TALAT Lecture 4702: Factors Influencing the Strength of Adhesive Joints

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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.

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TALAT Lecture 4702: Factors Influencing the Strength of Adhesive Joints

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

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