Effect of inclusion size on the high cycle fatigue strength and failure mode of a high V alloyed powder metallurgy tool steel

The fatigue strength of a high V alloyed powder metallurgy tool steel with two different inclusion size levels, tempered at different temperatures, was investigated by a series of high cycle fatigue tests. It was shown that brittle inclusions with large sizes above 30μm prompted the occurrence of subsurface crack initiation and the reduction in fatigue strength. The fracture toughness and the stress amplitude both exerted a significant influence on the fish-eye size. A larger fish-eye area would form in the sample with a higher fracture toughness subjected to a lower stress amplitude. The stress intensity factor of the inclusion was found to lie above a typical value of the threshold stress intensity factor of 4 MPa.m^1/2. The fracture toughness of the sample with a hardness above HRC 56 could be estimated by the mean value of the stress intensity factor of the fish-eye. According to fractographic evaluation, the critical inclusion size can be calculated by linear fracture mechanics.

Effects of the shape and size of rectangular inclusions on the fatigue cracking behavior of ultra-high strength steels

The fatigue cracking behavior of ultra-high strength steels containing rectangular inclusions of small sizes were investigated based on in situ observations by scanning electron microscopy （SEM）. The size and shape of rectangular inclusions affect markedly the initiation site and propagation path of a fatigue crack. Especially, the initiation site of a fatigue crack depends strongly on the angle between the long-axis of a rectangle inclusion and the loading direction, and the length/width ratio of this rectangle inclusion because the residual stress distribution fields vary with these conditions. The results coincide very well with those of finite element analysis.

LOW CYCLE FATIGUE BEHAVIOR OF AZ91HP ALLOY IN AS HIGHP RESSURE DIE CASTING

Fully reversed total strain-controlled low-cycle fatigue tests were conducted at room temperature in air to study the fatigue behavior of AZ91HP magnesium alloy in as high pressure die casting and subsequently heat treatment. All the specimens in different heat histories exhibit cyclic strain hardening in different degrees. It is difficult to distinguish the fatigue behavior of the die casting specimens from that of the solution aging specimens. The solution treated specimens show longer fatigue life at high strain amplitude and shorter fatigue life at low strain amplitude than the die casting and solution aging specimens though they have the lowest yield strength with higher strain hardening. Fatigue fracture surfaces for strain amplitude larger than 0.005 show very similar to those found by monotonic tensile tests. The SEM examination reveals that the regions of fatigue crack growth and final fracture can be characterized by quasi-cleavage mechanisms, but some shallow dimples, slip bands and secondary cracks are found on the fracture surface in the fracture crack growth areas.

Micro-mechanism of metal magnetic memory signal variation during fatigue

Tensile fatigue tests were designed to study the relation between the tangential magnetic memory signal and dislocations. According to experimental results, in the early stage of fatigue, the magnetic signal and the dislocation density rapidly increase; while in the middle stage, the magnetic signal gradually increases, the dislocation density remains steady, and only the dislocation structure develops. On the other hand, in the later stage, the magnetic signal once again increases rapidly, the dislocation structure continues to develop, and microscopic cracks are formed. Analysis reveals that the dislocations block the movement of the domain wall, and the area of dislocation accumulation thus becomes an internal magnetic source and scatters a field outward. In addition, the magnetic memory field strengthens with increasing dislocation density and complexity of the dislocation structure. Accordingly, the dislocation pinning factor related with the dislocation density and the dislocation structure has been proposed to characterize the effect of dislocations on the magnetic memory signal. The magnetic signal strengthens with an increase in the dislocation pinning factor.

Improvements in the microstructure and fatigue behavior of pure copper using equal channel angular extrusion

In this study, annealed pure copper was extruded using equal channel angular extrusion （ECAE） for a maximum of eight passes. The fatigue resistance of extruded specimens was evaluated for different passes and applied stresses using fatigue tests, fractography, and metallography. The mechanical properties of the extruded material were obtained at a tensile test velocity of 0.5 mm/min. It was found that the maximum increase in strength occurred after the 2nd pass. The total increase in ultimate strength after eight passes was 94%. The results of fatigue tests indicated that a significant improvement in fatigue life occurred after the 2nd pass. In subsequent passes, the fatigue life con-tinued to improve but at a considerably lower rate. The improved fatigue life was dependent on the number of passes and applied stresses. For low stresses （or high-cycle fatigue）, a maximum increase in fatigue resistance of approximately 500%was observed for the extruded material after eight passes, whereas a maximum fatigue resistance of 5000%was obtained for high-applied stresses （or low-cycle fatigue）. Optical microscopic examinations revealed grain refinements in the range of 32 to 4 μm. A maximum increase in impact energy absorption of 100%was achieved after eight passes. Consistent results were obtained from fractography and metallography examinations of the ex-truded material during fatigue tests.

Fatigue Test of Homemade Z Direction Steel Welded T Tubular Joints

This paper introduces the process and result of fatigue test of steel (Z direction steel) welded T tubular joints used in offshore engineering. Detailed measurement of stress concentration factor, stress distribution, fatigue life and crack development has been performed. Through analysis, an empirical formula of stress concentration factor for T tubular joints, fatigue S-N curve and crack propagation rule are obtained.

BEHAVIOR OF SHORT FATIGUE CRACKS AT BLUNT NOTCHES IN A MEDIUM CARBON STEEL

The behavior of part-through and through short cracks in single edge blunt notched specimens of a medium carbon steel was investigated by a replication method. It is found that the fatigue failure of these notched specimens is caused mainly by the growth of short surface cracks originating from the surface of notch root. More than 70% of the fatigue life is spent in the regimes in which short surface cracks initiate and propagate as part-through cracks before joining up to form a single through-thickness crack. The effect of original crack profile which is formed through the coalescence of multiple part-through cracks is the main reason causing the 'anomalous' propagation behavior of the through-thickness crack in its early stage.

Tensile and fracture properties of Ti-62A alloy plate with different microstructures

Ti-62A alloy plates with three different types of microstructure, fully equiaxed, bimodal, and Widmanstatten, were obtained by various heat treatments to investigate the effects of microstructure on the tensile and fracture properties at room temperature. The results reveal that Widmanstatten microstructure exhibits good damage tolerance behavior considering strength, fracture toughness, and fatigue crack growth behavior, while the bimodal microstructure shows good comprehensive properties considering the plasticity synthetically. Optical microscopy (OM) and scanning electron microscopy (SEM) microstructure analyses on fracture and fatigue crack path demonstrate that the dependence of mechanical properties and fatigue crack growth behavior on microstructural feature are attributed to the α lamellae width and the α colony size. ? The Nonferrous Metals Society of China and Springer-Verlag Berlin Heidelberg 2012.

Shot Peening and Solid Lubricating on Ti6 Al4 V Fretting Fatigue

The effects of three factors i.e. compressive residual stress, surface roughness and microstructure hardening in shot peened surface on fretting fatigue (FF) were evaluated. The results show that the residual stress and the surface roughness are predominant FF with the rectangular contact pad. Two kinds of commercial lubricants for post peening coating can not improve the FF resistance, since their processing reduce the compressive residual stress.

STUDY ON DISLOCATION STRUCTURES IN WELDED METALS UNDER FATIGUE LOAD

With transmission electron microscope, it has been observed and analyzed that the dislocation arrangements and the fatigue damage of eutectoid present in welded metals which are loaded at different fatigue loads and load cycles. The results indicated that the character of fatigue damage in ferrites of welded metals is the dislocation arrangement change and eutectoids of welded metals are the broken cementites. This may produce microcracks.

Copper foils with gradient structure in thickness direction and different roughnesses on two surfaces fabricated by double rolling

Copper foils with gradient structure in thickness direction and different roughnesses on two surfaces were fabricated by double rolling. The two surface morphologies of double-rolled copper foils are quite different, and the surface roughness values are 61 and 1095 nm, respectively. The roughness value of matt surface can meet the requirement for bonding the resin matrix with copper foils used for flexible printed circuit boards, thus may omit traditional roughening treatment; the microstructure of double-rolled copper foils demonstrates an obviously asymmetric gradient feature. From bright surface to matt surface in thickness direction, the average grain size first increases from 2.3 to 7.4 μm and then decreases to 3.6 μm; compared with conventional rolled copper foils, the double-rolled copper foils exhibit a remarkably increased bending fatigue life, and the increased range is about 16.2%.

Laser peening of aluminum alloy 7050 with fastener holes

The fatigue properties of laser peened aluminum alloy 7050 specimens with fastener holes were investigated. The surface profile and residual stress induced in the shock affected zone were characterized. Then, the fatigue specimens with notch were treated by laser peening （LP）, and the fatigue lifes of LP-treated specimens were measured and compared with base materials without LP. The results indicated that LP improved the fatigue lifes of all tested specimens. The average fatigue lifes of specimens treated by LP before hole-drilling were 173% longer than those of untreated samples and had better effects than those specimens treated by LP after hole-drilling.