Testing, tough-brittle transition impact

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. 

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. 

Charpy impact testing, also known as Charpy V notch testing, is performed on materials to determine toughness properties, usuaUy at low temperature. CIT enables one to determine the transition temperature between brittle and ductile faUure for any material or material specimen. CIT is also a good indication of a materials abiUty to absorb shock loads at low temperatures. CIT is used predominantly for carbon and low aUoy steels. It is not used for stainless steels because stainless steels do not fail in a brittle manner until extremely low temperatmes. 

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). 

The widespread use of Izod and Charpy impact tests to evaluate plastics is, to an unprejudiced eye, rather difficult to justify. Many structural polymers us in load-bearing applications do show a range of fracture behaviour from ductile to brittle . Most thermoplastics can show either kind of behaviour, and may suffer an abrupt tough-to-brittle transition with any of a number of parameters — one of which is the rate of loading at a notch. In order to select a polymer for a specific application it may be important to know its sensitivity to this kind of impact embrittlement. However, it is difficult to see how one might learn this fiem conventional impact strength data. 

PET. The behavior of crystalline PET at plane strain can be explained if its yield locus is similar to that of PS and PMMA (9, 10) where a craze locus intercepts the shear yield locus. The transition at plane strain to a craze locus would account for the brittleness. This transition, which takes place quite sharply at W/t = 23 (W/b = 8), is probably the cause for the low impact strength (< 1 ft-lb/inch) of the Vs-inch thick notched bars. The plane strain brittleness can be avoided if the geometric constraints can be removed, such as making the notch less sharp or making the test bar thinner. In fact, unnotched bars of PET, equivalent to having an infinite notch radius, are quite tough. The notch sensitivity of PET is typical of crystalline polymers. 

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