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3 secondary voids are nucleated at finely distributed

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3. Secondary voids are nucleated at finely distributed precipitates within the deforming ligaments. 72 Acoustic Emission Testing F IGURE 27. Sequence of micromechanisms involved in the growth of a ductile crack: (a) the process begins with disbonding at inclusion-to-matrix interfaces; (b) voids grow at the interface and plastic deformation localizes between adjacent voids; (c) intense deformation nucleates further microvoids at precipitate interfaces; (d) coalescence of the microvoids results in a ductile crack. (a) (b) (c) (d) Disbond Inclusion Void Void Localized plastic deformation Intense plastic deformation Microvoid Microvoid coalescence Void 10 μm (4 × 10 –4 in.)
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4. The secondary voids grow and link up, resulting in a crack whose dimension is the interinclusion spacing. The linking of voids may occur by one of two modes. In materials with low yield strength and high work hardening capacity, the decrease in net load supporting area (because of void growth) is balanced by the increased flow stress (because of work hardening) of the deforming intervoid ligament. Under this stable condition, hole growth continues almost until the voids overlap. This mode of coalescence is likely to be undetectable because of the very low crack growth velocity and small intercarbide ligament thickness. In materials with high yield strength and limited work hardening capacity, hole growth causes a loss of load supporting area that cannot be compensated by work hardening. Unstable strain ensues and results in a premature shear coalescence. This latter process may occur at intermediate velocity over distances determined by the interinclusion separation, 10 to 100 μm (0.0004 to 0.004 in.). The alternating shear is a potentially detectable source of acoustic emission. The design of structures is usually based on the premise that crack growth occurs by stable microvoid coalescence. Efforts are made during materials selection and design to ensure that insufficient stress develops during service for this mode of fracture to exist. Should undetected discontinuities exist in construction of materials, or if a manufactured component is subjected to greater than anticipated stress, ductile crack advance may occur. In tough low strength steels, only intermittent inclusion fractures indicate this. If these inclusions are absent or if the region ahead of the crack was appreciably prestressed in the past (during proof testing) so that inclusions are already fractured, the possibility exists for silent crack growth. This phenomenon is disconcerting to those interested in acoustic emission for nondestructive testing. Given the quality of materials and fracture mechanics analyses, the likelihood of a ductile fracture failure is becoming rare. Rather, the emerging concern is that some kind of embrittling phenomenon occurs because of environmental effects. This reduces the material’s resistance to crack growth and changes the mechanism of fracture from a ductile to a brittle one that is often detectable.
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  • Fall '19
  • Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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