Ductile-brittle behavior

Fig. 11-25. Ductile-brittle behavior in impact resistance [20], The iransilion between the zones varies with rate of impact and type of test.

To see how the fracture energy may be used in the initiation of chemical reactions, the concepts of fracture mechanics are introduced, including the strain rate and temperature dependence of the ductile-brittle behavior. The starting point is the Griffith theory which in its simplest form applies to perfectly brittle materials and states that for a crack to form, the elastic strain energy available must be at least sufficient to provide the energy of the new surfaces formed [74]. 

There are two classes of factors which influence the ductile—brittle behavior of an alloy. These are metallurgical factors and service factors. 

METALLURGICAL FACTORS. Typical metallurgical factors influencing ductile—brittle behavior are temper, microstructure, and welds. In general, the stronger the condition of a given alloy, the more susceptible it becomes to brittle behavior. As strength increases, ductility decreases. There are some exceptions... 

Carpick, R.W., Enachescu, M., Ogletree, D.F. and Salmeron, M., Making, breaking, and sliding of nanometer-scale contacts. In Beltz, G.E., Selinger, R.L.B., Kim, K.-S. and Marder, M.P., (Eds.), Fracture and Ductile vs. Brittle Behavior-Theory, Modeling and Experiment. Materials Research Society, Warrendale, PA, 1999, pp. 93-103. 

Fig. 2-29 Typical creep-rupture ductile-to-brittle behavior of TPs.

The main considerations of mechanical properties of metals and alloys at low temperatures taken into account for safety reasons are the transition from ductile-to-brittle behavior, certain unconventional modes of plastic deformation, and mechanical and elastic properties changes due to phase transformations in the crystalline structure. 

Time and energy can be saved if one recognizes that there is only one qualitative difference between a linear and a tridimensional polymer the existence in the former and the absence in the latter of a liquid state (at a macroscopic scale). For the rest, both families display the same type of boundaries in a time-temperature map (Fig. 10.1). Three domains are characterized by (I) a glassy/brittle behavior (I), (II), a glassy/ductile behavior, and (III) a rubbery behavior. The properties in domain I are practically... 

The location of Tp with respect to ambient temperature, Ta, in the timescale under consideration. In a first approach, one can distinguish between the cases where Tp < Ta (epoxy, phenolics) and Tp > Ta (UP, VE, PI). The networks belonging to the first family have relatively low moduli (E < 3 GPa, G < 1 GPa), and can display a ductile or semiductile behavior. The networks belonging to the second family have relatively high moduli (E > 3 GPa, G > 1 GPa), and generally exhibit a brittle behavior. 

Impact tests can be performed at various temperatures, especially at low temperatures (where there is a combination with the high speed), in order to determine the ductile-brittle transition. This transition is very important for characterizing the polymer behavior, and is determined usually at a constant speed and changing the temperature. Although it is less usual, it is possible to fix the temperature and to vary the speed. 

The ductile-brittle transition is clearly related to the yielding behavior of the thermoset in static experiments, (see Fig. 12.5). 

At room temperature, well below Tg, a brittle failure is generally observed. The ductile behavior appears when yielding becomes a competitive mechanism of deformation. At high speeds the brittle stress is not too much affected but ductile-brittle transition to higher temperatures. 

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...

This conclusion was only partly confirmed by scanning electron microscopy micrographs of RuC>4 stained surfaces taken at the crack tip of deformed specimens at 1ms-1, where the non-nucleated and /3-nucleated materials showed, respectively, a semi-brittle and semi-ductile fracture behavior. While some limited rubber cavitation was visible for both resins, crazes—and consequently matrix shearing—could not develop to a large extent whether in the PP or in the /1-PP matrix (although these structures were somewhat more pronounced in the latter case). Therefore, a question remains open was the rubber cavitation sufficient to boost the development of dissipative mechanisms in these resins ... 

This review has highlighted several aspects of the toughness of / -nucleated PP. Although its long-term behavior because of its relevance in pipe applications has been outlined, the focus has been on dynamic fracture properties, a topic which has been largely documented in the literature. Ductile-brittle... 

A shortening in relaxation time in the critically strained region makes some materials tough. The shift of relaxation time is attributed to strain-induced dilatation and can reach as much as five decades. Thermal history, on the other hand, dictates the initial state from which this dilatation starts and may be expressed in terms of excess entropy and enthalpy. The excess enthalpy at Tg is measurable by differential scanning calorimetry. Brittle to ductile transition behavior is determined by the strain-induced reduction in relaxation time, the initial amount of excess entropy, and the maximum elastic strain that the material can undergo without fracturing or crazing. 

The Transition from Ductile to Brittle Behavior of a Semicrystalline Polyester by Control of Morphology... 

Significant variation of the ultimate mechanical properties of poly(hexamethylene sehacate), HMS, is possible by con-trol of thermal history without significant variation of percent crystallinity. Both banded and unbanded spherulite morphology samples obtained by crystallization at 52°C and 60°C respectively fracture in a brittle fashion at a strain of r O.Ol in./in. An ice-water-quenched specimen does not fracture after a strain of 1.40 in./in. The difference in deformation behavior is interpreted as variation of the population of tie molecules or tie fibrils and variation of crystalline morphological dimensions. The deformation process transforms the appearance of the quenched sample from a creamy white opaque color to a translucent material. Additional experiments are suggested which should define the morphological characteristics that result in variation of the mechanical properties from ductile to brittle behavior. 

Recently Moore and Petrie (5) have demonstrated that control of sample thermal history can result in transition from ductile to brittle behavior for polyethylene terephthalate. This transition in behavior was related to volume relaxation of the glassy state. 

The effects of morphology (i.e., crystallization rate) (6,7, 8) on the mechanical properties of semicrystalline polymers has been studied without observation of a transition from ductile to brittle failure behavior in unoriented samples of similar crystallinity. Often variations in ductlity are observed as spherulite size is varied, but this is normally confounded with sizable changes in percent crystallinity. This report demonstrates that a semicrystalline polymer, poly(hexamethylene sebacate) (HMS) may exhibit either ductile or brittle behavior dependent upon thermal history in a manner not directly related to volume relaxation or percent crystallinity. 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

The ductile vs. brittle behavior depends on the relationship between the equations... 

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