Fracture energy balance approach

Sometimes the failure occurs by propagation of a crack that starts at the top and travels downward until the interface is completely debonded. In this case, the fracture mechanics analysis using the energy balance approach has been applied [92] in which P, relates to specimen dimensions, elastic constants of fiber and matrix, initial crack length, and interfacial work of fracture (W,). 

Griffith confirmed this equation using experimental data on glass as shown in Figure 18.6. Although the energy balance approach works well, kinetic effects may also be present during fracture as demonstrated for the fracture of mica flake. 

Advantages of the energy balance approach are firstly that from Equations 7.3 and 7.4 an unambiguous value of Gc may be deduced. This is true whether a thin or thick adhesive layer is employed and whether the crack propagates deep in the adhesive layer or at, or near, an interface. As will be seen later, determination of the fracture toughness, Kc, of a joint is not always so straightforward. Secondly, the value of Gc may be related to the intrinsic... 

The energy balance approach is generally the most applicable to flexible joints, since away from the crack tip the adhesive or substrates may not exhibit linear-elastic behaviour and so the stress intensity factor is invalid. Of course, in those cases where linear-elastic behaviour is observed then either approach may in principle be adopted. Essentially, to obtain an expression for the value of the adhesive fracture energy, Gc, for a particular geometry requires a solution to the left-hand side of Equation 7.3 to be deduced. 

An approach suggested by Williams et al. (117) uses an energy balance equation for initiation of flaw growth in a linearly viscoelastic material. A spherical flaw geometry was selected for simplicity since the expressions for the critical values of applied stress to cause fracture were similar for several flaw geometries. The critical conditions are based on a power (energy rate) balance... 

Griffith s approach is an energy balance A crack will propagate when the additional energy created by the formation of the fracture surfaces, E, is... 

A general approach to the quantitative characterization of fracture may now be given in terms of the energy balance required to propagate a crack or flaw. 

Fracture mechanics A J KINLOCH Basis energy balance and stress-intensity factor approaches... 

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