AE02.pdf

From cleavage to intergranular fracture and 2

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from cleavage to intergranular fracture and (2) appreciable upward shifts in the ductile-to-brittle transition temperature. Glassy grain boundary phases in ceramics such as alumina are also linked to the formation of intergranular fracture in ceramics. In aluminum alloys, liquid metals such as indium and gallium promote intergranular fracture. Once an intergranular crack is nucleated in these systems, it can propagate rapidly over great distances because of the absence of crack arresting features in the microstructure. Only when the crack reaches a grain boundary triple point and must branch radically from the maximum tensile stress plane is arrest possible in materials with uniformly weak grain boundaries. If the distribution of embrittling agents is not uniform, as occurs in some ceramic systems, then the dimensions of the embrittled region act to control the crack advance distance. Systems exhibiting brittle intergranular fracture are often copious emitters of detectable acoustic emission. Crack radii are often five to ten times those of cleavage microcracks and crack velocities are at least as high as those during cleavage. Acoustic emission strengths of 20 to 100 times those of cleavage are possible. In composites, fiber delamination can be likened to intergranular fracture because an interface disbond is the basic crack advance mechanism and because the crack advance distance can be many fiber diameters. Because fiber cleavage and disbonding are two of the main damage accumulation processes in composite materials, it is clear that their damage evolution has a high acoustic emission detection probability and is the reason for the success of many acoustic emission monitoring activities in these materials. 70 Acoustic Emission Testing
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Particle Microfracture Some inclusions and precipitates in engineering alloys are brittle at ambient temperature and will undergo brittle (cleavage) fracture under tensile loading. The interface at these discontinuities is often weak and frequently fails at low stress. The generation of acoustic emission from these microstructure constituents depends on the intrinsic properties of the particles, the strength of their interfaces and the particle dimensions. 71 Fundamentals of Acoustic Emission Testing F IGURE 26. Schematic diagram of three micromechanisms of cleavage crack nucleation in iron carbon alloys: (a) nucleation by a grain boundary iron carbide film; (b) grain boundary edge dislocation pile-up nucleation; (c) nucleation by intersection of edge dislocations on {011} slip planes. Ferrite ( α iron) Iron carbide Microcrack {001} Cleavage microcrack (a) (b) Edge dislocation pile-up of plastic energy Grain boundary Cleavage crack nucleated (c) Slip plane Edge dislocation Cleavage crack nucleated {011} {001} {011}
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Inclusions in Steels In steels, the fracture of manganese sulphide particles appears to be an important emission source, particularly in the absence of cleavage or intergranular failure modes. 98 Studies found that acoustic emission from manganese sulphide inclusions has a strong orientation dependence. During hot
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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