Testing Adhesive Joints E-Book
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- Herausgeber: Wiley-VCH
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- Sprache: Englisch
Joining techniques such as welding, brazing, riveting and screwing are used by industry all over the world on a daily basis. A further
method of joining has also proven to be highly successful: adhesive bonding. Adhesive bonding technology has an extremely broad range
of applications. And it is difficult to imagine a product - in the home, in industry, in transportation, or anywhere else for that matter - that
does not use adhesives or sealants in some manner. The book focuses on the methodology used for fabricating and testing adhesive and bonded joint specimens. The text covers a wide range of test methods that are used in the field of adhesives, providing vital information for dealing with the range of adhesive properties that are of interest to the adhesive community. With contributions from many experts in the field, the entire breadth of industrial laboratory examples, utilizing different best practice techniques are discussed. The core concept of the book is to provide essential information vital for producing and characterizing adhesives and adhesively bonded joints.
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Veröffentlichungsjahr: 2012
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Table of Contents
Related Titles
Title Page
Copyright
About the Editors
List of Contributors
Chapter 1: Manufacture of Quality Specimens
1.1 Preparing Bulk Specimens by Hydrostatic Pressure
1.2 Preparing Bulk Specimens by Injection
1.3 Preparing Bulk Specimens by Pouring
1.4 Preparing Lap Joints with Flat Adherends
1.5 Simple Methods for the Preparation of Single Lap Joints Specimens
1.6 Preparing Thick Adherend Shear Test Specimens
1.7 Modified Thick Adherend Shear Test
1.8 Preparing Butt Joints
1.9 Preparing Napkin Ring Specimens
1.10 Preparing T Joint Specimens
1.11 Preparing Flexible-to-Rigid Peel Specimens
1.12 Preparing Specimens for Fracture Properties: Double Cantilever Beam and Tapered Double Cantilever Beam
1.13 Preparing Bonded Wood Double Cantilever Beam (DCB) Specimens
1.14 Modified Arcan Test
References
Chapter 2: Quasi-Static Constitutive and Strength Tests
2.1 Quasi-Static Testing of Bulk Tensile Specimens
2.2 Uniaxial and Bulk Compression
2.3 Quasi-Static Testing of Bulk Compression on Flat Specimens
2.4 Iosipescu (V-Notched Beam) Test
2.5 Arcan (V-Notched Plate) Test
2.6 Quasi-Static Testing of Butt Joints in Tension
2.7 Shear Properties of Adhesives Measured by Napkin Rings and Solid Butt Joints in Torsion
2.8 Quasi-Static Testing of Thick Adherend Shear Test Specimens
2.9 Modified Thick Adherend Shear Test
2.10 Quasi-Static Testing of Lap Joints
2.11 Modified Arcan Test
2.12 Pin-and-Collar Test Method
References
Chapter 3: Quasi-Static Fracture Tests
3.1 Measuring Bulk Fracture Toughness
3.2 Quasi-Static Fracture Tests: Double Cantilever Beam and Tapered Double Cantilever Beam Testing
3.3 End-Notched Flexure
3.4 Mode II Fracture Characterization of Bonded Joints Using the ELS Test
3.5 The Notched Torsion Test to Determine the Mode III Fracture Properties of Adhesives
3.6 Other Mixed Mode Adhesive Fracture Test Specimens
3.7 Compact Mixed Mode (CMM) Fracture Test Method
3.8 Mixed Mode Bending (MMB) with a Reeder and Crews Fixture
3.9 Mixed Mode Fracture Testing
3.10 Fracture of Wood Double Cantilever Beam (DCB) Specimens
3.11 The T-Peel Test
3.12 Peel Testing at 180 °
3.13 The Floating Roller Peel Test
3.14 Climbing Drum Peel Test
3.15 The Analysis of Peel Tests
References
Chapter 4: Higher Rate and Impact Tests
4.1 Dynamic Elastic Modulus
4.2 The Pendulum Impact Test for Adhesives and Adhesive Joints
4.3 Higher Rate and Impact Tests: Fracture at High Rates
4.4 High-Strain-Rate Testing of Adhesive Specimens and Joints by Hopkinson Bar Technique
4.5 Clamped Hopkinson Bar
4.6 Testing of Adhesive Bonds under Peel and Shear Loads at Increased Velocities
References
Chapter 5: Durability
5.1 Measurement of the Diffusion Coefficient
5.2 Tests with Moisture
5.3 Durability Testing Using Open-Faced Specimens
5.4 Tests with Temperature
5.5 The Wedge Test
5.6 Fatigue
5.7 Mixed-Mode Fatigue Testing of Adhesive Joints
5.8 Measurement of Time-Dependent Crack Growth
5.9 Curvature Mismatch Fracture Test for Subcritical Debonding
References
Chapter 6: Other Test Methods
6.1 Thermal Characterization
6.2 Dynamic Mechanical Analysis with a Vibrating Beam Method
6.3 Bimaterial Curvature Method for Residual Stress and Thermal Expansion Coefficient Determination
6.4 The Pull-Off Test
6.5 Shaft-Loaded Blister Test
6.6 Tests under Hydrostatic Pressure
References
Index
Related Titles
Habenicht, G.
Applied Adhesive Bonding
A Practical Guide for Flawless Results
2009
ISBN: 978-3-527-32014-1
Brockmann, W., Geiß, P. L., Klingen, J., Schröder, B.
Adhesive Bonding
Materials, Applications and Technology
2009
ISBN: 978-3-527-31898-8
Possart, W. (ed.)
Adhesion
Current Research and Applications
2005
ISBN: 978-3-527-31263-4
Packham, D. E. (ed.)
Handbook of Adhesion
2005
ISBN: 978-0-471-80874-9
The Editors
Prof. Lucas F.M. da Silva
Faculty of Engineering
University of Porto
Department of Mechanical Engineering
Rua Dr. Roberto Frias
4200-465 Porto
Portugal
Prof. David A. Dillard
Virginia Tech
Engin. Science & Mechanics Dept
Blacksburg, VA 24061
USA
Prof. Bamber Blackman
Imperial College London
Dept. of Mechanical Engin.
South Kensington Campus
London SW7 2AZ
United Kingdom
Prof. Robert D. Adams
University of Bristol
Dept. of Mechanical Engin.
University Walk
Bristol BS8 1TR
United Kingdom
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About the Editors
Dr. Lucas F.M. da Silva is Assistant Professor at the Department of Mechanical Engineering of the Faculty of Engineering of the University of Porto (FEUP) where he is also Head of the Materials and Technological Group. His research focuses on adhesive bonding and is currently the President of the Portuguese Adhesion Society. He received his degree in Mechanical Engineering from the FEUP in 1996 and his Master of Science from the same institution in 1999. He received a PhD related to adhesive bonding in 2004 from the University of Bristol (UK) under the supervision of Prof. R.D. Adams. He has published 13 books, approximately 90 ISI journal papers and more than 100 papers in proceedings of conferences. He recently won the SAGE Best Paper Award 2010 and the Donald Julius Groen Prize 2010 (both from the Institution of Mechanical Engineers).He is member of the editorial board for the International Journal of Adhesion and Adhesives, Journal of Adhesion Science and Technology and The Journal of Adhesion, and reviewer of 24 international journals.
Prof. David A. Dillard is the Adhesive and Sealant Science Professor of Engineering Science and Mechanics at Virginia Tech, having received his BS and MS degrees from the University of Missouri-Rolla, and his PhD from Virginia Tech. With industrial experience at McDonnell Douglas prior to entering graduate school, he has also worked during summers and sabbaticals at General Motors, NASA (Langley and Ames), National Taiwan University, and Oak Ridge National Laboratory. With over 30 years of experience in adhesive bonding he regularly teaches academic courses in viscoelasticity, adhesion science, and sustainability, and has co-authored over 135 refereed journal articles and several book chapters. He served for five years as Director of the Center for Adhesive and Sealant Science and served as Founding Director of Macromolecules and Interfaces Institute, both at Virginia Tech. He recently completed a term as President of the Adhesion Society, in which he is a Robert L. Patrick Fellow, and serves on the Editorial Board of the Journal of Adhesion.
Bamber R.K. Blackman is a Reader in the Mechanics of Materials in the Department of Mechanical Engineering at Imperial College London. He is author to over 50 refereed papers and book chapters in the area of structural adhesives and composites and has presented his research to a wide international arena. He received an ‘Elsevier most cited author award’ in 2009 for an original research contribution on mode II fracture mechanics of adhesive joints. He chaired the US Adhesion Society's Structural Adhesives Division from 2006–2008 and has served as guest editor to the international journal, Engineering Fracture Mechanics on three occasions and has sat on the Scientific Advisory Committees for several leading international conferences. He is a chartered mechanical engineer and chaired the UK Institution of Mechanical Engineers, Structural Materials and Technology Group from 2008–2010. He is secretary to the European Structural Integrity Society's technical committee on ‘Polymers, Composites and Adhesives’ (ESIS TC4) where he leads the structural adhesives activity. His research interests include the effects of test rate and environmental ageing on the performance of adhesively–bonded fibre composite materials, the effects of impact and blast loading on engineering structures, the effects of surface treatments, the development of fracture mechanics test standards for adhesive joints. He was a keynote speaker at the 4th World Congress on Adhesion and Related Phenomena (WCARP-IV) in Arcachon, France in September 2010 and was a plenary speaker at the US Adhesion Society Annual Conference in New Orleans in Feb 2012.
Prof. Robert D. Adams is an Emeritus Professor of Applied Mechanics at the University of Bristol and a Visiting Professor of the University of Oxford. He started his academic career some 35 years ago and his work covers a wide range of topics. He is one of the most respected researchers in the field of mechanics of adhesively bonded joints. He has pioneered the use of finite element analysis from the early 1970s. He is Joint Editor-in-Chief of the International Journal of Adhesion and Adhesives, which has currently an impact factor of 1.678. He has organized (as Chair, member of Organising Committee, or member of Scientific Committee) approximately 50 conferences. He has written or edited several books on adhesives, the latest being ‘Adhesive Bonding, Science, Technology and Applications’, published at Woodhead in 2005. According to ISI Web of Science, he has 134 journal papers, being the total number of independant citations of 1611, and his h-index of 24.
List of Contributors
Chapter 1
Manufacture of Quality Specimens
Lucas F.M. da Silva, Stefanos Giannis, Robert D. Adams, Edoardo Nicoli, Jean-Yves Cognard, Romain Créac'hcadec, Bamber R.K. Blackman, Hitendra K. Singh, Charles E. Frazier, Laurent Sohier, and Bernard Gineste
1.1 Preparing Bulk Specimens by Hydrostatic Pressure
1.1.1 Introduction
Lucas F.M. da Silva
There are many test methods for the determination of failure strength data. Basically, they can be divided into two main categories: tests on neat resin or bulk specimens and tests in a joint or in situ. Tests in the bulk form are easy to perform and follow the standards for plastic materials. However, the thickness used should be as low as possible to represent the thin adhesive layer present in adhesive joints. Tests conducted on in situ joints more closely represent reality, but there are some difficulties associated with accurately measuring the very small adhesive displacements of thin adhesive layers. There has been intense debate about the most appropriate method and whether the two methods (bulk and in situ) can be related. Some argue that the properties in the bulk form may not be the same as those in a joint because the cure in the bulk form and the cure in a joint (thin film) may not be identical. In effect, the adherends remove the heat produced by the exothermic reaction associated with cure and prevent overheating. To minimize this problem, cure schedules should be selected to ensure that the thermal histories of the materials are similar in each case. Dynamic mechanical thermal analysis (DMTA) or differential scanning calorimetry (DSC) measurements can be made to compare the final state of cure of the materials (Section 6.1).
Bulk specimens are usually manufactured by pouring or injecting the adhesive in a mold with the final shape (Section 1.2), or by pressure between plates. The first method is suited to one-part adhesives that are relatively liquid. The mold can be open but can also be a closed cavity, in which case the adhesive needs to be injected. When the adhesive is viscous, in the form of a film or of two components, the second method generally gives better results. If the adhesive is viscous or in film form, the pouring (or injection) phase is difficult or impossible. On the other hand, the mixing of two-part adhesives can introduce voids. If the adhesive is liquid, the air bubbles can be removed by vacuum. da Silva and Adams [1] have used an “open” vacuum release technique to produce void-free specimens with limited success. If the adhesive is viscous, recent sophisticated machines in which the mixing is done at high speed under vacuum can ensure that the adhesive is void free. If the voids have been removed properly, the adhesive can be manufactured by pouring or injection, taking care not to introduce voids during this operation. A number of simple techniques have been used to reduce void incorporation during mixing and dispensing, such as mixing in a sealed bag by kneading and then snipping off a corner to dispense or preparing the adhesive in a syringe to minimize air entrainment. If not, the voids can be removed by high pressures and an excess of adhesive to compensate for the voids.
Although voids and other defects in bulk specimens may have minimal effects on averaged properties, such as modulus and other constitutive properties, they can have a significant effect on failure properties such as various strength metrics and strain at break. In reducing voids, it is important to understand where their source is. Voids can result from outgassing of the adhesive during cure, from air entrained in the adhesive during mixing or dispensing, and from absorbed water that evaporated at the elevated cure temperatures. Although some adhesives intentionally outgas to create foams, most adhesives do not outgas significantly. Especially when the cure temperature exceeds 100 °C, absorbed water in the adhesive can be vaporized, leaving the adhesive riddled with voids. Adhesive components that are known to absorb water may need to be dried before mixing to reduce voiding due to this latter mechanism.
1.1.2 Principle
The technique described in the French standard NF T 76-142 works particularly well for producing plate specimens without porosity [1, 2]. It provides a technique for curing plates of adhesive in a mold with a silicon rubber frame under high pressure (2 MPa or 20 atm). The pressure is calculated using the external dimensions of the silicone rubber frame. The technique, shown schematically in Figure 1.1, consists of placing in the center part of the mold a quantity of adhesive slightly greater (5% in volume) than the volume corresponding to the internal part of the silicone rubber frame. There is a gap, at the beginning of the cure, between the adhesive and the silicone rubber frame. This gap enables, at the moment of application of the pressure, the adhesive to flow (until the mold is completely filled) and to avoid gas entrapment. Note that there is an external metallic frame to keep the silicone rubber frame in place. If the adhesive is a film, the lid is placed on top of the layers, but the load is applied only when the adhesive has the lowest viscosity to ease the adhesive flow. If the adhesive is a paste, the pressure is applied right from the beginning of the cure. However, an internal metallic frame is necessary to keep the adhesive at the center of the mold when being poured to guarantee that there is a gap between the adhesive and the silicone rubber frame. This frame is, of course, removed before the application of pressure. This technique is suitable for any type of adhesive, that is, liquid, paste, or film. Standard ISO 15166 describes a similar method (Figure 1.2) of producing bulk plate specimens. However, this standard does not include a silicone rubber frame and just uses spacers to control the adhesive thickness. The bulk specimens obtained have a poor surface finish and contain voids, especially for two-part adhesives.
Figure 1.1 Adhesive plate manufacture according to NF T 76-142.
Figure 1.2 Adhesive plate manufacture according to ISO 15166.
1.1.3 Metallic Mold
Standard NF T 76-142 recommends using a metallic frame with an area of 150 × 150 mm2 and a silicone frame with a width of 50 mm and a thickness of 2 mm to produce an adhesive plate of 100 × 100 × 2 mm3. A pressure of 2 MPa is applied on the external dimensions of the silicone frame, that is, in this case, 20 kN. The thickness of the silicone rubber frame gives the final thickness to the adhesive plate. However, a thin metallic frame may deform easily under pressure or due to an incorrect use, which must not occur if perfectly plane plates are to be obtained. Therefore, a more robust metallic support to keep the silicone frame in place is advised. For example, the mold represented in Figure 1.3 can be used. It consists of a base and a lid with a working area of 195 × 90 mm2 (external dimension of the silicone frame) for the application of the 2 MPa pressure (35.1 kN). That area is sufficient to machine two dogbone specimens for tensile testing. However, smaller or larger areas can be used. Four metallic pieces are put around the base and bolted to form a metallic box that fits the base and the lid. The metallic box keeps the silicone frame in place and also enables excess of adhesive to escape. Figure 1.4 shows an exploded view of the mold with the metallic frame, the silicone rubber frame, and the adhesive plate. All the pieces of the mold have a good finish (ground), especially the base and the lid, because they will dictate the surface finish of the adhesive plate. The metal used to build the metallic mold can be carbon steel (e.g., 0.45% C) in the annealed condition. It is cheap, easy to machine, and guarantees good heat dissipation.
Figure 1.3 Dimensions in mm of the metallic mold.
Figure 1.4 Exploded view of the mold to produce plate specimens under hydrostatic pressure.
1.1.4 Silicone Frame
The silicone rubber frame seals the adhesive very tightly enabling the application of hydrostatic pressure to the adhesive. The silicone rubber frame also serves as a thickness control since the thickness of the adhesive plate is equal to that of the silicone frame because of the incompressibility of the silicone. Generally, a thickness of 2 mm is used. Larger thicknesses can be used, but the exothermic reaction during cure can cause adhesive burning for some adhesives. Thicknesses of up to 16 mm have been produced with the mold presented in Figure 1.4. Thick plates can be used to produce round specimens. The silicone used is a room temperature vulcanizing (RTV) silicone. The hardness of the silicone is not critical, and soft (20 shore A) to hard silicones (70 shore A) can be used. In general, a silicone with 50 shore hardness is used. Sheets of silicone rubber can be purchased easily in drugstores. Alternatively, a sheet of silicone can be manufactured with uncured silicone using the metallic mold described above. The silicone can be cut to the final dimensions with a cutter. The width of the silicone rubber used for the mold presented above is 22.5 mm, which means that the internal space of the silicone rubber frame (also the dimensions of the adhesive plate) is 140 × 45 × 2 mm3. The width of the silicone rubber frame should not be less than 15 mm.
1.1.5 Adhesive Application
Before the adhesive application, the release agent must be applied to the metallic mold. It is not necessary to apply the release agent to the silicone rubber frame because most adhesives do not usually bond well to silicone (unless silicone is used). The release agent should be well cured before the adhesive is applied to avoid interaction of the release agent with the adhesive. It is very important to precisely control the quantity of adhesive to deposit in the mold. An adhesive in excess of 5% in volume should be applied. The amount of adhesive to apply is calculated from the volume of the adhesive plate (140 × 45 × 2 mm3) plus 5% and the adhesive density. The adhesive is weighted carefully in a precision scale and then transferred to the mold (Figure 1.5a,b). In the case of two-component adhesives, the mixture of the resin and hardener introduce voids, but this technique eliminates most of these voids. To reduce even further the quantity of voids, a toothpick can be used to burst the air bubbles before the application of pressure. If the adhesive is a liquid, the adhesive can be degassed with vacuum.
Figure 1.5 Manufacture of bulk specimens under hydrostatic pressure. (a) Weighing of the adhesive, (b) adhesive application in the mold, (c) cure in a hot press, (d) adhesive plate removal from the mold, (e) final adhesive plate, and (f) dogbone specimens obtained machined from the adhesive plate.
1.1.6 Cure
The high pressure is best applied using a hot press (Figure 1.5c). In case the adhesive cures with temperature, the hot press is also the most practical equipment to apply temperature in a short time. However, it is advised to use a thermocouple close to the adhesive and to count the cure time from the moment the adhesive reaches the cure temperature.
After the adhesive cure, the cooling rate should be slow to guarantee a uniform temperature in the mold and avoid residual stresses. Also, the plate should not be removed before the mold has reached room temperature; otherwise, the residual stresses can deform the plate permanently.
After cure, the lateral parts of the mold are unbolted and the adhesive plate is easily removed (Figure 1.5d). The surface finish of the adhesive plate is excellent because of the high pressure, as can be seen in Figure 1.5e.
1.1.7 Specimen Machining
The plates are then machined according to the dimensions used for mechanical or physical testing with any type of geometry (Figure 1.5f ) (see Sections 2.1, 2.3, 2.4, 2.5, 3.1, 3.5, 4.4, 5.1, 5.4, 6.1 and 6.6). The standard ISO 2818 gives details on how to machine specimens from adhesive plates. The use of coolants should be avoided because they can diffuse into the adhesive and influence the adhesive's mechanical behavior. In that case, the adhesive should be dried before being tested. It is better to use an air coolant. Machining might not be possible with very flexible adhesives. In that case, sharp dies can be used to cut out dogbone or other shapes from flexible adhesive sheets. Alternatively, injection or pouring techniques can be used. The specimens should be conditioned under controlled temperature and humidity because these factors influence the mechanical properties of the adhesive.
1.1.8 Results
The strain to failure is highly dependent on the presence of defects such as voids and microcracks. In tension, once a crack is triggered next to a void, the specimen often fails there because of the high stress concentration. Generally, the strain to failure can vary widely unless the manufacture is very well controlled. Figure 1.6 shows tensile stress–strain curves of a two-part epoxy adhesive that was manufactured with the technique described above. This adhesive is relatively brittle and therefore, particularly sensitive to voids. Four curves are presented, and the difference between them is barely perceptible, even in terms of strain to failure, which shows that the hydrostatic pressure technique is well suited to manufacture bulk specimens.
Figure 1.6 Tensile stress–strain curve of a two-part epoxy adhesive manufactured with the hydrostatic pressure technique.
1.2 Preparing Bulk Specimens by Injection
Stefanos Giannis
1.2.1 Introduction
During the process of manufacturing of bulk specimens of adhesives and/or sealants for mechanical characterization, and to use for modeling adhesive joints, important issues can arise, and they are well addressed in the literature [3]. These are mainly voids that result from the inclusion of air, especially when mixing two-part components. Curing conditions can also introduce some issues, since curing of most adhesives is exothermic and, in thick layers such as those used for bulk specimens, additional heat may be present, which results in a different curing temperature compared to that of a joint configuration where the adhesive is present in a very thin layer and the effect of the additional heat is negligible.
Techniques for manufacturing sheets of bulk material are presented in Section 1.1. This method was found to work very well for a number of paste and film adhesives, by producing consistent void-free flat sheets of cured material from which tensile specimens can be cut. However, for tensile dumbbell-shaped specimens, an adequate way of cutting them out of the sheets has to be used. When manufacturing flat sheets of sealant materials, because the fully cured sealant is very soft, machining to the right dimensions is not a feasible option. In this case, cutting specimens out of the flat sheet, by means of a die having the appropriate geometry, is the main option. Nevertheless, potential edge effects due to die cutting could possibly affect the test results. An alternative method would be to mold the specimens in the desired shape. Some researchers used a centrifuging technique to manufacture bulk specimens of viscous cold-cure adhesive [4]. Although they addressed all the issues associated with void formation during mixing of their adhesives and managed to create a smooth paste, free of air bubbles, by putting the adhesive into syringes and centrifuging at 3500 rpm for 10 min; they did not take into account void formation while injecting the material into the molds. An alternative methodology would involve injecting the material into the molds and centrifuging the whole mold. This technique was used in Ref. [5] and is presented here.
1.2.2 Mold
The metallic mold shown in Figure 1.7 can be used. It consists of three individual parts: (i) the base part, which is used as the support and is attached to the centrifuge, (ii) the middle part, which determines the shape and the thickness of the specimen, and (iii) the top part, which seals the mold. To ensure that the liquid material (e.g., adhesive or sealant) does not stick to the mold during curing, it is recommended to apply three layers of a release agent (e.g., Frekote 55-NC, Henkel) before the injection of the uncured material. For the application of the release agent, the metallic molds can be heated to 80 °C. The three parts are then fitted together with bolts, which are tightened to 10 Nm. The uncured material is injected through a 2.5 mm threaded hole, as shown in Figure 1.7, at one side of the mold, while a 1.5 mm release hole at the other side is used to ensure that the interior is filled with material. When the mold is filled, both these holes are closed with screws.
Figure 1.7 Schematic representation of the mold used to manufacture the tensile specimens.
One- or two-part paste adhesives can be injected, after mixing, using a syringe. However, there are a number of material systems that are packaged in special plastic cartridge assemblies, which store, mix, and apply multiple-component adhesives, sealants, and other materials. The plastic cartridges assure accurate proportioning of the materials since the premeasured components are stored in separate compartments within the cartridges. These cartridges can be fitted in an air gun, and the mixed material can be injected directly from the mixing nozzle into the sealed steel mold under pressure (usually 2 bars).
1.2.3 Centrifuge
For the centrifuging process, six metallic molds are placed in a radial configuration (Figure 1.8) on a centrifuge, and they are centrifuged at 1500 rpm for ∼30 min. An enclosed lathe can be used as a centrifuge. It is difficult to estimate a universal required time for centrifuging for all material systems, since this would depend on a number of factors such as the void size, the viscosity of the material, the temperature of the centrifuge, the cure temperature and kinetics, and so on. Therefore, the centrifuge time needs to be evaluated by trial and error, judging from the quality of the specimens, in terms of the voids present, at the end of the process. During the centrifuging, the uncured material, under the action of the centrifugal force, moves toward the outer end of the mold and the entrapped air toward the inner end. Following the end of the centrifuging process, the molds are placed in a vertical position for the adhesive or sealant to cure. The manufacturer's common practice is followed during this phase with regard to choosing the appropriate temperature and humidity levels.
Figure 1.8 Schematic representation of the centrifuging configuration used to manufacture tensile specimens of sealants.
1.2.4 Cure
For adhesives that cure at room temperature, the mold should be placed in an air circulating oven at ambient conditions. Some control of the humidity level within the oven is desirable, as the cure of a number of adhesives can be affected by moisture in the environment. In the case that curing takes place at temperatures higher than the ambient room temperature, this should be monitored with a thermocouple placed as close as possible to the adhesive layer. One of the threaded holes on the top part of the mold can be used for this reason. Actual curing time should be measured from the moment that the adhesive reaches the curing temperature. Cooling should be slow, and the specimens should be allowed to reach ambient conditions before being removed from the mold. Most sealants cure at room temperature, so it is essential to control the humidity levels of the environment where curing takes place. Most laboratories have constant temperature and humidity levels, but using an oven at ambient temperature and humidity is recommended.
1.2.5 Final Specimen Preparation and Testing
At the end of the curing process and after demolding, the edges of the specimens are cut using a sharp knife. The resultant specimens used for tensile testing are shown in Figure 1.9. Any shape, following the principles of different international standards, can be produced by changing the middle part of the molds. The sealant specimens manufactured by this technique in Ref. [5] were 3.5 mm in thickness.
Figure 1.9 Geometry of the specimens for tensile testing of sealants based on ASTM D412.
Dumbbell-shaped specimens can then be tested for tension following common practices (Section 2.1). In Figure 1.10, the experimental results for a rubbery sealant material tested in tension are presented. Specimens were produced by following the principles described in this Section 1.2. Experimental results are presented as the tensile stress, both engineering and true stress, as a function of the measured engineering strain. Strain in this case was measured using a noncontacting laser extensometer. Very good reproducibility of results was found between the specimens manufactured with this technique.
Figure 1.10 Experimental tensile stress–strain curves for a sealant material.
1.3 Preparing Bulk Specimens by Pouring
Robert D. Adams
1.3.1 Introduction
It is usually necessary to understand the mechanical and physical properties of adhesives so that they can be used successfully in adhesive joints. The preparation of these adhesives in a useful form for testing is not easy if the properties are to resemble those of the actual joints, which are commonly in a thin film between two sheets of metal or composite. Using algebraic or numerical mathematical tools (such as finite element analysis), modulus, strength, and ductility values are necessary if predictions of joint strength are to be made. These values are measured on bulk specimens of such a size that is suitable for insertion into a test machine and in which the strains or displacement can be measured. Physical constants needed for diffusion mechanisms or for measuring the glass-transition temperature (Tg) are also usually measured on specimens whose dimensions are in the order of millimetres rather than micrometres. The need is therefore to be able to make specimens which give reliable values.
It has long been realized that the properties of bulk specimens may not be exactly the same as those in the thin film form. There are two main reasons for this. First, there is often a physicochemical action between the adherend surface and the adhesive closely adjacent thereto. The nature of any such action depends on the surface and the adhesive, but it will be more important in very thin films. Second, the chemical cure of many adhesives results in an exotherm. In thin films between metallic sheets, the heat produced by this exotherm is conducted away. However, in bulk specimens, the heat cannot be conducted away so easily and the temperature of the adhesive can rise to much higher than the nominal cure temperature.
The objective of this note is to explain how bulk specimens can be made (by pouring) and cured, bearing in mind the above cautions, for structural adhesives such as epoxies, acrylics, and so on. But experimenters should bear in mind the comments of Gillham [6] who used an impregnated braid as a torsion pendulum to study the cure of adhesives. He made it quite clear that it is not generally possible to achieve the same cure state by slow cure as by quick cure.
1.3.2 Nature of Adhesives Supplied
Adhesives come in many forms. They may be in a single part, which will be cured by heating to some temperature, or they may be in two parts, which must be mixed in agreed proportions and applied within some specified time, with or without the application of heat. Single-part adhesives can also be supplied as a thin film, often containing some supporting medium such as a woven web. If there is no supporting web, these films can be melted at an elevated temperature and poured. Adhesives are supplied in a variety of forms; they may be low-viscosity liquids or highly viscous pastes. When heated, most adhesive systems become less viscous for a time, but they cure and begin to gel, thus increasing in viscosity. However, with care and experience, it is possible to get most adhesives in a form that can be poured.
1.3.3 Mixing
One-part adhesives are mixed by the manufacturer and stored in containers until needed. These containers may be large, containing 500 kg or so of adhesive, or they may be small cartridges for manual application. Depending on how the adhesives are mixed and stored, they may contain air and other gases. In laboratory conditions, one-part adhesives may be stirred in vacuum and this can remove most of the entrapped air. However, the process is not easy or cheap.
Two-part adhesives also contain trapped air, and the separate components can also be stirred in vacuum to release all or most of this air. But two-part adhesives need to be mixed thoroughly just before use. This mixing process can be done in vacuum, but usually it is not. It therefore needs to be done with great care to avoid introducing air, and hence voids, in the cured adhesive.
It is possible to degas a mixed two-part adhesive, but again there are problems. It is to be remembered that such adhesives begin to cure as soon as they are mixed and the viscosity increases with time. Experience shows that degassing to remove air introduced by mixing causes bubbles to form on the top surface of the adhesive. These bubbles may remain trapped in the adhesive, depending on the viscosity and surface tension of the adhesive.
1.3.4 Pouring
Pouring the liquid adhesive into a mold must be done with care to avoid trapping air in the process. Personal experience of pouring a one-part adhesive in vacuum showed that while air was not trapped, voids could be created as the poured column is unstable. The application of pressure after pouring usually solves this problem, but if a pressing sequence is not used, voids can reappear.
1.3.5 Effect of Size
It has been mentioned earlier that there are problems of exothermic reaction with thick specimens. The thicker the specimen, the more likely is overheating due to exothermic reaction likely. The magnitude of the problem depends on the thickness dimension and cure chemistry. Making specimens of the order of 10–15 mm thick is difficult. Dr. Hua Yu working in my laboratory in Bristol cured specimens of epoxy in a metal tube in an oil bath at 60 °C. The specimens were 12.7 mm in diameter and 50 mm long. Thermocouples were placed in the adhesive and oil bath. The oil bath and outer radius of the adhesive showed temperatures of 60 °C as expected. However, a thermocouple at the center of the specimen reached a temperature of 160 °C and the cured material was distinctly brown in the middle. However, the same adhesive with a nominal (oil bath) temperature of 25 °C only reached 26.14 °C inside the specimen. This example is given to show that very high temperatures can be generated if there is an unacceptable combination of geometric dimensions, cure chemistry, and initial (nominal) cure temperature.
1.3.6 Specimen Production
Having mixed and then poured the adhesive into a suitable mold, it is necessary to try and eliminate any voids. One method is to centrifuge the system so that any trapped air can be caused to migrate to one end of the mold. A suitable system for doing this is described in Section 1.2. A rotational speed of 3500 rev/min (which is achievable in a workshop lathe) for 10 min is quite sufficient, and the use of a high-speed centrifuge is not necessary. The centrifuge technique can also be applied to the mixed adhesive before pouring into a mold. The best results can be achieved by centrifuging the adhesive and then pouring it into a closed mold, which can be subject to hydrostatic pressure. Such a mold is that described in the French standard NFT 76-142 referred to by Jeandrau [7] and described in Section 1.1.
It then remains to select a suitable cure cycle to produce a “bulk” specimen.
1.4 Preparing Lap Joints with Flat Adherends
Lucas F.M. da Silva
1.4.1 Introduction
Tests with thin sheet adherends, and in particular the single lap joint (SLJ) test, are very common in the industry. This is because the SLJ reproduces joints encountered in aeronautical structures, which were the pioneers of adhesive bonding technology. Several standards for preparing joints with flat adherends such as the SLJ (ISO 4587 or ASTM D 1002) recommend machining the specimens from two plates bonded together (Figure 1.11). However, this technique has disadvantages such as the effect of cutting, which might introduce cracks in the adhesive, and that cutting fluids might influence the bond. It might be preferable to machine the substrates to the correct dimensions before bonding. There are several geometric parameters that are very important to control rigorously because the adhesive properties to be measured depend on them. The main geometrical aspects to control are substrate alignment, adhesive thickness, and adhesive spew fillets. Generally, molds are used for that effect.
Figure 1.11 Two plates bonded together (a) for the manufacture of single lap joints and (b) according to standard ASTM D 1002 (dimensions in mm).
1.4.2 Mold
There are various devices to fix the substrates in place such as springs, clamps, weights, presses, vacuum bags, autoclaves, molds, and so on. Teflon molds are ideal because they do not require a release agent. However, when high-temperature cures are required, it is better to use molds made of the same material as the substrates to reduce the residual thermal stresses. The mold presented in Figure 1.12 can keep the substrates in place and guarantees the substrates' alignment, the overlap length, and the adhesive thickness. It can produce any type of lap joint with flat adherends (SLJ, double lap joint, laminated joints, etc.). The material used to build the mold was carbon steel (e.g., 0.45% C) in the annealed condition. It is cheap, easy to machine, and guarantees good heat dissipation. This mold can be used for any substrate provided the cure is done at room temperature. When bonding aluminum specimens at high temperatures, it is recommended to use an aluminum alloy for the mold. Up to six specimens with a width of 25 mm can be manufactured with this mold. The overlap length can be varied through blocks located laterally.
Figure 1.12 Mold for lap joints with flat adherends.
1.4.3 Substrate Preparation and Mounting
The substrates are generally prepared to remove dust, dirt, oil, oxides, or release agents in order to improve the interfacial bonding. To decrease joint misalignment, tab ends may be used, although the effect of spacers is relatively small [8]. They can be adhesively bonded to the substrates during the adherend bonding or used only during the test by means of a pin (Figure 1.13). If high-strength materials are used, such as heat-treated steel, these cannot be gripped and therefore the adherends must be loaded through a pin. Materials that are easy to grip (low hardness) such as aluminum, mild steel, or composites are better tested through gripping because the pin force might be sufficient to break the adherend before the joint fails (Section 2.10).
Figure 1.13 Tabs ends bonded (a) and fixed by a pin (b).
The lower parts of the joints (substrates, tab ends, and shims) are positioned in the mold (Figure 1.14a–c). Before the adhesive application, release agent must be applied to the metallic mold and the shims. The shims have three purposes: to control the overlap, to control the adhesive thickness, and to control the adhesive fillet (Figure 1.15). There are various practical methods to control the adhesive thickness such as use of glass spheres (Figure 1.16), calibrated wires (Figure 1.17), shims, fabric, and so on. Many adhesives come with microspheres in their formulation for an easy means of bondline thickness control. The same principle is used with supported film adhesives. However, the presence of glass spheres, wires, or the fabric may interfere with the adhesive behavior and should be taken into account. The wires should be removed before joint testing and therefore may not be very practical.
Figure 1.14 Manufacture of lap joints with flat adherends using a mold. The lower parts of the joints, substrates (a), shims (b), and tab ends (c) are positioned in the mold. Adhesive application on the area to bond (d). Substrates assembly (e). Application of the lid to the mold (f). After cure, the specimens are carefully removed from the mold (g). The separation is best done with a saw and not by breaking the adhesive (h). Adhesive excess on the sides of the joints is removed with a file (i).
Figure 1.15 Control of adhesive thickness by the use of shims.
Figure 1.16 Control of adhesive thickness by the use of glass spheres.
Figure 1.17 Control of adhesive thickness by the use calibrated wires.
1.4.4 Adhesive Application and Assembly
The adhesive application depends on the adhesive form. For liquid adhesives, no special care is required, as the adhesive easily flows through the whole area to bond. In the case of film adhesives too the adhesive application is straightforward, even though the gaps between the film and the substrate can lead to voids in the adhesive, as shown in Figure 1.18. In that case, applying vacuum and releasing it when the adhesive is liquid can minimize voids [1, 9] (Figure 1.19). The joint is placed in an oven under vacuum without the upper adherend (point 1 in Figure 1.19). The temperature is increased to a point at which the adhesive is most fluid, so that when the vacuum is released the voids collapse to a negligible volume (point 2 in Figure 1.19). The vacuum release technique is applied without the upper adherend to facilitate the evacuation of air. After the vacuum is released, the adhesive is largely free of voids and in its most fluid state so that it can wet the upper adherend adequately. The upper adherend is then placed on top and the joint allowed to cure. It is to be noted that the details of this method are likely to be system dependent, as elevated temperature will accelerate cure. The method may not be suitable for some chemistries.
Figure 1.18 Void formation in a film adhesive during various stages. (a) Initial air entrapment, (b) partial displacement, and (c) complete displacement.
Figure 1.19 Fabrication of adhesives in the bulk (plate) form and in a joint.
For paste adhesives applied with a gun, the scheme described in Figure 1.20 should be followed to avoid air entrapment. In case of paste adhesives applied with a spatula, a generous amount of adhesive should be applied on the area to bond (Figure 1.14d). For two-part adhesives that are mixed manually, the adhesive flow when the top adherend is placed eliminates part of the voids. However, for best results, the mixing should be done so that there are no voids in the adhesive. Recent sophisticated machines in which the mixing is done at high speed can ensure that the adhesive is relatively void free.
Figure 1.20 Application of paste adhesives.
The substrates should be assembled in a way to reduce the appearance of voids. There are basically two ways to put in contact two flat substrates, as shown in Figures 1.14e and 1.21. It is preferable to join the two substrates progressively by rotation as it leads to fewer voids. After complete assembly, the lid is applied to the mold (Figure 1.14f ). In the case of thick bondlines with liquid adhesives, special care must be taken to avoid the adhesive to spread out of the overlap. In that case, the overlap area must be sealed so that the adhesive stays in place.
Figure 1.21 Substrates assembly by translation and by rotation.
(Source: Adapted from [10].)
1.4.5 Cure
To guarantee that there is a good contact between all parts of the mold, it is better to apply, in addition to the lid, a weight on the mold or a pressure with a press. In case the adhesive cures with temperature, the hot press is the most practical equipment to apply temperature in a short time. However, it is advised to use a thermocouple close to the adhesive and to count the cure time from the moment the adhesive reaches the cure temperature.
After the adhesive cure, the cooling rate should be slow to guarantee a uniform temperature in the mold and reduce residual stresses. Also, the specimens should not be removed before the mold has reached room temperature; otherwise, the residual stresses may reduce the joint strength.
1.4.6 Specimen Cleaning
After cure, the specimens are carefully removed from the mold (Figure 1.14g). Owing to an excess of adhesive, the joints might be joined to one another in places. The separation is best done with a saw and not by breaking the adhesive, which might introduce cracks (Figure 1.14h). The packing shims should be carefully removed to avoid breaking the joint. The adhesive excess on the sides of the joints is removed with a file (Figure 1.14i). The specimens should be conditioned under controlled temperature and humidity until testing because these factors may influence the mechanical properties of the adhesive.
1.5 Simple Methods for the Preparation of Single Lap Joints Specimens
Edoardo Nicoli
1.5.1 Introduction
The single lap joint (SLJ) specimens have traditionally been used for characterizing adhesively bonded joint systems. This kind of specimen is still among the most widely used for gathering important mechanical information of adhesively bonded systems, namely, the shear strength. Methods for evaluating these measures are described, for example, in the ASTM D 1002 [11] for SLJs (for calculating the apparent shear strength). SLJ specimens are commonly used because they are rather simple to construct and resemble the geometry of many practical applications. Nevertheless, the construction and the testing of this geometry can present some problems. The correct alignment of the specimens and the control of the bondline thickness can be issues for producing quality specimens. These technical aspects, if not properly considered, can lead engineers and researchers to strive with specimens' preparation and test setup for tests that are standardized and, therefore, are expected to be straightforward.
In this section, a fixture for overcoming the problems of specimen alignment and bondline control in SLJs is presented. This simple solution was implemented by the author during a specific project that required the production of a large number of quality SLJs [12]. The fixture is now used as a laboratory tool for the quick and accurate fabrication of specimens. Other techniques to produce SLJs are presented in Section 1.4.
1.5.2 Single Lap Joint (SLJ) Specimens
Although different lap specimen configurations can be used for evaluating the mechanical properties of bonded joints, the most common geometry is the SLJ configuration recommended in ASTM D1002. This standard describes how the apparent shear stress can be measured in bonded metal specimens. The specimens are two metal strips of dimensions 101.6 × 25.4 × 1.6 mm (4 × 1 × 1/16 in.) that are bonded together along an overlap of 12.7 mm (1/2 in.) as shown in Figure 1.22. The specimens are then tested in a traditional load frame and the value of the ultimate applied load at failure, divided by the extension of the bonded area, gives the apparent shear strength of the joint.
Figure 1.22 Schematic view of single lap joint (SLJ) specimen.
Important factors for preparation of specimens, such as bondline thickness, surface preparation of the adherends, and adhesive curing procedure are not described in the ASTM D 1002 standard, but usually indicated in the technical documentation of the adhesive product. Nevertheless, adhesive layer thickness, surface preparation, and curing procedure influence the shear strength of the specimens and should be controlled when preparing specimens.
1.5.3 Traditional Methods for SLJ Bonding
The SLJ specimens are often prepared by clamping together the two metal parts after the adhesive application and keeping this configuration during the curing. The clamping is traditionally done in a number of different ways such as using binder clips and C-clamps. There are four major problems that can arise when using these clamps.
1.5.4 The Idea for a New Fixture for SLJ Bonding
The need for developing a new fixture for constructing SLJ arose with a project that compared seven different adhesives in SLJs, where an electrical insulator layer also had to be embedded within the adhesive layer [12]. This activity required testing of a large number of SLJ specimens. The crucial aspect was being able to construct specimens with consistent geometrical characteristics, limiting the specimen to specimen variability, in an efficient and non-time-consuming way. Because of the importance of preparing specimens with good alignment and a consistent bondline thickness, the bonding fixture was developed as the initial step of the project. The goals for this fixture are as follows:
The idea for fixture is schematically shown in Figure 1.23. The idea was to place all the specimens in a jig, separated by spacers, and to control the bondline thickness with a changeable shim that imposes an offset to the parts to be bonded. The shim thickness was equal to the sum of thicknesses of one adherend and the desired bondline. The bondline thickness of all the specimens was nominally controlled by this single shim.
Figure 1.23 Schematic view of the jig and specimens.
1.5.5 The Fixture
The fixture that was developed is an aluminum-made jig, and in the configuration opened before the specimen's placement, is shown in Figure 1.24.
Figure 1.24 SLJ bonding fixture with two of the later bars removed.
The fixture consists of 50 × 19 mm (2 × 3/4 in.) aluminum bars connected together. Four of these bars form the frame of the jig, and the other four are lateral constraints that control the proper alignment of the SLJs. Two of the lateral bars are removed during the specimen's positioning and then placed once the specimens are placed inside the jig. Once the adhesive is distributed on the metal parts to be bonded in the SLJ in correspondence to the area of the overlap, the parts are placed in the jig and aligned as illustrated in Figure 1.25, with aluminum blocks used as spacers between the different specimens. The dimensions of the jig and the spacers allow for bonding 10 specimens at a time, but a similar design can be implemented for a different number of specimens.
Figure 1.25 SLJ bonding fixture during specimens' loading.
Once all the specimens are positioned inside the jig, the other two lateral bars of the jig are put in place and the specimens are compressed with the two screws at the top of the jig. As shown in Figure 1.26, the specimens are positioned inside the jig without any residual degree of freedom. The two screws on the top of the fixture are tightened with an Allen wrench after the placement of the specimens and the lateral bars. All the parts of the fixture that are in contact with the specimens and that can possibly get in contact with adhesive are covered with tape for facilitating cleaning of the fixture after use and avoiding unwanted bonding of specimens with the fixture. Little changes in the aluminum bars of the jig would allow the construction of SLJ with geometries different from that designated by ASTM D 1002, if needed. A positive aspect of this configuration is that once the jig is closed, it can be moved with all the specimens without issues related to their alignment. The dimensions of the jig allow it to fit inside most of the industrial ovens used for curing.
Figure 1.26 SLJ bonding fixture after specimens' loading.
1.6 Preparing Thick Adherend Shear Test Specimens
Lucas F.M. da Silva
1.6.1 Introduction
The thick adherend shear test (TAST) is one of the most popular types of failure strength test because it is easy to make and test the specimens. The conventional single lap shear (SLS) joint, which is mostly used for comparing and quality control of adhesives, puts the adhesive in a complicated state of stress (Section 2.10). Therefore, it is not suitable for the determination of the true adhesive properties. When using stiff and thick metallic adherends, the adhesive is in a state of essentially uniform shear over most of the overlap and peel stresses are reduced. Two forms of the TAST are used, as developed by Krieger [13] (ASTM D3983), in the United States, and Althof and Neumann [14] (ISO 11003-2), in Europe. The main difference between the two tests is the size of the specimen: the Althof specimen is half the size of Krieger's. ISO 11003-2 recommends machining the specimens from two plates bonded together, as shown in Figure 1.27. However, this technique has several disadvantages. The machining (cut into bars and then into transverse slots) needs to be done without coolant, as it may react with the adhesive or damage the interface and the increase in temperature may affect the adhesive properties. Cutting in the highly stressed region at the end of the adhesive layer may introduce cracks in the adhesive. Cutting the slot is difficult to control and may affect the load transfer. The bonded plates may have an irregular shape or surface, leading to irregular adhesive thicknesses. Last, the specimens produced by this method cannot be reused, which may be an important factor in terms of cost. Adams and coworkers [15] have proposed that a better solution is to machine the adherends to the correct dimensions before bonding with the geometry shown in Figure 1.28. The bending stiffness is higher than when the joint is composed of two bonded bars and therefore reduces the peel stresses. This technique is described in the following discussion.
Figure 1.27ISO 11003-2-recommended specimen manufacture.
Figure 1.28 Thick adherend shear test (TAST) geometry (a) and dimensions in millimetres (b) (ISO 11003-2).
1.6.2 Mold
A specially built jig was designed for aligning and holding the specimens (Figure 1.29). To control the overlap and fillet, steel shims are inserted into the gaps once the adherends have been brought together (Figure 1.30
