Fractography

About: Fractography is a research topic. Over the lifetime, 5043 publications have been published within this topic receiving 86068 citations.

Effects of different surface modifications on the fatigue life of selective laser melted 15–5 PH stainless steel

Abstract: Effects of machining, electro-polishing and laser surface re-melting (LRM) on the fatigue life of Selective Laser Melted (SLM) 15–5 precipitation hardening (PH) stainless steel are reported. Electro-polished surface showed ~97.4% reduction of surface roughness (Ra) and hence improved the fatigue life drastically (~100%) as compared to as-built specimens. Synergic effect of both compressive residual stress and reduction in surface roughness caused drastic improvement (~138%) in fatigue life of machined SLM specimens when compared to its as-built counterpart. Laser Surface Re-melting is found to be an effective technique to reduce the surface roughness (~91.2%) as well as sub-surface defects of geometrically complex as-built SLM specimen and thus improved its fatigue life by ~119%. Fractography analysis showed surface roughness, sub-surface defects and micro notches as the primary crack initiating factors for SLM specimens. High surface roughness in as-built part causes multiple crack initiation sites for specimens failed under both low and high stress amplitude. However, for machined, electro-polished and LRM SLM specimens distinct and multiple crack initiation sites could be observed when specimen failed under high cycle and low cycle regime respectively. However, for all the cases, fatigue life is found to be less compared to its wrought counterpart. Present study could be used as a guideline to select proper surface modification methods for SLM 15–5 PH stainless steel for a desired fatigue life.

Dependency of fracture toughness on the inhomogeneity of coarse TiN particle distribution in a low alloy steel

Abstract: An investigation of the effect of the coarse TiN particle distribution on the fracture toughness of a steel, as determined by crack-tip opening displacement (CTOD), was carried out using a range of samples from a Ti-treated steel that had been thermally cycled to simulate a coarse grained heat-affected zone (CG HAZ) microstructure. Experimental results from tests carried out at room temperature showed that the inhomogeneous spatial distribution of the coarse TiN particles in the microstructure ahead of the fatigue precrack caused the samples to fail with significantly different CTOD values. Detailed fractographic investigation showed that, with an increased number of overall fracture initiation sites (FISs) and number density of local cleavage initiation sites (CISs) caused by coarse TiN particles, the fracture toughness CTOD values generally decreased. The increase in FIS number and CIS number density has been related to the inhomogeneous coarse TiN distribution ahead of the fatigue precrack and so the sampling of microstructural areas with a high number density of coarse TiN particles. The mechanism by which the coarse TiN particles cause cleavage fracture initiation is discussed.

Creep deformation behavior of 9Cr1MoVNb (ASME Grade 91) steel

Abstract: Detailed investigations have been performed to examine the creep deformation behavior and microstructural evolution of modified 9Cr-1Mo steel which is widely used in high temperature power plant component. The creep data were analysed in terms of the temperature compensated power law and Monkman-Grant relation. The creep activation energy of Grade 91 steel was determined with 543±30 kJ/mol without threshold stress compensation, while after correcting the threshold stress; the activation energy is decreased to 303±15 kJ/mol. This value is close to the activation energy of creep in α-Fe. The calculated threshold stress showed a strong dependence on temperature. The creep behavior of the steel was described by the modified Bird-Mukherjee-Dorn relation. The rate controlling creep deformation mechanism was identified as the edge dislocation climb with stress exponent of n=5. Further, the value of creep damage tolerance factor (λ) and stress exponent was used to identify the cause of creep damage, showed significant difference in the high and low stress regimes. The fracture surface morphology of the ruptured specimens was studied by scanning electron microscopy to further elucidate the failure mechanisms. Whereas deformed microstructure was examined by transmission electron microscopy. The significant decrease in creep strength in the alloy has been attributed to microstructural degradation associated with precipitates and dislocation substructure.

Deformation and fracture behavior of two Al-Mg-Si alloys

Abstract: Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing, transmission electron microscope (TEM), and scanning electron microscope (SEM) observation. Tensile test results show that the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of artificial aging and are only 0.045 and 0.06, respectively, in the overaged condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the rapid decrease of elongation to failure at the early stage of artificial aging. In fully aged conditions, dislocations are concentrated in narrow bands during plastic deformation of these alloys, which is responsible for the very low n values of the Al-Mg-Si alloys in peak aged and overaged conditions. The Si particles formed in the interior of grains of the higher Si containing alloy reduce the inhomogeneous deformation behavior. The TEM results show that large precipitates and precipitate-free zones (PFZs) along grain boundaries are formed in peak aged and overaged conditions, and SEM observations demonstrate that the tensile fracture modes of the two alloys in these aging conditions are completely intergranular with many small cusps decorated on facets of the fractured grain boundaries. Thus, the fracture process of both alloys is suggested to be that in which the high local stresses, built up where the slip band impinges on the grain boundaries, nucleate voids at the grain boundary precipitates by decohesion of the particle/PFZ interface, and then coalescence of these voids within the PFZ leads to the final fracture of these alloys.