Brittle-mode transition

Impact modifiers for PET are generally elastomeric compounds that increase impact strength and elongation while usually decreasing modulus. An effective way to enhance the impact strength and to induce a brittle/ductile transition of the fracture mode, is by the dispersion of a rubber phase within the PET matrix. The... 

The brittle-ductile transition temperature depends on the characteristics of the sample such as thickness, surface defects, and the presence of flaws or notches. Increasing the thickness of the sample favors brittle fracture a typical example is polycarbonate at room temperature. The presence of surface defects (scratches) or the introduction of flaws and notches in the sample increases Tg. A polymer that displays ductile behavior at a particular temperature can break in the brittle mode if a notch is made in it examples are PVC and nylon. This type of behavior is explained by analyzing the distribution of stresses in the zone of the notch. When a sample is subjected to a uniaxial tension, a complex state of stresses is created at the tip of the notch and the yield stress brittle behavior known as notch brittleness. Brittle behavior is favored by sharp notches and thick samples where plane strain deformation prevails over plane stress deformation. 

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

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... 

In contrast to the compatible PPO blends, the transitional blends exhibit a sigmoidal dependence of a on Vpp (Fig. 10). A minimum is reached between 10 and 20% PPO while at 60 to 80% PPO a maximum appears to exist at the same level and in the same composition range as found for the compatible blends. The embrittlement transition is again apparent at high VppQ but more samples fail in the brittle mode at between 60 and 80% PPO than at corresponding blend compositions in the case of the compatible blends. 

The incompatible blends of PpClS/PPO, exhibit a much broader minimum in a (Fig. 11) but again a appears to reach a S3mergistic level at 80% PPO. More samples fail in the brittle mode at corresponding compositions than do the compatible and transitional blends in the embrittlement region. Additional data is required... 

Brittle-Ductile Transition The temperature at which the mode of fracture changes from brittle to ductile fracture. 

Fig. 17.12 Image representing failure modes in impact specimen, (a) A complete break into two pieces for brittle sample, (b) a mixed mode with hinged or a complete break for samples near brittle-ductile transition, and (c) a hinged break for tough samples (Tiwari and Paul 201 Ic)...

It can be argued that the intersection points in the second and fourth quadrants of Figure 11.10 constitute a brittle-ductile transition, both yielding mechanisms being coexistent. The brittle-ductile transition is a transition in the sense of which mode is predisposed to happen first in a given stress field (14). 

The extremely low ductility values at ambient temperature and the increased ductility with increasing temperatures strongly influence the observed fracture mode. Tfensile and fatigue specimens indicate that the predominant fracture modes are cleavage at low temperatures due to dislocation pile-up and intergranular fracture above the brittle-ductile transition temperature (Ref 13-17). 

All of that stated above allows it to be supposed that the brittle-ductile transition in the considered epoxy polymers is controlled by the Ludwig-Davydenkov criterion [72]. This very simple criterion assumes that the brittle-ductile transition controls the relation of fracture and yielding stresses, namely if the fracture stress is smaller than the yield stress then the material breaks in a brittle manner, if the other way around by the ductile mode. 

The following topics are addressed in this chapter simple fracture (both ductile and brittle modes), fundamentals of fracture mechanics, fracture toughness testing, the ductile-to-brittle transition, fatigue, and creep. These discussions include failure mechanisms, testing techniques, and methods by which failure may be prevented or controlled. 

For thermoplastic polymers, both ductile and brittle modes are possible, and many of these materials are capable of experiencing a ductile-to-brittle transition. Factors that favor brittle fracture are a reduction in temperature, an increase in strain rate, the presence of a sharp notch, an increase in specimen thickness, and any modification of the polymer structure that raises the glass transition temperature (T ) (see Section 15.14). Glassy thermoplastics are brittle below their glass transition temperatures. However, as the temperature is raised, they become ductile in the vicinity of their T s and experience plastic yielding prior to fracture. This behavior is demonstrated by the stress-strain characteristics of poly(methyl methacrylate) (PMMA) in Figure 15.3. At 4°C, PMMA is totally brittle, whereas at 60°C it becomes extremely ductile. 

The early study of brittle failures, notably those of the Liberty ships, indicated a temperature dependence. This can be illustrated by plotting both fracture stress (of) and yield stress (Oy) against temperature (Fig. 8.81). Below a certain temperature some materials exhibit a transition from ductile to brittle fracture mode. This temperature is known as the ductile-brittle transition temperature DBTT. 

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. 

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]...

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