Adhesives fracture energy

ISO, Standard test method for mode I interlaminar fracture toughness, G/c, of unidirectional fibre-reinforced polymer matrix Composites. ISO 15024 2001. Blackman, B.R.K., H. Hadavinia, A.J. Kinloch, M. Paraschi and J.G, Williams, The calculation of adhesive fracture energies in mode I revisiting the tapered double cantilever beam (TDCB) test. Engineering Fracture Mechanics 2003. 70 p. 233-248. BSI, Determination of the mode I adhesive fracture energy, Gic, of structural adhesives using the double cantilever beam (DCB) and tapered double cantilever beam (TDCB) specimens. 2001. BS 7991. 

The TDCB test configuration was employed to determine the adhesive fracture energy, G. Tests were conducted at different crosshead speeds between 10 and 1 m/s. A schematic of the test specimen is illustrated in Fig. 1(a). Due to symmetry only half of the specimen is considered in the numerical analysis, Fig. 1(b). The profile of the arms is machined such that the rate of compliance increases linearly with the crack length and hence the derivative of the compliance with crack length remains constant. The beams are contoured to the profile described by Eqn. 1 [2], where h is the height of the beam, a is the crack length and m is a constant (m = 2000 m" in the present work). 

The thickness of the TDCB specimens (S = 10 mm) is sufficient to ensure plain strain conditions. It should be noted that during the test the arms remain within their elastic limit. Therefore, from simple beam theory [7], and by the use of linear elastic fracture mechanics, the strain energy release rate of the adhesive can be obtained using Eqn. 2, where P is the load at failure and E, is the substrate modulus. The calculated adhesive fracture energy was employed in the simulation of the TDCB and impact wedge-peel (IWP) tests. 

Table I. Adhesive Fracture Energy in a Blister Test in Various Environments...

FIG. 6—The adhesive fracture energy as a function of exposure time. 

Standards such as the British Standards Institute BS 7991, Determination of Mode I Adhesive Fracture Energy, GIC, of Structural Adhesives Using The Double Cantilevered Beam (DCB) and Tapered Double Cantilevered Beam (TDCB) apply to these tests. 

Determination of the mode I adhesive fracture energy, Gjc, of structural adhesives using the double cantilever beam (DCB) and tapered double cantilever beam (TDCB) specimens Essential work of fracture (EWF)... 

In the first experiments, the crack behavior for uniform rubber material was measured (Fig. 16.7(a)). As the force F applied to the crack increased, so did the crack speed. Identical behavior was observed for different rubber thicknesses and also for the stiffened rubber, demonstrating the simplicity of the peel crack system in which the peel force F depends only on the adhesive fracture energy R at a given crack speed, as shown by the peel equation (Chapters 7 and 13). 

Figure 16.12. Four tests of fracture energy (a) Griffith test for cohesive fracture energy, (b) T tear test, (cMap test for adhesive fracture energy, and (d) peel tost.

Figure 16,13. (a) Fracture energy results at different crack speeds for cohesive and adhesive failure 30min interface crosslinking, (b) Crack behavior at the interface for different adhesive fracture energies. 

The rate of energy input needed to advance a crack unit area, the adhesive fracture energy G, can be written in a form that emphasises the contribution of... 

Cleavage strength BS 5350 Part Cl 1986 ASTM 01062-72(83) ASTM 03433-75(85) ASTM 03762-79(83) ) Compact tension specimen ) Parallel- or tapered doublecantilever-beam joint for determining the adhesive fracture energy, G c Wedge cleavage test (for aluminium adherends)... 

Tables. Adhesive Fracture Energy Data on Elastomer-Modified Polyimides...

This work shows how the type and magnitude of interfacial forces can be deduced from the results of adhesion tests. For this simple polymer, the adhesive fracture energy is made up of an intrinsic term Go related to the interfacial forces, and a term related to the viscoelastic energy losses during testing. The viscoelastic loss can be expressed as an additive term (see Eqns. 1 and 3) or, for this system, as a multiplicative factor, Eqn. 4. Although for SBR at all normal rates and temperatures of test the viscoelastic term xj/ is much greater than the interfacial term W, W exerts a profound influence on the measured adhesion because of the multiplicative relationship. 

To determine the valne of the adhesive fracture energy, it is necessary to decide the mode of deformation of the pressurized layer. In the case of a relatively thin blister, the mode of deformation is considered to be mainly that of tensile deformation of the blister, and the blister is then modelled as an elastic membrane. Alternatively, in the case of a relatively thick blister, the pressurized layer is considered to deform mainly by bending, and this is modelled as an elastic circular plate with a built-in edge constraint. A further contribution to the stored elastic energy, which is available to assist growth of a debond, arises from an internal stress inherent in the test specimen.Snch stresses may be inlrodnced during... 

Recently, there have been reported several developments of the common blister test method." These include the island-blister test and the inverted-blister test developed by Fernando and Kinloch. Both of these test methods are designed to enable the adhesive fracture energy to be measured for a coating of thin adhered film where the coating or film has insufficient strength to resist the pressure needed for debonding if the standard blister test was employed. Wan and Mai have described a blister test in which the crack is driven by the expansion of a fixed mass of gas and stable growth ensues. ... 

Peel test Assuming the strain in the tab is negligible (e.g. if (1) the peel forces are very low or (2) a fabric- or plastic-backed rubbery adhesive or a relatively thick metallic substrate is the peeling member) and plastic bending of the tab does not occur, then adhesive fracture energy (critical strain energy release rate) is given by... 

Simple-extension test-piece Adhesive fracture energy is given by... 

In contrast to polyethylene and EVAs, Epoxide adhesives are thermosets and are much stiffer and less ductile. The adhesion fracture energy (Gc, see Fracture mechanics), between both unmodified and rubber-toughened epoxies (see Toughened adhesives) and several metals has generally been found to be much higher when microfibrous surfaces were involved (Table 2) ... 

A completely different set of tests are referred to as Fracture mechanics test methods. In these tests, attempts are made to measure true material properties of the adhesive joint independent of the geometry test. Such test methodologies require careful preparation of samples so that all experimental variables are controlled. A typical parameter measured for such joints would be the adhesive fracture energy, and normally, this would be determined as a function of some parameter such as the rate or temperature of testing. 

A further important parameter is the adhesive thickness within a joint. In certain adhesive systems such as rubber-toughened epoxies, there is an optimum thickness of adhesive within which energy-dissipation processes can take place. Above and below this critical size, the adhesive fracture energy will be lower. 

Flat and contoured cantilever-beam specimens for determining the adhesive fracture energy, 0 ... 

Trantina [55] applied fracture mechanics to adhesive joints with some success and applied the failure criteria to a finite element model to find adhesive fracture energies. The influence of the glueline thickness was not accounted for. Hu [56] used a shear lag analysis and applied failure criteria in terms of Jc and it was shown that this gave good predictions of failure and was also able to account for the adhesive thickness. It was noted that this is consequently a good method of predicting failure for adhesive materials loaded in shear. 

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