ASM Metals HandBook Vol. 8 - Mechanical Testing and Evaluation
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the accuracy of torque measured for achieving clamp load could be 2% of the reading (50 N · m) and a fifth off-
torque accuracy of 5% (1.1 N · m). In this case, testing could be done with a single transducer for all torque
measurements.
Frequency Response Analysis
All test specifications should call for accuracy of measurements at the speeds at which the test is run rather than
contain statements that specify reading rates or 3 dB points on filters, which can introduce errors as high as
30%.
A typical example is shown in Fig. 57, where a 30° hard joint is used to evaluate fastening tools according to
ISO 5393. With a 100 rpm tool and the filter specified in the standard, an error of about 30% would be expected
because the filter will have reduced the measured signal at least 30% for equivalent waveforms above 500 Hz.
To properly capture the peak torque for a 1000 rpm tool, the system needs to be flat to 500 Hz, not down 3 dB
in response at 500 Hz.
Fig. 57 Frequency response analysis
In this case, the intent of the standard was to minimize errors due to electrical noise and perhaps due to a
perception that higher-frequency components did not contribute to the tightening process. The 3 dB point
specified at 500 Hz in the standard clearly places a limitation on the upper rpm for the testing of tools.
Measurement Resolution
The measurement resolution and accuracy for a test should be verified at the specified testing speed. The
example shown in Fig. 58 illustrates several important features verifying measurement capability. In the
following test, two torque transducers in series are driving an M12 prevailing nut at 100 rpm. The initial
position of the nut is such that the nylon patch is not engaged. The transducers have full-scale ranges of 68 and
9 N · m.
Fig. 58 Measurement resolution using transducers with full-scale ranges of 68 and 9 N · m
The maximum applied torque is slightly less than 3 N · m, or about 2%, of full scale on the larger capacity unit
and 30% of full scale on the smaller-capacity unit. It is clear from the recorded data plot that at 100 rpm, the
transducers are reading precisely the same torque values.
This test verifies that the measurement system is capable of simultaneously capturing readings at the 100 rpm
test speed. It also illustrates the possibility of the use of the high-capacity torque sensor to make valid
measurements in the range of 2% of full-scale capacity when used with this measuring system.
ISO 9000 Registration Versus Laboratory Accreditation
There are significant differences between laboratory accreditation using ISO 25 and quality system registration.
The key difference can be summarized in that the essence of ISO 25 is to ensure the validity of test data,
whereas technical credibility is not addressed in ISO 9000.
ISO 9000 and QS 9000 registrations require that procedures are in place, well documented, and followed. ISO
25 goes further in that it requires the science of the laboratory measurements be understood and that the tester is
capable of demonstrating that valid measurements are the product of his or her testing procedures.
All testing procedures should be evaluated to account for certain basic considerations. American testing
standards for fasteners may not receive international recognition unless they can be shown to meet the basic
requirements of ISO 25. The most important questions to answer when reviewing standards are the following:
• Will the procedure produce accurate results?
• How have the procedures been validated to ensure accuracy?
• Does the tester understand the science behind the test procedures?
• Are the limitations of the procedures known?
The science of measurement engineering is a critical area of technology needed to properly qualify both the
specifications and the measurement systems used to verify the quality of threaded fasteners. The pure
acquisition of data for its own sake is one of the greatest technical crimes of this era. All measurements must
reflect a technical knowledge of the testing process, as well as a full understanding of the capability and
limitations of the measuring system.
Mechanical Testing of Threaded Fasteners and Bolted Joints
Ralph S. Shoberg, RS Technologies, Ltd.
Reference
1. “Standard Test Methods for Determining the Mechanical Properties of Externally and Internally
Threaded Fasteners, Washers, and Rivets (Metric),” F 606M, Annual Book of ASTM Standards, ASTM
Mechanical Testing of Threaded Fasteners and Bolted Joints
Ralph S. Shoberg, RS Technologies, Ltd.
Selected References
Fastener Design and Application Engineering
• J.H. Bickford, An Introduction to the Design and Behavior of Bolted Joints, 3rd ed., Marcel Dekker,
Inc., New York, 1995
• A. Blake, Design of Mechanical Joints, Marcel Dekker, Inc., New York, 1985
• A. Blake, What Every Engineer Should Know about Threaded Fasteners: Materials and Design, Marcel
Dekker, Inc., New York, 1986
• V. Moring Faires, Design of Machine Elements, 3rd ed., The Macmillan Co., 1955
• P.G. Forrest, Fatigue of Metals, Pergamon Press, Addison-Wesley Publishing Co., Inc., 1962
• Fastener Standards, 5th ed., Industrial Fasteners Institute, Cleveland, OH, 1970
• Metric Fastener Standards, 2nd ed., Industrial Fasteners Institute, Cleveland, OH, 1983
• R.C. Juvinall, Engineering Consideration of Stress, Strain, and Strength, McGraw-Hill Book Co., 1967
• R.C. Juvinall and K.M. Marshek, Fundamentals of Machine Component Design, 3rd ed., John Wiley &
Sons, 1999
• G.L. Kulak, J.W. Fisher, and J.H.A. Struik, Guide to Design Criteria for Bolted and Riveted Joints, 2nd
ed., John Wiley & Sons, 1987
• J.E. Shigley, Mechanical Engineering Design, 5th ed., McGraw-Hill Book Co., 1989
• R.S. Shoberg, Torque-Angle Signature Analysis, Fastener Technology International, Vol 19 (No. 1),
1996
• R.S. Shoberg, Engineering Fundamentals of Torque-Turn Tightening, Fastening Technology 1996,
March 1996 (Cleveland, OH), Clemson University College of Engineering & Science, 1996
• R.S. Shoberg, Analyzing Torque-Angle Signatures For Reliable Bolted Joints, Assembly Technology
Expo '96 (Chicago, IL), Sept 1996
• R.S. Shoberg, Coordination of Bolted Joint Design and Test Methods, Fastening Design & Application
Engineering Conference (Novi, MI), Oct 1996
Fastener Testing
• “Metric Screw Threads—M Profile,” B1.13M-1995, American Society of Mechanical Engineers
• “Standard Test Methods for Determining the Mechanical Properties of Externally and Internally
Threaded Fasteners, Washers, and Rivets,” F 606, Annual Book of ASTM Standards, ASTM
• Determination of Coefficient of Friction of Bolt/Nut Assemblies under Specified Conditions, DIN 946,
Deutsches Institut für Normung e.V., Berlin, Germany, 1990
• “Steel Metric Threaded Fasteners Torque/Clamping Force Performance,” Ford WZ100, Ford Motor Co.,
1993
• “Torque Tension Test,” GM 9064P, General Motors Corp., 1998
• IFI 101, 100/107, and 543 standards (drafts), Industrial Fastener Institute, Cleveland, OH
• “Torque-Tension Test Procedure for Steel Threaded Fasteners—Inch Series,” SAE J 174, Society of
Automotive Engineers, 1996
• “Systematic Calculation of High Duty Bolted Joints, Part I,” VDI 2230, Translation of the German
Edition 7/1986, C. Junker and J. Newnham, SPS Laboratories, Naas, Ireland, 1988
Measurement Accuracy
• “Rotary Tools for Threaded Fasteners—Performance Test Method,” ISO 5393, 2nd ed., 1994-0501, Ref
No. ISO 5393:1994(E), International Organization for Standardization
• “Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results,” NIST
Technical Note 1297, 1994
• F.R. Schraff, Defining Data Accuracy, Sensors, Vol 15 (No. 6), 1998
• R.S. Shoberg, “Special Report: Measurement Resolution for Locknut Testing,” RS Technologies, Ltd.,
April 1994
• P.K. Stein, Measurement Engineering, Vol 1, Stein Engineering Services, Inc., 1962
• J.L. Taylor, Measurement Uncertainty: A Basis for Assessing and Improving the Quality of
Measurement Processes, Sverdrup Technology, May 1998
• P.S. Unger, ISO/IEC Guide 25 Versus ISO 9000 for Laboratories, American Association for Laboratory
Accreditation, 1995
• “Test for Evaluating the Torque-Tension Relationship of Both Externally and Internally Metric
Threaded Fasteners,” USCAR Fastener Strategic Standards Technical Committee, draft, May 1998
• C.P. Wright, Applied Measurement Engineering, Prentice Hall, 1995
Testing of Adhesive Joints
K.L. DeVries and Paul Borgmeier, University of Utah
Introduction
MOST ENGINEERING DESIGNS require the connecting or joining of component parts. Means available to
accomplish these goals can be broadly classified as mechanical connections, welding, and adhesives.
Mechanical connections such as bolts, pins, and rivets have the advantages of being easy to install, they can be
inspected often, and they can be repaired or replaced. On the other hand, such connections require making holes
in the component members, which act as stress risers (with typical stress concentration factors of 3 or greater).
Structures that experience dynamic loading, such as airframes, often fail from fatigue cracks originating at their
stress concentrations. The heads of connections such as nuts and bolts can affect aesthetics, streamlining, and
other parameters. Another type of mechanical connection that is becoming increasingly popular is the “snap”
connector. These connections are wedge-like tabs and grooves that, when forced together, snap into place to
hold containers or housings together. Snap connectors greatly facilitate assembly but have limited strength, are
easily damaged in assembly (or even more likely during disassembly), and may deteriorate with time and wear.
In welding, the joining segments of the component parts are locally melted, brought together, and fused
(additional material may be supplied from a welding rod). Welded joints have the advantage of being relatively
continuous, resulting in high strength. They have the disadvantage of requiring a relatively expensive process.
The required heating can damage other parts of the structure or its contents. Not all combinations of materials
can be welded. Most metals and plastics can be welded only to themselves or to a limited number of other
similar materials. Some materials do not lend themselves to welding at all.
Adhesive joints involve joining parts by bonding component parts together with an adhesive. In some plastic
applications, one of the parts may act as the adhesive, and in some cases, a solvent may be used to
dissolve/soften the materials, which are then bonded by diffusion of the polymer chains into the respective
parts, forming essentially one material. Examples of this type of bonding include the solvent bonding of
polyvinyl chloride (PVC) pipe and acrylic materials. Very dissimilar materials can, in principle, be joined by
adhesives, for example, wood to plastic, metal to ceramics, and aluminum to steel. Likewise, it appears that the
“load” could be distributed over a larger area compared with joining materials with, for example, bolts. This
aspect can be even further enhanced by appropriate design as, for example, in the familiar finger-scarf joint
used to increase the length of lumber. This type of joint design accomplishes several beneficial goals: it
increases the bonded area, changes what for a butt joint would be largely tensile stresses to more shear stress,
and produces a smooth, attractive (almost invisible) joint.
Along with their advantages, adhesive joints pose their own design problems. Prediction of load-carrying
capacity is often neither straightforward nor reliable. Adhesive joints also have problems with inspection
because the bonded surfaces are usually not visible after assembly. Stresses in an adhesive joint are generally
not uniformly distributed on the bonded surfaces, and in fact, elastic stress analysis often exhibits singularities
analogous to the stress risers associated with the other methods of joining already mentioned. Because adhesive
joints usually involve very dissimilar materials in the adherends and adhesives, cure stresses, thermostresses,
deformation mismatches, and so on may cause problems.
Despite these drawbacks, the advantages and positive features of adhesive joints are so attractive that the use of
adhesives has enjoyed phenomenal growth over the last 50 years. One compilation of adhesives lists more than
5000 different commercial adhesives available to the U.S. designer (Ref 1). Adhesive selection, testing, use in
design, and application is a very complex subject covered in a variety of large reference books (for example,
Ref 2, 3, 4, 5, 6 and Adhesives and Sealants, Volume 3 of the Engineered Materials Handbook published by
ASM International) and literally thousands of journal articles; dedicated journals include the Journal of
Adhesion and the International Journal of Adhesion Science and Technology. Articles on adhesives and
adhesion also appear in journals or publications of the following organizations: American Society for Testing
and Materials, Society for Experimental Mechanics, American Society of Mechanical Engineers, Materials
Research Society, and many others. The Society for Adhesion, the Gordon Research Conferences, and the
American Chemical Society hold annual or periodic conferences dedicated to the subject of adhesives. Despite
this extensive research and study, there is still much to be done, and adhesive joint design is still as much art as
it is science.
Brazing and soldering are methods of joining that have similarities to adhesives and welding. These methods
might be viewed as falling between welding and adhesive bonding. Brazing is closer to welding in that, while it
uses a different material for “bonding” the “adherends,” its application typically involves extensive heating and
perhaps some interpenetration of the brazing and host materials. The material used for brazing is melted in a
puddle, in which there may be some alloying with the host material(s) on fusion. In soldering, on the other
hand, usually only the solder material melts, and if it wets the host material(s), it attaches by adhering to the
joined pieces. It might be viewed as a metallic form of hot-melt adhesive.
Adhesive science and technology remains an important area of active research. New adhesives continue to be
developed, and work continues on the understanding of stresses; methods of analyzing stresses, strains,
displacement, and load-carrying capacity; improved experimental techniques; and so on.
References cited in this section
1. Adhesives, Edition 6, D.A.T.A. Digest International Plastics Selector, 1991
2. R.L. Patrick, Ed., Treatise on Adhesion and Adhesives, Vol 1–6, Marcel Dekker, 1966–1988
3. G.P. Anderson, S.J. Bennett, and K.L. DeVries, Analysis and Testing of Adhesive Bonds, Academic
Press, 1977
4. A.J. Kinlock, Adhesion and Adhesives, Chapman and Hall, 1987
5. A. Pizzi and K.L. Mittal, Ed., Handbook of Adhesive Technology, Marcel Dekker, 1994
6. K.L. Mittal, Adhesive Joints, Plenum Press, 1984
Testing of Adhesive Joints
K.L. DeVries and Paul Borgmeier, University of Utah
Purpose of Testing Adhesive Joints
In no area of materials is testing more important than it is for adhesives. In the United States, the largest
organization devoted to standardizing test procedures is the American Society for Testing and Materials
(ASTM). ASTM and its counterparts in other countries work through the International Organization of
Standardization (ISO) in an effort to coordinate testing on an international scale. All these activities are very
important and time consuming, entailing countless hours of volunteer efforts in the United States alone. Other
organizations, besides ASTM, have established testing procedures that relate to the testing of adhesive joints.
Such organizations include the military (MIL specs), other professional societies, such as the Society of
Automotive Engineers (SAE), the American Society of Mechanical Engineers (ASME), and industrial
organizations.
Many examples of the wide variety of standardized tests for adhesives can be found in the Annual Book of
ASTM Standards, Volume 15.06, which is updated and republished annually (Ref 7). Except where specifically
noted, the standards and practices listed in this treatise can be found in this volume. There are also many other
less formalized tests used by various organizations and a few specialized tests for adhesives found in other
ASTM volumes. ASTM practices, specifications, and test methods cover a wide variety of topics ranging from
measurements of shelf life and pot life through resistance to mold or insect attack to determination of viscosity
and various aspects of strength. This article concentrates on tests and methods used in the measurement of
adhesive joint strength. A logical question in the discussion relative to the selection of the specific adhesive
might be “How strong is it compared with the other available adhesives?” The answer to such a query is neither
as simple nor as straight-forward as might be assumed. The reasons for this complexity are addressed in this
article.
Testing of adhesive joint strength might be conducted for a number of reasons, including the following:
• Quality control: To ensure that changes such as adhesive age, mix method, and surface preparation that
will affect bond quality have not been introduced
• Comparative analyses: To determine which of a series of adhesives, primers, surface preparations, cure
methods, and so on is best suited for a given bond application
• Generation of engineering design data: To predict the load-carrying capability of a given bonded joint
The geometry of the test specimen selected is very important even for quality control tests. In fact, an adhesive
that exhibits “high strength” in a tensile test may have poor peel strength and vice versa. In fact, many variables
may affect bond strength. Each variable may affect the bond strength in a different manner, and the manner in
which the bond strength is affected may be different for each test geometry. The amount of adhesive used in a
mixing batch and the delay time between mixing and casting will likely affect strength and may affect peel
strength differently than it affects tensile strength. As another example, bond thickness often affects lap shear
strength in a different way than it affects tensile button strength. If a series of lap shear joint test are completed
for various adhesive thickness is optimum for a given joint unless the geometry, adhesive, and loading are
nearly identical to those of the lap shear joints tested. Thus, even for comparative analyses, a great deal of care
must be used in applying laboratory test data to joints whose geometries differ in even subtle ways from that of
the laboratory test specimens.
There are many standard adhesive tests available to the engineer for determining the strength of adhesive joints.
In the example just discussed, reference was made to tensile and peel specimens. Indeed, most standard tests for
adhesive joints fall within three general categories, namely, peel tests, lap shear tests, and tensile tests. It is
important to recognize, however, that the stresses at the tip of the debond region are not necessarily of the
character implied by the name. In fact, the stresses are usually of mixed character. For example, as discussed in
the section “Lap Shear Tests”, the stress in the critical region of the so-called lap shear specimens are apt to be
more crack opening (tensile) in nature than shear.
One purpose for obtaining adhesive joint engineering data is in an effort to ensure that a bonded joint will
withstand the loads for which it was designed. This might be accomplished by “proof testing” every joint, that
is, by loading every joint manufactured to its design load. However, this approach has its problems. First, such
procedures are apt to be very costly. Furthermore, the joint may withstand such loading but be damaged by the
loading such that it subsequently fails when subjected to aging, creep, and fatigue loading or the environment.
In addition, when a joint is in place in a structure, it is often difficult to apply the actual or even a simulated
load. The load may be prohibitively large and/or in an awkward location, making testing difficult, very
expensive, or even impossible. There is also a danger that applying such proof loads might damage neighboring
or adjoining parts of the structure.
For these reasons, an engineer is generally required to obtain data from laboratory size samples and use this
data to infer the strength of a given “practical” joint. For many of the standard adhesive joint tests, the results
are reported as the load at failure divided by bonded area. In one straightforward design approach, this stress is
then compared with the average stress that exists in the joint being evaluated when its maximum load is
applied. However, if the joint geometry, loading time, and other conditions are not identical to the laboratory
test conditions, the direct comparison can lead to unsafe joint designs. This problem is discussed in subsequent
sections (“Lap Shear Tests” and “Adhesive Fracture Mechanics Tests”).
Reference cited in this section
7. Adhesives, Annual Book of ASTM Standards, Vol 15.06, ASTM (updated annually)
Testing of Adhesive Joints
K.L. DeVries and Paul Borgmeier, University of Utah
Factors Influencing Mechanical Strength of Adhesive Joints
Before proceeding with a description of specific tests, a few general comments might be helpful. The strength
of an adhesive joint should not be viewed as being essentially (or even largely) an inherent property of a given
adhesive. Typically, strength depends on many factors. The nature of the adherends may have a dramatic effect
on joint strength. A given adhesive may not even “adhere” to some potential substrates. Even where the
inherent adhesion may be good, its quantitative value is typically highly dependent on proper cleaning, surface
treatment, and details of curing. The presence of coupling agents and surface roughness can affect strength.
Thickness of adhesive and adherends and other geometric factors also play strong roles in the strength of a
joint. In short, it is safe to say that the strength of an adhesive joint is a system property that depends on many
factors beyond the chemical and physical nature of the adhesive per se.
Some ASTM and other standards and practices address these other factors. For example, the following ASTM
standards cover surface treatments for various materials:
• D 2093, “Standard Practice for Preparation of Surfaces of Plastics Prior to Adhesive Bonding”
• D 2651, “Standard Guide for Preparation of Metal Surfaces for Adhesive Bonding”
• D 2674, “Standard Test Method for Analysis of Sulfochromate Etch Solution Used in Surface
Preparation of Aluminum”
• D 3933, “Standard Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding
(Phosphoric Acid Anodizing)”
Coupling agents such as silanes, titanates, zirconates, and chrominates are sometimes applied to the surface
before application of the adhesive. The agents are also sometimes incorporated into the adhesive. In at least
some cases, they are thought to form covalent bonds between the adhesive and the adherend surfaces (Ref 8).
As with most materials, aging can affect the properties of an adhesive and the strength of an adhesive joint. It is
often not convenient (or possible) to simply wait to see how time and exposure to the elements might cause
deterioration. Accelerated aging tests are an effort to infer from short-term tests, usually under very harsh
conditions, how well materials might hold up under longer-term, more realistic service conditions. A
comparison of how different adhesive joints in wood exposed to boiling water for several hours or days might,
for example, be used to estimate their relative weatherability (ASTM D 3434, D 5572). While the comparisons
might not be perfect, they are likely better than nothing. Several other standards that address accelerated testing
of adhesives include ASTM D 1101, D 1183, D 2559, D 3632, and D 4502.
Reference cited in this section
8. E.P. Plueddemann, Silane Coupling Agents, Plenum Press, 1982
Testing of Adhesive Joints
K.L. DeVries and Paul Borgmeier, University of Utah
Qualitative Tests
Most “adhesive strength tests” are quantitative in nature. The test results are typically given as “average stress”
at failure (tensile and lap joints) or force per unit width (peel tests). The preparation and testing of quantitative
samples are expensive and time consuming. It would often be advantageous to have a means of making a
qualitative determination of adherence to ascertain if the cost of conducting the quantitative tests would likely
be justified. Such quick screening of candidate adhesive/adherend pairs might result in significant time and cost
savings by eliminating unlikely candidates and assisting in selecting those worthy of further study.
The first effort along this line with which the authors are familiar is found in the patent literature (U.S. Patent
4,025,159, Cellular Retroreflective Sheeting). In this patent, Dr. J.M. McGrath of 3M Corporation explores
means of increasing the bond strength between the cover sheet and the base sheet (polymer binder with
embedded reflective glass spheres) in retroreflective sheeting. This sheeting is used in stop signs and other
reflective signs. To accomplish this goal, McGrath proposes curing (cross linking or chain extension) the
sheeting after the thermoforming operation. He points out that the proposed process does not work for all
potential pairs of cover sheet and base materials. As an aid in selecting materials for further study, he suggests
casting a small amount of candidate base materials on potential cover sheets and curing them in place. After
curing, McGrath proposes a single-edged razor blade be used to lift, scrape, or otherwise separate the base
material from the cover sheet. The relative effort required to facilitate this separation is accessed and used as
guide in the selection of candidates for further study.
More recently, Committee D-14 of ASTM formalized and adopted ASTM D 3808 “Standard Test Method for
Qualitative Determination of Adhesion of Adhesives to Substrates by Spot Adhesion.” The stated purpose of
the document is to provide “a simple qualitative procedure for quickly screening whether an adhesive will,
under recommended application conditions, bond to a given substrate without actually making bonded
assemblies.” In this test method, spots of adhesive are placed onto a substrate using the application procedure
and curing conditions acceptable to the user and supplier of the adhesive. To test adhesion, the document
recommends the use of “a thin stainless steel spatula or similar probe” as a prying lever. It states “If the results
are acceptable, then standard quantitative adhesive test procedures can be used to obtain quantitative
measurement of the adhesive's performance.” The authors believe such methods of preliminary screening are
certainly worthy of consideration in many instances. They can often save time and testing costs.
Testing of Adhesive Joints
K.L. DeVries and Paul Borgmeier, University of Utah
Peel Tests
Peel tests are easy to visualize albeit their analysis and the interpretation of peel test results is neither so easy
nor straightforward. When a strip of adhesive tape is placed partially on paper or on another substrate and then
removed by pulling the free end (the portion of the tape that is not attached), in essence, a peel test is being
conducted, as shown schematically in Fig. 1. Several quantitative observations might readily be made from
such a test (it is important to note that the tape is a very flexible material). The force required to propagate the
peel failure is a strong function of the angle at which the peel force is applied. If the angle is small (i.e., close to
the direction of the substrate surface), large peel surfaces are required. As the angle increases to the point where
the peel angle is 90°, perpendicular to the surface, the force is reduced significantly. As the angle is further
increased, it reaches a limit at 180° (ASTM D 903 describes a 180° peel test for a “flexible” peel specimen).
The force per unit width required to facilitate peel is called the peel force or the stripping strength. It can also be
observed that the forces required to sustain peeling depend on the rate of peel. For tapes bonded with pressure-
sensitive adhesives, very slow peel rates require relatively small peel forces, while at extremely high rates, the
peel forces are much more substantial. It can also be noted in this simple peel test that the force required to
initiate the peel may differ from that required to sustain the peel once it has started. It may also be observed that
the force required to peel the tape from the substrate depends on the nature of the surface to which the tape is
attached. For example, it may adhere very tightly to a clean glass surface but hardly at all to moist or oily glass.
Fig. 1 Schematic of the peel test
ASTM has formalized a variety of different peel tests in its Annual Book of ASTM Standards, Volume 15.06.
Several of the standard peel test geometries (described in detail in ASTM D 903, D 3167, and D 1876) are
illustrated in Fig. 2 (Ref 7). ASTM D 1781 describes the climbing drum peel test, which is used to determine
the peel resistance of adhesive bonds between a relatively flexible adherend and a rigid adherend.
Fig. 2 Typical peel test specimens. (a) Stripping strength specimen (ASTM D 903). (b) Roller drum peel
test specimen (ASTM D 3167). (c) T-peel test specimen (ASTM D 1876). Source: Ref 7
Several other peel tests are in common usage that have not been standardized by organizations such as ASTM.
For example, the authors have found a test they have adopted from researchers at 3M Corporation to be very
useful for measuring peel strength. This method makes use of a test jig incorporating a platform that uses rollers
that allow the platform to move horizontally with extremely low friction. It is used to test the adhesion between
two thin flexible sheets. For testing, one of the sheets is bonded to a thin aluminum sheet with a strong
adhesive. A region of debond along the adhesive between the thin flexible sheets is initiated with a razor blade
or by other means, leaving a loose tab. The aluminum sheet with the attached bilayer sheeting is inserted into
slots on the jig platform, or otherwise held fixed to it. The jig is then attached to the lower crosshead of a
universal-testing machine such that the platform is horizontal. The loose tab is then attached by a grip to the
upper crosshead. During testing, motion between the two crossheads then peels the upper sheet from the lower.
The peel angle is maintained at very nearly 90° as the platform moves horizontally (facilitated by the low
friction rollers), keeping the peel region directly below the upper grip. In some cases where the adherence
between the sheetings is high and the sheeting cohesive strength is relatively low, it has been helpful to
reinforce the upper sheet by applying strapping tape that is gripped and peeled along with the upper sheet. It is
important to note, however, that even though the peel failure may follow the same path, it would not be
anticipated that the peel force would be the same as for the nonreinforced sheet. Adhesive joint strength is a
“system property,” and the addition of another layer can modify the energy absorption during the peel process,
thereby altering the associated peel forces. It is, therefore, important to compare the results from tests on
reinforced materials with those of other materials that are similarly reinforced and vice versa.