Transition to brittle fracture

Fig. 5.75 SEM images of worn surface of AZ15 a before transition to brittle-fracture-controiied wear (load 260 N speed 0.23 ms paraffin oil lubricant sliding distance 2068 m) b after transition to brittle-fracture-controlled wear (load 340 N, speed 0.23 ms paraffin oil lulnicant sliding distance 2.55 m) [13]. With kind permission of Elsevier...

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. 

In this paper the recently developed techniques to characterize the mar resistance of coating systems were presented. The techniques base on methods that create a single scratch onto a surface. Characteristic values like the critical load as a measure for the transition from plastic behaviour to brittle fracture can be determined and used to rank different clearcoat systems and to compare these results with other physical properties. In the field of mar resistance the cross-linking density of the... 

The second part of the investigation was directed toward the understanding of the crack propagation stages. Two questions were especially addressed (i) the determination of the transition from fatigue to brittle fracture (ii) the determination of the crack arrest conditions. Within the frame of linear elastic fracture mechanics, such an analysis requires the... 

Table 7.8 Gradual transition from one mechanism of failure to another, intergranular corrosion to brittle fracture...

Higher temperature may result in a weaker Rehbinder effect as well. This occurs due to the facilitation of a plastic flow at elevated temperatures. Thermal fluctuations result in the relaxation of deformational microheterogeneities. As a result, at elevated temperatures local concentrations of stresses are too low to initiate the formation of primary microcracks. An increase in temperature thus often leads to a transition from brittle fracture in the presence of adsorption-active medium to plastic deformation. The decrease in the rate of deformation of a solid has an analogous effect slow deformation also results in an increased probability of the thermally activated relaxation of locally concentrated deformations and stresses. 

The Nil-Ductility Transition (NDT) temperature, which is the temperature at which a given metal changes from ductile to brittle fracture, is often markedly increased by neutron irradiation. The increase in the NDT temperature is one of the most important effects of irradiation from the standpoint of nuclear power system design. For economic reasons, the large core pressure vessels of large power reactors have been constructed of low carbon steels. 

The metallurgy of the steel used for storage tanks, transport vessels, and pipelines is important but rather too complex for detailed explanation here. However, two points should be noted. Each type of steel has a transition temperature below which it is subject to brittle fracture. The brittle fracture is initiated at a notch or crack, usually near a weld where a stress occurs. Once... 

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]. 

Figure 4 illustrates the effect of particles on the relative fracture strain of filled polymers. The fracture strain depends on the polymer matrix. The most typical dependence is step-like, and a moderate reduction in fracture strain at low filler contents is followed by a sharp drop of fracture strain in a comparatively narrow interval of filler contents. The magnitude of the strain decrease is approximately 100-fold, much higher than the decrease in strength. The drop in fracture strain is caused by the transition from a ductile to brittle fracture. Filled PTFE behaves differently and remains ductile up to < ) = 0.35. 

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). 

In the long-term and under stress PE exhibits slow crack and brittle fracture in non-aggressive environments (e.g., air and water). This is of particular importance because PE is used extensively in applications that demand long-term durability (cable, pipe, landfill linings, geotextiles). At ambient temperature for high-quality grades the transition to brittle failure will occur after many years under stress. 

The effect of the notch, with its stress concentration and increased strain rate, on the transition temperature zone of the SAE1034 steel plate was quite marked. The transition temperature zone was raised by at least 200 F, to above 0 F, Occasional brittle service failures would also be expected with plain carbon structural steels such as this in the range of lower ambient temperatures. The transition temperature zone of the steel weld deposit is quite broad and extends well below 0 F, indicating that the weld deposit was more resistant to brittle fracture than the steel plate. 

CHARPY KEYHOLE IMPACT TESTS. The results of the Charpy keyhole impact tests are shown in Fig. 8. This test is also a severe test, combining all three of the embrittling service conditions. It is one of the laboratory tests most likely to produce a ductile—brittle transition in a material that is susceptible to brittle fracture. 

When some normally ductile metal alloys are cooled to relatively low temperatures, they become susceptible to brittle fracture—that is, they experience a ductile-to-brittle transition upon cooling through a critical range of temperatures. These Liberty ships were constructed of steel that... 

Giovannola and Finnic [175] discussed the balance between failure strength (Tf and hypothetical yield stress ay of the material. For normal brittle behavior brittle fracture, Uf should be greater than ay, which is possible at a local scale, especially at the tip of a tool which causes high-temperature promoting flow. This fact substantiates the assumption that a critical load exists at which a transition from plastic flow to brittle fracture will occur [321]. 

Nickel/gold component terminations and solder pad surface finishes are the source for Au in solder joints where Sn Au intermetallics are formed. The formation of a brittle intermetallic compound (Au,Ni)Sn4 is a characteristic feature of these Pb-free alloys. The boundary between Ni3Sn4 and (Au,Ni)Sn4 is weak, and hence provides a low-energy path for crack propagation and the opportunity for the ductile-to-brittle fracture transition to occur. Subsequent aging indicates transgranular cracks at the (Au,Ni)Su4 layer [13]. Increasing the solder volume or solder joint size... 

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