Physically based fatigue short crack growth concept

The paper contains the main results of an investigation of an effect of shot peening on residual fatigue life of specimens made of an aircraft V-95 Al-alloy (equivalent to 7075 alloy) containing physically short fatigue cracks. In the first part, an adaptation and verification of direct current potential drop method for detection and measurement of short crack growth is described and discussed. The specific properties of the material and quite large dimensions of flat specimens with side necking with a low stress concentration factor had to be considered when the position of electrodes was specified. The specimen type and dimensions were proposed considering the investigation of shot peening effects. Physically short fatigue cracks of the length from 0.2 mm to more than 3 mm, most of them between 0.8 – 1.5 mm, were prepared under high cycle fatigue loading of the constant nominal stress amplitude ±160 MPa. Specimens with existing short fatigue cracks were shot peened using two different groups of parameters. Crack growth characteristics after shot peening were measured and compared with crack growth in specimens without shot peening. Retardation of crack growth was significant particularly for cracks shorter than 2 mm. Results are also discussed form the viewpoint of fracture mechanics approach. For the specific stress amplitude, evaluated results enable to estimate threshold length of defects, which after the application of shot peening will be reliably arrested.

Strain visualization of growing short fatigue cracks in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint: Intergranular cracking and crack closure

An investigation of the effects of mean and alternating stresses on short fatigue crack growth under uniaxial loading is reported. Tests under stress‐controlled conditions with different combinations of mean and alternating stresses were conducted at room temperature using a 1.99% NiCrMo steel. The fatigue process was divided into three phases: Plasticity Localization, Microstructural Short Crack (MSC) growth, and Physically Short Crack (PSC) growth. The effect of mean stress on the above three phases is studied and a set of equations is obtained which can predict the early stage and growth behaviour of short cracks under various mean and alternating stresses. Satisfactory predictions of fatigue endurance curves at different mean stresses have been achieved. This approach provides an alternative method, based on the physics of the fatigue process, for use by designers who currently rely on conventional Goodman‐type diagrams.

The short-crack propagation behaviour of 8090 Al–Li alloy under different ageing conditions has been investigated. The effect of notch geometry on short fatigue crack growth was also studied. The results show that the geometrical configuration of the notch significantly affects the growth behaviour of the short crack, the growth rates of notched short cracks being much higher than those of long cracks at the same stress intensity factor range ΔK level. The orientations of the specimens had a stronger effect on the growth rate of long cracks than on that of short cracks.

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.

Effect of microstructure on short fatigue crack growth of wire arc additive manufactured Ti-6Al-4V

Microstructurally short fatigue crack initiation and growth in Ti-6.8Mo-4.5Fe-1.5Al

Prediction of short fatigue crack propagation behaviour by characterization of both plasticity and roughness induced crack closures

This paper presents a new technique for studying the growth and morphology of fatigue cracks. The technique allows short fatigue crack growth, crack depth, aspect ratio (crack depth/half crack length), and crack front configuration to be measured using a Confocal Scanning Laser Microscope (CSLM). CSLM measurements of the initial stage of crack growth in Al 2024-T351 revealed that microstructurally short fatigue cracks grew initially along a plane inclined to the applied stress. The angle of the inclined plane (Stage I crack growth) was found to be about 45 degrees to the axis of the applied tensile load. Aspect ratio and the angle of maximum shear plane (Mode II), obtained using the CSLM technique, showed a good agreement with those obtained using a Surface Removal (SR) technique. The aspect ratios obtained using the CSLM technique were found to remain constant with increasing crack length in Al 2024-T351 and SAE 1045 Steel at 0.83 and 0.80, respectively. Optical sectioning along the length of a crack revealed that the crack front in the interior of the materials has a semi-elliptical shape. These results are in good agreement with results obtained using the SR technique. The CSLM technique was employed to characterize the fracture surface of fatigue cracks in an SAE 1045 Steel. CSLM image processing of the fracture surface near the crack tip constructed a three dimensional profile of fracture surface asperities. The heights of asperities were obtained from this profile. Optical sectioning from a post-image-processed crack provided crack depth and crack mouth width at every point along the crack length for each load level. The crack opening stress was taken as the stress level at which the crack depth stopped increasing with increases in applied stress.

The Effect of a Complex Stress State on Fatigue Crack Propagation and the Orientation of the Cracking Plane in VT3-1 Aeronautical Titanium AlloyThe subject of the paper is the investigations of fatigue crack imitation and propagation in notched specimens made of the VT3-1 aeronautical russian titanium alloy under combined bending - torsion loading. The presence of short cracks was revealed at various ratios of bending to torsion. Experimental courses of short and long crack growth rates have been proved by the SEM and TEM micrographs which illustrated the changes in the mechanism of cracking in the examined specimens. The attempt was undertaken in order to explain partly brittle fracture that was observed in the range of fatigue short crack growth in the VT3-1 titanium alloy specimens. The results of the study of atmospheric hydrogen absorption capability and its ability for penetration inside the faces of nucleated and propagated microcracks in the surface layer allowed for suggestion that the cleavage mechanism of fracture found in the regime of short crack growth in the VT3-1 titanium alloy specimens was induced by hydrogen.

Short fatigue crack growth under mixed mode loading in Ni 3Al alloy single crystals

A multi-scale damage model for fatigue accumulation due to short cracks nucleation and growth

This paper deals with the prediction of short and long fatigue crack growth in notched members using linear elastic fracture mechanics (LEFM) and includes the stochastic nature of the fatigue process. The probabilistic approach to modelling of crack growth in the depth of components links up with the authors’ previous work, which considered surface crack growth in notched and unnotched samples. The capability of the discussed method in terms of crack growth prediction and lifetime estimation has been verified using experimental data gained for the (a+s) Ti-6Al-SMo-2Cr (Vt3-1) alloy tested under reversed torsion and reversed bending, at room temperature, and in the range of high cycle fatigue (HCF). Characteristic features of crack initiation and propagation in the examined titanium alloy components with a circumferential notch are shown. Micro-observations of fracture surfaces using both a SEM and TEM allowed us to establish the mechanism of cracking.

The short fatigue crack growth behaviour of a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post-mortem analysis of fracture surfaces and crack profiles. Crack-microstructure interactions have been clearly delineated using a combination of optical microscopy (OM), scanning electron microscopy (SEM) and three dimensional (3D) X-ray microtomography (SRCT). Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy

In this study, the initiation and early growth behaviour of short fatigue cracks in En 7A steel with a high content of elongated MnS inclusions was investigated, by generating and evaluating data o...