Why is brittleness important

Engineers need to understand fracture mechanisms. However, for ductile fracture, the presence of plastic deformation gives warning that failure is imminent, allowing preventive measures to be taken. On this basis, it is possible to divide materials into two broad categories; namely:. The effect of temperature on the nature of the fracture is of considerable importance.

Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures. The temperature above which a material is ductile and below which it is brittle is known as the ductile—brittle transition temperature DBTT , nil ductility temperature NDT , or nil ductility transition temperature. This temperature is not precise, but varies according to prior mechanical and heat treatment and the nature and amounts of impurity elements.

It can be determined by some form of drop-weight test for example, the Charpy or Izod tests. The ductile—brittle transition temperature DBTT is the temperature at which the fracture energy passes below a predetermined value e. Ductility is an essential requirement for steels used in the construction of reactor components, such as the reactor vessel.

Therefore, the DBTT is of significance in the operation of these vessels. In this case, the size of the grain determines the properties of the metal. For example, smaller grain size increases tensile strength, tends to increase ductility and results in a decrease in DBTT. Grain size is controlled by heat treatment in the specifications and manufacturing of reactor vessels.

The DBTT can also be lowered by small additions of selected alloying elements such as nickel and manganese to low-carbon steels. Typically, the low alloy reactor pressure vessel steels are ferritic steels that exhibit the classic ductile-to-brittle transition behaviour with decreasing temperature. This transitional temperature is of the highest importance during plant heatup. In some materials, the transition is sharper than others and typically requires a temperature-sensitive deformation mechanism.

For example, in materials with a body-centered cubic bcc lattice the DBTT is readily apparent, as the motion of screw dislocations is very temperature sensitive because the rearrangement of the dislocation core prior to slip requires thermal activation. This can be problematic for steels with a high ferrite content.

This famously resulted in serious hull cracking in Liberty ships in colder waters during World War II, causing many sinkings. The vessels were constructed of a steel alloy that possessed adequate toughness according to room-temperature tensile tests. It must be noted that low-strength FCC metals e. On the other hand, many high-strength metals e. DBTT can also be influenced by external factors such as neutron radiation , which leads to an increase in internal lattice defects and a corresponding decrease in ductility and increase in DBTT.

This phenomenon, known as irradiation embrittlment , results in the steadily increase in DBTT. It is not likely that the DBTT will approach the normal operating temperature of the steel. However, there is a possibility that when the reactor is being shut down or during an abnormal cooldown, the temperature may fall below the DBTT value while the internal pressure is still high.

Therefore nuclear regulators require that a reactor vessel material surveillance program be conducted in watercooled power reactors. See also: Neutron Reflector. Irradiation embrittlement can lead to loss of fracture toughness. Neutron irradiation tends to increase the temperature ductile-to-brittle transition temperature at which this transition occurs and tends to decrease the ductile toughness. One of the most serious metallurgical problems and one that is a major concern in the nuclear industry is stress-corrosion cracking SCC.

Stress-corrosion cracking results from the combined action of an applied tensile stress and a corrosive environment , both influences are necessary. SCC is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. It tends to propagate as stress opens cracks that are subject to corrosion, which are then corroded further, weakening the metal by further cracking.

The cracks can follow intergranular or transgranular paths, and there is often a tendency for crack branching. Failure behavior is characteristic of that for a brittle material, even though the metal alloy is intrinsically ductile.

SCC can lead to unexpected sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature.

SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments.

Stress-corrosion cracking may cause, for example, a failure of nuclear fuel rod after inappropriate power changes, rod movement and plant startup. Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides and mild steel cracks in the presence of alkali boiler cracking. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions.

Nickel-based alloys, however, are not effected by chloride or hydroxide ions. An example of a nickel-based alloy that is resistant to stress-corrosion cracking is inconel. Special Reference: U. Department of Energy, Material Science. January Cladding prevents radioactive fission products from escaping the fuel matrix into the reactor coolant and contaminating it. There are various fuel failure root causes , that have been identified in past.

One of possible causes is also the pellet-cladding interaction PCI , which may be caused by stress-corrosion cracking. We next gathered uniaxial compression test data of different rock types from the literature [ 8 , 39 — 42 ], including 58 igneous rocks, 21 metamorphic rocks, and 48 sedimentary rocks.

The data were fitted with equation 22 method 1 , equation 24 method 2 , and equation 25 method 3 to predict K. Figure 7 and Table 4 show that the K values of igneous, metamorphic, and sedimentary rocks predicted by the three methods are all within reasonable ranges.

Values of K for igneous and metamorphic rocks obtained from the three methods are distributed over a relatively large range from 0. In method 1, K values for sedimentary rocks are concentrated between 0. The three prediction methods present average K values of 0. Similarly, the average K values for metamorphic are 0. The results are consistent with values reported by Wen Tao of 0. However, the average K for sedimentary rocks obtained by the fitting methods differ slightly from the statistical experimental results.

The primary cause of the differences is that the fitting of equations 24 and 25 can introduce errors owing to a lack of experimental data for sedimentary rocks. The above analysis clearly demonstrates that the presented equations for brittleness index can reliably predict K.

In this study, we determined the relationship between brittleness index and crack initiation stress ratio K by fracture mechanics theory and indoor uniaxial compression tests. After theoretical derivation, the brittleness index B 1 is found to have an inversely proportional relationship with K and brittleness index B 2 has a variant inverse proportional relationship with K , whereas no relationship is observed between brittleness indexes B 3 and B 4 and K.

Analysis of experimental data from igneous, metamorphic, and sedimentary rocks shows that B 1 displays a power relationship with K , B 2 has a linear relationship with K , and no relationship is observed between B 3 and B 4 and K. Seventy different types of uniaxial compression test data were collected from igneous, metamorphic, and sedimentary rocks, and consistent behavior is observed within the same rock types.

K values were estimated on the basis of uniaxial compression test data for different rock types. The results show that K concentrates between 0. In method 1, K values for sedimentary rocks concentrate between 0. Average K values for all rock types obtained by the three prediction methods are consistent with the experimental data.

The redefined brittleness index formula can therefore be used for reliable predictions of K. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Article of the Year Award: Outstanding research contributions of , as selected by our Chief Editors. Read the winning articles. Journal overview. Special Issues. Academic Editor: Hugo Rodrigues. Received 10 Jul Revised 22 Dec Accepted 10 Feb Published 24 Apr Introduction The rock brittleness index is of great significance for mining engineering, tunnel engineering, oil and gas exploration, and rock burst susceptibility and has received extensive attention in the literature.

Crack Initiation Mechanism of Rock under Uniaxial Compression Lockner [ 21 ] argued that deformation and failure is a progressive process characterized by the initiation, propagation, and coalescence of microcracks for most rock samples.

Figure 1. Stress-strain diagram of granite showing the stages of crack development after [ 30 ]. Figure 2. Table 1. Figure 3. Relationship between crack initiation stress ratio K of granite and different brittleness indexes: a B 1 ; b B 2 ; c B 3 ; d B 4.

Table 2. Figure 4. Relationship between crack initiation stress ratio K of gneiss and different brittleness indexes: a B 1 ; b B 2 ; c B 3 ; d B 4. Table 3. Summary of mechanical parameters of limestone samples. Figure 5. Relationship between crack initiation stress ratio K of limestone and different brittleness indexes: a B 1 ; b B 2 ; c B 3 ; d B 4. Figure 6. Relationship between crack initiation stress ratio K and different rock types of rocks with different brittleness indexes: a B 1 ; b B 2 ; c B 3 ; d B 4.

Figure 7. Prediction of K value for igneous, metamorphic, and sedimentary rocks using a method 1, b method 2, and c method 3. Table 4. Percentage of K value range distribution of different rock types under three prediction methods. References A. Obert and W. Hucka and B. View at: Google Scholar R. Tarasov and Y.

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