Critical brittleness temperature

Design of assemblies and their positioning above the core result in non-uniform irradiation conditions and the number of specimens irradiated to similar neutron fluence is insufficient for a reliable determination of the critical brittle fracture temperature shift. 

In Table 4.3, critical temperatnre data related to polymers are quoted. These include heat distortion temperatnre, brittleness temperature, melt temperature, mold temperature, recommended maximnm operating temperature, and performance at low temperatnre. Heat distortion temperatnres, for example, range from 10-50 MPa (low-density polyethylene, polynrethane) to above 250 MPa (polyphenylene sulfide, urea formaldehyde-diallyl phthalate, polyether ether ketone, and polyamide-imide). 

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]. 

Brittle Temperature n Temperature at wliich a material transforms from being ductile to being brittle, i.e., the critical normal stress for fracture is reached before the critical stress for plastic deformation. 

The alloy niobium titanium (NbTi) and the intermetaUic compound of niobium and tin (Nb.3 Sn) are the most technologically advanced LTS materials presently available. Even though NbTi has a lower critical field and critical current density, it is often selected because its metallurgical properties favor convenient wire fabrication. In contrast, Nb.3Sn is a veiy brittle material and requires wire fabrication under very well-defined temperature conditions. 

The properties of these brittle ceramics depend critically on the preparative conditions. Intimate mixtures of the oxides, carbonates or nitrates of the relevant metals in the required proportions are heated at temperatures of 900-I000°C. For YBa2Cu307 j , all compositions in the range 0 < jc < 0.5 superconduct and the highest Tc is found where jc 0. For others, the oxygen content must be stringently controlled. In all cases, the most... 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

Little is known about the variation of the critical stress ", with structure and temperature. For the polyethylene discussed abovedecreased from 620 psi at 22X to 39general trend with all polymers. Turner (84) found that the value of (r(. for polyethylenes increased by a factor of about 5 in going from a polymer with a density of 0.920 to a highly crystalline one with a density of 0.980. Reid (80,81) has suggested that for rigid amorphous polymers. ", should be proportional t° (Tt - T) For brittle polymers, the value of ", may be related to the onset of crazing. 

Creep leads ultimately to rupture, referred to as creep-rupture, stress-rupture or static fatigue. Creep-rupture of thermoplastics can take three different forms brittle failure at low temperatures and high strain rates ductile failure at intermediate loads and temperatures and slow, low energy brittle failure at long lifetimes. It is this transition back to brittle failure that is critical in the prediction of lifetime, and it is always prudent to assume that such a transition will occur [1], A notch or stress concentration will help to initiate failure. 

The mechanical properties of Nafion materials have not been of the most critical importance, as in the case of commercial thermoplastics or composite materials that are expected to be load-bearing. Rather, the primary focus has been on transport properties. To be sure, the mechanical integrity of membranes as mounted in cells, and under the perturbation of pressure gradients, swelling-dehydration cycles, mechanical creep, extreme temperatures, and the onset of brittleness and tear resistance, is important and must be taken into consideration. 

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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