Ductility-promoting effect

Additive levels below a certain threshold make the bath more snsceptible to imparity effects. For example, iron contamination of 100 mg/L without the recommended concentration of ductility-promoting agents was found to produce a columnar grain strnctnre at the hole corners. Similarly, organic contaminants such as photoresist can prodnce laminar deposits. 

The observation that an increase in temperature or a decrease in rate both result in the same fracture response points toward a viscoelastic influence on thermoset fracture behavior, especially crack initiation. This characteristic behavior of epoxies has been explained qualitatively by consideration of the temperature and strain rate effects on the plasticity of the material at the crack tip . In effect, test conditions which promote the formation of a so-called crack tip plastic zone, or blunt the crack by a ductile process, promote unstable crack propagation. This aspect of unstable fracture is subsequently discussed in more detail. 

Many austenitic stainless steels display a maximum in hydrogen embrittlement susceptibility at a temperature near 200 K and, like the ferritic alloys, are less susceptible at elevated temperature. The former behavior is illustrated in Fig. 3.7, which shows that the relative ductility of several austenitic stainless steels is lowest between 200 and 250 K [11] (a relative ductility value of 1 implies no hydrogen embrittlement). One effect of low temperature is to promote strain-induced martensite in metastable austenitic stainless steels, which enables hydrogen embrittlement. While a maximum in hydrogen embrittlement at moderately low temperatures is common for austenitic stainless steels, it is not a universal trend at least one alloy shows a minimum near room temperature [41, 42]. 

Trachte and DiBenedetto, 1971 Wambach et ai, 1968). Since PPO is much more ductile at 25 C than the epoxy resins mentioned, the effects of filler and adhesion promoter on PPO should tend to resemble the effects on an epoxy resin in a ductile state (e.g., at 130°C). Indeed this is the case. The point is that a filler tends to increase surface roughness and hence y in an otherwise brittle matrix, especially if the filler-matrix adhesion is poor, but tends to inhibit plastic deformation (by constraints or by simple volume replacement) in an otherwise ductile matrix. Such effects are not accounted for in Nielsen s simple treatment (Section 12.1.2.3) and conceivably may occur as competitive mechanisms (see Figure 12.20). A useful summary of such competitive factors is given in Table 12.3 for the glass-bead-epoxy systems (DiBenedetto and Wambach, 1972) the discussion should be relevant to other cases as well. 

The specimen used in Izod test must be notched. The reason for notching the specimen is to provide a stress concentration area that promotes a brittle rather than a ductile failure. A plastic deformation is prevented by such type of notch in the specimen. The impact values are seriously affected because of the notch sensitivity of certain types of plastic materials. This effect was discussed in detail in Section 2.7.2.B. 

---