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Fracture Mechanisms in Metals
Published in T.L. Anderson, Fracture Mechanics, 2017
The fracture toughness of ferritic steels can change drastically over a small temperature range, as Figure 5.28 illustrates. At low temperatures, steel is brittle and fails by cleavage. At high temperatures, the material is ductile and fails by microvoid coalescence. Ductile fracture initiates at a particular toughness value, as indicated by the dashed line in Figure 5.28. The crack grows as load is increased. Eventually, the specimen fails by plastic collapse or tearing instability. In the transition region between ductile and brittle behavior, both micromechanisms of fracture can occur in the same specimen. In the lower transition region, the fracture mechanism is pure cleavage, but the toughness increases rapidly with temperature as cleavage becomes more difficult. In the upper transition region, a crack initiates by microvoid coalescence but ultimate failure occurs by cleavage. On initial loading in the upper transition region, cleavage does not occur because there are no critical particles near the crack tip. As the crack grows by ductile tearing, however, more material is sampled. Eventually, the growing crack samples a critical particle and cleavage occurs. As fracture toughness in the transition region is governed by these statistical sampling effects, the data tend to be highly scattered. Wallin [47] has developed a statistical model for the transition region that incorporates the effect of prior ductile tearing on cleavage probability.
Effect of Al2O3, SiO2 and carbon nanotubes on the microstructural and mechanical behavior of spark plasma sintered aluminum based nanocomposites
Published in Particulate Science and Technology, 2020
B. Sadeghi, P. Cavaliere, A. Perrone
The fine microstructure of Al-2%Al2O3 and Al-2%CNTs is revealed and confirmed by tensile fracture surfaces. The coarser grain structure of Al-2%SiO2 composite is revealed by tensile fracture surfaces. The nucleation of microvoids can be caused by particle cracking or interfacial failure between the reinforcements and the matrix. Microvoids grow during applied tensile forces and microvoids coalesce when adjacent microvoids link together or the material between microvoids experiences necking. Accordingly, microvoid coalescence leads to fracture. The reinforcements increase propensity to grain boundary fracture through formation of continues brittle phase alone grain boundaries, this confirms the reduction of ductility in composites with respect to pure SPSed aluminum. The high concentration of residual porosity would has provided a preferred path for crack propagation, thereby providing a condition for embrittlement. By adding ceramic nanoparticles or CNTs the fracture is mainly transgranular because of grain boundary weaking and porosity coalescence. Higher local ductility is experienced by pure aluminum.