Transition test speeds

Toughness assessment of ductile polymers is still a matter of debate. A sensitive way to characterise the mechanical performance of these materials, and to rank them, is to determine their ductile-brittle transitions. Test speed can thus be varied over several decades of test speed, while keeping the temperature constant, or a wide range of temperature can be scanned in controlled steps at given velocity. In the first case, the higher the speed at which the tough-to-brittle transition occurred, the better the grade in terms of fracture resistance. In the latter case, the lower the temperature at which the brittle-to-ductile transition occurred, the more suited the material for impact applications. 

In most thermoplastics, transitions from ductile to brittle behaviour may be induced by increasing the test speed. For the reasons already invoked in the introduction, this is of particular concern in iPP, whose impact proper-... 

Fig. 21 The critical stress intensity for mode I crack initiation at different temperatures as a function of test speed in a iPP with Mw of 248 kg mol1 and a polydispersity of 5.2 and a similar material containing approximately 80 wt% y3 phase. The arrows mark ductile-brittle transitions in the y3 modified specimens [24]...

Fig. 5 Evolution of the fracture energy, Gtot, with the temperature, T, for non-nudeated and /S-nucleated resins with different flowabilities a MFR 0.3 dgmin-1 and b MFR 2 dgmin-1. The ductile-brittle transition temperature was chosen in a somewhat arbitrary manner as the temperature corresponding to half of the maximum of Gt01 in the considered MFR range. It reflects the transition from a semi-ductile to a fully ductile behavior, without breaking of the tested specimen. The test speed was about 1.5 ms-1, the specimens were injection molded...

The fracture test on three-point-bend samples revealed that the fracture behavior of ABS remains ductile for temperatures above -80 °C at a crosshead speed of 5mm/min. The maximum fracture energy at crack initiation is also observed around -80 °C. Figure 27.20 shows that above this temperature, G, decreases continuously with increasing temperature. When the test speed is increased to lOOmm/min, the temperature at brittle ductile transition is shifted to about -40 °C as shown in Figure 27.21. At an impact velocity of 2.5 m/s, the brittle-ductile transition occurs at around -20 °C as shown in Figure 27.22. The results also confirm that the fracture energy at crack initiation is maximum at the brittle-ductile transition. 

Fig. 2. Evolution of the apparent toughness, Kj k, with the logarithm of the test speed, v, at room temperature for iPP/EPR-1 and iPP/EPR-2. The arrows indicate the test speed at which the ductile-brittle transitions occur.

Both criteria are exemplified in Table 2 and 3 for iPP/EPR-1 tested at room temperature. Table 2 shows (i) to be violated when the mode of failure is ductile (i.e at 0.001 m/s), whereas it remains valid, as expected, in case of brittle fracture (i.e 6 m/s). Table 3 highlights that plane stress conditions prevail roughly up to speeds higher than one decade of test speed tthan the ductile-brittle transition. 

Fig. 8. Apparent toughness, K] , , plotted against the test speed, v, for different ligament length. The shadowed regions correspond to the transition zones. Material iPP/EPR-1 tested at room temperature.

Because of the dependence of the ductile-brittle transition with the crack length, it would be advantageous to define it in terms of rp or Ketr- Their evolution over the investigated range of test speeds is given in Fig. 9. To define an unequivocal ductile-brittle transition is, however, ambitious does it occur at 0. 4 m/s, the maximum of Keir, or at 0.7 m/s, the inflection in rp ... 

Fig. 9. Size of the plastic zone, rp, and effective acture resistance, K tr, plotted against the logarithm of the test speed. Grey zone transition zone, incertainty about the values tp and K fp. Material iPP/EPR-1 tested at room...

Indeed, as obvious from both exemples given in Fig. 2, the transition could thus be determined accurately within 0.1-0.2 decades of test speeds with few samples in a relative short time frame. Moreover, as the apparent values (Kimax) are always lower than the effective parameters (Keff), none of the material descriptor would be overestimated. In addition, since Kjmax-values have been shown to provide a semi-quantitative evaluation (in terms of test speed or temperature) of fracture resistance parameters, a coherent material comparison would be possible over the whole investigated range. This remark remains true as long as the grades have similar rp. For iPP grades, it should be checked (and considered with more caution) when materials exhibit different particle and matrix melt flow rates, or different crystalline structures. It should also be investigated in detail when different polymer families (ABS versus HIPS or rubber modified iPP) are compared. 

Table I. Transitions and Fracture Behavior of 2 L RTPMMA over the Range of Testing Speeds Investigated...

The final transition, Transition II, occurs at a test speed about two decades above that at which Transition I is seen. The unstable fracture already occurs in the linear domain of the force-displacement curve (a in Figure 3). This transition is related to the total disappearance of toughening effects. The absence of a whitened zone on the specimen is noticeable. Under these conditions, the KImax = Klc and GImax = Glc values measured for the 2 L15 system are even lower at high test speeds than those measured for the neat PMMA. 

It is important to note that for all 2 L RTPMMAs, the whitened area of the fracture surface decreases continuously between transitions 0 and II when the test speed is raised, and it does not present the abrupt variation that can be observed when the temperature is varied. 

Because the highest testing speed reached in our experiment setup was 14 m/s, it is not clear whether Transition II for 30% and 45% RTPMMA was fully completed at the highest velocities investigated. If it was not, a further decrease in fracture resistance may occur at higher velocities. Transitions 0 and I of 2 L15 occur at speeds close to those of the transitions in the neat ma-... 

Table III. Test Speeds at Which Transitions Occur, and Crack Speed for Neat PMMA and 2 L RTPMMAs...

Our investigations over a wide range of test speeds have shown that the fracture behavior is a fully time-dependent process, which involves the glassy matrix as well as the rubbery particles. The transitions result from the progressive disappearance of the ability of the material to develop large-scale plasticity triggered by the presence of the particles. They appear then to be mainly controlled by matrix-relaxation behavior together with the nature of the particles. 

The mechanical behavior of polymers is well recognized to be rate dependent. Transitions from ductile to brittle mode can be induced by increasing the test speed. The isotactic PP homopolymer with high molecular weight is ductile at low speed tensile tests. It is brittle at tension under high test speeds at room temperature. Grein et al. (62) determined the variation of Kiq with test speed for the a-PP CT samples (Fig. 11.22). The force-displacement (F-J) curves and the schematic diagrams of the fracture surfaces of CT samples are presented in Fig. 11.23. At a very low test speed of 1 mm s , the F-d curve exhibits a typical ductile behavior as expected. At 10 mm s, the F-d curve stiU displays some nonlinearity before the load reaches its maximum value, but this is substantially suppressed as test speeds increase further. The samples fail in brittle mode at test speeds >500 mm s . From Fig. 11.22, the Kiq values maintain at 3.2 MPam at test velocities from 1 to... 

Figure 11.26 Variation of Kjq with the crack-tip loading rate, dKJdt, for ct-PP and (3-PP CT specimens tested at (a) —30°C, (b) —5°C, (c) 25°C, and (d) 60°C. The upper scale gives an indication of the test speed. The arrows indicate the ductile-brittle transition in (3-PP. (From Reference 31 with permission from Elsevier.)...

---