The Positive Role of Nanometric Molybdenum–Vanadium Carbides in Mitigating Hydrogen Embrittlement in Structural Steels

The influence of hydrogen on the fracture toughness and fatigue crack propagation rate of two structural steel grades, with and without vanadium, was evaluated by means of tests performed on thermally precharged samples in a hydrogen reactor at 195 bar and 450 °C for 21 h. The degradation of the mechanical properties was directly correlated with the interaction between hydrogen atoms and the steel microstructure. A LECO DH603 hydrogen analyzer was used to study the activation energies of the different microstructural trapping sites, and also to study the hydrogen eggresion kinetics at room temperature. The electrochemical hydrogen permeation technique was employed to estimate the apparent hydrogen diffusion coefficient. Under the mentioned hydrogen precharging conditions, a very high hydrogen concentration was introduced within the V-added steel (4.3 ppm). The V-added grade had stronger trapping sites and much lower apparent diffusion coefficient. Hydrogen embrittlement susceptibility increased significantly due to the presence of internal hydrogen in the V-free steel in comparison with tests carried out in the uncharged condition. However, the V-added steel grade (+0.31%V) was less sensitive to hydrogen embrittlement. This fact was ascribed to the positive effect of the precipitated nanometric (Mo,V)C to alleviate hydrogen embrittlement. Mixed nanometric (Mo,V)C might be considered to be nondiffusible hydrogen-trapping sites, in view of their strong hydrogen-trapping capability (~35 kJ/mol). Hence, mechanical behavior of the V-added grade in the presence of internal hydrogen was notably improved.

Friction Stir Processing Influence on Microstructure, Mechanical, and Corrosion Behavior of Steels: A Review

Friction stir processing (FSP) technology has received reasonable attention in the past two decades to process a wide range of materials such as aluminum, magnesium, titanium, steel, and superalloys. Due to its thermomechanical processing nature, FSP is used to alter grain structure and enhance mechanical and corrosion behavior in a wide range of steels. The refinement in grains and phase transformations achieved in steel after FSP affects hardness, tensile properties, fracture toughness, fatigue crack propagation rate, wear resistance, and corrosion resistance. A number of review papers are available on friction stir welding (FSW) or FSP of nonferrous alloys. In this article, a comprehensive literature review on the FSP/FSW of different types of steels is summarized. Specifically, the influence of friction stir processing parameters such as advancing speed, rotational speed, tool material, etc., on steels’ performance is discussed along with assessment methodologies and recommendations.

ON THE LAW OF DISTRIBUTION OF THE RATE OF FATIGUE FRACTURE

The article discusses the correction of the law of development of fatigue cracks based on the application of the Fokker–Planck–Kolmogorov equation. The coefficient of variation of the fatigue crack propagation rate was chosen as the correction criterion. Its decrease brings the descriptions of the fatigue fracture process closer to engineering. The latter is especially important for specialists dealing with the assessment of the fatigue strength of metal-consuming structures, where mass formation of fatigue cracks is inevitable.

Mechanical Behavior and Morphological Study of Polytetrafluoroethylene (PTFE) Composites under Static and Cyclic Loading Condition

The key goal of this study was to characterize polytetrafluoroethylene (PTFE) based composites with the addition of bronze particles and mineral fibers/particles. The addition of individual fillers was as follows: bronze—30–60 wt.%, glass fibers—15–25 wt.%, coke flakes—25 wt.% and graphite particles—5 wt.%. Both static and dynamic tests were performed and the obtained results were compared with the microscopic structure of the obtained fractures. The research showed that the addition of 60 wt.% bronze and other mineral fillers improved the values obtained in the static compression test and in the case of composites with 25 wt.% glass fibers the increase was about 60%. Fatigue tests have been performed for the compression-compression load up to 100,000 cycles. All tested composites show a significant increase in the modulus as compared to the values obtained in the static compression test. The highest increase in the modulus in the dynamic test was obtained for composites with 25 wt.% of glass fibers (increase by 85%) and 25 wt.% of coke flakes (increase by 77%), while the lowest result was obtained for the lowest content of bronze particles (decrease by 8%). Dynamic tests have shown that composites with “semi-spherical” particles are characterized by the longest service life and a slower fatigue crack propagation rate than in the case of the long glass fibers. In addition, studies have shown that particles with smaller sizes and more spherical shape have a higher ability to dissipate mechanical energy, which allows their use in friction nodes. On the other hand, composites with glass fiber and graphite particles can be successfully used in applications requiring high stiffness with low amplitude vibrations.

Numerical Modeling of Plasticity-Induced Fatigue Crack Growth Retardation Due to Deflection in the Near-Tip Area

This article studies the retardation effect in plasticity-induced fatigue crack growth rate for a low-medium strength steel, due to the appearance of microdeflections in the crack path. To this end, the finite element method was used to model the crack with its kinked tip under several stress intensity factor (SIF) ranges. The results allowed a calculation (after a small number of cycles) of the fatigue crack propagation rate for the multiaxial and uniaxial fatigue configurations at the microscopic level. It was observed that the retardation effect rose with an increase in the initial kinked crack tip angle, an increase in the initial projected kinked crack tip length, and with a decrease in the SIF range.

Research on stress intensity factor and fatigue crack propagation rate of the general-purpose gondola car body

In order to study the crack propagation characteristics of key parts of the general-purpose gondola car body, according to the structural characteristics and crack statistics results of the car body, the floor along the cross bearer weld was taken as the research object. The stress intensity factors under different conditions of the floor were studied, based on linear elastic fracture mechanics and finite element analysis. In addition, the crack propagation rates of the floor were studied by the Paris formula. These research results will provide a reference not only for formulating maintenance procedures for the general-purpose gondola car body, but also for evaluating the safety and reliability of wagon bodies.

Influence of microstructural evolution and localized delta ferrite number on high-cycle fatigue crack opening and propagation rate

In this research, percentages of nickel as austenizer and chromium as ferritizer, in the molten metal pool of SS304L weldments are varied to analyze the effect on the produced delta ferrites morphology and volume. The resulting localized precipitation of delta ferrite and its effect on the fatigue crack propagation rate of the material are examined in this work. The estimation of localized delta ferrite number for a particular weld zone is assessed through the development of a MATLAB code for image processing. It is observed that a reduction in the percentage of nickel in filler alloy yields randomly scattered granular ferrites in the fusion zone and equiaxed grain growing in the heat-affected zone with the development of blocky ferrites on the grain boundaries, whereas increased percentage of chromium in the filler causes the precipitation of dendritic clusters and vermicular ferrites in fusion zone and heat-affected zone of weldments, respectively. The effect of ferrite numbers and their morphology on the crack opening displacements and the corresponding fatigue crack propagation rate in different weld zones are analyzed. It is observed that the ferrite number ranging between 15 and 19 provides optimum fatigue strength for SS 304L. Paris curves are plotted from the test data, which shows that the weldment attained with filler alloy having Cr 26.60% and Ni 10.92% depicts the highest fatigue life compared to other studied weldments.

Crack Propagation in the Spur Gear of Tubler Gear Mechanism of Lathe Machine

An estimation of the spur gear service life and crack path analysisis proposed in this paper. Ip theory using numerical solution is used to find the fatigue crack propagation rate, service life, and crack path due to bending fatigue. A spur gear of tumbler gear mechanism of lathe machine, is implemented for this analysis. An experimental test on gear specimens was carried out to analyze the crack propagation due to fatigue. Besides, service life estimated by the theory was compared with the most common Paris law. Hence further, extended finite element approach (Ansys)is implemented for the simulation of gear crack propagation trajectory. The simulation results are verified with experimental test. The service life and crack trajectory of the gear is obtained with ‘Ip’ theory and found in good agreement with experiment.In overall, this study signifies the gear failure prediction methodology during crack propagation in respect of life cycles and trajectory path.

Experimental Study on Fatigue Crack Propagation of High-Strength Steel Wire with Initial Defects for Bridge Cables

In order to study the effect of initial defects on fatigue crack propagation law, a test method to identify fatigue crack propagation rate and path based on load waveform variation was presented, and a new test device was designed to apply fatigue pulsation loads to multiple wires for bridge cables simultaneously in this paper. To simplify the corrosion defect formation process, a machine-cut notch was used to describe the initial defect on the steel wire surface. Firstly, fatigue crack propagation tests were conducted on the surface notched steel wire specimens. By using crack front marking technique, the “beach-like patterns” visible to the naked eyes on the cross sections of the steel wires were formed, and the process of fatigue crack propagation can be tracked and reproduced. Then Autodesk Computer Aided Design (AutoCAD) software was used to describe the morphology of “beach-like patterns” and accurately measure the depth and width of cracks. Finally, the influence of initial defect morphology on fatigue crack propagation rate was investigated according to the relationship between fatigue cracks depth and cyclic loading numbers. The results show that the test device designed in this paper can effectively realize the synchronous fatigue crack propagation test of multiple wires, and significantly shorten the fatigue test period. By observing and analyzing the change of load waveform, the moment of fatigue crack propagation can be directly and accurately determined. The larger the depth, the smaller the width and the sharper the morphology of initial defect, the faster the crack propagation rate and the shorter the life of notched wire specimens under the combined action of fatigue loads and corrosive medium.