Brittle-tough transition

The presence of notches or sharp angles or of a few holes, voids, particle inclusions or small inserts tends to concentrate the stress. Different polymers vary in their notch sensitivity and this is presumably a reflection of how close they are to their tough-brittle transitions. The aim of the designer and processor must be to reduce such stress concentration to a minimum. 

Figure 10.8. Effects of melt flow index and density on the room temperature tough-brittle transition of polyethylene. (Reproduced by permission of ICI)...

It may be seen from Fig. 2.80 that some plastics experience the change from ductile to brittle behaviour over a relatively narrow temperature range. This permits a tough/brittle transition temperature to be quoted. In other plastics this transition is much more gradual so that it is not possible to attribute it to a single value of temperature. In these circumstances it is common to quote a Brittleness Temperature, rg(l/4). This temperature is defined as the value at which the impact strength of the material with a sharp notch (1/4 mm tip radius) equals 10 kJ/m. This temperature, when quoted, gives an indication of the temperature above which there should be no problems with impact failures. It does not mean that the material should never be used below Tb(1/4) because by definition it refers only to the sharp notch case. When the material has a blunt notch or is un-notched its behaviour may still be satisfactory well below Tb(1/4). 

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. 

Therefore the clear, tough-brittle transition exists, with reference to the temperature variation, when the notch was cut on the edge of... 

Tough—Brittle Transition in Cloth Laminates. For cloth specimens notched edgewise transition of fracture mode occurred with a variation of specimen size although the law of similarity held for each region having the same fracture mode. Figure 5 shows this transition for bidirectional cloth specimens at room temperature. With the increase of remain-... 

Both of these individual strengths increase with increase in the rate at which strain is applied. Yield strength has heen found to be more sensitive to change in strain rate than brittle strength, and thus increases more quickly. For a material showing tough fracture at a low strain rate, this means that there comes a point at which Oy and are equal, and this corresponds to the tough-brittle transition. Further increase in the strain rate causes a, to... 

Orovan E (1948-49) Fi acture and strength of solids. Kept Prog Phys 12 185-232. Vincent P I (1960) The tough-brittle transition in thermoplastics, Po/ymer 1 425-444. Ward I M and Hadley D W (1993) An Introduction to the Mechanical Properties of Solid Polymers, Wiley, Chichester, New York, pp. 271-276. 

Figure 9.65 Variation of tear strength with melt flow index showing the tough-brittle transition for LDPE, HDPE, and EV copolymers at 10% volume fraction of silica. (From Ref. 61.)...

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