Fracture ductile-brittle transitions

From this therefore it is evident that the failure stress, ductile/brittle transitions which may be observed in plastics. According to line B, as the fiaw size decreases the failure stress tends towards infinity. Clearly this is not the case and in practice what happens is that at some defect size ([Pg.132]

Steels are normally ductile at ambient temperatures, although they are often close to brittle behaviour, as is indicated by the ductile-brittle transition temperature. If the conditions at the tip of a sharp crack are considered, it can be seen that brittle fracture will occur if it is easier to break the atomic bond at the tip of the crack than it is to emit a dislocation to blunt the crack (see Thompson and Lin ). As dislocation emission is more temperature sensitive than the bond strength it becomes more difficult at low temperatures and brittle fracture occurs. The very severe effects of hydrogen on the performance of steels can be attributed to its role in allowing brittle fracture... 

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. 

Dual nickel, 9 820—821 Dual-pressure processes, in nitric acid production, 17 175, 177, 179 Dual-solvent fractional extraction, 10 760 Dual Ziegler catalysts, for LLDPE production, 20 191 Dubinin-Radushkevich adsorption isotherm, 1 626, 627 Dubnium (Db), l 492t Ductile (nodular) iron, 14 522 Ductile brittle transition temperature (DBTT), 13 487 Ductile cast iron, 22 518—519 Ductile fracture, as failure mechanism, 26 983 Ductile iron... 

Low Test Temperature. The possibility of brittle fracture shall be considered when conducting leak tests at metal temperatures near the ductile-brittle transition temperature. 

To avoid brittle fracture during operation, maintenance, transportation, erection, and testing, good design practice shall be followed in the selection of fabrication methods, welding procedures, and materials for vendor furnished steel pressure retaining parts that may be subjected to temperature below the ductile-brittle transition point. 

Yielding and fracture are two very important properties of materials, particularly for thermosets. Both aspects can be associated by considering the ductile-brittle transition temperature, TB (Fig. 12.5). 

Thus, the understanding of thermosets fracture needs the complete description of the yielding and the influence of both experimental variables, (T, e), on the one hand, and the relationship with structural parameters, on the other hand. Unfortunately, few results are available in the literature dealing with the ductile-brittle transition of thermosets. Very often it is stated that thermosets are more brittle than thermoplastics but this depends only on the location of the test temperature compared with the ductile-brittle transition temperature. 

The fracture energy cannot be related to the failure of chemical bonds which may contribute only with a few Jm-2. Furthermore, the possibility of crazing is not allowed in thermosets because fibrils cannot exist due to the high crosslink density. So, in the case of high-Tg cross-linked materials the main source of energy absorption before failure is the yielding of the network. This assumption is obviously valid only above the ductile-brittle transition temperature (Fig. 12.5), where yielding is temperature-dependent. ... 

The introduction of rubber particles increases the fracture energy of the networks at room temperature, but also decreases the temperature of the ductile-brittle transition (Van der Sanden and Meijer, 1993). This ductile-brittle transition is strongly dependent on the nature (and Tg) of the rubber-rich phase and the amount of rubber dissolved in the matrix. The lowest ductile-brittle transition is obtained with butadiene-based copolymers (Tg — 80°C), compared with butylacrylate copolymers (Tg —40°C). 

Abstract The fracture properties and microdeformation behaviour and their correlation with structure in commercial bulk polyolefins are reviewed. Emphasis is on crack-tip deformation mechanisms and on regimes of direct practical interest, namely slow crack growth in polyethylene and high-speed ductile-brittle transitions in isotactic polypropylene. Recent fracture studies of reaction-bonded interfaces are also briefly considered, these representing promising model systems for the investigation of the relationship between the fundamental mechanisms of crack-tip deformation and fracture and molecular structure. 

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...

Interestingly, the ductile-brittle transition observed for the MIM system provided an opportunity to assess the material fracture toughness, which was not possible using classical fracture mechanics tests due to the intrinsic brittleness of the MIM system. The measurement of the critical crack length, Lc, in the contact plane at the onset of brittle propagation allows estimation of a fracture toughness K C = a x+JnLc in the order of 0.85 MPa m1/2, i.e. much less than that of a poly(methylmethacrylate) homopolymer (1.20 MPa m1/2). 

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. 

In scheme A, step 3 must combine in principle a fracture property molar mass relationship and a molar mass critical value M c corresponding to the ductile-brittle transition. 

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. 

Selecting the ductile - semi-ductile transition as ductile-brittle transition was attempting but not realistic it lead admittedly to substancial reduction of the global fracture energy, but it is known to be highly geometry. Chosing between the semi-ductile - semi-brittle transition or... 

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. 

LeGrand, D.G. Crazing, yielding, and fracture of polymers 1. Ductile-brittle transition in polycarbonate. J. Appl. Polym. Sci. 1969, 13, 2129-2147. 

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