Microstructural Characterization and Crack Propagation Behavior of a Novel β-Solidifying TiAl Alloy

Novel β-solidifying TiAl alloys have great potential for engineering applications in the aerospace and automotive industries. The introduction of the β0 phase will inevitably affect crack propagation. However, the related mechanism is unclear. In this study, the crack propagation behavior of different β0-containing microstructures was systematically investigated by three-point bending tests. The results show that the coarse γ/α2 lamellar microstructure exhibits better fracture toughness than the fine-grain microstructure because large numbers of γ/α2 lamellar boundaries can effectively hinder crack propagation. The propagation direction depends largely on the orientation of the γ/α2 lamellae. When the angle between the crack propagation direction and the γ/α2 lamellar boundary is small, the crack tends to propagate along γ/α2 lamellae. When the angle is close to 90°, the crack generally propagates by the trans-lamellar mode. Moreover, the crack tends to traverse across the fine β0/γ duplex region due to the low resistance of fine grains in the crack propagation. The transgranular and intergranular modes are the main fracture mechanisms in the microstructure of the fine β0/γ grains. Some shear ligaments can also be identified in the lamellar microstructure and these can consume propagation energy. The enlarged image shows that the crack propagation direction can be changed by the β0 phase, owing to its high hardness. The crack tends to stop at the β0 phase region.

Determination of the Crack Propagation Direction in Mixed-Mode Missions due to Cyclic Loading

The evaluation of cyclic crack propagation due to missions with varying mixed-mode conditions is an important topic in industrial applications. This paper focuses on the determination of the resulting propagation direction. Two criteria are analyzed, the dominant step criterion and the averaged angle criterion, and compared with experimental data from tension-torsion tests with and without phase shift. The comparison shows that the dominant step criterion yields better results for small to moderate values of the phase shift. For a large phase shift of 90°, the experimental results are not very consistent, and therefore, no decisive conclusions can be drawn.

Crack Arresting With Crack Deflecting Holes in Steel Plates

Experimental and numerical studies of the effect of crack deflecting holes in steel plates under high cycle fatigue are presented in this paper. The experimental studies show that with the careful location of the holes, crack propagation can be arrested. A numerical model is provided and validated against the experimental work. The numerically predicted crack propagation direction and crack growth rate were in good agreement with the crack propagation obtained in the experimental work.

Measurement and Analysis of Hydrogen Distribution in Stress Environment Using Vickers Microhardness Technique

In this paper, the stress distribution field in front of the crack tip was obtained by loading a modified WOL specimen using a bolt. Considering the relationship between microhardness and hydrogen content or internal stress in the metal, a model based on the change of microhardness increment is proposed to describe the trend of hydrogen concentration distribution in the stress environment. The agreement between theoretical model and experimental results is verified by the Vickers microhardness tester. Based on the model, there is a simple additive relationship between the hydrogen-induced microhardness increment and the stress-induced microhardness increment. Therefore, the microhardness tester can be employed to evaluate the hydrogen distribution in metals quantitatively. The experimental results demonstrated that the Vickers microhardness method has accurately revealed the hydrogen concentration behavior accurately in a known equibiaxial stress environment. The hydrogen distribution of specimens in the stress environment was analyzed by taking the change of the microhardness increment along the crack propagation direction of specimens as the indicator.

Study of Mode I-II Mixed Crack Propagation in Electrofusion Joint of Polyethylene Pipe

Electrofusion joint plays an important role in connecting polyethylene (PE) pipe. In our previous study, penetrating crack failure through the fitting with an angle of about 70° was observed, and axial stress was found to be an important factor in the crack propagation. In this paper, experiments were carried out to study the crack propagation phenomena of the electrofusion joint of PE pipe. Digital Image Correlation (DIC) method was used to measure the displacement on specimen’s surface, as well as full-field strain distribution, based on which the J-integral of the crack tip was calculated. Besides, a finite element numerical simulation was conducted, and its accuracy was verified by experimental J-integral value. Through combination of experimental observations and finite element method, the phenomenon that the angle between crack propagation direction and tube axial is about 70° is detailed analysed. By comparison and analysis of the testing results, critical J-integral value during crack propagation is determined. Furthermore, critical J-integral value of crack propagation in electrofusion joint is predicted.