Bonding test specimen geometry

The test specimen geometry impacts the ability to discriminate between the bond strength of different formulation and adhesive systems. Of the methods used. Method A seems to be the least discriminating and the other tests should probably be used preferentially, depending on what kind of application is being evaluated. Method F appears to have the most consistent ability to discriminate differences in response [22]. 

Apart from the short beam shear test, which measures the interlaminar shear properties, many different specimen geometry and loading configurations are available in the literature for the translaminar or in-plane strength measurements. These include the losipescu shear test, the 45°]5 tensile test, the [10°] off-axis tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled tube torsion test. Since the state of shear stress in the test areas of the specimens is seldom pure or uniform in most of these techniques, the results obtained are likely to be inconsistent. In addition to the above shear tests, the transverse tension test is another simple popular method to assess the bond quality of bulk composites. Some of these methods are more widely used than others due to their simplicity in specimen preparation and data reduction methodology. 

Lap shear tests are close to peel tests however, this method is more quantitative in the base. Two sheets are bonded to each other by toughened adhesives (acrylic) in the test configuration and tensile tester applies load at a rate of usually 1 mm/min [96]. The lap shear specimen represents the most utilized geometry for studies of adhesive bonding because specimens as single- or double-lap shears (Fig. 8.13a, b) are easy for the measmement [97]. 

Table 2.5 summarises typical values of these bulk mechanical properties at around 20 °C for a range of epoxy adhesives. Of particular note is the low strength and stiffness of the epoxy polyamides and polysulphides as compared to those with aliphatic polyamine hardeners. For the design of bonded assemblies, joint tests are often used to determine the relevant mechanical properties. It must be remembered, however, that the results will be highly dependent on the specimen geometry and testing conditions and... 

Standard test methods for the determination of impact resistance (ASTM D 950-82, ISO 9653 1998 and BS 5350, Part C4) specify the specimen geometry shown in Fig. 1, see also Appendix. While providing a simple basis for comparisons between adhesives, a number of disadvantages are associated with this configuration for example, it does not reproduce the differential strain effects often present in real joints and it does not readily permit the evaluation of bonds to many important coated substrates. 

The wood-to-wood test specimen has a smaller bond area, decreasing the probability of failure occurring in the wood. The specimen geometry is shown in Figure 2. The specimens are loaded by U-shaped grips attached to a mechanical testing instrument, which help insure specimen alignment. 

Material Variability, including Anisotropy, Porosity, and Surface Condition Test Specimen Size, Fibo" Architecture, and Gage Section Geometry Effects Gripping and Bonding Failures... 

A first step in the validation of this approach is to test simple specimens under controlled conditions and to compare predictions with measured failure load values. First lap shear geometries were examined, then an L-geometry was studied in more detail. The bond-line in these small specimens was very similar to that in the quasi-unidirectional fracture specimens as the small dimensions allow panels to be pressed uniformly after assembly (which is not the case for industrial top-hat stiffeners). 

Fracture mechanics characterisation tests have been performed to determine the mixed mode fracture envelope of an epoxy bonded glass/epoxy composite. Analysis of lap shear, and L-stiffener geometries has shown that for this relatively brittle adhesive reasonable first estimations of failure loads can be obtained for both cracked and uncracked specimens. An image analysis technique has been developed which enables failure mechanisms to be... 

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