Fracture critical temperature

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

The properties of the Nicalon /SiC (PIP) system followed a similar pattern (A/c(ou) = 400°C, A rc(omc) = 500°C), though this system failed through an interlaminar shear failure process (delamination) and the property reduction saturated at A T= 600°C. The Nicalon /SiC (CVI) system failed by fracture through fibre planes but its properties (ou, omc, WOF) had the same critical temperature difference, A Tc = 700°C. The pre- and post-quench stress-displacement curves for this material can be seen in Fig. 15.9. However, measurement of the Young s modulus of this system before and after quenching by means of a dynamic mechanical resonance technique showed the onset of decrease at ATC(E) = 400°C, i.e. significantly lower than the A 7C of the other properties. 

This effect may also cause an oscillatory instability of the front propagation in the case of the isothermal mechanism of the auto wave process. However, in this case a jumplike onset of dispersion in the next layer of the solid matrix will be connected not with the critical temperature gradient of thermal fracture, but with the critical concentration of the final product accumulated at the boundary of this layer (fracture due to a local alteration of the density). 

Select low-temperature steels for fracture-critical structural members designed for tensile stress levels greater than a ksl (40 MPa) and specify a minimum Charpy V notch Impact energy absorption of 20 ft-lb (27 J) for base metal, heat-affected zones (HAZs), and welds when the structures are exposed to low-ambient temperatures. Fracture-critical members are those tension members whose failure would have a significant economic impact. 

The phases are readily identified by the colour of polymer traces present in the crystal, blue above and red below 142 K Sharp phase boundaries between the phases have been identified by Bloor et al. as the (031) and (041) crystallographic planes of the two structures. Larger crystals tend to fracture during the transition due to nucleation of several low temperature phases at crystal defects. The large change in crystal density ( 4%) renders the transition temperature sensitive to hydrostatic pressure At a pressure of about 3 10 MPa the critical temperature is shifted... 

As an example, Fig. 8 shows the fracture toughness for PMMA and Fig. 9 the fringe pattern transition at the critical temperature, whereas Fig. 10 shows the lateral face of the sample with the crack-tip above and below the critical temperature. It has also been shown that neither the bulk modulus nor the craze stress varies near the critical temperature (Fig. 11 and 12). It seems that the local material property varying near that particular temperature is the craze stiffness, as shown in Fig. 13. 

Fig. 8. Fracture toughness K, (at about lOjim/s crack velocity) for PMMA over a wide temperature range. Note the sharp increase of K, below the critical temperature = —20 C, below which multiple crazing occurs at the crack tip. Above the critical temperature, Kj remains almost constant versus temperature. From Ref courtesy of Chapman and HaU, Ltd.

Recently, a modified small punch testing (MSP) method has been developed for mechanical evaluation of ceramic and ceramic/metal composite materials used in FGMs [5]. In the present study, the MSP method was applied to evaluate TiC-based ceramics, which are employed as ceramic-side materials in a TiC/Ni3Al FGM. Because the TiC/Ni3Al FGMs apparently show different damage behavior at temperatures from 1208 to 1373 K, the fracture strength and deformation of TiC-based ceramics were measured at these two critical temperatures. The differences in the behaviors of different TiC sintered bodies and their... 

Thus if the temperature difference AT becomes larger than AT, defined by Equation (8), the fracture permeability will increase drastically. AT, is referred to the critical temperature difference. Note that AT is independent of the size of the cooled region, i.e. d. 

Figure 8.62 Influence of test temperature on fracture toughness. The transformation zone size increases with decreasing temperature, as the tetragonal particles become less stable. At some critical temperature, however, the transformation can become spontaneous.

Mechanical properties of SiC-AlN-Y203 composites (SiC 50%wt-AlN 50%wt), pressureless-sintered with an innovative and cost-effective method, were determined before and after oxidation performed at 1300°C for 1 h. As a consequence of the oxidative treatment, fracture toughness increased from 4.6 MPa m to 6.6 MPa m, flexural strength from 420 MPa to 488 MPa, Weibull modulus from 4.5 to 5.3 and thermal shock resistance (expressed as critical temperature difference) from 3I0°C to 380°C. First of all, these results demonstrated that a pre-oxidation treatment is needed to increase the mechanical resistance and reliability of SiC-AlN-Y203 components. Secondarily, the beneficial effects of the oxidation on the mechanical properties could be explained in terms of compressive residual stresses and crack healing ability. 

Creep rate spectroscopy was also successfully used as the method allowing one to estimate the ability of steels to be inclined to brittle fracture. The temperature position and height of the CR peak within the range between —60°C and 20 °C was the distinct characteristics of their comparative tendency towards the brittle fracture and was of use for prediction of the critical brittle-ductile transition temperature [336]. 

The thermal shock fracture behavior of ceramics has been investigated using traditional testing methods such as the water quench in which the critical temperature difference, where samples are subjected to severe damage, is used as the thermal shock resistance. Recently, new experimental methods are proposed and applied to the investigation of macroscopic crack propagation process during thermal shock fracture, based on fracture mechanics. ... 

Aerogel Method. It is believed that the most reliable method for drying bulk wet gel without occurrence of cracks and fracture is to use supercritical temperatures aud pressures of the solvent. Critical temperatures and critical pressures ofrelevaut substauces are shown in Table 6-3. Under the supercritical condition, the solvent is present as one phase fluid, which is not a liquid nor a gas, showing no interface between the liquid and gas. Since there is no surface (interface) which distinguishes the liquid from gas, no surface tension or capillary force works. Accordingly, the wet gel can be easily and rapidly dried without cracks and fracture. 

Note Forgings were processed at 30 °C (50 °F) above the P transus temperature, 30 min, fan cooled+40 °C (70 °F) below the P transus, 1 h, AC + 540 °C (1000 °F), 8 h. Source G.W. Kuhlman et aL, "Charactmization ofTi-6-22-22S High-Strength Alpha-Beta Titanium Alloy for Fracture Critical Applications, Proc. 7th Int. Titanium Conf., San Diego, TMS/AIME, June 1992, to be published... 

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