenicht
alu
Types of Stresses on Adhesive Joints 4702.01.02
Training in Aluminium Application Technologies
Summary of influencing parameters
The combined action of the influencing factors and their parameters are the basis for the
production of an optimal adhesive joint and govern its attainable strength (Figure
4702.01.03).
Strength of Adhesively Joined Metals
Adhesive Layer Joining Material Geometric Design Stress Type
Modulus of Modulus of
Overlap Length, lü Mechanical
Elasticity, EK Elasticity, EF
Tensile Strength,
Shear Modulus, G Overlap Width b Physical
Rm
Poisson´s Ratio, Yield Strength, Joining Part
Chemical
µK Re Thickness, s
Stress-Shearing- 0.2 % Offset Yield Complex Mech.,
Adhesive Layer
Phy., Chem.,
Behaviour Strength, Rp0,2 Thickness, d
Time-Dependent
Poisson´s
Contraction
Source: Habenicht
alu Parameters Influencing the Strength
Training in Aluminium Application Technologies of Adhesive Joints in Metals 4702.01.03
TALAT 4702 4

5. 4702.02 Stress Distributions in Lap Joints
• Load distribution in adhesive sheet joints
• Stress distribution in a brittle and an elastic-plastic adhesive layer
• Distribution of stress during peeling
Load distribution in adhesive sheet joints
The load distribution in one-sided lap joints subjected to tensile-shear loading, depends
on the stiffness of the joint parts and the deformability of the adhesive layer. The
amount of the relative movement of the joint parts is a result of the deformation capacity
of the adhesive layer (Figure 4702.02.01).
Stress Distribution in Adhesive Sheet Joints
τα
Unloaded Joint
A B
σα C D
Loaded Rigid Joint and
Shear Stress Distribution
Loaded Unrigid Joint and Normal Stress Distribution
alu
Stress Distribution in Adhesive Sheet Joint 4702.02.01
Training in Aluminium Application Technologies
Stress distribution in a brittle and an elastic-plastic adhesive layer
Adhesives of the elastic-plastic type cause only low stress peaks at the overlap ends, in
spite of the large relative movements of the joint parts (Figure 4702.02.02).
TALAT 4702 5

6. Stress of Adhesive Joints in Metals
a) τmax b) τmax
F F
F F
v1 v2
v1 < v2
alu Stress Distribution in a Brittle (a) and
an Elastic - Plastic (b) Adhesive Layer 4702.02.02
Training in Aluminium Application Technologies
Distribution of stress during peeling
The occurrence of possible peeling stress during loading has a very major influence on
the strength of adhesive joints. They occur both in tensile tests conducted on lap joints
(due to eccentric loading) as well as in pure peeling tests (with extremely high stress
peaks) (Figure 4702.02.03).
Stress of Adhesive Joints in Metals
F
X
X
σmax
alu
Training in Aluminium Application Technologies
Distribution of Stress during Peeling 4702.02.03
TALAT 4702 6

7. 4702.03 Effects of Geometric Parameters
• Influence of overlapping on adhesive joints
• Correlation of overlap length and joining part elongation
Influence of overlapping on adhesive joints
The strength of narrow (≤ 5 mm) overlapped joints is a result of solely adhesion and
cohesion forces in the adhesive layer. At overlapping lengths exceeding a certain
amount and depending on the geometry and strength of the joint parts and on the
deformation capacity of the adhesive layer, stress peaks, in excess of the strength of the
adhesive layer, occur at the overlap ends causing the strength to fall (Figure
4702.03.01).
Strength of Adhesive Joints in Metals
80
Nmm -2 1
Adhesive Strength τ B
60
2
3
Adhesive Strength τ B
40
20
0 5 10 15 20 25 30 35 40 45 50
Length of Overlap lü mm
Length of Overlap lü 1 = Epoxy Dicynanoamide
2 = Phenolic Polyvinyl Formal
Influence of Overlap Length 3 = PMMA - Neoprene / Styrene
on Adhesive Strength ( schematic )
Adhesive Strength as a Function of
Overlap Length in Adhesive Joints
with Different Deformation Behaviours
Source: Matting
alu
Training in Aluminium Application Technologies
Influence of Overlapping on Adhesive Joints 4702.03.01
Structural materials should not be subjected to stresses exceeding their proportionality
limit. In practice, the limiting stress should be lower than the 0.2 % yield strength.
Correlation of overlap length and joining part elongation
Consequently, the optimal overlapping length, lo2, of adhesive joints is so chosen that
overloading the structural to more than the limiting stress causes a rupture of the
adhesive layer (Figure 4702.03.02).
TALAT 4702 7

8. Strength of Adhesive Joints in Metals
τ Bmax
τ Bm τ Bmax
τ Bm
τ Bm
F F F
l ü1 F l ü2 F l ü3 F
σ
R p 0,2
l ü1 l ü2 l ü3 lü
alu Correlation of Overlap Length and
Training in Aluminium Application Technologies Joining Part Elongation 4702.03.02
4702.04 Effects of Stiffness and Strength of the Joining Parts
• Correlation between adhesive strength and joining part thickness
• Correlation between adhesive strength and strength of joint parts
Correlation between adhesive strength and joining part thickness
The joint part thickness increases the strength of the adhesive joint by increasing both
the stiffness as well as the bending moment of the joint. An increased thickness of the
joint parts also increases the adhesive joint strength (Figure 4702.04.01).
Stress peaks occurring at the overlap ends are lower for thicker joint parts because the
latter leads to a higher rigidity allowing the adhesive layer to accommodate a larger part
of the load.
TALAT 4702 8

9. Strength of Adhesive Joints in Metals
60
Nmm -2
1
45
Adhesive Strength τ B
2
3
30
1 = Epoxy Dicynanoamide
2 = Phenolic Polyvinyl Formal
3 = PMMA - Neoprene / Styrene
15
0 1,5 3 4,5 mm 6
Thickness of Joint, s
Source: Brockmann
alu Correlation of Adhesive Strength and 4702.04.01
Training in Aluminium Application Technologies Joint Thickness
Correlation between adhesive strength and strength of joint parts
These remarks apply in a similar manner to the strength of the joint components (Figure
4702.04.02).
Strength of Adhesive Metal Joints
Examples for One-Sided Overlap Joints in Different Aluminium Alloys
40
[Nmm-2]
AlCuMg2
31
Adhesive Joint Strength TB
30
AlMg3
23
20
Al s = 10 mm lÜ = 7.0 mm
12 Adhesive EPOXY RESIN
10
110 230 480
0 100 200 300 400 [Nmm-2] 500
Tensile Strength Rm
Source: Krekeler, Litz
alu Correlation Between Adhesive Strength and
Training in Aluminium App lication Technologies Joining Parts Strength; Examples 4702.04.02
4702.05 Effects of Ageing under Stress and Humidity
• Behaviour of aluminium alloy 6060 - T6 under stressing
Behaviour of aluminium alloy 6060 - T6 under stressing
The strength of an adhesive joint depends on the thermal and mechanical stress as well
as on the humidity of the environment.
TALAT 4702 9

10. The combined occurrence of both types of stresses is especially harmful (Figure
4702.05.01).
Strength of Adhesive Metal Joints
a) b)
Nmm -2
14,0
Adhesive Joint Strengtht τ B
28
Nmm -2
Phenolic Resins, warm hardening
21 10,5
Stress σ
7,0 Phenolic Resins,
14
warm hardening
Epoxy Resins, cold hardening
7 3,5
Epoxy Resins,
cold hardening
0 100 300 500 700 days 10 10 2 10 3 10 4 10 5 min
Stressing Time
7 70 days
Behaviour of Aluminium Alloy 6060 - T6 under Stressing: Time to Failure of Joint
a) 52° C, 100 % rel. Humidity, without Mechanical Stressing
b) 52° C, 100 % rel. Humidity, with Mechanical Stressing
Source: Minford
Behaviour of Aluminium Alloy 6060 - T6
alu
under Stressing 4702.05.01
Training in Aluminium Application Technologies
4702.06 Fatigue Behaviour of Adhesive Joints
• Deformation behaviour of adhesive layers under repeated stress
• Correlation between fatigue strength under repeated stress and a number
of cycles for different strengths of joining parts
Deformation behaviour of adhesive layers under repeated stress
Adhesive joints with sufficient deformation capacity have a longer operational life than
those with lower deformability. Fatigue strength increases with the static strength of the
adhesive.
At 107 cycles, the fatigue strength is equal to about 14 % of the static short-time strength
(Figure 4702.06.01).
TALAT 4702 10

11. Strength of Adhesive Metal Joints
18
Nmm-2
1
14 1 = Epoxy - Nylon
2 = Epoxy - Polyaminoamide
12
!0
3 = Phenolic - Polyvinyl Formal
Maximum Stress
2
10
9
3
8
7.2
7
1- ! = 50 Nmm-2
B
6 2- B! = 43.4 Nmm-2 6.1
5.7
3 - ! = 37.4 Nmm-2
B
5
104 5 105 5 106 5 107 5 108
Number of Cycles N
Source: Matting, Draugelates
alu Deformation Behaviour of Adhesive Layers
Training in Aluminium Application Technologies under Repeated Stress 4702.06.01
Correlation between fatigue strength under repeated stress and a number of cycles
for different strengths of joining parts
High-strength materials, adhesively joint, attain higher life-cycles under dynamic
loading due to the slight deformation of the adhesive layer. This means that the stress
peaks are lower and the load distribution is more favourable (Figure 4702.06.02).
Strength of Adhesive Metal Joints
15
[Nmm-2]
Adhesive: Epoxid - Phenol
10
Repeated Stress Tschw
X 10 CrNiNb 18 9
AlCuMg 2 pl
5
0
103 104 105 106 107 108
Number of Cycles N
Source: Althof
alu Correlation Between Fatigue Strength and Number of Cycles
Training in Aluminium Application Technologies for Different Strength of Joining Parts 4702.06.02
TALAT 4702 11

12. 4702.07 Literature/References
1. Habenicht, G.: Kleben. Springer-Verlag Berlin-Heidelberg-New York 1990.
2. Crocombe, A.D. and Adams, R.D.: Influence of the spew fillet and other
parameters on the stress distribution in the single lap joint. J.of Adhesion 13 (1981),
pp. 141-155
3. Dorn, L. and Liu, W.: The stress state and failure properties of adhesive-bonded
plastics/metal joints. Int. J. Adhesion and Adhesives Vol. 13 (1993), No.1, pp.
21-31
4. Adams, R.D. , Coppendale, V., Mallick, Al-Hamdan, H.: The effect of
temperature on the strength of adhesive joints. Int. J. Adhesion and Adhesives Vol.
12 (1992), No.3, pp.185-190
5. Bigwood, D.A. and A.D. Crocombe: Non-linear adhesive bonded joint design
analyses. Int. J. Adhesion and Adhesives Vol. 10 (1990), No.1, pp. 31-41
6. Minford, J.D.: Adhesives. In: Durability of Structural Adhesives, Applied Science
Publishers, New York and London 1983, pp. 135-214
7. Matting, A. und Draugelates, U.: Die Schwingfestigkeit von
Metallklebverbindungen. Adhäsion 12 (1968), H. 1, S. 5-22; H. 3, S. 110-134, H. 4,
S. 161-176.
8. Eichhorn, F. und Stockhausen, G.: Langzeit- und Alterungsverhalten hochwertiger
Schmelzklebverbindungen von Metallen. AIF-Abschlußbericht Nr. 5234, Aachen
1984.
9. Hahn, O. und Wender, B.: Beanspruchungsanalyse von geometrisch und
werkstoffmechanisch „unsymmetrischen“ Metallklebverbindungen mit der Finite-
Elemente-Methode. Forschungsbericht des Landes NRW, Nr. 3187. Westdeutscher
Verlag, Opladen 1984.
10. Brockmann, W.: Grundlagen und Stand der Metallklebtechnik. VDI-Verlag,
Düsseldorf 1971.
11. Matz, C.: Klärung der adhäsiven Bindungsmechanismen von strukturellen
Aluminium-Klebverbindungen. Forschungsbericht (LFF 8350 6) des BMFT, 1985
12. Brockmann, W., Hennemann, O.-D., Kollek, H. und Matz, C.: Adhäsion in
Aluminiumklebungen des Flugzeugbaus. Adhäsion 30 (1986) H. 7/8, S. 31-38; H. 9,
S. 24-35; H.10, S. 20-35.
13. Althof, W.: Festigkeit von Metallklebverbindungen bei schwingender
Beanspruchung. DFBO-Mitt. 19 (1968) 5, S. 48-51.
TALAT 4702 12

13. 4702.08 List of Figures
Figure No. Figure Title (Overhead)
4702.01.01 Factors Influencing the Strength of Adhesive Joints
4702.01.02 Types of Stresses on Adhesive Joints
4702.01.03 Parameters Influencing the Strength of Adhesive Joints in Metals
4702.02.01 Stress Distribution in Adhesive Sheet Joints
4702.02.02 Stress Distribution in a Brittle (a) and an Elastic-Plastic (b) Adhesive
Layer
4702.02.03 Chemical Reacting Adhesives
4702.03.01 Influence of Overlapping on Adhesive Joints
4702.03.02 Correlation of Overlap Length and Joining Part Elongation
4702.04.01 Correlation of Adhesive Strength and Joint Part Thickness
4702.04.02 Correlation between Adhesive Strength and Joining Parts Strength;
Examples
4702.05.01 Behaviour of Aluminium Alloy 6060 - T6 under Stressing
4702.06.01 Deformation Behaviour of Adhesive Layers under Repeated Stress
4702.06.02 Correlation Between Fatigue Strength and Number of Cycles for Different
Strengths of Joining Parts
TALAT 4702 13