Fracture critical energy release rate

In a first testing series, the fracture behavior of the neat, fully crosslinked epoxy network was studied. A fully unstable crack propagation behavior was observed and the critical stress intensity factor, Kj (0.82 MPaxm ), and the critical energy release rate, Gj (0.28 kj/m ), were determined [87]. These are typical values for highly crosslinked epoxy networks prepared with DGEBPA and aromatic or cycloaliphatic diamines. 

The question rise whether the behaviour described can be evaluated on a fracture mechanics basis. The critical energy release rate associated with the formation of a transverse crack is technically difficult to measure. As a first approximation, it is possible to use the critical energy release rate obtained from a double cantilever beam fracture mechanics test (DCB). This test concerns the growth of a delamination between two layers (mostly oriented ai 0°) in opening mode I. Tests performed on the same carbon-polyetherimide at 0°/0° interface as in this study were reported recently [9] and gave a value of 1200 J/m. ... 

The results based on a fracture mechanics analysis show that the experiments were able to give an (expensive) approximation of the critical energy release rate for transverse cracking in carbon-polyetherimide under mode I. Limitation is that the choice of an initial crack length is critical. This should be of less importance when considering multiple transverse cracking. 

The work of fracture may be characterized by a critical energy-release rate for cleavage G . A crack propagates in the absence of a plastic strain if the equality ( deav 2y holds, where ys is the surface energy. If a nucleation and motion of dislocations occur at the tip of the crack the stress concentration relives. A critical... 

Approximating the fracture surface energy as half the critical energy release rate Gc (Gc = Kf/E) for crack formation (two surfaces), the specific energy due to fracture becomes... 

Again, once the critical energy release rate /nc is determined, the Mode 11 fracture toughness under the plane stress condition, Knc, can be found through the critical energy release rate as... 

Of course, an SLB specimen provides more information than the pure Mode I and Mode II fractures, that is, the mixed fracture mode. Therefore, we need to define a new parameter to evaluate the mixed mode healing efficiency. Because the energy release rate is a scalar, we can define the total critical energy release rate, Jc, during the mixed mode fracture process as... 

In order to determine fractme toughness, stress intensity approach and energy approach are applied. The stress intensity approach yields fracture toughness The energy approach provides a critical energy release rate (G. ). Fractme toughness of particle-filled polymer composites shows a very complex variation with increasing particle fraction. The particle size distribution parameter is the one aspect which plays a decisive role on the structmal and mechanical properties of the components and the... 

Fig. 8. Critical energy release rate (fracture toughness) of glass-fiber/epoxy composites, with the fibers being heat-treated (bare), and ABS- and APS-treated glass (29).

Plastics toughened against fracture by increasing the critical energy release rate. 

The left-hand side of the equation is again a material parameter. For any fixed a, it determines the load at which the component will fail by crack propagation. The right-hand side is (for the case Y = 1) equal to that of equation (5.4). Fracture toughness and critical energy release rate are therefore related by the following equation ... 

In the previous sections, we introduced several important material parameters The fracture toughness Ki., the critical energy release rate t/j, and the crack-growth resistance curve. We now want to see how these quantities are measured. 

---