Józef Kuczmaszewski Fundamentals of metal-metal adhesive joint design Lublin University of Technology Polish Academy of Sciences, Lublin Branch Lublin, 2006 Reviewer: prof. dr hab. inż. Robert Sikora Translated by: Tomasz Bylica Cover designed by: Radosław Dolecki © Politechnika Lubelska ISBN 83-89293-11-0 Printed by: WSCHÓD Agencja Usługowa LUBLIN, ul. Długa 5 www.wschod.lublin.pl List of contents 1. Theoretical foundations of adhesion ...........................................................9 1.1. Basic terms .....................................................................................................9 1.2. Adhesion in engineering ..............................................................................10 1.3. Major theories of adhesion ...........................................................................11 1.4. Adhesion in molecular-kinetic approach ......................................................27 1.5. Adhesion in thermodynamic approach .........................................................28 References ....................................................................................................35 2. Adhesives .....................................................................................................39 2.1. General characteristics .................................................................................39 2.2. Classification of adhesives ...........................................................................42 2.3. Applications of adhesives ............................................................................46 2.4. Sealants and renovation putties ....................................................................47 2.5. Adhesive components and modification ......................................................49 2.6. Special adhesives .........................................................................................51 2.7. Conclusions ..................................................................................................54 References ....................................................................................................55 3. Bonding operations ....................................................................................57 3.1. Typical bonding operations...........................................................................57 3.1.1. Preparation of bonded surfaces ..........................................................58 3.1.2. Choice of adhesive .............................................................................79 3.1.3. Preparation of adhesives .....................................................................80 3.1.4. Application of adhesive ......................................................................83 3.1.5. Curing of adhesive ..............................................................................84 3.1.6. Setting (fixing) of bonded elements ...................................................85 3.1.7. Finishing and bond inspection ............................................................86 3.2. Principles of selection of technological parameters of bonding ...................87 References ....................................................................................................88 4. Producibility of bonded structures ...........................................................91 4.1. Factors determining producibility in bonding operations ............................91 4.2. Examples of typical assembly operations ..................................................100 4.3. Conclusions ................................................................................................104 References ..................................................................................................105 5. Strength of bonded joints ........................................................................107 5.1. Stress analysis in bonded joints ..................................................................107 5.1.1. Effect of edge line length .................................................................114 5.1.2. Effect of chamfering of overlap ends ...............................................115 5.1.3. Effect of adhesive flash at overlap ends ...........................................118 5.2. Strength of face-face bonds ........................................................................119 5.3. Strength of trunnion-sleeve bonds ..............................................................122 5.4. Tear-off strength .........................................................................................123 5.5. Compound cases of bonded joint loading ...................................................125 6 J. Kuczmaszewski 5.5.1 Resistance to cracking ......................................................................125 5.5.2. Thermal shock fatigue ......................................................................137 5.5.3 Eccentricity of load ...........................................................................139 5.6. Creep in bonded joints ................................................................................140 5.7. Fatigue strength of bonded joints ................................................................146 5.8. Ageing of adhesive materials ......................................................................147 References ..................................................................................................159 6. Special tooling for bonding operations ...................................................163 6.1. Typical procedures in bonding operations ..................................................163 6.2. Dosage and mixing equipment ...................................................................163 6.3. Adhesive applicators ...................................................................................165 6.4. Fixtures and alignment tools .......................................................................169 6.5. Equipment for application of clamping pressure .......................................171 6.6. Heating equipment ......................................................................................174 6.7. Important tips for engineers designing bonding fixtures ............................175 References ..................................................................................................176 7. Design of bonded joint structures ............................................................177 7.1. Input data for bonded joint design ..............................................................177 7.2. Analysis of stress status ..............................................................................179 7.3. Heuristic model for strength estimation of bonded metal-metal joints .......180 7.4. Safety assessment of bonded structures ......................................................186 7.5. Determination of dimensions of overlap bond ............................................191 References ..................................................................................................192 8. Quality inspection/testing of bonded joints ............................................193 8.1. Testing of adhesives ....................................................................................193 8.2. Testing of adhesive properties of surface layer of bonded elements ..........194 8.3. Non-destructive testing of bonded joints ....................................................197 8.4. Destructive testing of bonded joints ...........................................................201 References ..................................................................................................204 9. Conclusion .................................................................................................207 List of symbols used E – coefficient of elongation elasticity of bonded material m E – coefficient of elongation elasticity of adhesive when cured k G – coefficient of shape elasticity of adhesive k n - Poisson coefficient of adhesive k n – Poisson coefficient of bonded material m q- wetting angle R – tensile strength of bonded materials r g – free surface energy of adhesive l g – free surface energy of bonded material s h - viscosity H – hardness s – internal stresses w t – time R – radius p – pressure K – Boltzmann constant T – temperature P – force S – surface area W – work of adhesion ad V – volume PAC – Poliaminoamid C hardener E57 – Epidian 57 epoxy adhesive t – crippling shear stress n g – thickness of bonded element m g – thickness of adhesive layer k l – overlap length z l – limit overlap length gr k – permissible tensile stress r b – overlap width M – bending moment g J – integral J r – distance between charges G – coefficient of shape elasticity of bonded material W – work of cohesion k Z-1 – triethylenetetramine hardener s – tensile breaking stress m R – mean arithmetic deviation of profile from median line a Chapter 1 Theoretical foundations of adhesion 1.1. Basic terms Etymologically, the term „adhesion” is derived from the Latin word „adhaesio” i.e. tacking or joining. In this context adhesion should be treated as a surface phenomenon that leads to the formation of a new system – an adhesive bond with a complex of specific characteristics determined by the properties of the adhesive and of the materials bonded, and by the occurrence of a division between them [1.6, 1.20]. Cohesion means the inner coherence that a material, the adhesive in the case under consideration, achieves as a result of a variety of physicochemical processes taking place in the course of curing. Adhesive is an organic or inorganic substance that is capable of permanent joining of materials as a result of various physical or physicochemical phenomena occurring at the adhesive-bonded material phase boundary (adhesion) or within the adhesive mass (cohesion). Adhesives can be single- or multi-component preparations. The composition of the adhesive mass, apart from the basic component, may include curing agents or hardeners (for chemically cured adhesives), solvents and thinners, thixotroping agents, dyes, antioxidants, fillers, antistatic agents, softeners, stabilising agents, and others. Curing agents or hardeners are substances which, added to a polymer, enter in a chemical reaction with it to ultimately impart to the substance the state of a solid. Solvents are fluids capable of dissolving other substances, i.e. of forming with those substances a homogeneous system or mixture. Thinners are fluids thinning down the concentration of a substance solution, without having capacity to dissolve. Stabilising agents are compounds that slow down reactions taking place in the processes of ageing of materials, especially of polymers. This concerns primarily the processes of degradation or destruction occurring under the effect of heating, shearing, radiation, or influenced by moisture, fungi [1.16] etc. Plastifiers or softening agents are liquid or solid chemically inert organic substances that interact physically with polymers to reduce intermolecular forces, which leads in consequence to lowering of the brittle point and glass temperature of polymers, of the softening point, hardness and tensile strength, with simultaneous increase of elasticity [31]. Fillers and carriers are auxiliary agents whose function is to modify the properties of the polymer. They are materials of natural or synthetic origin, introduced into the polymer as a matrix and forming with it a suitable composite material with properties altered in the desired direction. 10 J. Kuczmaszewski Antistatic agents are good electrical conductors that are used to reduce or eliminate electrostatic charges on the products. Their action consists in reducing the number of electrostatic charges on material surfaces. Thixotroping agents are substances introduced to polymers in order to impart to them thixotropic properties. Their effect consists in that adhesives in repose are characterised by greater viscosity than during mixing or shaking. [1.16] Separators (anti-adhesive agents) are substances that, introduced into the mass of a polymer or onto its surface, prevent the adhesion of the material to moulds and other equipment; in many cases they also prevent corrosion processes. Antipyrenes are substances reducing combustibility; they are capable of effecting spontaneous extinguishing through the emission of large amounts of non- flammable gases. Adhesion is usually considered in two fundamental systems: solid-liquid, i.e. a system leading in consequence to the formation of an adhesive joint, that is an area of joining of two materials, and solid-liquid-solid, i.e. a system leading to the formation of an adhesive bond, i.e. a connection made up of two joints, where the layer of adhesive material acts as the link. Classic adhesive joints are formed between solid substrates and paints, enamels, printing inks, metallic plastic coatings, plastic coats on metal surfaces, etc. Typical adhesive bonds are connections based on glues, putties, laminated composite materials, sandwich structures, layered composites, etc. 1.2. Adhesion in engineering Adhesion is a phenomenon whose importance in numerous processes is dominant or significant. Special importance characterizes such adhesive bonds in which adhesion plays a dominant role in the constitution of their properties [1.3, 1.22]. This concerns especially such technologies as structural bonding, sealing, and cementing. Such bonds are used more and more frequently as substitute for welding, spot welding, soldering, or forced-in joints [1.10, 1.18, 1.27]. Adhesive bonding as a method of joining has a very long history and dates back to ancient times. Glues of animal origin, especially bone, hide and casein glues, were used to bond papyrus, among other things. Also common were masonry mortars which in essence were glues. Adhesion concerns also numerous technologies related to surface engineering, such as paint and enamel coating, or application of coats with special properties. Moreover, adhesion as a phenomenon plays an important role in such technologies as bimetal production, powder metallurgy, flotation processes, production of laminated composites, and many others. Adhesion is also highly important in the interpretation of numerous processes in engineering. This concerns in particular the techniques and processes of diagnostics as well as the processes of friction and wear [1.1, 1.7, 1.14, 1.25]. Fundamentals of metal-metal adhesive joint design 11 The technological processes under analysis mostly require „high” adhesion, while in the design of kinematic sets of drive units we are interested in such a selection of materials and such a constitution of their properties as to obtain as low levels of adhesive bond forces as possible. What is required, therefore, is adequate knowledge on the possibility of control of adhesive properties of surface layers of structural materials. This problem will be presented in greater detail in subsequent chapters. 1.3. Major theories of adhesion The literature of the subject, often even highly specialized, contains frequent inconsistency in the terminology used. The term “adhesion” is sometimes taken to mean energy and sometimes strength, and “force of adhesion” can also be encountered. At present adhesive forces and bonds are classified as shown in Fig. 1.1 below. Adhesion Mechanical Proper Physical adsorption Chemical adsorption Hydrogen bonds Dispersive Induction Orientation Conformational Fig. 1.1. Classification of adhesive forces and bonds [1.20] Historically speaking, the first theoretical works on adhesion appeared in the nineteen twenties. One of the first hypotheses [1.6] assumes that the condition for the formation of an adhesive bond is the penetration of the adhesive into the micro- pores of the bonded elements and creation of mechanical links capable of transmitting loads. The idea of mechanical adhesion is presented schematically in Fig. 1.2. 12 J. Kuczmaszewski Fig. 1.2. Mechanical adhesion – schematic presentation: 1 – adhesive, 2 – entrapped air, 3 – bonded material Views on the importance of mechanical adhesion changed [1.6, 1.33, 1.36], as new theories were developed. For a period of time its importance was assumed to be negligible, but now it is accepted that in certain operations mechanical adhesion plays a significant role. The value of mechanical adhesion is constituted primarily by the topography of surface. The stereometric parameters of the surface layer play a significant role in the process of adhesive joining of elements. This follows not only from the fact of increase of the actual surface area through its mechanical „development”, but also results from changes in the degree of imbalance of intermolecular forces in the outer nanolayer of atoms and molecules, as shown schematically in Fig. 1.3. Fig. 1.3. Schematic illustration of the relation between the value of free surface energy and the stereometric features of the bonded elements [1.12 ] Fundamentals of metal-metal adhesive joint design 13 Parameter R , commonly used in engineering for the estimation of surface a roughness, does not, however, describe well the stereometric features of the surface from the viewpoint of adhesive properties. At present there are possibilities of more objective estimation of the stereometric features of the surface, employing spatial parameters for its description. It is, however, necessary to point out that excessive „development” of the surface may lead to a weakening of the joint as a whole. Fig. 1.4 presents schematically the surface layer of an aluminium alloy after the operation of etching. Fig. 1.4. Schematic presentation of the surface layer of an aluminium alloy after the operation of etching A surface layer formed as shown in Fig. 1.4 results from incomplete wetting and the formation of a “weakening” transition zone in an adhesive bond. Therefore, in such cases correct pressure in the course of bonding is important. The energy properties of thus prepared surface layer depend on the process parameters, not only of the etching operation but also of auxiliary operations, such as rinsing or drying. Rinsing may lead to closing of pores, and to the formation of hydrates (bayerite, boehmite) significantly changing the spatial parameters of the surface layer, and thus its energy properties. From among the spatial parameters especially important are the longitudinal and lateral spacing of irregularities and the apex angle of irregularities. The concept of free surface energy refers to the surface, but in reality its summary value is constituted by several atom layers, as illustrated schematically in Fig. 1.5.