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Types of Corrosion in the Offshore Environment
Published in Karan Sotoodeh, Coating Application for Piping, Valves and Actuators in Offshore Oil and Gas Industry, 2023
There are two types of cracks that can form in a material: ductile and brittle. Ductile cracks form more slowly and result in plastic deformation. A brittle fracture is a very rapid-forming crack that occurs without plastic deformation. A HISC crack is brittle and fast and occurs without any material deformation. Figure 1.49 illustrates hydrogen cracking or embrittlement in the form of a brittle crack in a piece of pipe. The reason why duplex and super duplex are susceptible to HISC is related to the ferritic phase inside these two materials. Duplex and super duplex have two phases: austenitic, which is ductile; and ferritic, which is brittle. Hydrogen makes the ferritic phase embrittle, while the austenitic phase acts as a crack barrier. The application of coating on the metal surface provides a barrier against hydrogen attack and prevents HISC. However, this method is not 100% effective in HISC mitigation due to possible defects in the coating. Even a small crevice in the coating where the metal is under high tensile stress can cause HISC. For this reason, the DNV standard does not accept coating as a means of preventing HISC. Instead, DNV proposes proper material selection and design with respect to stress and strain, even if the material is coated.
Ceramics and Composites
Published in Yip-Wah Chung, Monica Kapoor, Introduction to Materials Science and Engineering, 2022
Although slip along the [110] direction is allowed, the magnitude of the slip vector b (Burgers vector) in ceramics is larger than that in typical metals (e.g., 0.392 nm for sodium chloride versus 0.255 nm for copper). Since dislocation energy is proportional to |b|2, it is more difficult to create dislocations in ceramics compared to metals. Given the refractory nature of typical ceramics, it takes a large stress to move dislocations in ceramics, since moving dislocations requires the breaking of bonds. These factors – limited number of available slip systems, difficulty of generating the required dislocations, and large stress to move dislocations even when they are available – explain why plastic deformation is difficult in ceramics. Without plastic deformation, brittle fracture occurs suddenly and without warning.
Miscellaneous Topics
Published in Nicolae Lobontiu, Compliant Mechanisms, 2020
Mechanical components usually fail due to one or more of the following principal mechanisms: yield failure, fracture failure or fatigue failure. The failure phenomenon is currently seen as an irreversible process that develops locally and through which the macrostructure and operational performance of the component are negatively altered. Ductile materials (especially metals) fail by yielding, whereby the deformations exceed the proportionality limit and reach the non-recoverable plastic domain. Ductile materials, such as typical metals including mild steel, aluminum, titanium, copper, magnesium, and some of their alloys, as well as nonmetals like Teflon, are capable to undergo large plastic deformations before fracture. Brittle materials, such as cast iron, concrete, glass or ceramic compounds, silicon and silicon-based compounds, fail by fracture as they incur relatively small deformations before breaking apart, compared to ductile materials. Fatigue failure applies to both ductile and brittle materials and is caused by stresses less than the ultimate strength. Such stresses might occur statically on a less frequent basis, when the loads are not applied repeatedly but failure manifests itself in time because cracks, for instance, develop to critical sizes. More frequently though, the fatigue failure is generated by cyclic loading, whereby loads and deformations are applied to the mechanical component more than once in a repetitive manner.
Plasma nitriding effects in fatigue behaviour of the welded 4340 steel
Published in Surface Engineering, 2023
P. S. P. Garcia, A. S. M. Cardoso, R. Chales, B. B. Almeida
Regarding the fracture surfaces, Figure 11 shows the fracture surface of the SAE 4340 AR sample, which exhibits appreciable area reduction and dimples, indicating a typical ductile fracture. Figure 12(a) presents the micrography representing the crack initiation in the W sample from 4340 steel, which was located at the surface of the weld centre. This region presented voids caused by materials vaporization during the laser beam pass and plasma formation. Figure 12(b) shows more details about the referred void, highlighting the martensitic needle-like formation and dendritic structure caused by rapid cooling, both evidence that the crack started at the welding bead. As stress concentration factors, these defects led to a brittle fracture by cleavage mechanisms. Finally, for the WPN and PN cases, fracture occurred in regions close to the edge, and outside the weld region for the WPN condition. Figure 13 shows a transition between fragile and ductile fracture mode in the PN 4340 sample condition.
Effect of banana skin powder and coir fibre on properties and flexural behaviour of precast SCC beam
Published in International Journal of Sustainable Engineering, 2021
Muhammad Tahir Lakhiar, Noridah Mohamad, Abdul Aziz Abdul Samad, Khairunisa Muthusamy, Md Azree Othuman Mydin, W. I. Goh, Steafenie George
From the above reviews, it is proven that agricultural wastes such as BSP and POFA have a great potential as new innovative materials to enhance the properties of concrete. Meanwhile, SCC is a high-performance concrete that can be compacted into every corner of a formwork from its own weight, without the need for vibrating process (Okamura and Ouchi 2003). SCC with a similar water cement ratio has a slightly higher strength compared to conventional concrete, due to the lack of vibration which gives an improved interface between the aggregate and hardened paste. Its advantages include faster construction, lesser manpower, easier placing, non-segregated, uniform and complete consolidation, better surface finishes, improved durability, and increased strength (Uysal 2018; Okamura and Ouchi 1998). With these advantages, it is thus expected that SCC when incorporated with agricultural wastes would have higher strength with high potential to be developed as structural element. However, SCC, like conventional concrete, is a brittle material, which fractures when subjected to stress. It also has a little tendency to deform before rupture. It possesses high compressive strength but weak in tensile and flexural strength. One way to resist this brittle failure is by increasing its tensile and flexural strength.
A study on the impact and fatigue failure of AZ91E–Ni coated alumina composites
Published in Canadian Metallurgical Quarterly, 2020
D. Sameer Kumar, K. N. S. Suman, Palash Poddar
The impact testing is used to assess the material when it is subjected to high rates of quick loading. The energy absorbed by the material before it breaks or fractures is noted and the quantity of energy is considered as a parameter to understand the physical nature of the materials. If the material is brittle, it absorbs a lesser quantity of energy and breaks more readily while if the material is ductile, it needs more energy for fracture. The impact strength in terms of the impact energy absorbed in Joules during the IZOD impact test of the monolithic AZ91E alloy and the composites prepared with 2 wt-% uncoated and nickel-coated alumina was tabulated in Table 3.