Fracture energy determining factors

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. 

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). 

THE DETERMINATION OF ADHESIVE FRACTURE TOUGHNESS FOR LAMINATES BY THE USE OF DIFFERENT TEST GEOMETRY AND CONSIDERATION OF PLASTIC ENERGY CORRECTION FACTORS... 

INVESTIGATION OF ENERGY CORRECTION FACTORS IN THE DETERMINATION OF ADHESIVE FRACTURE TOUGHNESS. 

By measuring water uptake, the diffusion coefficient and equilibrium concentration of water for the bulk adhesive were obtained at different temperatures. A value of 37 kJ/mol was also calculated for the activation energy of diffusion. A value for the plane-strain stress intensity factor, Kic, for the bulk adhesive was obtained using compact tension specimens. Tensile butt joints were prepared from mild steel blocks bonded with the epoxy adhesive and the fracture stress determined as a function of time of exposure to water at the different temperatures. An activation energy of 32 kJ/mol was calculated for joint failure, in close agreement with that obtained for the diffusion of water. This supports the view that the processes responsible for loss of joint strength are controlled by water diffusion. It was found that joints exposed to 20°C/55% RH showed no reduction in strength, even... 

In nearly all cases fracture is initiated by surface damage. A rather obvious, and yet very general and determining, factor is the fact that nucleation and the development of surface damage serve as a precondition for the initiation of local fracture processes leading to the loss of stability in solids. This, in turn, means that a solid surface is most vulnerable to the action of the ambient medium. In other words, the surface is the source of both the free energy excess due to the presence of uncompensated chemical bonds and the exposure to the active components of the medium [10]. 

Fracture mechanics has been applied to model some of the fracture characteristics asbestos-cement composites, especially those related to crack bridging by the fibres. Mai [12] analysed the behaviour of both notched beams in flexural loading and double cant lever beams (DCB) to determine the critical stress intensity factor, /Cc and the specific work of fracture, WF, which measures the average fracture energy per apparent unit crack surface over the entire fracture process. The effect of fibre content on these parameters is shown in Figure 9.15. The trends resemble those observed for the effect of fibre content on strength (Figure 9.14). 

Step 2. After a contact time t, the material is fractured or fatigued and the mechanical properties determined. The measured properties will be a function of the test configuration, rate of testing, temperature, etc., and include the critical strain energy release rate Gic, the critical stress intensity factor K[c, the critical... 

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 fracture behaviour of polymers, usually under conditions of mode I opening, considered the severest test of a material s resistance to crack initiation and propagation, is widely characterised using linear elastic fracture mechanics (LEFM) parameters, such as the plane strain critical stress intensity factor, Kic, or the critical strain energy release rate, Gic, for crack initiation (determined using standard geometries such as those in Fig. 1). LEFM... 

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