Fatigue Characterization and Fractographic Analysis of Aluminium 6063 Alloy

Aluminium and its alloy are widely employed in various automobile and aircraft areas because of their unique specific strength and formability. Al alloys that have been employed in aerospace structural components will undergo dynamic loading, which leads to fatigue due to mechanical stress and thermal conditions. Considering studies toward the low cycle fatigue behaviour of Al alloys are significantly narrowed, this chapter sighted to the analysis of fatigue behaviour of Al 6063 alloy at the various total strain amplitude (TSA) of 0.4% and 0.8%, which performed through the low cycle fatigue testing machine at the frequency rate of 0.2 Hz. The test results show that for 0.4% TSA, the number of cycles to failure (N) is 1786, whereas as the TSA increases, N got reduced. For 0.8% TSA, the cycle to failure is 291 and samples undergone cyclic softening during the test. The rate of cyclic plastic strain raised up with the increase in the TSA. Crack propagation was observed along with the quasi-cleavage fracture for 0.4% TSA and cleavage fracture for 0.8% TSA.

Investigating of Strain Effect on Cleavage Fracture for Reactor Pressure Vessel Material

Failure mechanism of 20MnMoNi55 steel in the lower self of ductile to brittle transition (DBT) region is considered as brittle fracture but it has been observed from the experimental analysis of stress-strain diagram that clear plastic deformation is shown by the material before failure. Therefore, strain correction is implemented in the cleavage fracture model proposed by different researchers in the lower self of the DBT region with the help of finite element analysis. To avoid a huge number of experiments being performed, Monte Carlo simulation is used to generate a huge number of random data at different temperatures in the lower self of the DBT region for calibration of the cleavage parameters with the help of the master curve methodology. Fracture toughness calculated after strain correction through different models are validated with experimental results for the different probability of failures.

Study on Microstructure and Fatigue Properties of FGH96 Nickel-Based Superalloy

In this study, using synchrotron radiation X-ray imaging, the microstructure, tensile properties, and fatigue properties of FGH96 nickel-based superalloy were tested, and the fatigue damage mechanism was analyzed. An analysis of the experimental results shows that the alloy structure is dense without voids or other defects. It was observed that the primary γ′ phase is distributed on the grain boundary in a chain shape, and the secondary γ′ phase is found inside the crystal grains. The X-ray diffraction (XRD) pattern indicates that no other phases were seen except for the γ and γ′ phases. The tensile strength of the alloy is 1570 MPa and the elongation is 12.1%. Using data fitting and calculation, it was found that the fatigue strength of the alloy under the condition of 5 × 106 cycles is 620.33 MPa. A fatigue fracture has the characteristics of secondary crack, cleavage step, fatigue stripe, tire indentation, and dimple. The fracture is a mix of cleavage fracture and ductile fracture. Through a three-dimensional reconstruction of the alloy synchrotron radiation imaging area, it was found that the internal defects are small and mostly distributed at the edge of the sample. The dimple morphology is formed by cavity aggregation and cavity germination resulting from defects in the material itself, fracture of the second-phase particles, and separation of the second-phase particles from the matrix interface. By analyzing the damage mechanism of fatigue fractures, it is concluded that the cleavage step is formed by the intersection of cleavage planes formed by branch cracks, with the main crack of the confluence extending forward to form a cleavage fracture. The crack propagation path was also analyzed, and under the action of cyclic load and tip passivation, the crack shows Z-shaped propagation.

Relating matrix stress to local stress on a hard microstructural inclusion for understanding cleavage fracture in high strength steel

Macroscale cleavage fracture toughness of high strength steels is strongly related to the fracture of hard microstructural inclusions. Therefore, an accurate determination of the local stress on these inclusions based on the matrix stress is necessary for the statistical modelling of macroscale cleavage fracture. This paper presents analytical equations to quantitatively estimate the stress of the microstructural inclusions from the far-field stress of the matrix. The analytical equations account for the inclusion shape, the inclusion orientation, the far-field stress state and matrix material properties. Finite element modelling of a representative volume element containing a hard inclusion shows that the equations provide an accurate representation of the local stress state. The equations are implemented into a multi-barrier model and compared with CTOD experiments with two different levels of constraint.