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Case studies in adhesives selection
Journal of Materials Processing Technology 143–144 (2003) 219–224
Case studies in adhesives selection J.C. Suárez b , I. Diez de Ulzúrrun b , M.V. Biezma c , J.M. Ruiz Román d , M.A. Mart´ınez a,∗ , J.C. del Real e , F. López a a
Escuela Politécnica Superior de la Universidad Carlos III de Madrid, Madrid, Spain ETS de Ingenieros Navales de la Universidad Politécnica de Madrid, Madrid, Spain c Escuela Superior de la Marina Civil, Universidad de Cantabria, Madrid, Spain d ETS de Ingenieros de Minas de la Universidad Politécnica de Madrid, Madrid, Spain e ETS de Ingenier´ıa (ICAI), Universidad Pontificia Comillas de Madrid, Madrid, Spain b
Abstract The selection of an adhesive for a particular application is not as easy as endeavour as it might originally appear. To achieve optimum performance when bonding two materials, one must carefully plan every stage of the bonding process. The selection of an adhesive is a critical factor that will influence each step. The adhesive selection will be dependent primarily on: • The type and nature of substrates to be bonded. • The method of curing that are available and practical. • The expected environments and stresses that the joint will undergo in service. The adhesive selection process is difficult because many factors must be considered, and there is no universal adhesive that will fulfil every application. It is usually necessary to compromise when selecting a practical adhesive system. Adhesive properties limit performance. We need a way of surveying properties, to get a feel for the values design-limiting properties can have. One property can be displayed as a ranked list or bar chart. But it is seldom that the performance of a component depends on just one property. Almost always it is a combination of properties, for instance, of the strength-to-shear modulus ratio. This suggests the idea of plotting one property against another, mapping out the fields in property-space occupied by each adhesive type. The resulting charts are helpful in many ways. They condense a large body of information into a compact but accessible form; they reveal correlations between adhesive properties, which aid in checking and estimating data. In this paper, we have also used the methodology from Professor Michael F. Ashby of Cambridge University in order to solve general practical cases where we had chosen adhesive bonding as a joining process. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Materials; Joints; Bonding; Adhesives; Selection
1. Introduction
2. Specific use
The selection of an adhesive for a given structural case must be based on scientific considerations [1,2]. The type and state of an adhesive, its working properties, and the temperature and time required for its cure influence the choice of process for producing a structure. These factors and others pertaining to the material and form of the structure, influence, specifically, the method of application and setting of the adhesive, as well as the assembly and fixturing of the structure [3,5].
Whether a part to be assembled represents a production item or a prototype, or is still in a stage of development, may considerably influence adhesive selection. The cost of the adhesive may sometimes be cause for rejection in a particular production situation. Because the application of heat and/or pressure is not usually convenient or even available in the field, adhesives that are needed for field repair usually must perform without either [3].
3. Application ∗ Corresponding author. Tel.: +34-91-336-7135; fax: +34-91-336-7135. E-mail address: [email protected] (M.A. Mart´ınez).
Although the major function of adhesives is mechanical, that is, to fasten, sometimes they are also required to seal
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00428-X
220
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
and insulate. As already stated, formulations that are good electrical and/or thermal conductors are also available. Further, adhesives prevent electrochemical corrosion in joints between dissimilar metals and resist vibration and fatigue. In addition, unlike mechanical fasteners, adhesives do not generally change the contours of the parts that they join [3].
Having in mind the nature of the substrates, there are tables where the designer can find the adhesive indicated by substrate to be joined, but these adhesives are only candidates, being necessary to make another considerations for the choice of the best adhesive for a given situation.
6. Joint design 4. Manufacturing conditions Primer: This will involve a separate stage in manufacture. Some adhesives will only function on correctly primed surfaces. This is an issue quite separate from the question of whether a primer is desirable for special environmental conditions [7]. Bonding pressure: Choice is restricted if pressure cannot be applied to the joint. Bonding temperature: Room temperature curing adhesives never give as high a lap-shear strength as is obtained if curing is at elevated temperature. The best results demand the correct temperature specified for the adhesive. If the temperature is determined by other factors, then the adhesive must be chosen to fit the temperature. Pot life: Must be checked in relation to process, facilities available, and competence of staff. Form of adhesive: Solid, liquid or paste, if solid in film form is it supported on a carrier? Pastes sometimes contain balotini to ensure correct spacing of the adherends and thus void a ‘starved’ joint. Primers are always low viscosity solutions from which solvent must be allowed to evaporate; they may need a separate cure or partial cure.
5. Materials to be joined Materials to be joined (adherends) influence the choice of adhesive with respect to their mechanical and physical properties, as well as the surface preparation they require prior to joining. Flexible materials should not be joined with a hard, brittle adhesive, nor should critical shapes be joined with a solvent–base adhesive that would tend to distort them. However, an adhesive that lightly attacks one or both of the materials to be joined may be used to reduce normally required surface preparation [3]. The heat and environmental resistance are often of great importance, especially in the case of structural adhesives. Selecting a specific adhesive from a table of general properties is difficult because formulations within one class of adhesive may vary widely in physical properties. General physical data for several common structural metal adhesives are presented in tables. These tables may prove useful in making preliminary selections or eliminating obviously unsuitable adhesives. Once the candidate adhesives are restricted to a few types, the designer can search more efficiently for the best bonding system.
The performance of an adhesive in a joint is very much a function of the type of joint and the mode of loading. Joints that provide a more uniform stress distribution between members are generally stronger and more reliable than those that produce stress concentrations. For this reason, joints are better when loaded in shear and compression than in peel and cleavage. Tough adhesives respond more favorably and resist fracture better than hard, brittle materials under high strain conditions. It is highly desirable that joints be designed specifically for adhesive bonding. Direct conversion of a conventional, mechanically fastened joint to adhesive bonding is usually a poor idea [6]. Ideally, if there is a failure, it should be away from the bondline and in the substrates and at some stress level much higher than the expected load requirements. If failure does not occur in the substrate, then it should occur cohesively within the adhesive material. Possibly, the worst mode of failure is an adhesion failure at the interface. If toughness is required, relatively flexible adhesives with low glass transition temperatures may be chosen. Unfortunately, these adhesives do not posses the high degree of temperature and chemical resistance that more rigid formulations possess. The thickness and strength of the substrates are important factors in selecting an optimum adhesive. Flexible materials, such as rubbers or thin metal, plastic films, etc. are often subject to flexure in service and should not be bonded with rigid, brittle adhesives. Differences in flexibility or thermal expansion between adherends can introduce internal stresses into the glue line. Such stresses can lead to premature bond failure. To a certain extent, stresses can be minimised through joint design, but the performance of the bond is still affected by them. When minimum stress between adherends of the same materials is the objective, it is desirable to choose an adhesive that is similar with respect to rheological properties, thermal expansion, and chemical resistance. This assumes that all other adhesive requirements are met. Frequently, other adhesive properties are more critical for the joint than ultimate bond strength. Generally, rigid adhesives are selected for applications requiring high shear strength where the joint is designed so that the load is usually in shear. These resins are generally highly crosslinked thermosets such as epoxies, acrylics, phenolics and other such polymers. Often an elevated temperature cure is required. These adhesives have very high
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
lap shear strengths but at the expense of low peel and impact strength. Peel and impact strengths of adhesives are related in that adhesives with good peel properties generally will have good impact strength. Thermosetting adhesives tend to form rigid and, therefore, peel strength and impact strength will be lower than for tough rubber/elastomer based and thermoplastic adhesives. Of the epoxies, the polysulfide-epoxies, epoxy-nylons, and polyamide-epoxies have excellent peel properties. Urethanes also have very high peel strength and flexibility. Thickness of the adhesive in the joint (i.e. bond-line thickness) can be a significant parameter in applications requiring a high strength requirement. The highest tensile and shear strength are obtained with high modulus adhesives when the film thickness is a minimum. For maximum peel strength and cleavage, elastic adhesives are chosen. Optimum strengths in these modes of stresses are usually achieved with increased bond thickness. Limitations may be placed on the temperature and/or pressure than can be used because of temperature or pressure sensitive elements in the assembly to be joined.
7. Service conditions Maximum and minimum service temperatures influence adhesive selection because adhesives are temperaturesensitive materials. Although some available adhesives can withstand 400 ◦ C, a limitation of 100 ◦ C is more usual for most types, even thermosetting adhesives, because strengths fall at temperatures considerably above this range. Low service temperatures, on the other hand, lead to the embrittlement of many adhesives and internal stressing of most joints [4]. The adhesive should have a glass transition temperature above the normal operating temperatures. Most adhesives can tolerate excursions of the order of tens of degrees above their glass transition temperature, depending on the loads and duration of time at temperature. However, aerospace companies classify as non-allowable any part having exceed the glass temperature. Usually adhesives that are more rigid have the higher glass transition temperatures. The total amount of time at elevated temperatures is an important criterion. If the joint will only make several excursions of limited time to its maximum operating temperature, then a less temperature resistant adhesive may suffice. If the joint is kept most of its time at the maximum temperature, the elevated temperature resistance and resistance to high temperature oxidation will be an important criteria in the choice of an adhesive. Most probably all adhesives are adversely affected by moisture, especially when in a stressed condition. However, performance under conditions of moisture varies considerably among adhesives. The effect of any fluids
221
present, such as oils, solvents, or hydraulic fluids, must be considered. Adhesives must also be selected with regard to the type of stress and environmental conditions to which they will be exposed. These factors may be the most important criteria in deciding which adhesive to use. Depending on the nature of the stress and the environments expected, these factors could drastically limit the number of candidate adhesives. The designer must be able to confidently predict all the conditions that the adhesive bond will see in service. He or she must also be able to determine the possible effects of these conditions on the bond, in the long term as well as the short term.
8. Mechanical requirements Shear strength: When it is a limiting feature of the design so that adhesives of the highest shear strength must be used, choose a suitable modified epoxy and consider whether peel strength is also required. Cleavage and tension: If these stresses are high then look for an adhesive with high peeling strength. Be prepared to sacrifice some shear strength to obtain it. Impact strength: Look for high peel strength if impact resistance is not separately given in the manufacturer’s data sheets. Two-component adhesives that separate into two phases will, in general, give the highest impact resistance. These are frequently described as ‘toughened’. Deformation: For most designs, deformation caused by shear within the adhesive can be ignored, the deformation of the joint being solely that of the adherends. However, for some precisely dimensioned designs, the modulus may need to be known and the possibility considered that the shear deformation of the adhesive may progressively increase, i.e. it may creep.
9. Charts for the selection of adhesives The selection of the proper adhesive for a given situation depends basically upon materials to be joined, the service requirements, method of adhesive application and costs competitive with other joining methods. The service requirements must be studied thoroughly. Several factors to be considered in describing the bonding application are: type of loading, operating temperature range, chemical resistance, weather and environmental resistance, flexibility, differences in thermal expansion rates, odor or toxicity problems, etc. [8,9]. The adhesive selection process is difficult because many factors must be considered, and there is no universal adhesive that will fulfil every application. It is usually necessary to compromise when selecting a practical adhesive system. Adhesive properties limit performance. We need a way of surveying properties, to get a feel for the values
222
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
Fig. 1. Shear modulus versus fracture toughness.
design-limiting properties can have. One property can be displayed as a ranked list or bar-chart. But it is seldom that the performance of a component depends on just one property. Almost always it is a combination of properties, for instance,
of the strength-to-shear modulus ratio, σ/G, or the fracture toughness-to-shear modulus ratio, KIIC /G. This suggest the idea of plotting one property against another, mapping out the fields in property-space occupied by each adhesive class,
Fig. 2. Shear modulus versus tensile strength.
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
223
Fig. 3. Glass temperature chart.
and the sub-fields occupied by individual adhesives, Figs. 1 and 2. The resulting charts are helpful in many ways. They condense a large body of information into a compact but acces-
sible form, they reveal correlations between adhesive properties, which aid in checking and estimating data and they lend themselves to a performance-optimizing technique, which becomes the basic step of the selection procedure, Figs. 3–5.
Fig. 4. Water absorption chart.
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J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
Fig. 5. Thermal expansion chart.
10. Conclusions
References
The charts for the selection of adhesives are an appropriate method for helping designers to cut through a huge bulk of data and go straight forward to the most promising candidate for a specific application. To obtain the performance index needed for the selection, it is necessary to analyse the particular application to see what are the mechanical requirements on the joint, and the constraints that have to be considered. For the sake of selection, you do not need to analyse in detail the whole structure but rather use a simplified model of the joint and the service conditions. The hard task of the designer is just to imagine a meaningful simplification. The rest of the process of selection is straight forward using the charts.
[1] M.F. Ashby, Materials Selection in Mechanical Design, 2nd ed., Pergamon Press, New York, 1999. [2] D. Cebon, M.F. Ashby, L. Lee-Shothaman, Cambridge Engineering Selector, Version 3.0, User’s Manual, Granta Design Ltd., 1999. [3] L.H. Sharpe, Adhesives and sealants, in: Overview: Adhesives Technology, ASM International, Materials Park, OH, 1990. [4] E.M. Petrie, Handbook of Adhesives and Sealants, McGraw-Hill, New York, 2000. [5] A.V. Pocius, Adhesion and Adhesives Technology, Carl Hanser, Munich, 1997. [6] A.J. Kinloch, Adhesion and Adhesives, Science and Technology, Chapman & Hall, London, 1990. [7] R.D. Adams, W.C. Wake, Structural Adhesive Joints in Engineering, Elsevier, New York, 1986. [8] W.C. Young, Roark’s Formulas for Stress and Strain, 6th ed., McGraw-Hill, New York, 1989. [9] J.C. Suárez, F. López, in: J.M. Mart´ın (Ed.), Uniones Adhesivas Estructurales, vol. VIII.D, CYTED, Red Temática, 2000.
Case studies in adhesives selection J.C. Suárez b , I. Diez de Ulzúrrun b , M.V. Biezma c , J.M. Ruiz Román d , M.A. Mart´ınez a,∗ , J.C. del Real e , F. López a a
Escuela Politécnica Superior de la Universidad Carlos III de Madrid, Madrid, Spain ETS de Ingenieros Navales de la Universidad Politécnica de Madrid, Madrid, Spain c Escuela Superior de la Marina Civil, Universidad de Cantabria, Madrid, Spain d ETS de Ingenieros de Minas de la Universidad Politécnica de Madrid, Madrid, Spain e ETS de Ingenier´ıa (ICAI), Universidad Pontificia Comillas de Madrid, Madrid, Spain b
Abstract The selection of an adhesive for a particular application is not as easy as endeavour as it might originally appear. To achieve optimum performance when bonding two materials, one must carefully plan every stage of the bonding process. The selection of an adhesive is a critical factor that will influence each step. The adhesive selection will be dependent primarily on: • The type and nature of substrates to be bonded. • The method of curing that are available and practical. • The expected environments and stresses that the joint will undergo in service. The adhesive selection process is difficult because many factors must be considered, and there is no universal adhesive that will fulfil every application. It is usually necessary to compromise when selecting a practical adhesive system. Adhesive properties limit performance. We need a way of surveying properties, to get a feel for the values design-limiting properties can have. One property can be displayed as a ranked list or bar chart. But it is seldom that the performance of a component depends on just one property. Almost always it is a combination of properties, for instance, of the strength-to-shear modulus ratio. This suggests the idea of plotting one property against another, mapping out the fields in property-space occupied by each adhesive type. The resulting charts are helpful in many ways. They condense a large body of information into a compact but accessible form; they reveal correlations between adhesive properties, which aid in checking and estimating data. In this paper, we have also used the methodology from Professor Michael F. Ashby of Cambridge University in order to solve general practical cases where we had chosen adhesive bonding as a joining process. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Materials; Joints; Bonding; Adhesives; Selection
1. Introduction
2. Specific use
The selection of an adhesive for a given structural case must be based on scientific considerations [1,2]. The type and state of an adhesive, its working properties, and the temperature and time required for its cure influence the choice of process for producing a structure. These factors and others pertaining to the material and form of the structure, influence, specifically, the method of application and setting of the adhesive, as well as the assembly and fixturing of the structure [3,5].
Whether a part to be assembled represents a production item or a prototype, or is still in a stage of development, may considerably influence adhesive selection. The cost of the adhesive may sometimes be cause for rejection in a particular production situation. Because the application of heat and/or pressure is not usually convenient or even available in the field, adhesives that are needed for field repair usually must perform without either [3].
3. Application ∗ Corresponding author. Tel.: +34-91-336-7135; fax: +34-91-336-7135. E-mail address: [email protected] (M.A. Mart´ınez).
Although the major function of adhesives is mechanical, that is, to fasten, sometimes they are also required to seal
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00428-X
220
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
and insulate. As already stated, formulations that are good electrical and/or thermal conductors are also available. Further, adhesives prevent electrochemical corrosion in joints between dissimilar metals and resist vibration and fatigue. In addition, unlike mechanical fasteners, adhesives do not generally change the contours of the parts that they join [3].
Having in mind the nature of the substrates, there are tables where the designer can find the adhesive indicated by substrate to be joined, but these adhesives are only candidates, being necessary to make another considerations for the choice of the best adhesive for a given situation.
6. Joint design 4. Manufacturing conditions Primer: This will involve a separate stage in manufacture. Some adhesives will only function on correctly primed surfaces. This is an issue quite separate from the question of whether a primer is desirable for special environmental conditions [7]. Bonding pressure: Choice is restricted if pressure cannot be applied to the joint. Bonding temperature: Room temperature curing adhesives never give as high a lap-shear strength as is obtained if curing is at elevated temperature. The best results demand the correct temperature specified for the adhesive. If the temperature is determined by other factors, then the adhesive must be chosen to fit the temperature. Pot life: Must be checked in relation to process, facilities available, and competence of staff. Form of adhesive: Solid, liquid or paste, if solid in film form is it supported on a carrier? Pastes sometimes contain balotini to ensure correct spacing of the adherends and thus void a ‘starved’ joint. Primers are always low viscosity solutions from which solvent must be allowed to evaporate; they may need a separate cure or partial cure.
5. Materials to be joined Materials to be joined (adherends) influence the choice of adhesive with respect to their mechanical and physical properties, as well as the surface preparation they require prior to joining. Flexible materials should not be joined with a hard, brittle adhesive, nor should critical shapes be joined with a solvent–base adhesive that would tend to distort them. However, an adhesive that lightly attacks one or both of the materials to be joined may be used to reduce normally required surface preparation [3]. The heat and environmental resistance are often of great importance, especially in the case of structural adhesives. Selecting a specific adhesive from a table of general properties is difficult because formulations within one class of adhesive may vary widely in physical properties. General physical data for several common structural metal adhesives are presented in tables. These tables may prove useful in making preliminary selections or eliminating obviously unsuitable adhesives. Once the candidate adhesives are restricted to a few types, the designer can search more efficiently for the best bonding system.
The performance of an adhesive in a joint is very much a function of the type of joint and the mode of loading. Joints that provide a more uniform stress distribution between members are generally stronger and more reliable than those that produce stress concentrations. For this reason, joints are better when loaded in shear and compression than in peel and cleavage. Tough adhesives respond more favorably and resist fracture better than hard, brittle materials under high strain conditions. It is highly desirable that joints be designed specifically for adhesive bonding. Direct conversion of a conventional, mechanically fastened joint to adhesive bonding is usually a poor idea [6]. Ideally, if there is a failure, it should be away from the bondline and in the substrates and at some stress level much higher than the expected load requirements. If failure does not occur in the substrate, then it should occur cohesively within the adhesive material. Possibly, the worst mode of failure is an adhesion failure at the interface. If toughness is required, relatively flexible adhesives with low glass transition temperatures may be chosen. Unfortunately, these adhesives do not posses the high degree of temperature and chemical resistance that more rigid formulations possess. The thickness and strength of the substrates are important factors in selecting an optimum adhesive. Flexible materials, such as rubbers or thin metal, plastic films, etc. are often subject to flexure in service and should not be bonded with rigid, brittle adhesives. Differences in flexibility or thermal expansion between adherends can introduce internal stresses into the glue line. Such stresses can lead to premature bond failure. To a certain extent, stresses can be minimised through joint design, but the performance of the bond is still affected by them. When minimum stress between adherends of the same materials is the objective, it is desirable to choose an adhesive that is similar with respect to rheological properties, thermal expansion, and chemical resistance. This assumes that all other adhesive requirements are met. Frequently, other adhesive properties are more critical for the joint than ultimate bond strength. Generally, rigid adhesives are selected for applications requiring high shear strength where the joint is designed so that the load is usually in shear. These resins are generally highly crosslinked thermosets such as epoxies, acrylics, phenolics and other such polymers. Often an elevated temperature cure is required. These adhesives have very high
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
lap shear strengths but at the expense of low peel and impact strength. Peel and impact strengths of adhesives are related in that adhesives with good peel properties generally will have good impact strength. Thermosetting adhesives tend to form rigid and, therefore, peel strength and impact strength will be lower than for tough rubber/elastomer based and thermoplastic adhesives. Of the epoxies, the polysulfide-epoxies, epoxy-nylons, and polyamide-epoxies have excellent peel properties. Urethanes also have very high peel strength and flexibility. Thickness of the adhesive in the joint (i.e. bond-line thickness) can be a significant parameter in applications requiring a high strength requirement. The highest tensile and shear strength are obtained with high modulus adhesives when the film thickness is a minimum. For maximum peel strength and cleavage, elastic adhesives are chosen. Optimum strengths in these modes of stresses are usually achieved with increased bond thickness. Limitations may be placed on the temperature and/or pressure than can be used because of temperature or pressure sensitive elements in the assembly to be joined.
7. Service conditions Maximum and minimum service temperatures influence adhesive selection because adhesives are temperaturesensitive materials. Although some available adhesives can withstand 400 ◦ C, a limitation of 100 ◦ C is more usual for most types, even thermosetting adhesives, because strengths fall at temperatures considerably above this range. Low service temperatures, on the other hand, lead to the embrittlement of many adhesives and internal stressing of most joints [4]. The adhesive should have a glass transition temperature above the normal operating temperatures. Most adhesives can tolerate excursions of the order of tens of degrees above their glass transition temperature, depending on the loads and duration of time at temperature. However, aerospace companies classify as non-allowable any part having exceed the glass temperature. Usually adhesives that are more rigid have the higher glass transition temperatures. The total amount of time at elevated temperatures is an important criterion. If the joint will only make several excursions of limited time to its maximum operating temperature, then a less temperature resistant adhesive may suffice. If the joint is kept most of its time at the maximum temperature, the elevated temperature resistance and resistance to high temperature oxidation will be an important criteria in the choice of an adhesive. Most probably all adhesives are adversely affected by moisture, especially when in a stressed condition. However, performance under conditions of moisture varies considerably among adhesives. The effect of any fluids
221
present, such as oils, solvents, or hydraulic fluids, must be considered. Adhesives must also be selected with regard to the type of stress and environmental conditions to which they will be exposed. These factors may be the most important criteria in deciding which adhesive to use. Depending on the nature of the stress and the environments expected, these factors could drastically limit the number of candidate adhesives. The designer must be able to confidently predict all the conditions that the adhesive bond will see in service. He or she must also be able to determine the possible effects of these conditions on the bond, in the long term as well as the short term.
8. Mechanical requirements Shear strength: When it is a limiting feature of the design so that adhesives of the highest shear strength must be used, choose a suitable modified epoxy and consider whether peel strength is also required. Cleavage and tension: If these stresses are high then look for an adhesive with high peeling strength. Be prepared to sacrifice some shear strength to obtain it. Impact strength: Look for high peel strength if impact resistance is not separately given in the manufacturer’s data sheets. Two-component adhesives that separate into two phases will, in general, give the highest impact resistance. These are frequently described as ‘toughened’. Deformation: For most designs, deformation caused by shear within the adhesive can be ignored, the deformation of the joint being solely that of the adherends. However, for some precisely dimensioned designs, the modulus may need to be known and the possibility considered that the shear deformation of the adhesive may progressively increase, i.e. it may creep.
9. Charts for the selection of adhesives The selection of the proper adhesive for a given situation depends basically upon materials to be joined, the service requirements, method of adhesive application and costs competitive with other joining methods. The service requirements must be studied thoroughly. Several factors to be considered in describing the bonding application are: type of loading, operating temperature range, chemical resistance, weather and environmental resistance, flexibility, differences in thermal expansion rates, odor or toxicity problems, etc. [8,9]. The adhesive selection process is difficult because many factors must be considered, and there is no universal adhesive that will fulfil every application. It is usually necessary to compromise when selecting a practical adhesive system. Adhesive properties limit performance. We need a way of surveying properties, to get a feel for the values
222
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
Fig. 1. Shear modulus versus fracture toughness.
design-limiting properties can have. One property can be displayed as a ranked list or bar-chart. But it is seldom that the performance of a component depends on just one property. Almost always it is a combination of properties, for instance,
of the strength-to-shear modulus ratio, σ/G, or the fracture toughness-to-shear modulus ratio, KIIC /G. This suggest the idea of plotting one property against another, mapping out the fields in property-space occupied by each adhesive class,
Fig. 2. Shear modulus versus tensile strength.
J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
223
Fig. 3. Glass temperature chart.
and the sub-fields occupied by individual adhesives, Figs. 1 and 2. The resulting charts are helpful in many ways. They condense a large body of information into a compact but acces-
sible form, they reveal correlations between adhesive properties, which aid in checking and estimating data and they lend themselves to a performance-optimizing technique, which becomes the basic step of the selection procedure, Figs. 3–5.
Fig. 4. Water absorption chart.
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J.C. Su´arez et al. / Journal of Materials Processing Technology 143–144 (2003) 219–224
Fig. 5. Thermal expansion chart.
10. Conclusions
References
The charts for the selection of adhesives are an appropriate method for helping designers to cut through a huge bulk of data and go straight forward to the most promising candidate for a specific application. To obtain the performance index needed for the selection, it is necessary to analyse the particular application to see what are the mechanical requirements on the joint, and the constraints that have to be considered. For the sake of selection, you do not need to analyse in detail the whole structure but rather use a simplified model of the joint and the service conditions. The hard task of the designer is just to imagine a meaningful simplification. The rest of the process of selection is straight forward using the charts.
[1] M.F. Ashby, Materials Selection in Mechanical Design, 2nd ed., Pergamon Press, New York, 1999. [2] D. Cebon, M.F. Ashby, L. Lee-Shothaman, Cambridge Engineering Selector, Version 3.0, User’s Manual, Granta Design Ltd., 1999. [3] L.H. Sharpe, Adhesives and sealants, in: Overview: Adhesives Technology, ASM International, Materials Park, OH, 1990. [4] E.M. Petrie, Handbook of Adhesives and Sealants, McGraw-Hill, New York, 2000. [5] A.V. Pocius, Adhesion and Adhesives Technology, Carl Hanser, Munich, 1997. [6] A.J. Kinloch, Adhesion and Adhesives, Science and Technology, Chapman & Hall, London, 1990. [7] R.D. Adams, W.C. Wake, Structural Adhesive Joints in Engineering, Elsevier, New York, 1986. [8] W.C. Young, Roark’s Formulas for Stress and Strain, 6th ed., McGraw-Hill, New York, 1989. [9] J.C. Suárez, F. López, in: J.M. Mart´ın (Ed.), Uniones Adhesivas Estructurales, vol. VIII.D, CYTED, Red Temática, 2000.