Thick-adherend lap-shear test

For thick adherend lap shear test specimens a surface treatment was performed. The aluminium adherends were 10 mm thick and 25 mm wide. After the specimens were cured and postcured, the adherends were cut to form a test area with an 8 mm lap length. 

Thick adherend lap shear tests (TALST) were not based on any standard. However, standard practice used in the tests was followed. The displacement rate used was 0.3 mm/min. The adhesive layer displacement was measured using two in-house built KGR-1 type extensometers. 

Several testing techniques are possible which use different specimen configurations, different loading modes, and various loading frequencies. Marceau et al. have described results for thick adherend lap shear and double cantilever beam (DCB) specimens exposed to 140 F/100%... 

Most adhesives are still characterized by the average shear stress measured on standard lap-shear test coupons (ASTM D-1002) in a variety of environments. This can be misleading in the context of joint design since this so-called adhesive shear strength varies with the adherend material, their thickness [usually 1.3 mm (0.063 in.)], and with the length of the overlap, as described by Hart-Smith (1993). Such coupons should never he used for anything other than quality-control tests, for which their complex mixture of variable shear and peel stresses makes them very useful. 

Adhesives by the Thick Adherend Tensile Lap Specimen D 4027 Test Method for Measuring Shear Properties of Structural Adhesive by the... 

ASTM D 3983-81—Measuring strength and shear modulus of nonrigid adhesives by the thick adherend tensile lap specimen [thick adherend test]... 

As with tensile tests, it is sometimes necessary to test dissimilar materials in shear or materials which cannot be easily fabricated into adherends. For these cases, ASTM D3164-73 describes the testing of a lap shear sandwich joint. The test specimens consist of two metal adherends, typically 0.064 inch thick aluminum, the structural adhesive, and a 0.010-inch thin film of plastic. The sandwich joint is shown in Figure 8. 

Test Method for Measuring Strength and Shear Modulus of Nonrigid Adhesives hy the Thick-Adherend Tensile-Lap Specimen-, and ASTM D2979, Standard Test Method for Pressure-Sensitive Tack of Adhesives Using an Inverted Probe Machine. 

ASTM (2007), D5656-04E01, Standard test method for Thick-adherend metal lap-shear joints for determination of the stress-strain behaviour of adhesives in shear by tension loading , in Annual Book of ASTM Standards, 2007 edn. West Conshohocken, PA ASTM International. 

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The thick adherend shear test also provides data on the modulus of the adhesive. If we use this data along with a knowledge of the modulus and thermal expansion of the two materials being bonded together, it is possible to compute the stress levels experienced by a simple lap joint. 

First, we must (with one exception) disqualify as shear tests the single-lap joint and its allies described earlier. The exception is the thick adherend shear test. As discussed above, provided proper precautions are taken, this test can be used to determine the shear properties of an adhesive, although the ultimate strength cannot be attained (para. 4.2 of ASTM D 3983-81). 

The various forms of lap joint described in Section 3.2.1 do not really provide a true measure of the adhesive shear properties, with the possible exception of the thick adherend test, although this cannot give the ultimate shear strength. 

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