Direct fracture energy method

A number of methods have been applied to evaluate the experimental data recorded during the tests, and they may be classified into two types compliance method and direct fracture energy method. 

An alternative to the compliance methods is a direct fracture energy measurement. In this method, the crack extension is related directly to the area, AG,... 

Some of the more important methods of failure studies include stress-strain, impact loading, and fatigue. Creep and stress relaxation (Chapter 10) may cause serious damage to engineering materials, but they normally do not result in fracture per se except for creep rupture. Emphasis in this chapter will be on the study of fracture energy, kinetics of crack growth, and molecular mechanisms. The reader is directed to Chapter 13 for a fuller discussion of plastic toughening. 

The relationship (4.45) between the final radius ap of the delamination zone and the corresponding separation energy F was established by adapting some general concepts of fracture mechanics. The same result can be obtained by direct appeal to energy methods. The total elastic energy in the film between the free surface at r = 6 and some remote circular boundary, say at r = i > a, is... 

General Considerations. Ideally, a test method should have the following features. First, it should employ simple, easily prepared specimens. Secondly, the failure force should remain constant, at least in principle, as the line of separation is forced to advance over long distances, so that fluctuations in the failure force can be attributed to variations in the strength of adhesion. And, finally, the fracture energy should be obtained directly from the dimensions of the specimen, its stiffness during loading, and the applied force at which failure takes place. No other measurements should be needed to calculate G. ... 

Peeling. One of the simplest and most widely used methods of measuring adhesion is by peeling a thin adhering layer away from a rigid substrate (Fig. 14a) or from an identical layer (Fig. 14b). The peel force is a direct measure of fracture energy, at least in the simplest cases, as the following analysis shows. 

It should be pointed out that while the indirect methods provide a qualitative method of ranking the adhesion between fiber and matrix and the composite lamina test methods actually measure fiber-matrix interface sensitive composite properties, the direct methods not only provide a measure of fiber-matrix adhesion but can also provide information about fiber-matrix failure mode and a method to measure the energy involved in fracture of the fiber-matrix interface. This last parameter is important in relating fiber-matrix adhesion to composite toughness. 

Nakayama J. Bending method for direct measurement of fracture surface energy of brittle materials. Japan J App Phys 1964 3(7) 422-423. 

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