Fractography

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

Short crack initiation and growth at 600 °C in notched specimens of Inconel718

Abstract: The natural initiation and growth of short cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short-crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life.

A critical-strain criterion for hydrogen embrittlement of cold-drawn, ultrafine pearlitic steel

Abstract: A stress-modified, critical-strain model of fracture-initiation toughness has been adapted to the case of hydrogen-affected pearlite shear cracking, which is a critical event in transverse fracture of cold-drawn, pearlitic steel wire. This shear cracking occurs via a process of cementite lamellae failure, followed by microvoid nucleation, growth, and linkage to create shear bands that form across pearlite colonies. The key model feature is that the intrinsic resistance to shear-band cracking at a transverse notch or crack is related to the effective fracture strain at the notch root. This fracture strain decreases with the logarithm of the diffusible hydrogen concentration (C H). Good agreement with experimental transverse fracture-initiation-toughness values was obtained when the sole adjustable parameter of the model, the critical microstructural dimension (l*), was set to the mean dimension of shearable pearlite colonies within this steel. The effect of hydrogen was incorporated through the relationship between local effective plastic strain (ɛ eff f ) and C H, obtained from sharply and bluntly notched tensile specimens analyzed by finite-element analysis (FEA) to define stress and strain fields. No transition in the transverse fracture-initiation morphology was observed with increasing constraint or hydrogen concentration. Instead, shear cracking from transverse notches and precracks was enabled at lower global applied stresses when C H increased. This shear-cracking process is assisted by absorbed and trapped hydrogen, which is rationalized to either reduce the cohesive strength of the Fe/Fe3C interface, localize slip in ferrite lamellae so as to more readily enable shearing of Fe3C by dislocation pileups, or assist subsequent void growth and link-up. The role of hydrogen at these sites is consistent with the detected hydrogen trapping. Large hydrogen-trap coverages at carbides can be demonstrated using trap-binding-energy analysis when hydrogen-assisted shear cracking is observed at low applied strains.

Effects of microporosity on tensile properties of A356 aluminum alloy

Abstract: The effect of microporosity on the tensile properties of A356 alloy was investigated through systematic experimental approaches, with a constitutive prediction that takes into account the strain rate sensitivity and strain-hardening exponent. The strain rate sensitivity was measured through the incremental strain rate change method, and the volumetric porosity and fractographic porosity were obtained from the measurements of bulk density and the quantitative fractography analyses on the fractured surface, respectively. The UTS and elongation exhibit a strong dependence upon the variation in microporosity, with a linear and inverse parabolic relationship, respectively. The constitutive prediction based on the fractographic rather than the volumetric porosity can more accurately predict the overall tensile properties of A356 alloy. The constitutive model should necessarily take into account the strain rate sensitivity and strain-hardening exponent for an exact theoretical prediction of the tensile properties.

Effect of Grain Size on Mechanical Properties of Submicrometer 3Y-TZP: Fracture Strength and Hydrothermal Degradation

Abstract: This paper considers fracture strength, fracture origins, and hydrothermal degradation of 3Y-TZP with grain sizes in the range of 110–480 nm. Biaxial fracture strength testing was used to show that the fracture strength increases with grain size and is governed by the concurrent change in fracture toughness. Hydrothermal degradation was studied by means of fractography, Raman microscopy and its effect on fracture strength. Up to 200 nm grain size, hydrothermal degradation of strength is limited. Larger grain sizes exhibit either premature failure or an increase in strength. A surface transformation zone was found to be responsible for both phenomena.