Absolute fracture energy

The coupling of Fg with the other loss terms ilr (cf. Eq. 10) means that even a modest absolute increase in Fg may lead to a much larger increase in fracture energy F. 

The three types of force-time behavior noted for HiPS fractured at different temperatures also apply to other polymer blends and grafts, where values of the impact strength (or fracture energy) were measured as a function of temperature. Such behavior has been observed by Bucknall and Street (1967) not only for ABS (Figure 3.17), but also for rubber-modified PVC, HiPS, and a methacrylate-butadiene-styrene (MBS) copolymer. Not surprisingly, the concentration of rubber is important with respect to both the absolute value of impact strength (Figures 3.16 and 3.17) and the type... 

The pre-treatment of a filler surface changes fhe interface, and thus it is expected to affect the properties of adjacent phases extending some way into the bulk. There are examples of using the various surface modifiers such are acids and acid precursors, alkoxysilanes, organofitanates and related compounds, stearic acid and others." How the surface pre-treatment of CaCOs filler by sodium stearate changed fhe adhesion parameters in the PVAc composite is illustrated in Table 1 The adhesion parameters could be used to relate the interactions at the interface to the mechanical properties of the composite. For example, the small absolute decreases in the work of adhesion after the pre-treatment can lead to the proportionate large absolute decreases in fracture energy. 

In Fig. 10.12 the dependences of fracture and fracture surface fiactal dimensions, calculated according to the Eqs. (1.9), (4.50), (4.51) and (5.17) and the values D, calculated according to the Eq. (10.17) are adduced as a notch length function for HDPE samples in Sharpy impact tests. The values D and very good correspondence is quite obvious by both the dependence on the course and absolute values. This means, that the fracture energy U value for HDPE is defined by polymer structural state, which is characterized by fractal dimension d. The coupling of U (or A ) with the value d at fracture type (mechanism) correct choice is determined by the dimensions d and d intercommunication, expressed through Poison s ratio value [41]. 

Depending on plasticity degree a polymers behavior in impact tests is described by either the Eq. (10.21) or the Eq. (10.22). Let us note, that the Eq. (10.21) was derived from the conditions of elastic energy in sample accumulation and dissipation and therefore, fracture process itself in the obvious form (i.e., new surfaces formation) is not taken into consideration at the derivation [23]. The Eq. (10.22) was obtained from the modified energetic Griffith criterion, in which a new surfaces formation at crack If ont advancement is directly taken into consideration [22]. At such treatment it is supposed, that crack edges are absolutely flat, though experimental observations... 

The work 7" of fracture was identified hy Dupr and Griffith with the surface energy of solids. In reality work is spent on the deformation leading to rupture rather than on the Break itself. In simple instances it is equal to the work needed to extend a column of material (of unit cross-section) in front of the growing crack until the column snaps. This extension may he purely elastic. The new concept contraiy to the old (a) indicates the analogy Between the Breaks of a liquid and a solid (B) accounts for the absolute value of y/ (c) agrees with the effect of cross-linking on (h) is compatible with the rate dependence of... 

Experimental results of a quasi-unidirectional CMC revealed different creep behavior in tension and compression. Due to the porous matrix, the samples with 90° fiber orientation showed the largest creep deformation rates. In tension creep the fracture strain was 0.4-1% for 90° samples whereas in 0° fiber orientation creep strain above 8% was feasible. At the beginning of compression experiments, the absolute creep rate was highest and decreased continually. Creep parameters, i.e. temperature and stress dependencies, were deteimined in case of compression stress. Thereby, the activation energy averaged to 700 kJ/mol and the stress exponent varied with the fiber orientation from 3 to 1.9. The variation in the stress exponent was presumably caused by different stress exponents of fibers and matrix and quite likely due to the effect of matrix compaction. Latter one could be visualized by optical micrographs. 

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