Ductile-mode transition

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. 

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]...

Two families of transparent polycarbonate-silicone multiblock polymers based on the polycarbonates of bisphenol acetone (BPA) and bisphenol fluorenone (BPF) were synthesized. Incorporation of a 25% silicone block in BPA polycarbonate lowers by 100°C the ductile-brittle transition temperature of notched specimens at all strain rates silicone block incorporation also converts BPF polycarbonate into a ductile plastic. At the ductile-brittle transition two competing failure modes are balanced—shear yielding and craze fracture. The yield stress in each family decreases with silicone content. The ability of rubber to sustain hydrostatic stress appears responsible for the fact that craze resistance is not lowered in proportion to shear resistance. Thus, the shear biasing effects of rubber domains should be a general toughening mechanism applicable to many plastics. 

Ductile-brittle transitions are more accurately located by variable temperature tests than by altering impact speed in an experiment at a fixed temperature. This is because a linear fall in temperature is equivalent to a logarithmic increase in straining rate. The ductile-brittle transition concept can be clarified by sketches such as that in Fig. 11-25. In the brittle region, the impact resistance of a material is related to its LEFM properties, as described above. In the mixed mode failure... 

Both criteria are exemplified in Table 2 and 3 for iPP/EPR-1 tested at room temperature. Table 2 shows (i) to be violated when the mode of failure is ductile (i.e at 0.001 m/s), whereas it remains valid, as expected, in case of brittle fracture (i.e 6 m/s). Table 3 highlights that plane stress conditions prevail roughly up to speeds higher than one decade of test speed tthan the ductile-brittle transition. 

Comprehensive structural study of Ti-3Al-5Zr-Si-alloys, as-cast and deformed, confirmed the features found with the binary Ti-Si-system described above. The transition from polygonal to dendritic structure takes place between 2- and 4-wt.% Si. Alloy with 2-wt.% Si fails with intergranular (but ductile) mode whereas alloys with 4- and 6-wt.% fail with mixed mode where dendritic structure may be recognized. In any case, eutectic areas, in contrast to dendrite or polygonal bodies, which are of a-phase failing with cleavage microcracking, fail with ductile mode - with voids coalescence (Fig. 8). Hot plastic deformation transforms the alloys studied into ductile or semi-ductile materials, which fail only with ductile void coalescence mode [1],... 

Syn, C.K. and Taylor, J.S., Ductile-brittle transition of cutting mode in diamond of single crystal silicon and glass. Poster session 1989 ASPE/IPES conference, Monterey (1989). 

Notched impact specimens fail in three different modes based on the ability to absorb impact energy a hinged break for tough samples, a complete break into two pieces for brittle samples, and a mixed mode, either a hinged or a complete break, for samples near the ductile-brittle transition composition. These three failure... 

Morphological studies have indicated that failures of all BRHA composites are brittle in nature while WRHA composites experience ductile failiu-es [2]. It appears that the incorporation of the WRHA does not alter the ductile mode of failure of the polypropylene matrix. On the other hand, slight addition of the BRHA results in a marked transition of the failure mode (to brittle fracture). 

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 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. 

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... 

Ni-20Cr 60 ppm Ce 0.90 rate, transition from brittle intergranular to ductile transgranular fracture mode ... 

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

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