Papers by Keyword: Fatigue Crack Initiation

Abstract: Titanium alloy Ti6Al4V was oxidation annealed in the beta-phase region (1050 °C/3 hours + WQ) in a furnace under a non-protective atmosphere. The above treatment caused the formation of an alpha-case layer on the surface. The above layer, because of its high hardness and strength, has a significant effect on the surface properties of the alloy. However, undesirable effects include the formation of cracks in this layer and a change in the mechanism of initiation and propagation of fatigue cracks. Based on the above findings, it is also very complicated to predict the fatigue life of Ti alloys processed in this way because of the presence of cracks in the alpha-case layer and the varying thickness of this layer. From the fractography and microstructural point of view, the initiation of fatigue cracks in the heat-treated alloy is realized by transcrystalline cleavage and the formation of pronounced fracture facets as a consequence of surface cracks in the alpha-case layer. Fatigue crack propagation (in the alpha-case layer region) is realized along the interface of the alpha-phase needles and the beta-phase primer grain without the significant presence of so-called fatigue striations.

Abstract: Fatigue tests were conducted under several stress ratios, including negative maximum stress to elucidate the fatigue crack initiation mechanism of a magnesium alloy, AZ31. The specimen surface near the crack initiation site was analyzed by EBSD. On the basis of the results of EBSD analysis, it is concluded for an alternating cyclic stress condition (fully reversed cyclic stress) that fatigue cracks formed from grains where both the grain size and Schmid factor of the basal slip system are large, and that the crack initiation mechanism is based on irreversible slipping and unrelated to twinning. Under compression-compression fatigue test (R=10), cracks were formed along boundary of grains with large Schmid factor and misfit of both side grain are large. At the tip of the initiated crack, twin bands were observed.

Abstract: Axially push-pull cyclic tests of a low strength rotor steel were performed up to the very high cycle fatigue regime at ambient environment under ultrasonic frequency. Fatigue tests were interrupted at selected number of cycles for surface morphology observation and roughness measurement with the help of a 3D surface measurement system (Alicona InfiniteFocusSL). The fatigue extrusions and slip band developed on the specimen surface were recorded. The influence of stress level on the number and morphology of slip band was discussed. The surface roughness of fatigue specimens was found to be increased with the increasing of fatigue cycles. The fatigued specimens were finally cracked from surface or interior micro-defects after observation of fracture surface by scanning electron microscopy. The internal damage behavior consists of crack initiation and early propagation from micro-defect, crack growth within the fish eye, and fast crack growth. It is observed that there exists a competition between surface and internal fatigue damage in the very high cycle fatigue regime, i.e., surface damage is gradually developed with the increasing of fatigue cycles, while the critical interior micro-defect can be dominant for fatigue cracking.

Abstract: Fatigue crack initiation stage occupies a large proportion of total fatigue life in modern engineering materials and structures which are often designed under lower service loading conditions. In this paper, the fatigue crack initiation behavior from a micro-void in a small-scale specimen was studied in-situ in SEM. Surface morphologies were monitored in-situ and images were taken during interrupted tests at selected number of cycles, and displacement and strain map around the void was calculated based on digital image correlation (DIC) technique. The results indicated that the strain evolution near the micro-void could be divided into stages, before crack initiation. The strain increasing rate was fast in the early stage and slower in the second stage. A critical cyclic strain value for fatigue crack initiation from the micro-void was obtained around 9%, and was believed to be the dominant factor for early stage of fatigue damage.

Abstract: The present work is focused on the study of crack initiation during low cycle fatigue (LCF) loading of the second generation nickel-based superalloy MAR-M247 treated with hot isostatic pressing. LCF tests were conducted on cylindrical specimens in symmetrical push-pull cycle under strain control with constant total strain amplitude and strain rate at 800 °C in air atmosphere. Selected specimens were electrolytically polished to facilitate surface relief observations. Crack initiation sites were studied by means of scanning electron microscopy (SEM) in dual beam microscope TESCAN LYRA 3 XMU FESEM equipped with focus ion beam (FIB). The microstructure of the material is characterised by coarse dendritic grains with numerous carbides and small casting defects. The average grain size was 2.1 ± 0.3 mm. Fractographic analysis revealed the fatigue crack initiation sites and their relation to the casting defects and material microstructure. Casting defects, carbide inclusions and interdendritic areas were found to be important crack nucleation sites. Specimens’ surface observations revealed the formation of pronounced surface relief with short worm-like markings. Fatigue crack initiation in these places is documented and discussed.

Abstract: Fatigue damage in a metallic material during very high cycle fatigue can strongly be correlated to the microstructure. This paper provides a review and a discussion on the micro damage behaviors in a nickel base alloy and three steels during very high cycle fatigue using micro plasticity and material mechanics. The results show that cyclic plastic deformation in these materials can occur very locally even with an applied stress that is much lower than the yield strength. The fatigue damage occurs mainly at grain or twin boundaries due to local impingement and interaction of slip bands and these boundaries. The crystallographic properties, Schmid factors and orientations of grain and boundaries have very important roles to the fatigue damage. Subsurface fatigue crack initiation in the matrix is one of the mechanisms for very high cycle fatigue. In the fine granular area, high plastic strain localization and cyclic plastic deformation can lead to dislocation annihilation and consequently formation of vacancies, or eventually nanopores at the subcell boundary that leads to fatigue crack initiation and propagation.

Abstract: Two fatigued materials with f.c.c. lattice, i.e. pure polycrystalline copper and austenitic Sanicro 25 stainless steel, were subjected to the study of the persistent slip markings (PSMs) developed on the surface of the suitably oriented grains. They were observed using scanning electron microscopy (SEM) and thin surface FIB lamellae were prepared and studied by transmission electron microscopy (TEM). The aim was to correlate the specimen surface profile with the underlying internal dislocation structure. The localization of the intensive cyclic slip into persistent slip bands (PSBs) of the material was observed and associated with the PSMs on the specimen surface. Extrusions, intrusions and the dislocation structure appertaining to them were analysed, documented and discussed in relation to the models of fatigue crack initiation.

Abstract: Drilling holes in the vicinity of the crack tip turns the crack into a notch and reduces the crack tip stress intensity factor. In this paper, a new idea is used in which instead of a single hole, two symmetric and interconnected holes are drilled at the crack tip. The main concept of double stop-hole method is to reduce the stress concentration at the edge of stop-holes in the cracked structural elements. The double stop drill hole method can be used to increase the fatigue life of the cracked components. The fatigue crack growth retardation is examined using an experimental investigation coupled with a stress analysis on the efficiency of proposed double stop-holes. The distance between the hole centers is considered as the main parameter affecting the efficiency of this method. The results show that the fatigue life extension caused by the double stop-hole method is significantly more than the conventional single stop-hole method.

Abstract: The top 10 most influential articles in Very high cycle fatigue (VHCF) have been indentified from web of science data. The attributes of the top 10 papers have been discussed. It was found that specialty area of fatigue called as “VHCF” is an emerging field. The most cited papers discussed the two the fatigue crack mechanism in fatigue. It was found that crack initiation shifts from surface to subsurface if the material beyond 10⁷ cycles. There are some models which can predict the fatigue life of the material however the exact estimation is still challenging. Hence, it was found that still further efforts are required in the field to accurately understand the VHCF behavior.

Abstract: The most influential articles in Very high cycle fatigue (VHCF) during last 20 years have been identified from web of science data. It has been found that VHCF is an important category of fatigue damage and has produced the highest impact in the field. The number of articles and their citations is continuously increasing in the overall published papers in the field of fatigue. In VHCF field, it was found that the participants of VHCF conferences from VHCF 1 to 5 are the main contributors in all the VHCF papers and in its top 100 influential papers. Majority of the articles were experimental studies. The fatigue crack initiation in the VHCF domain is considered the most important area in the field. In VHCF, the most influential papers till date have been identified. This may prove helpful to trainees mastering the most influential literature of the field as well as more established professionals searching for starting points for new investigations