Testing of Adhesive Bonds
Adhesive tests are used for a variety of reasons including comparison of properties, quality checks for a “batch” of adhesives to determine whether the adhesives are still up to standard, checking the effectiveness of surface and/or other preparation, and determination of parameters useful in predicting performance.
Keywords
Tensile; shear; cleavage; adhesive bonds; fatigue
12.1 Introduction
Adhesive tests are used for a variety of reasons including [1]:
• Comparison of properties (tensile, shear, peel, flexural, impact and cleavage strength, durability, fatigue, environmental resistance, conductivity, etc.)
• Quality checks for a “batch” of adhesives to determine whether the adhesives are still up to standard
• Checking the effectiveness of surface and/or other preparation
• Determination of parameters useful in predicting performance (cure conditions, drying conditions, bond-line thickness, etc.).
Testing is important in all aspects of materials science and engineering, but it is especially so in adhesives [2]. Such tests evaluate not only the inherent strength of the adhesive but also the bonding technique, surface cleanliness, effectiveness of surface treatments, etchings of surfaces, application and coverage of the adhesive, and the curing cycle [3].
This chapter discusses in a general manner the various types of testing carried out on adhesive joints. Only the more important types are covered. Following this discussion, a compilation of 53 subject areas is listed, with all relevant ASTM methods and practices and SAE Aerospace Recommended Practices (ARPs) [4].
12.2 Tensile
Pure tensile tests are those in which the load is applied normal to the plane of the bond line and in line with the center of the bond areas (Figure 12.1). ASTM D897 is one of the oldest standards still used for testing adhesive bonds. The specimens and grips called for require considerable machining and, because of the design, tend to develop edge stresses during the test. Because of these limitations, D897 is being replaced by D2095 on rod and bar specimens. These specimens, prepared according to ASTM D2094, are simpler to align and, when correctly prepared and tested, more properly measure tensile adhesion [6].
Tensile tests are among the most common tests used for evaluating adhesives, despite the fact that, where possible, joint designs are used that load the adhesive in other than a tensile mode. Most structural materials have high tensile strengths when compared to the tensile strengths of structural adhesives. One of the advantages of the tensile test is that it yields fundamental and uncomplicated tensile strain, modulus, and strength data [1].
12.3 Shear
Pure shear stresses are those that are imposed parallel to the bond and in its plane (Figure 12.1). Single-lap shear specimens do not represent pure shear, but are practical and relatively simple to prepare. They also provide reproducible, usable results. The preparation of specimens of this type and method of testing are described fully in ASTM D1002. Two types of panels for preparing multiple specimens are described [6].
Shear tests are very common because samples are simple to construct and closely duplicate the geometry and service conditions for many structural adhesives. As with tensile tests, the stress distribution is not uniform and, while it is often conventional to give the failure shear stress as the load divided by the bonding area (Table 12.1), the maximum stress at the bond line may be considerably higher than the average stress. The stress in the adhesive may also differ from pure shear. Depending on such factors as adhesive thickness and adherend stiffness, the failure of the adhesive “shear” joint can be dominated by either shear or tension [1].
Table 12.1
Effect of Overlap and Plasma Type on Joint Strength of High-Density Polyethylene (2.5-cm-Wide Tape) [7]
| Treatment Type | Plasma Exposure Time (min) | Load to Failure (kg) | ||
| 12.5 mm Overlap | 6.3 mm Overlap | 3.2 mm Overlap | ||
| Oxygen plasma | 30 | 100.9 | 94.5 | 77.3 |
| Oxygen plasma | 1 | 105.9 | 98.2 | 78.2 |
| Helium plasma | 1 | 105.4 | 90 | 76.4 |
| Control, solvent wiped | – | 10.4 | – | – |
Methods other than ASTM D1002 are in use. ASTM D3163 describes an almost identical test configuration, except for thickness. This method helps alleviate the problem of adhesive extruding out from the edges of the sample. ASTM D3165 describes how a specimen can be prepared to determine the strength properties of adhesion in shear by tension loading of laminated assemblies. The double lap shear test offers the advantage of reducing the cleavage and peel stresses found in the single-lap shear test [1].
Compression shear tests are also commonly used. ASTM D2182 (withdrawn 1983) describes sample geometry similar to that of the lap-shear specimen and the compression-shear-test apparatus. ASTM D905 describes a test for determining the shear properties of wood (hard maple, etc.). ASTM E229 (withdrawn 2003) determines the shear strength and shear modulus by torsional loading. With proper sample construction and alignment, the adhesive in E229 is subjected to a more homogeneous stress distribution in this configuration than with lap-shear specimens [1].
12.4 Peel
Peel tests intended for flexible adhesives are designed to measure resistance to highly localized stresses (Figure 12.1). Peel forces are therefore considered as being applied to linear fronts. The more flexible the adherend and the higher the adhesive modulus, the more nearly the stressed area is reduced to linearity. The stress then approaches infinity. Since the area over which the stress is applied is dependent on the thickness and modulus of the adherend and the adhesive, and is therefore very difficult to evaluate exactly, the applied stress and failing stress are reported as linear values, i.e., pounds per linear inch. Probably, the most widely used peel test for thin-gauge metal adherends is the T-peel test (ASTM D1876). In this test, the entire load applied is transmitted to the bond. This type of peel thus tends to provide the lowest values of any peel test [6].
With elastomeric adhesives, peel strength is dependent on bond thickness. The elongation characteristics of these adhesives permit a greater area of the bond to absorb the applied load as the bond thickness increases. The T-peel test is probably the most widely used peel test since it uses only one thickness of metal. The Bell peel test is designed to peel at a constant radius around a 1-in. (2.52 cm) steel roll and, for this reason, provides more reproducible results. ASTM D1781 uses a metal-to-metal climbing drum in an attempt to achieve this same constant peel radius by peeling around a 4-in.-diameter rotating drum. While the fixtures used with the Bell and drum-peel tests help stabilize the angle of peel, the ideal of a fixed radius of peel is not achieved because the high modulus of the metal tends to resist close conformation to the steel roll or drum. In both methods, considerable energy is used in deforming the metal so that they provide higher peel values for a given adhesive than the T-peel method [6].
ASTM D3167 is a test for determining the floating-roller peel resistance of adhesives. The specimens for this test are made by bonding a flexible material to a comparatively rigid one. The method is of particular value for acceptance and process control testing. It may be used as an alternative to ASTM D1781 (Climbing Drum Test). This method should be considered more severe, since the angle of peel is greater.
ASTM D903 uses a 180° peel to determine the peel or stripping strength. In this method, one of the adherends must be flexible enough so that it can essentially fold back on itself.
12.5 Cleavage
Cleavage is a variation of peel in which the two adherends are rigid. The load is applied normal to the bond area at one end of the specimen (Figure 12.1). The prying forces of cleavage stress are exerted perpendicularly and away from the plane of the bond line. Cleavage stress typically is concentrated on one edge. ASTM D3807 describes how to measure “cleavage peel” of adhesives used with engineering plastics.
12.6 Creep
Often when a bonded structure is subject to a permanent load in service, especially in the presence of vibration, the resistance of the adhesive to creep is important. Two ASTM methods are used to measure creep, ASTM D2293 involving compression loading, and ASTM D2294, involving tensile loading. ASTM D1780 is a standard practice on conducting creep tests [6].
12.7 Fatigue
While static strength tests are useful in screening and selecting adhesives for most bonding applications, they do not cover the rigorous conditions of intermittently applied stress, or fatigue. The test used is ASTM D3166. Although intended for metal/metal joints, the test can be used for plastic adherends. The single-lap shear specimen of ASTM D1002 is used. The specimen is tested on a special tensile-testing machine capable of imposing a cyclic or sinusoidal stress on it. Ordinarily, the test is carried out at 1,800 cycles/min. The number of cycles to failure at a given level is recorded and a so-called S–N curve constructed.
12.8 Impact
Impact tests measure the ability of an adhesive to attenuate or absorb forces applied in a very short time interval. Essentially, these tests measure the rate sensitivity of an adhesive to an applied load. ASTM D950 describes a pendulum method for applying an impact load to a shear specimen. The results are reported as foot pounds of energy absorbed in failing the bond of a 1-square inch specimen. Some machines use gravity to accelerate the given load that strikes the test specimen. A variation of the gravity-impact methods uses a weight multiplied by the distance dropped series. Other more sophisticated apparatuses use compressed air to decrease the time of load application to as little as 10−5 s [6].
12.9 Durability
A number of ASTM tests and practices involve durability, but one of the most important is the Wedge Test, ASTM D3762. In this method, a wedge is forced into the bond line of a flat-bonded aluminum specimen, thereby creating a tensile stress in the region of the resultant crack tip. The stressed specimen is exposed to an aqueous environment at an elevated temperature or to any other desired environment. The resultant crack growth with time and failure modes is then evaluated. The test is primarily qualitative, but it is discriminatory in determining variations in adherend surface preparation parameters and adhesive environmental durability.
12.10 Compilation of Test Methods and Practices
12.10.1 Aging (Permanency)
ASTM D1183: Standard test methods for resistance of adhesive to cyclic aging conditions.
ASTM D1581 (withdrawn 1991): Standard test method for bonding permanency of water-or solvent-soluble liquid adhesives for labeling glass bottles.
ASTM D1713 (withdrawn 1990): Standard test method for bonding permanency of water- or solvent-soluble liquid adhesives for automatic machine sealing top flaps of fiberboard specimens.
ASTM D1877 (withdrawn 1984): Standard test method for permanency of adhesive-bonded joints in plywood under mold conditions.
ASTM D3632: Standard practice for accelerated aging of adhesive joints by the oxygen-pressure method.
12.10.2 Amylaceous Matter
12.10.3 Ash Content
12.10.4 Biodeterioration
ASTM D1382 (withdrawn 1990): Standard test method for susceptibility of dry adhesive film to attack by roaches.
ASTM D1383 (withdrawn 1990): Standard test method for susceptibility of dry adhesive film to attack by laboratory rats.
ASTM D1877: Standard test method for permanence of adhesive-bonded joints in plywood under mold conditions.
ASTM D4299: Standard test methods for effect of bacterial contamination of adhesive preparations and adhesive films.
ASTM D4300: Standard test methods for effect of mold contamination on permanence of adhesive preparation and adhesive films.
12.10.5 Blocking Point
12.10.6 Characterization
12.10.7 Chemical Reagents
12.10.8 Cleavage
12.10.9 Cleavage/Peel Strength
See also peel strength (Section 12.10.30).
12.10.10 Corrosivity
12.10.11 Creep
ASTM D1780: Standard recommended practice for conducting creep tests of metal-to-metal adhesives.
ASTM D2293: Standard test method for creep properties of adhesives in shear by compression loading (metal-to-metal).
ASTM D2294: Standard test method for creep properties of adhesives in shear by tension loading.
12.10.12 Cryogenic Temperatures
12.10.13 Density
12.10.14 Durability (Including Weathering)
ASTM D1151: Standard test method for effect of moisture and temperature on adhesive bonds.
ASTM D1828: Standard practice for atmospheric exposure of adhesive-bonded joints and structures.
ASTM D2918: Standard practice for determining durability of adhesive joints stressed in peel.
ASTM D2919: Standard practice for determining durability of adhesive joints stressed in shear by tension loading.
See also wedge test (Section 12.10.52).
12.10.15 Electrical Properties
12.10.16 Electrolytic Corrosion
12.10.17 Fatigue
12.10.18 Filler Content
12.10.19 Flexural Strength
12.10.20 Flow Properties
12.10.21 Fracture Strength in Cleavage
12.10.22 Gap-Filling Adhesive Bonds
12.10.23 Grit Content
Federal Test Method Std. 175B, Method 4041.1: Grit, lumps, or undissolved matter in adhesives [8].
12.10.24 High Temperature Effects
12.10.25 Hydrogen Ion Concentration
12.10.26 Impact Strength
12.10.27 Low and Cryogenic Temperature
12.10.28 Nonvolatile Content
12.10.29 Odor
12.10.30 Peel Strength (Stripping Strength)
ASTM D903: Standard test method for peel or stripping strength of adhesive bonds.
ASTM D1781: Standard method for climbing drum peel test for adhesives.
ASTM D1876: Standard test method for peel resistance of adhesives (T-peel test).
ASTM D2918: Standard practice for determining durability of adhesive joints stressed in peel.
ASTM D3167: Standard test method for floating roller peel resistance.
12.10.31 Penetration
12.10.32 pH
See hydrogen ion concentration (Section 12.10.25).
12.10.33 Radiation Exposure (Including Light)
12.10.34 Rubber Cement Tests
12.10.35 Salt Spray (Fog) Testing
12.10.36 Shear Strength (Tensile-Shear Strength)
ASTM E229 (withdrawn 2003): Standard test method for shear strength and shear modulus of structural adhesives.
ASTM D905: Standard test method for strength properties of adhesive bonds in shear by compression loading.
ASTM D906: Standard test method for strength properties of adhesives in plywood type construction in shear by tension loading.
ASTM D1002: Standard test method for strength properties of adhesives in shear by tension loading (metal-to-metal).
ASTM D1144: Standard practice for determining strength development of adhesive bonds.
ASTM D2182 (withdrawn 1983): Standard test method for strength properties of metal-to-metal adhesives by compression loading (disk shear).
ASTM D2295: Standard test method for strength properties of adhesives in shear by tension loading at elevated temperatures (metal-to-metal).
ASTM D2339: Standard test method for strength properties of adhesives in two-plywood construction in shear by tension loading.
ASTM D2557: Standard test method for strength properties of adhesives in shear by tension loading in the temperature range from −267.8°C to −55°C.
ASTM D2919: Standard practice for determining durability of adhesive joints stressed in shear by tension loading.
ASTM D3163: Standard recommended practice for determining the strength of adhesively bonded rigid plastic lap-shear joints in shear by tension loading.
ASTM D3164: Standard recommended practice for determining the strength of adhesively bonded plastic lap-shear sandwich joints in shear by tension loading.
ASTM D3165: Standard test method for strength properties of adhesives in shear by tension loading of laminated assemblies.
ASTM D3166: Standard test method for fatigue properties of adhesives in shear by tension loading (metal/metal).
ASTM D3528: Standard test method for strength properties of double lap shear adhesive joints by tension loading.
ASTM D3931: Standard practice for determining strength of gap-filling adhesive bonds in shear by compression loading.
ASTM D3933: Standard practice for measuring strength and shear modulus of non-rigid adhesives by the thick adherend tensile lap specimen.
ASTM D4027: Standard practice for measuring shear properties of structural adhesives by the modified-rail test.
12.10.37 Specimen Preparation
See also surface preparation (Section 12.10.44).
12.10.38 Spot Adhesion Test
12.10.39 Spread
12.10.40 Storage Life
12.10.41 Strength Development
12.10.42 Stress-Cracking Resistance
12.10.43 Stripping Strength
See peel strength (Section 12.10.30).
12.10.44 Surface Preparation
ASTM D2093: Standard recommended practice for preparation of surfaces of plastics prior to adhesive bonding.
ASTM D2651: Standard practice for preparation of metal surfaces for adhesive bonding.
ASTM D2674: Standard methods of analysis of sulfochromate etch solution used in surface preparation of aluminum.
ASTM D3933: Standard practice for preparation of aluminum surfaces for structural adhesive bonding (phosphoric acid anodizing).
ARP 1524: Surface preparation and priming of aluminum alloy parts for high durability structural adhesive bonding, phosphoric acid anodizing.
12.10.45 Tack
12.10.46 Tensile Strength
ASTM D897: Standard test method for tensile properties of adhesive bonds.
ASTM D1144: Standard practice for determining strength development of adhesive bonds.
ASTM D1344 (withdrawn 1985): Standard method of testing cross-lap specimens for tensile properties of adhesives.
ASTM D2095: Standard test method for tensile strength of adhesives by means of bar and rod specimens.
12.10.47 Torque Strength
12.10.48 Viscosity
12.10.49 Volume Resistivity
12.10.50 Water Absorptiveness (of Paper Labels)
12.10.51 Weathering
See durability (Section 12.10.14).