In situ SEM study on fatigue short crack initiation and propagation in microscale single-crystal copper with different loading conditions

Simulation of stage I-crack growth using a hybrid boundary element technique

Microstructural effects on short fatigue crack propagation in cast Al-Si alloys

Influence of environment on low cycle fatigue damage in Ti [sbnd]6Al [sbnd]4V and Ti 6246 titanium alloys

Herein, a short crack propagation in a martensitic spring steel by means of in situ fatigue tests in a confocal laser microscope and additional electron back‐scattered diffraction analyses to link the local crystallographic orientation of the hierarchical martensitic microstructure with the short crack propagation behavior are investigated. For this purpose, different stress levels are imposed at a constant R ratio of −1 and a frequency of 10 Hz applying a sinusoidal command signal. It is found that the early fatigue damage evolution is characterized by the formation of slip bands, which subsequently serve as crack initiation sites. Most of the slip bands and correspondingly most of the short fatigue cracks initiate at or close to prior austenite grain boundaries. A strong dependence of the crack density, used as a parameter for fatigue damage, on the applied stress amplitude could be observed. The prior austenite grain boundaries can be identified to act as obstacles to short crack propagation because of the corresponding strong change in the crystallographic orientation, leading to an oscillating short crack propagation rate.

Short crack propagation from cracked non-metallic inclusions in a Ni-based polycrystalline superalloy

Improved high-temperature fatigue performance of laser directed energy deposited Ni-based superalloy by regulating the heat treatment

2D cohesive zone modeling of crack development in cementitious digital samples with microstructure characterization

Diffraction and phase contrast tomography techniques were successfully applied to an austenitic–ferritic duplex stainless steel representing exemplarily a metallic material containing two phases with different crystal structures. The reconstructed volumes of both phases were discretized by finite elements. A crystal plasticity finite-element analysis was executed in order to simulate the development of the experimentally determined first and second order residual stresses, which built up due to the manufacturing process of the material. Cyclic deformation simulations showed the single-grain-resolved evolution of initial residual stresses in both phases and were found to be in good agreement with the experimental results. Solely in ferritic grains, residual stresses built up due to cyclic deformation, which promoted crack nucleation in this phase. Furthermore, phase contrast tomography was applied in order to analyze the mechanisms of fatigue crack nucleation and short fatigue crack propagation three-dimensionally and nondestructively. The results clearly showed the significance of microstructural barriers for short fatigue crack growth at the surface, as well as into the material. The investigation presented aims for a better understanding of the three-dimensional mechanisms governing short fatigue crack propagation and, in particular, the effect of residual stresses on these mechanisms. The final goal was to generate tailored microstructures for improved fatigue resistance and enhanced fatigue life.

This study presents the effects of the geometry of the notches on the fatigue life, crack initiation, and growth in mixed mode and first mode of the fracture. For this purpose, some samples were tested with U-shaped notches with different radii and depths and various orientations with respect to the loading directions under fatigue loads. Using the fatigue crack growth rates, the Paris law coefficients were obtained for the used material in different conditions. It was shown that these coefficients are independent of the geometry of the samples. Fatigue crack growth behaviors in the mixed mode tests were also in good agreement with the result of the numerical simulations. Predictions of the maximum tangential stress criterion and numerical simulation for the position of the crack initiation on the notch root were compared with the tested observations, which showed good agreement. The fatigue life of the test samples was compared with the analytical results provided by the Manson-Coffin law. It was shown that a 25% decrease in notch depth increases the fatigue life by 4 to 6 times, also, an increase of 0.5 mm in the notch radius increases the fatigue life by 40%. Finally, the fracture surface of the samples was checked using an optical microscope. This study showed that the fracture surfaces have one or two lateral shear lips and plane stress conditions were established.

Soil failure associated with crack propagation for an agricultural tillage tool

Fatigue propagation of short and long cracks in gaseous hydrogen environment in 3.5NiCrMoV steel

High frequency push-pull fatigue experiments on the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318LN) revealed that crack nucleation and crack propagation through the first grain determine significantly the lifetime of the material. Only in very few cases it was observed that fatigue samples which endured one billion load cycles without failure (run-out samples) contain microcracks which reached or overcame the first microstructural barrier (phase or grain boundary). This leads to the conclusion that in most cases the highest macroscopic stress or strain amplitude which does not lead to fatigue crack propagation through the entire first grain can be considered as the fatigue limit of the material. The present study documents that the experimentally identified fatigue mechanisms can be represented in mesoscopic finite element simulations by taking into account the effects of anisotropic elasticity, crystal plasticity, macro and micro residual stresses, plastic strain concentration in form of slip bands, crack nucleation and short crack propagation through the first grain. The current investigation shows that such simulations enable the determination of the fatigue limit of both real and synthetic microstructures. By means of real microstructures, containing slip traces and microcracks, the calculations can be verified and the required microstructural parameters can be determined.

The damage of metals and alloys under cyclic straining occurs through the progressive development of surface roughness and short crack nucleation and growth before the beginning of fatal crack propagation in the hulk. During the major part of the fatigue lifetime (> 80% N F ), a development of short cracks preceeding the fatal crack propagation takes place. Thus, fatigue lifetime is mainly determined by the resistance against short crack propagation. In aqueous solutions, the reduction of fatigue lifetime results often from complex corrosion-deformation interactions which can lead to the activation of new damage mechanisms which involve generally local electrochemical reaction An experimental method based on the monitoring of current or potential transients during cyclic straining is proposed. Data related to the elementary damage mechanisms and to the damage development during the test can be collected from the shapes and amplitudes of current (potential) peaks within individual cycles and from the evolution of corresponding peak values versus number of cycles. Examples of applications illustrate the possibilities of the method in the field of corrosion fatigue studies of passivating alloys. First, the damage localization in a duplex stainless steel and relationship between strain rate and damage evolution in a single phase c.c stainless steel in a chloride solution are discussed. Then, the application of the method to identify the iodine induced corrosion fatigue mechanisms in Zircaloy-4 is presented. In all cases, the stage of short crack nucleation as well as transitions between different damage mechanisms can be precisely determined.

Bonded joints exhibit high mechanical performances but also suffer from poor durability in humid and/or hot environmental conditions. Indeed, delayed failures can be observed under stationary loading conditions due to chemical aggression but also due to creep failure. Despite bonded joints are generally loaded in shear, discussion on crack initiation and propagation condition under mode II stationary loading condition are lacking. Then, a semi-analytical model is proposed here to determine response of a End loaded Split (ELS) like test specimen considering viscoelastic-viscoplastic interface behavior, stationary loading condition and steady-state crack propagation regime. From these simulations, the evolution of crack propagation rate as a function of macroscopic loading intensity is obtained. The intrinsic nature of such “master curves” is finally discussed.

The propagation of fatigue short crack is a fractal curve, the irregular path can be described by fractal dimension. The fractal dimension of the crack implicit in many of the physical factors, such as microstructure of material, load conditions and so on. In this paper, the short crack for low cycle at high temperature is studied. The material 20# steel in the various stress state is experimentally studied, the initiation, propagation and coalescence influence of short crack are observed. By analyzing experiment results, characters of crack fractal dimensions evolving with the cycle process are as follows, the behavior of short fatigue cracks satisfies fractal feature, with the development of the fatigue process, the fractal dimension is increased.