Fracture process parameters

Tvergaard V, Hutchinson JW, The relation between crack growth resistance and fracture process parameters in elastic-plastic solids, J. Mech Physics of solids, 40, 6, 1992, 1377-97. 

Shogenov, V. M., Kozlov, G. V, Mikitaev, A. K. (1989). Prediction if Rigid-Chain Polymers Fracture Process Parameters. sokomolek. Soed. B, 31(11), 809-811. 

Hence, the stated above results have shown, that the stable crack in PASF samples, obtained from different solvents, can be treated as stochastic fi actal, whose dimension has clear physical significance - its value is reciprocal to stress concentration degree at a crack tip. The value is determined unequivocally by polymer structure fractal dimension and in its turn, influences on PASF samples fracture process parameters [8]. 

The stress-intensity factors are quite different from stress concentration factors. For the same circular hole, the stress concentration factor is 3 under uniaxial tension, 2 under biaxiai tension, and 4 under pure shear. Thus, the stress concentration factor, which is a single scalar parameter, cannot characterize the stress state, a second-order tensor. However, the stress-intensity factor exists in all stress components, so is a useful concept in stress-type fracture processes. For example. 

A basic aim of fracture mechanics is to identify fracture criteria such as G,c and KI(. and others discussed later which are independent of the geometry of the cracked body and, ideally, even of the test conditions, e.g. test temperature. Thus fracture mechanics parameters should greatly assist in developing a more fundamental understanding of the fracture process and should be of considerable use in the practical areas of material selection and development, engineering design and life prediction. 

The Role of Molecular Parameters in Contact Fracture Processes... 

Since Wf = U and A = It, Equation (14) is similar to Equation (12). It has therefore been argued [23] that the impact fracture energy at crack initiation in polymers with ductile behavior G is rather the essential fracture work we. Furthermore, the second parameter representing the variation of the impact fracture energy during stable crack growth, T, has been attributed to the work dissipated in the outer plastic zone and is not related to the fracture process [23]. 

In PMMA of different molecular weight the craze dimensions have been measured at moving crack tips in the speed range of 10" mm/s < a < 10 mm/s at room temperature using an experimental set-up especially developed to apply the microscopic interference technique under short time conditions thus enabling a complete characterization of all relevant parameters at any instant during the fracture process... 

A molecular transition model involving the -relaxation in PMMA has been first put forward by Johnson and Radon They explained the transition in crack speed behavior based on a correlation between the temperature dependence of a time to failure inferred from fracture experiments and the temperature variation of the reciprocal frequency of the P-relaxation peak. They thus assumed that the crack transition is caused when the P-process is fully active. Also the fracture mechanics parameter K, governing the transition from slow to fast crack growth, shows a time and temperature dependence equal to that of the p-transition... 

Many process parameters are responsible for transcrystallinity." The most extensive transcrystallinity is observed under rapid pulling of fibers and high cooling rates. Other parameters include the viscosity of the polymer melt, the rate of shear, the fiber/matrix wettability, and the temperature gradient between matrix and fiberFigure 10.15 shows the effect of interlayer thickness on the fracture en-... 

Equilibrium tests at the local deformation stage of a sample provide adequate changes of external forces to internal efforts of a material to resist with corresponding static development of the main crack. These tests are most appropriate when using bending or tension of the samples, because the fracture process will be defined by development of a unique breakaway type of crack, which allows determination of the actual surface area of the fracture. This means that the tests correctly provide the real physical processes of fracture of a concrete and the principles of nonlinear fracture mechanics with traditional mechanical characteristics of concrete and allows determination of a set of power and energy parameters of the material fracture. 

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