Testing, tough-brittle transition

The tough-brittle transition temperature is hard to define it is, of course, strongly dependent on the conditions, such as the time scale of the experiment, notch effects etc. The brittleness temperature is, in general, being determined by a series of standard impact tests, carried out at different temperatures when 50% of the samples are broken in a brittle way, then the brittleness temperature has been reached. 

Tough-Brittle Transition of Glass Fiber Composites by Impact Testing... 

Numerous studies have been made of the mechanical properties of fibrous composites these include recently published papers on impact properties by Izod (1,2, 3,4) and Charpy (5,6) and drop weight (7) tests. We reported the Charpy impact fracture behavior of various glass-polyester composites regarding the effects of temperature (8,9,10), specimen size (8), and fiber orientation (10). This paper describes the effects of the tough-brittle transition in the impact behavior of glass-polyester composites which occurs with a variation of temperature and specimen size. 

Interestingly, the ductile-brittle transition observed for the MIM system provided an opportunity to assess the material fracture toughness, which was not possible using classical fracture mechanics tests due to the intrinsic brittleness of the MIM system. The measurement of the critical crack length, Lc, in the contact plane at the onset of brittle propagation allows estimation of a fracture toughness K C = a x+JnLc in the order of 0.85 MPa m1/2, i.e. much less than that of a poly(methylmethacrylate) homopolymer (1.20 MPa m1/2). 

The effect of crystallinity on the PP fracture behaviour was observed from tests on the neat polymer, by using different crystallisation temperatures and annealing treatment spherulite sizes range from 20 pm to 80 pm, and crystallinity X. from 64% to 75% [20 -21]. As the crystallinity is increased, the elastic modulus is enhanced and the toughness (both critical energy Jq 2 and propagation energy) is considerably reduced a ductile to brittle transition is observed at Xg > 70% This is coherent with results from Ouedemi [22]. 

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. 

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.

The possibility to get geometry independent parameters constitutes a master trump to characterise properly a ductile polymer. However, whereas far from the ductile-brittle transition the evaluation of Kes is achieved easily, it is more challenging closer to it. The difficulties to determine reliable effective toughness values in this latter case and thus precise ductile-brittle transitions will be illustrated with grade iPP/EPR-1 tested at room temperature. 

The transition from plane-stress to plane-strain can also be brought about by impact rate (23), temperature (Fig. 15), molecular weight, and thermal history (24). The effects of thickness and rate are shown by an example in Figure 16. In this example notched polycarbonate (PC) specimens are tested at various bending speeds. The thick (6.4 mm) specimens are uniformly brittle, whereas the thin (3.2 mm) specimens are imiformly ductile. The intermediate thickness (4.4 mm) specimens exhibit a ductile-to-brittle transition at about 0.3 cm/s. In toughened PC even the 6.4-mm-thick specimens are tough up to about 40 cm/s (see Fig. 17). 

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