Fatigue Crack Nucleation Sites Research Articles

Mechanical properties of the hot-rolled Mg–12Gd–3Y magnesium alloy

Tensile and high cycle fatigue (HCF) properties of the hot-rolled Mg–12Gd–3Y (wt.%) magnesium alloy have been investigated. The magnesium alloy exhibits a fatigue strength of about 150 MPa, which is much higher than that of the commercial Mg–8Al–Zn alloy AZ80. Aging heat-treatment (T5) improved the fatigue life of the Mg–12Gd–3Y alloy. Fatigue cracks nucleated at the intense slip bands in the as-rolled alloy. After T5 treatment, however, the fatigue crack nucleation site shifted to the phase boundaries between MgGdY particles and Mg matrix. T5 heat-treatment retarded the crack initiation and thus improved the fatigue life of the Mg–12Gd–3Y alloy.

Effect of Shot Peening on Surface Characteristics and Fatigue Properties of T5-Treated ZK60 Alloy

The effects of shot peening on surface characteristics and high cycle fatigue (HCF) performance of T5-treated high-strength magnesium alloy ZK60 (named after ZK60-T5) were investigated. The glass bead with an average diameter of 0.35 mm was adopted for shot peening and the Almen intensity was arranged from 0.02 to 0.40 mmN. The surface microstructure and texture of ZK60-T5 are greatly changed by shot peening, and residual compressive stress is produced in the surface deformation layer. The magnesium alloy ZK60-T5 shows a pronounced overpeening effect. A marked improvement in fatigue life is obtained at low Almen intensities, namely the fatigue strength (at 10 7 cycles) increases from 150 to 195 MPa at the optimum Almen intensity of 0.05 mmN. The fatigue crack nucleation site of ZK60-T5 is also found from surface regions to the subsurface. [doi:10.2320/matertrans.MRA2008415]

Effects of grain boundary- and triple junction-character on intergranular fatigue crack nucleation in polycrystalline aluminum

The effects of grain boundary- and triple junction-character on intergranular fatigue crack nucleation were studied in coarse-grained polycrystalline aluminum specimens whose grain boundary microstructures were analyzed by SEM-EBSD/OIM technique. Fatigue crack nucleation occurred mainly along grain boundaries and depended strongly on both the grain boundary character and grain boundary configuration with respect to the persistent slip bands. However, it was little dependent on the geometrical arrangements between the grain boundary plane and the stress axis. Particularly, random boundaries become preferential sites for fatigue crack nucleation. The fatigue cracks were also observed at CSL boundaries when the grain-boundary trace on the specimen surface was parallel to persistent slip bands. On the other hand, no intergranular fatigue cracks were observed at low-angle boundaries. The fatigue cracks were observed at triple junctions as well as grain boundaries. Their nucleation considerably occurred at triple junctions where random boundaries were interconnected. The grain boundary engineering for improvement in fatigue property was discussed on the basis of the results of the structure-dependent intergranular and triple junction fatigue crack nucleation.

Failure analysis of belt/roll tribological pair used for the production of eucalypt fiber panels

The premature failure of a large agglomeration machine used for the annual production of 360,000 m 3 of eucalypt fiber panels was investigated to identify the nucleation and growth mechanisms of cracking in PH stainless steel belts (126 m × 2.9 m × 3.0 mm). These belts are used to compress a cushion composed of eucalyptus fibers and glue, being the pressure transmitted from the pistons by the action of numerous case-hardening steel rolls. Examination of the belt working interfaces (belt/rolls and belt/eucalypt fibers) indicated that the main cracking was nucleated on the belt/roll interface and that there is a clear relationship between the crack nucleation and the presence of superficial irregularities, which were observed on the belt/roll working surface. Used rolls showed the presence of perimetric wear marks and 2 μm silicon-rich encrusted particles (identified as silicon carbide). Lubricant residues contained the presence of helicoidal wires, which were originated by the release of the stainless steel cleaning brush bristles, and 15 μm diameter metallic particles, which were generated by material detachment of the belt. The presence of foreign particles on the tribological interface contributed to an increase of the shear stresses at the surfaces and, consequently, the number of the contact fatigue crack nucleation sites in the belt/roll tribo-interface. The cracking was originated on the belt/roll interface of the stainless steel belt by a mixed rolling/slip contact fatigue mechanism, which promoted spalling and further nucleation and growth of conventional fatigue cracks. Finally, the system lubrication efficiency and the cleaning procedure should be optimised in order to increase the life expectancy of the belt.

Effects of Mn content and texture on fatigue properties of as-cast and extruded AZ61 magnesium alloys

Fatigue behavior of as-cast and extruded AZ61 magnesium alloys in ambient air (20 °C–55%RH) was investigated. It was found that size and distribution of cast defect influenced tensile and fatigue performance of the as-cast alloy. Fatigue limit of the as-cast alloy was significantly low compared to the extruded alloy. The casting defects served as stress concentration sites for fatigue crack nucleation. Fatigue tests were also carried out on a high Mn content alloy. All of the specimens failed from an inclusion near the specimen surface. Fatigue limit of Mg alloy with high Mn content was lower compared to that of the low Mn content alloy. Further, investigation on the effect of texture on fatigue and fatigue crack growth behavior of the extruded AZ61 magnesium alloy plate was carried out. The results showed that fatigue strength in the longitudinal direction to the extruded direction was higher compared to those in the transverse and 45° directions. Significant effect of specimen orientation on fatigue crack growth behavior for both short and long cracks was found near the threshold region. However, regardless of specimen orientation, the d a / d N – Δ K eff curves for all three kinds of specimens were in a narrow band. It is suggested that the difference in the fatigue life among the specimen orientations will be mainly due to the difference in the crack closure behavior. A transition of fracture mechanism was found for a long crack. Slip fracture mechanism was dominant above the transition point, whereas below the transition point, slip fracture mechanism was associated with cleavage fracture.

Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength

The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was >10 7 cycles. A significant change in the slope of the S–N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 10 7 and 10 9 cycles are >99%.

A model of initial flaw sizes in aluminum alloys

It has been shown in a previous study that cracked constituent particles constitute the majority of the observed fatigue crack nucleation sites in several aircraft aluminum alloys. Distributions of these particle sizes may be modeled as initial crack sizes and used to predict fatigue life. Lognormal distributions have been fit to the measured lengths and widths of the constituent particles for 2024-T3, 2524-T3, and 7075-T6 aluminum alloys. Choleski decomposition was used to generate pairs of crack width and depth, resulting in a theoretical initial semi-elliptical crack size. The resulting distribution can be used to predict fatigue life distributions.

Elastic Anisotropy Effect of Crystals on Polycrystal Fatigue Crack Initiation

Fatigue bands have been observed in both monocrystalline and polycrystalline metals. Extrusions and intrusions at the free surface of fatigued specimens are favorable sites for fatigue crack nucleation. Previous studies (Lin and Ito, 1969; Lin, 1992) mainly concerned the fatigue crack initiation in aluminum and its alloys. The elastic anisotropy of individual crystals of these metals is insignificant and was accordingly neglected. However, the anisotropy of the elastic constants of some other metallic crystals, such as titanium and some intermetallic compounds, is not negligible. In this paper, the effect of crystal anisotropy is considered by using Eshelby’s equivalent inclusion method. The polycrystal analyzed is Ni3Al intermetallic compound. The plastic shear strain distributions and the cumulative surface plastic strain in the fatigue band versus the number of loading cycles were calculated, and the effect of crystal anisotropy on the growth of the extrusions was examined.

Observations of Slip Bands in the Study of Fatigue Fracture of High-Purity Iron

There are two basic problems about fatigue, one is how cracks are formed, and the other is how they grow. The answers to these questions have been sought by investigators for a long time, but even now the problems remain substantially unsolved. Since the dawn of the use of electron microscopy for observing microstructural changes in metals, recent works have begun to shed light on the nature of basic physical processes involved in fatigue, and, on the other hand, a study to unravel the relationship between microscopic and macroscopic behaviors of materials has been urgently required. It is considered that if plastic replicas and optical microscope were prepared in one and the same horizon it would be possible to obtain the correlation of microscopic characteristics with macroscopic aspects of fatigue deformation.In the present paper the report is given of the examination made, to clarify the basic mechanisms of fatigue fracture in b.c.c. metals, of the thin plate specimens of high-purity iron (99.996%) subjected to alternating bending stress. Particularly the microstructural changes that occurred around the fatigue cracks were examined by optical metallography and electron microscopy, using an improved replica technique.The results obtained in this study are summarized as follows:(1) The crossing points of the two slip bands are the preferred sites for fatigue crack nucleation. In these regions the aggregation of vacancies generated by the to-and-fro slip movement could form pores in the slip bands, and these in turn could grow into microcracks.(2) There was a rather pronounced grain size effect on the nucleation and propagation of fatigue cracks. In coarse grained specimens fatigue cracks were formed preferentially in the vicinity of the grain boundaries, then spread partly along the slip bands and partly along the grain boundaries. Both the nucleation and propagation of cracks in the fine grained specimens, however, were found only in the grain boundaries.(3) Observations with an electron microscopy disclosed that large numbers of microcracks were formed in the region where the first crack was nucreated. It is confirmed that the possible coalescence of random microcracks leads to fatal cracks.(4) The microcracks are formed at the tip of the growing crack by the shear strain field which precedes the crack propagation. When microcracks are numerous enough, they in turn coalesce into the cracks.(5) Observations on the surfaces have shown that substructure develops around the fatigue crack if the grain is favorably oriented, and crack propagation occurs preferentially along the subgrain boundaries.(6) In high-purity iron, extrusions and intrusions have been observed in slip bands, and deformation bands were also formed. It is suggested that the deformations in polycrystalline materials are much complicated because of the constraints of surrounding grains.