Impact fracture resistance

The method developed to its highest state in the ESIS test protocol [2] can be seen as a rigorous geometric correction which can give a test residt (Gg) portability to other geometries. The load/displacement trace up to the peak load point is characterised by an area /p. The impact fracture resistance is given as ... 

For a polyoxymethylene (POM), on the other hand, impact data (Fig. 3) do not conform to the thermd decohesion model at all. The minimum predicted impact fracture resistance of this material is f/D,min = 2.21 kJ m"2 at 23°C, but most impact fracture toughness results (Fig. 3) are at least 50% higher than this and there is no sign of a region of -2/3 power impact speed dependence, l ere is a further increase in at displacement rates far too low to be considered as impact or for die thermal decohesion mechanism to be viable. Viscoelastic crack blunting is a much more likely explanation. It seems most likely that the craze mechanics assumed by the thermal decohesion model simply do not apply to POM. 

Figure 3 — Impact fracture resistance data for POM bend specimens as a function of the impact configuration function.

Impact fracture resistance, Gq. The most probable trigger for initiation was identified... 

The closed-form expression for impact fracture resistance given here as Eqn. (4) was derived [4] on the basis of assumptions which were always recognised as restrictive. In particular, the specimen loading was assumed to be linear, at a rate given by the compliance of the initial crack, whilst the craze at its tip was assumed to be small and to correspond to that at the tip of a crack in a centre-cracked plate. 

The magnitude and nature of the load are considered in formulating the design. The load may be essentially quasistatic, cycHc, or impact. Many stmctural failures, for example, have been caused by supposedly innocuous stmctural details welded in place without any consideration given to their effect on fatigue properties. The service temperatures are also important, since they affect the fracture resistance of a material. 

Fig. 11a Notched impact strengths, NIS, at room temperature for non-nucleated, calcium pimelate nucleated, NU-100 nucleated and y-quinacridone-modified PP homopolymers over a wide range of MFR b degree of fracture resistance improvement with the different nucleating agents—the values are normalized with the NIS of the non-nucleated corresponding neat resin taken to evolve as follows MS(non-nucleated) =- 1.26 In(MFR) + 6.01. The test speed was about 3.8 ms1, the specimens were injection molded...

Fig. 25a provides a comparison of the Charpy notched impact strength (MS) at room temperature of the non-nucleated and of the /(-doped resins (fully -modified with 0.1 wt% calcium pimelate) as a function of the IV of the rubber. Both series show the same trends increasing the IV for the rubbery phase has a positive effect on the toughness. However, the delta between their respective fracture resistance at given IV, A(NIS), is not constant varying between 6% for an IV of 2 dgl-1 to 50% for an IV of 1.7 dg 1 1 ... 

For the DMA trace in Fig. 35—typical for almost all reported literature data—it is obvious that both ac- and -relaxations are more pronounced for the /1-nucleated material than for its non-nucleated counterpart suggesting both its amorphous and crystalline phases to be more mobile. Varga showed the progressive increase of the maximum of the loss factor, tan 8, as Kfj was progressively increased from 0 to 75% [137]. This improved damping obviously favors fracture resistance [48,72,77,190]. Detailed correlations between the intensity of the relaxations and the impact strength in -nucleated PP would be beyond the scope of this review they are published elsewhere [111, 191]. 

From the variation of Gt with temperature in Figure 27.7, it can be seen that Ta tends to increase as G decreases. Since the yield stress varies with temperature, there seems to be a correlation between these fracture resistance parameters and the plastic zone in front of the crack tip. Furthermore, at an impact speed of 2 m/s, because the thermal conductivity of polymers is relatively low, it could be expected that the heat generated in the plastic zone could not dissipate away. The rise in temperature at the crack tip could therefore result in a localized reduction in the yield stress and a blunting effect of the crack tip [24,25]. 

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