Effective Fracture Surface

For bulk specimens, Kic and Gic convey the same information. However, for adhesively bonded structures, such as the specimens described in this report, Kic and Gic differ. E in Equation 3 is the modulus of the adherend (the PMMA) rather than the adhesive, and so Kic is not characteristic solely of the PS layer but of the composite as a whole. The other expression for fracture behavior is the effective fracture surface energy, y. It is related to the fracture energy by... 

The chief problem is that plastic deformation effects at the crack tip usually obscure the process of surface formation. The work needed to propagate a crack is utilized not only to form two surfaces and to bend the cleaved material elastically but also for plastic deformation around the crack tip [16]. The energy needed for this plastic deformation is dissipated as heat. Hence the measured quantity, which is called the effective fracture surface tension yntts- is numerically larger than y. Reliable results can only be obtained at temperatures below 150 K when plasticity practically disappears 121,55]. 

TABLE 3 Effective Fracture Surface Energies (in 10"1 N/m) Determined by Cleavage Experiments. Below 15 C the Effective Fracture Surface Energy is Close to y... 

Miciophotographs of fractured surfaces at the lowest and highest frequencies of this study, i.e. 1 and 900 Hz are compared in Figure 2. There was practically no difference in the damage mechanisms between I and 100 Hz. and hence 100 Hz case is not shown for the sake of brevity. There were differences on the fracture surface, which can be attributed to test frequency effects. Fracture surface in the matrix region of specimen tested at the frequency of 1 Hz showed only a few large matrix cracks and debris, while its counterpart at the frequency of 900 Hz showed relatively more amount of smaller matrix cracks and debris. Figures 2(a) and (b). On the other... 

The development of fractnre mechanics aimed at consideration of nonelastic and non-linear effects is of particnlar importance for applications in composite materials in which the internal strncture is actually designed to control the cracking and to increase the effective fracture surface energy. A few non-linear fracture approaches are described below. 

One of the purposes of this paper is to differentiate between surface free energy and fracture surface energy. Thus, several basic principles of fracture mechanics are discussed with reference to the Griffith s energy-b J nce concept and the Irwin-Orowan s plastic-zone concept. We also define several frequently misused terms, e.g., effective fracture surface energy, fracture energy and fracture toughness. Actual values of related parameters are presented to illustrate the applicability of fracture mechanics to the selection of polymers for structural materials. 

In the following sections, we shall discuss three modes of fracture and several fracture criteria. Later, we shall enumerate several major variables affecting effective fracture surface energy, fracture energy and fracture toughness of polymers. 

We now define the effective fracture surface energy, r, as the energy in creating two new surfaces and it s quantity becomes... 

Since the effective fracture surface energy includes chiefly the dissipative component, or the work of plastic deformation, Wj, and a minor component, surface free energy of a solid, 7 we may write... 

IV. PAKAMETERS AFFECTING EFFECTIVE FRACTURE SURFACE ENERGY, AND FRACTURE TOUGHNESS OF POLYMETHYL METHACRYLATE... 

Table I. Effect of Molecular Weight on Effective Fracture Surface Energy and Fracture Toughness of Polymethyl Methacrylate,...

The effect of molecular weight on fracture energy and fracture toughness of polymethyl methacrylate (PMMA) is illustrated with the data calculate< j from the effective fracture surface energy obtained by Berry ... 

The effect of crack speed on fractur to g of PMMA has been tabulated by Marshall and Williams." From their data, we calculated the effective fracture surface energy and the frac-ture2 nergy as shown in Table II. In Fig. 6, fracture toughness ia.shown to increase with the increase in crack speed, a. Williams used the analytic method to relate K and the strain rate at the tip of a propagating crack by the rollowing equa-... 

Table II. Effective Fracture Surface Knergles> of Polymethylmethacrylate. ... 

Effect of Molecular Orientation on Effective Fracture Surface Energy... 

For the study of the effect of orientation on fracture surface nergy, PMMA sheets were biaxially oriented in their planes. Effective fracture surface energies for three... 

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