Microstructure cracks

Zhang, M.H. (1995) Microstructure, crack propagation and mechanical properties of cement pastes containing high volumes of fly ashes. Cement and Concrete Research... 

In case of the alumina-chromia brick RESISTAL RK30SR (Fig. 5) infiltration of the microstructure can be traced over the whole polished section (0-20 mm fi-om the hot face). Nevertheless the highest microstructural changes were observed in the area 0-2 mm from the hot face. In that brick area pore filling infiltration, recrystallization of Cr-corundum bearing matrix, corrosion of Zr-mullite and formation of Mg-Fe-Al-Cr-oxide took place. The immediate hot face is covered with a 1-2 mm thin reaction zone. In the infiltrated brick microstructure cracks formed parallel but also vertical to the hot face can be observed. Cracks are partly filled with slag. 

Examples of microstructural crack deflection are shown in Fig. 10 for the different crack orientations. In Fig. 10a an intact spraying splat deflects a segmentation crack, in Fig. 10b the delamination crack bounces between the two interfaces with the adjacent splats, leaving a serrated fracture surface. 

Microstructure on Surfaces. Gross cracks and voids are usually lined with microstmcture, as indicated by Figure 3. As the depth—width,... 

The surface from which the cracks originate may not be apparent without a microstructural examination. Stress-corrosion cracks invariably produce brittle (thick-walled) fractures regardless of the ductility of the metal. 

Microstructural examinations revealed that the cracks originated on the external surface. The cracks were branched and ran across the metal grains (transgranular). 

Microstructural examinations revealed that the tube was annealed brass. All cracks originated on the internal surface at pit sites. Cracks were fine, branched, and ran through the metal grains (transgranular). Crack density over the specimens examined averaged 40 cracks per linear inch (16 cracks/cm) of surface. 

Microstructural examinations revealed that the cracks originated on the external surface (Fig. 9.15). The cracks were highly branched and transgranular. The branched, transgranular character of these cracks is typical of stress-corrosion cracking of austenitic stainless steels. The thick-walled fracture faces are also typical of cracking by this mode. 

Microstructural examinations disclosed highly branched, predominantly transgranular cracks originating on the internal surface. Cracks of this form are typical of SCC in austenitic stainless steels. 

Microstructural examinations revealed branched, transgranular cracks originating on the external surface (treated cooling water). Analysis of material covering the crack surfaces revealed the presence of chlorine. 

Microstructural examinations revealed that branched cracks originated at shallow pit sites on the external surface. The pits, which may have formed during idle periods from differential oxygen concentration cells formed beneath deposits, acted as stress concentrators. The transverse (circumferential) crack orientation and the localization of cracks along just one side of the tube revealed that bending of the tube was responsible for the stresses involved. 

The second misconception involves the perception of what constitutes a defect. A defect is not simply a visually observable discontinuity such as a hole, lap, or seam in a component. Defects, from a failure-analysis standpoint, may also be such things as a high residual stress that may lead to cracking or unfavorable microstructures that can lead to either... 

Dents in tubing can induce erosion failures, especially in soft metals such as copper and brass. Welding and improper heat treatment of stainless steel can lead to localized corrosion or cracking through a change in the microstructure, such as sensitization. Another form of defect is the inadvertent substitution of an improper material. 

Types of damage can be classified as uniform or localized metal removal, corrosion cracking or detrimental effects to the environment from the corrosion products. Local attack can take the form of shallow pits, pitting, selective dissolution of small microstructure regions of the material or cracking. Detrimental effects are certainly not the case with buried pipelines, but have to be considered for environments in vessels and containers. It is usual, where different results of reactions lead... 

The few studies which addressed the fracture mechanics behavior of very small cracks generally revealed a quantitative departure from behavior determined at longer crack lengths. This result may be attributed to a departure from perfect mechanical and microstructural similitude between long and small cracks. 

Fig. 7. Microstructures of the three primary graphites used in this work (a) H-451, (b) IG-I I, and (c) AXF-5Q. [F]-filler particles, [P]-pores and [C] cracks.

The variability in fracture stress when small artificial flaws were controlling strength was particularly pronounced for H-451 graphite, as can be seen in Fig. 12. Here the crack dimensions and crack trip process zone dimensions are comparable to the microstructural dimensions. Consequently, local variations in microstmeture... 

T. Hoshide, K. Kusuura. Life prediction by simulation of crack growth in notched components with different microstructures and under multiaxial fatigue. Fatigue Fract Eng Mater Struct 27 201, 1998. 

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