Fracture energy factor

If contact with a rough surface is poor, whether as a result of thermodynamic or kinetic factors, voids at the interface are likely to mean that practical adhesion is low. Voids can act as stress concentrators which, especially with a brittle adhesive, lead to low energy dissipation, i/f, and low fracture energy, F. However, it must be recognised that there are circumstances where the stress concentrations resulting from interfacial voids can lead to enhanced plastic deformation of a ductile adhesive and increase fracture energy by an increase in [44]. 

Fig. 46. a Critical stress intensity factor, of solvent-modified and macroporous epoxy networks prepared via CIPS with various amounts of cyclohexane, b Fracture energy of sol-vent-modified and macroporous epoxy networks prepared via CIPS with various amounts of cyclohexane... 

Russell, A.J. and Street, K.N. (1984). Factors affecting the interlaminar fracture energy of graphite/epoxy laminates. In Proc. 4th Intern. Conf. on Composite Materials. (T. Hayashi, K. Kawata and S. Umekawa eds.), Japan Society of Composites Materials, Tokyo, p. 129. 

Various test geometries may be used to determine values of the fracture energy, G,c, and stress-intensity factor, KIc, at the onset of crack growth and the more common ones are illustrated in Fig. 1. 

Grillet et al. (1991) studied mechanical properties of epoxy networks with various aromatic hardeners. It is possible to compare experimental results obtained for networks exhibiting similar Tg values (this eliminates the influence of the factor Tg — T). For instance, epoxy networks based on flexible BAPP (2-2 - bis 4,4-aminophenoxy phenyl propane) show similar Tg values ( 170°C) to networks based on 3-3 DDS (diamino diphenyl sulfone). However, fracture energies are nine times larger for the former. These results constitute a clear indication that the network structure does affect the proportionality constant between ay and Tg — T. Although no general conclusions may be obtained, it may be expected that the constant is affected by crosslink density, average functionality of crosslinks and chain... 

Eq. (13.145) is called the Griffith equation for thin sheets, i.e. in plane stress, where Gic, known as the fracture energy, has replaced 2y. A frequently used parameter in plane stress is the critical stress energy factor Kq, which in the case of a wide sheet (plane stress) is defined as... 

A third method which recently provided considerable insight into the role of crazes in deformation and fracture of amorphous polymers is the optical interference measurement of crazes (preceding a crack). Since the pioneer work of Kambour, this method has been widely used to determine characteristic craze dimensions and critical displacements. W. Doll gives an overview on recent results and on their interpretation in terms of fracture mechanics parameters (stress intensity factor, plastic zone sizes, fracture surface morphology, fracture energy). 

Analysis of Failure Failure of "Flawless" Materials Fracture Mechanics Griffith Theory Stress Intensity Factors Fracture Energy Viscoelastic Effects Examples Fatigue Conclusion... 

On comparing Eqs. (10.3) and (10.6), we see that the critical stress intensity factor, Kc, and the fracture energy, or critical strain energy release rate, Gc, are related to each other and to the breaking stress at the crack tip, as follows ... 

Elastomers in the first category show the simplest tearing behavior and are therefore described first. For these materials, once fracture has been initiated, a tear propagates at a rate dependent on two principal factors the strain energy release rate, G, and the temperature, T. The former quantity represents the rate at which strain energy is converted into fracture energy as the crack advances. It is defined by a relation analogous to Eq. (10.4) ... 

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