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  • 1. 11 Testing of Adhesives K. L. DeVries and P. R. Borgmeier University of Utah, Salt Lake City, Utah, U.S.A.I. INTRODUCTION The molecular mechanisms by which materials can adhere to one another have not been determined unambiguously. To date, no one has been able to predict reliably the strength of an adhesive joint based purely on the properties of the adhesives and adherends. Rather, to determine the strength of a joint, one must resort to testing. As will hopefully be made clear in this chapter, testing is not always as straightforward as it might appear supercially. Testing is, however, an important aspect of adhesive science and technology. Tests are conducted for a wide variety of purposes. There are a large number of dierent standard adhesive tests available to the technologist, engineer, or scientist, depending on his or her goals. It is essential not only that the proper test be selected (or designed) and that care be exercised in conducting the test, but also that the results be interpreted properly. The latter aspect often requires considerable background and insight.II.STANDARD TESTSIn the United States, the American Society for Testing and Materials (ASTM) is the organization that has assumed the responsibility for the development of standards on characterization and performance of materials, product systems and services, and the promotion of related knowledges. The testing specications of other organizations (such as the military in their Mil-Specs) often parallel those specied by ASTM. ASTM operates as a source of voluntary consensus standards. Most other countries (or groups of countries) have similar organizations, and there is considerable interaction and interchange between these groups from the various countries. For example, many of the standards adopted by European and Asian groups nd their basis in ASTM, and vice versa. This is accomplished through coordinating committees from the various countries that meet frequently. While the standards of most of these countries could be cited in this chapter, the authors will rely almost exclusively on ASTM since these are the standards with which they are most familiar. The interested reader can usually nd comparable standards in his or her own country. ASTM publishes an Annual Book of Standards that updates the test methods and other details on a yearly basis. The changes are seldom revolutionary but rather, evolutionary. New standards do appear, however, and Copyright 2003 by Taylor & Francis Group, LLC

2. all standards are reviewed periodically, at which time they may be eliminated or updated. The responsibility for developing new standards, revising existing standards, and approving standards for publication falls on ASTM volunteer committees. Of the more than 30,000 members of ASTM, nearly two-thirds serve on such committees. This is no small task. The 1990 Annual Book of Standards was composed of 68 volumes, divided among 16 sections. It is not uncommon for these volumes to be more than 500 pages long and some have nearly twice this many pages. The D-14 committee has the primary responsibility for adhesives. Volume 15.06 is the ASTM publication that covers most adhesive standards. Including indexes and the like, it has some 485 pages covering 116 standards. To this point we have used the term standard in a very general sense. ASTM has a hierarchy of types of communications of this general nature, which are dened as follows: 1.2.3. 4.5.6.III.Classications are a systematic arrangement or division of materials, products, systems, or services into groups based on similar characteristics (origin, properties, composition, etc.) in which the instructions or options do not recommend specic courses of action. Guides provide a series of options or instructions but do not recommend a specic course of action. The purpose here is to oer guidance based on a consensus but not to establish xed procedures. Practices outline denitive procedures for conducting specic operations or functions, that do not produce specic test results (comparative test methods). Specications are a precise statement of a set of requirements to be satised by a material, system, service, and so on, and the procedures to be used to determine if the requirements are satised. Terminology is a document that helps standardize the terminology, their denitions, descriptions, symbols, abbreviations, acronyms, and so on. The relevant example here is D-907, Standard Terminology of Adhesives, which was originally published in 1947 and most recently approved by D-14 in 1990. This provides denitions for several hundred terms in common use in adhesive science and technology. Despite this very worthy eort, the use of many terms is somewhat ambiguous and a reader/researcher must seek to determine a words exact meaning by looking at the context in which it is used. Test methods are denitive procedures for identication, measurement, and/or evaluation of qualities, characteristics, or properties of materials, products, systems, or services. These will, in general, produce test results.SOME SELECTED STANDARDSTests are performed for a wide variety of dierent reasons. Many tests have as a goal to compare dierent materials, procedures, products, and so on. For such comparisons to be meaningful, it is important that some type of standard procedure be used to obtain the information that will be used for comparison. This is, of course, one of the most important reasons for having standards. The goal is to separate, as much as possible, the results obtained from dierences due to the laboratory or operator. In principle, one should be able to compare results from one operator (say, in Europe) with those in another place (say, in the United States). Furthermore, all tests are not conducted with equa1 rigor. Quantitative testing is generally expensive and time consuming. Testing organizations have at times, therefore, Copyright 2003 by Taylor & Francis Group, LLC 3. developed qualitative tests that might be used to eliminate obvious unlikely candidates quickly. ASTM-D-3808, Spot Adhesion Test, is a good example of such a test. It is well established both theoretically and empirically that all adhesivesubstrate pairs are not equally compatible. An adhesive that might tightly adhere to one substrate may form very weak bonds with other substrates with varying degrees between these extremes. To prepare and test standard quantitative test geometries (some of which will be described subsequently) for a large number of candidate adhesives for a given substrate could be prohibitively expensive. ASTM D-3808 suggests the following alternatives: 1. 2. 3.4.Prepare candidate substrates using techniques similar to those expected in service. Mix a quantity of the candidate adhesive according to the procedures specied by the adhesive manufacturer. Small spots of the adhesive (typically 510 mm in diameter) are placed on the substrates using application and curing techniques comparable to those expected in service or specied by the manufacturer. The operator then uses a thin stainless steel spatula (or similar probe) to pry or lift the spot from the substrate. The operator then uses his or her senses to assess ease of separating the spot from the substrate.Based on this operation, a decision is made as to whether this system is worthy of further quantitative analysis. Some quantitative tests are the subject of the next section. A.Tests for Adhesive Joint StrengthA relatively large number of tests have been proposed and formalized for evaluating the strength of an adhesive. Although there are some important exceptions (some covered in the following sections), the majority of the long-standing adhesive joint strength tests fall in three categories: tensile, shear, and peel. 1. Tensile Tests Given the choice, a designer seldom uses adhesives in a direct tensile loading mode. The primary reason for this is probably related to the fact that by overlapping, scarng, and so on, the contact area of the adhesive may be markedly increased over that of a simple butt joint. The nger joints used to produce longer pieces of lumber or in other wood construction are a familiar example of such techniques. Figure 1 clearly demonstrates the dierence in bond area between a butt joint and one of the types of nger joint assembliesFigure 1 Typical nger joint. Copyright 2003 by Taylor & Francis Group, LLC 4. described in ASTM D-4688. It is noted that this geometry also results in a change in the stress state. The butt joint appears to be dominated by tensile stresses (supercially at least). Dierences in elasticviscousplastic properties between the adhesive and adherends induce shear stresses along the bond line even for the case of the pure butt joint, but the dominant direct stresses are tensile. (This is perhaps an opportune point to note that, as explained in more detail later, the stress distribution in joints is almost never as simple as it might rst appear.) Clearly, for the tensile-loaded nger joint, shear stresses as well as tensile stresses are applied directly to the bond line. Another reason why tensile joints are avoided in design is because it has been observed empirically that many adhesives exhibit lower strength and high sensitivity to alignment when exposed to butt-type tensile stresses. However, a variety of standard tensile tests are available. Sample geometries for some of the more common of these are shown schematically in Fig. 2. Figure 2a is a schematic representation of the conguration from ASTM D-897. It is used (with slight geometric alterations) for various adherends, ranging from wood to metal. It is often called the, pi tensile test because the diameter is generally chosen to yield a crosssectional area of one square inch. Detailed specications for U-shaped grips machined to slip over the collar on the bonded spool-shaped specimen, to help maintain alignment during loading, are give

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