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.
13
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
Transcript
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
TALAT 4702 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 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).
Training in Aluminium Application Technologies
alu Factors Influencing the Strength of Adhesive Joints 4702.01.01
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 4
Training in Aluminium Application Technologies
alu Types of Stresses on Adhesive Joints
Source: Habenicht
Mechanical Stress:Shear
TensionTensile-Shear
Bending (Peeling)Compression
Torsion
S tre s s in g T y p e
Complex Stress:(Both Types)
Time-DependentStress
Short-Time
Impact, High-Rate Impat
Static
Long-Time
StaticReversedRepeated
Static Dynamic
Physical
Chemical
Environmental Stress:(Aging)
Temperature(Reaction)
Climate gen.Corrosive Climate
4702.01.02
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).
Training in Aluminium Application Technologies
alu 4702.01.03
Strength of Adhesively Joined Metals
Parameters Influencing the Strengthof Adhesive Joints in Metals
Source: Habenicht
Adhesive Layer Joining Material Geometric Design Stress Type
Overlap Length, lü Mechanical
Tensile Strength,Rm
Overlap Width b Physical
Yield Strength,Re
Joining PartThickness, s Chemical
0.2 % Offset YieldStrength, Rp0,2
Adhesive LayerThickness, d
Complex Mech.,Phy., Chem.,
Time-Dependent
Poisson´sContraction
Modulus of Elasticity, EK
Shear Modulus, G
Poisson´s Ratio,µK
Stress-Shearing-Behaviour
Modulus of
Elasticity, EF
TALAT 4702 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).
Training in Aluminium Application Technologies
alu
Stress Distribution in Adhesive Sheet Joints
Stress Distribution in Adhesive Sheet Joint 4702.02.01
Unloaded Joint
τα
Loaded Rigid Joint and Shear Stress Distribution
A B
C D
Loaded Unrigid Joint and Normal Stress Distribution
σα
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 6
Training in Aluminium Application Technologies
alu
Stress of Adhesive Joints in Metals
v1 < v2
Stress Distribution in a Brittle (a) and an Elastic - Plastic (b) Adhesive Layer 4702.02.02
FF
τmax
v2
b )
FF
τmax
v1
a )
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).
Training in Aluminium Application Technologies
alu
Stress of Adhesive Joints in Metals
4702.02.03Distribution of Stress during Peeling
σmax
X
X
F
TALAT 4702 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).
Training in Aluminium Application Technologies
alu
Adhesive Strength as a Function ofOverlap Length in Adhesive Joints with Different Deformation Behaviours
Influence of Overlap Length on Adhesive Strength ( schematic )
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 8
Training in Aluminium Application Technologies
alu
τ Bmax
l ü1 l ü2 l ü3
Strength of Adhesive Joints in Metals
l ü1 l ü2 l ü3
τ Bmaxτ Bm τ Bm
τ Bm
R p 0,2
σ
F
F
F
F
F
F
4702.03.02
l ü
Correlation of Overlap Length and Joining Part Elongation
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 9
Training in Aluminium Application Technologies
alu
Strength of Adhesive Joints in Metals
Source: Brockmann
4702.04.01
1
2
3
0 1,5 3 4,5 6mm
15
30
45
60Nmm-2
Adhe
sive
Stre
ngth
τ B
Thickness of Joint, s
Correlation of Adhesive Strength andJoint 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).
4702.04.02Correlation Between Adhesive Strength and Joining Parts Strength; Examples
Strength of Adhesive Metal Joints
Source: Krekeler, Litz
Adh
esiv
e Jo
int S
treng
th
T B
Tensile Strength Rm
Adhesive EPOXY RESIN
[Nmm-2]
s = 10 mm lÜ = 7.0 mm
AlCuMg2
Al
AlMg3
10
20
30
40
0 100 200 300 400 500[Nmm-2]
31
23
12
110 230 480
Examples for One-Sided Overlap Joints in Different Aluminium Alloys
alu
Training in Aluminium Application Technologies
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 10
The combined occurrence of both types of stresses is especially harmful (Figure 4702.05.01).
Training in Aluminium Application Technologies
alu
100 300 500 700 days
7
14
21
28
0
Phenolic Resins, warm hardening
Epoxy Resins, cold hardening
Stressing Time
Nmm -2
Adhe
sive
Joi
nt S
treng
thtτ
BStrength of Adhesive Metal Joints
Phenolic Resins,warm hardening
Epoxy Resins,cold hardening
3,5
7,0
10,5
14,0
107 70
10 2 103 104 10 5 min
daysTime to Failure of Joint
Nmm -2
Stre
ssσ
a) 52° C, 100 % rel. Humidity, without Mechanical StressingBehaviour of Aluminium Alloy 6060 - T6 under Stressing:
b) 52° C, 100 % rel. Humidity, with Mechanical Stressing
Behaviour of Aluminium Alloy 6060 - T6 under Stressing
Source: Minford
4702.05.01
a) b)
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 11
4702.06.01Deformation 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
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).
4702.06.02
Source: Althof
Number of Cycles N
Rep
eate
d St
ress
T s
chw
Strength of Adhesive Metal Joints15
10
5
0103
[Nmm-2]
Correlation Between Fatigue Strength and Number of Cycles for Different Strength of Joining Parts
Adhesive: Epoxid - Phenol
104 105 106 107 108
X 10 CrNiNb 18 9
AlCuMg 2 pl
alu
Training in Aluminium Application Technologies
TALAT 4702 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 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
