Peel tests fracture mechanics

Specific tests of adhesion are described in more detail under the following articles Blister test, Climbing drum peel test, Fracture mechanics. Napkin ring test. Peel tests. Rubber to metal bonding - testing. Shear tests. Tensile tests. Wedge test and in Refs. [1-5] see also Standards for adhesives and adhesion and Appendix. 

Many widely used Tests of adhesion can be applied to the rubber to metal bonds Blister test. Fracture-mechanics test specimens. Non-destructive testing of adhesively-bonded structures. Peel tests. Shear tests. Tensile tests and Wedge test. This particular article is concerned with those aspects that are of practical concern in the rubber-processing industry. 

The three principal forces to which adhesive bonds are subjected are a shear force in which one adherend is forced past the other, peeling in which at least one of the adherends is flexible enough to be bent away from the adhesive bond, and cleavage force. The cleavage force is very similar to the peeling force, but the former applies when the adherends are nondeformable and the latter when the adherends are deformable. Appropriate mechanical testing of these forces are used. Fracture mechanics tests are also typically used for structural adhesives. 

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. 

Thouless, M.D. and Jensen, H.M. Elastic fracture mechanics of the peel-test geometry. Journal of Adhesion 1992, 38, 185-197. 

Although important for structural adhesive bonds, fracture mechanics is not as critical for non-structural low load-bearing adhesives as used in most electronic modules. For the most part, passing minimum specification requirements for peel and tensile strengths both at ambient conditions and accelerated test conditions are sufficient. However, computer-simulated modeling and reliability analysis have been used for evaluating electrically conductive adhesives as used in electronics assembly. ... 

Tensile tests and so on. It can be a peel strength or a Fracture mechanics parameter. These measures are sometimes referred to as the practical adhesion of a particular joint they more or less satisfactorily answer the question, How strong is the joint ... 

Although for many straightforward comparative purposes it is often adequate to record the results as a peel force/width, it is easy to derive an expression for the peel energy from basic mechanical principles by equating the work done by the test machine to the work done on the sample. The result forms a basis for understanding and interpreting peel tests (see also Fracture-mechanics test specimens). 

The optimal adhesive layer thickness depends on a number of factors. Some bonds, notably Tensile test specimens, are stronger when the adhesive layer thickness is redn-ced. For fracture specimens, including double cantilever beam specimens bonded with structural adhesives (see Fracture mechanics test specimens), optimal bond thicknesses have been identified, although the optimal thickness depends on the loading rate and test temperature. " Enhanced ductility plays a role in this process, and a sufficient quantity of adhesive is desired to dissipate energy (see Peel tests). This latter mechanism is also important in the peel energy of Pressure-sensitive adhesives and other systems. 

There are numerous examples of the application of fracture mechanics to structural adhesive systems. Most notable are those of Mostovoy and his coworkers which have already been mentioned. " Bascom and coworkers have made significant contributions to the understanding of the effect of bondline thickness on fracture toughness. Kinloch and Shaw extend the work of Bascom to include rate effects and to develop mathematical models of the fracture resistance of adhesives. Hunston et al have used these methods to study viscoelastic behavior in the fracture process of structural adhesives.Mostovoy and Ripling used these techniques to determine the flaw tolerance of several adhesives,while Bascom and Cottington have studied the effect of flaws caused by air entrapment in structural adhesives." Finally it must be mentioned that one of the most simple, most widely used tests for strucural adhesives, the peel test, is actually a version of the double cantilever beam test. 

The calculation of G and for a number of geometries, such as peeling, double cantilever, double torsion, or blister test, can be found in textbooks. We will concentrate on the case of adherence of punches (and especially of a sphere) which is conceptually an important topic via which to understand the connection between adherence, mechanics of contact, and fracture mechanics, or, more simply, what is an area of contact. 

Clearly, the peel strength is not a fundamental property for an adhesive. The value of force per unit width required to initiate or sustain peel is not only a function of the adhesive type, but also depends on the particular test method, rate of loading, thickness and stiffness of the adherend(s) and adhesive as well as other factors. Thus, peel tests generally do not yield results that may be used in quantitative design. This does not imply, however, that the peel test is not a useful test. Peel tests provide quantitative comparisons between different adhesive systems, insight into rate and temperature effects, etc. Additionally, peel tests can be used to provide fracture mechanics information as will be discussed in the next section. In the author s opinion, the latter aspect of peel tests has been perhaps most adroitly exploited by Gent and Hamed [18-20] who used peel tests in conjunction with fracture mechanics to obtain insights into time-temperature effects, the role of plasticity, and many other aspects of adhesive fracture. 

The analytical methods of fracture mechanics (both cohesive and adhesive) are described in a number of references [21-24] and will not be repeated here. However, a brief outline of one simple approach provides some insight into the concepts, principles, and methodologies involved for the reader who is not familiar with fracture mechanics. In the previous discussion of peel tests, it was noted that Gent and Hamed [ 18-20] had performed some extremely informative fracture mechanics tests using peel specimens. We consider a simplified fracture mechanic analysis of the 90° peel test shown in Fig. 17. Here we assume that the substrate is rigid and the peel adherend is very flexible and perfectly elastic. The stress distribution in the vicinity of the 90° bend is complex and difficult to determine. If the material is perfectly elastic, however, this stress distribution is... 

Obviously, key questions which now arise are how good are these various analytical and finite element analysis methods at yielding a value of the adhesive fracture energy, Gc, (a) which is independent of the details of the peel test geometry, for example, independent of the peel angle and thickness of the peel arm and (b) which agree with results from other test methods, for example, with values of Gc from standard linear-elastic fracture-mechanics (LEFM) tests. 

As stated above, peel tests provide practical adhesion values that include polymer and substrate mechanical properties, stored stresses, plastic deformation, and other parameters. It has been demonstrated that an analysis of the peel test mechanics allows to extract the fundamental adhesion from the experimental data [71]. A method to calculate the interfacial fracture energy of a polymer bonded to a rigid substrate by using peel tests has also been presented [72]. 

However, Gent and Hamed [104] have shown that the theory of small bending deformations used to derive Equation 6.24 is only valid to peel mechanics when )8pmp sin a, i.e. when Kp = 1. The results from these analyses have not been widely used in the interpretation of peel test data and the fracture mechanics approach, based upon an energy balance argument, which avoids the necessity for developing a detailed stress analysis has been far more widely applied. This approach is considered in Section 7.3.2. 

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