Ductile fracture, characteristics

The first secondary transition below Tg, the so called fj-relaxation, is practically important. This became evident after Struik s (1978) finding that polymers are brittle below Tp and establish creep and ductile fracture between Tp and Tg. The p-relaxation is characteristic for each individual polymer, since it is connected with the start of free movements of special short sections of the polymer chain. In view of more recent data of Tp Boyer s relation, Eq. (6.29), is very approximate and fails completely for amorphous polymers with high Tg s (e.g. aromatic polycarbonates and polysulphones). Some rules of thumb may be given for a closer approximation. 

Figures 7.64 and 7.65 provide a view of the fracture surface of a bolt recovered following a fall of ground from a hardrock mine. This is at magnification x 10. A close-up of dimpled areas is presented in Figure 7.66, at a magnification x 1500. The dimples are shallow and inclined characteristic of ductile fracture. No sign of cleavage or quasicleavage, is observed (absence of river pattern that would be associated with brittle fracture). In the upper right part of the photo dimpled areas are masked by oxidation.

The characteristics of fracture surfaces of F-185 neat resin and those of F-185 matrix In the composites are similar to those reported In the literature ( ). The fact that the fracture surfaces of F-185 neat resin and F-185 matrix In the composites show typical ductile fracture behavior, while the unmodified HX-205 shows brittle fracture behavior, seems to Indicate the toughening effect of F-185 as a result of Incorporation of CTBN rubber. The fracture energies of these materials are being... 

The fracture surface after ductile fracture has a characteristic shape, one side resembling a cup and the other a cone (Figure 10.20b, for point D Figure 10.21). 

The close-up of dimpled areas is shown in Figure 5.61. The dimples are shallow and inclined characteristic of ductile fracture. No sign of cleavage or quasi-cleavage is observed. The absence of river pattern excludes the possibility of brittle fracture. 

As it is known, the work of fracture U, characterizing expenditure of energy on material deformation up to failure, is one of the most important plasticity characteristics. In paper [5] it was shown, that the fracture character of solids is determined by the fractal dimension df of their structure at 4-2. 50 brittle fracture is realized, at 4 2.50-2.67 - quasibrittle (quasiductile) fracture and at 4 2.70 - ductile fracture. This classification allows to suppose plasticity increase characterized by value U at raising df. Actually, the dependence U dj) shown in Fig. 3 confirms this assumption. This dependence is linear, the increase U is observed at raising df and zero value U is reached at 4=2.50, i.e., at brittle fracture. Since limiting (maximal) value df for real solid is equal to 2.95 then this allows to estimate... 

Work of fracture is the measure of the energy necessary to break an element made with a given material and is closely related to the kind of fracture brittle or ductile. Brittle fracture is characteristic for amorphous materials like glass, and ductile fracture is for crystalline materials like metals. Depending on conditions, the materials may behave in different ways, for example, metals are brittle at low temperature. 

FIGURE 10.17 The dependences of fracture surface fractal dimension d, calculated according to the Eq. (10.28), on characteristic ratio C at fixed dissipated energy fraction Tjj 0.2 (1), 0.5 (2) and 0.8 (3). The shaded Imes mdicate level for ductile fracture ( ) and super plasticity (2 ) [57]. 

Stress-assisted transformation permits some p alloys to achieve a degree of low-temperatvire ductility otherwise unexpected in a bcc structure. For example, Ti 50Nb, whose deformation at low temperature is discussed in Sections 8.1.2 and 12.14.1, when tested to failure at 4.2 K, reveals the finely dimpled fracture surfaces characteristic of microscopic ductile fracture (Fig. 6.2). 

The critical hydrogen content for the ductility loss increased with increasing hydrogen solubility in the alloy. The fracture surfaces were not characteristic of those found under conditions of SCC. In terms of hydrogen and deuterium solubility in a similar series of bcc alloys, the equilibrium constants were determined at infinite dilution as a function of temperature The free energy function was expressed in terms of the bound-proton model. 

One way of looking at the fracture characteristics of a ductile material is by measuring the amount of plasticity at a crack tip prior to crack propagation (Fig. 8.84). One test which measures this is the crack-tip opening displacement (CTOD), 5. Wells has found that 6 can be related to the strain energy release rate, G, by the formula ... 

A more universal fracture characteristic for use with ductile materials is the J integral . This is similar to CTOD but relates a volume integral to a surface integral and is independent of the path of the integral it can be classed as a material property. The J integral can also be used to predict critical stress levels for known crack lengths or vice versa. 

---