Iron-dust magnets

Fatigue crack initiation and propagation behavior in Al – 7075 alloy under in-phase bending-torsion loading

The fatigue crack initiation and propagation behaviors were investigated on the Ti-6Al-4V alloy specimens which were hydrogen charged in 3% sodium chloride or lN sulfuric acid aqueous solution. The number of cycles to crack initiation obtained for notched tpecimens, Ni, was smaller in the specimens tested under hydrogen charging than in those tested in air when the range of the apparent stress intensity factor ΔKa was relatively large, while when ΔKa was small, Ni was rather large for the charged specimens. The crack propagation velocity da/dN was smaller in the specimens tested under hydrogen charging than those tested in air. The decrease in da/dN of the former specimens corresponded to the increase in crack closure : i. e., the relation between the range of effective stress intensity factor ΔKeff and Nt for two conditions was expressed by the same line. According to an X-ray analysis, the main reason for the enhancement of crack closure in the hydrogen charged specimens seemed to be the expansion due to the hydride formation near the specimen surface but not on the crack propagation plane (a kind of phase-transformation-induced crack closure).

Effect of hydride precipitation on the fatigue cracking behavior in a zirconium alloy cladding tube

Effects of temperature on fatigue cracks initiation and propagation for a high-speed railway wheel rim steel

To clarify the mechanism of fatigue failure in eutectoid steel, fatigue crack initiation and propagation behaviors have been studied in detail under rotating bending fatigue tests using rail steels. The fatigue failure surface was also examined by using SEM.The main results obtained are as follows;(1) Fatigue cracks initiated at the interface between ferrite and pearlite in case of rail steel with pro-eutectoid ferrite. In rail steel without pro-eutectoid ferrite, fatigue cracks occurred at the inter-lamellar of pearlite due to the slip.(2) The fatigue crack preferentially propagated along the inter-lamellar of pearlite in the early stage. As it grew, however, it propagated perpendicularly to the stress axis by changing its direction at every pearlite block.(3) On the fatigue failure surface in eutectoid steel, the pearlite lamellar was observed, which looks apparently alike striation. But the typical striation pattern of fatigue was not observed.

To monitor fatigue crack initiation and propagation, and to judge the fatigue damage status of ferromagnetic material, fatigue bending tests of four-point single edge notch bend (SENB4) specimens were carried out. Metal magnetic memory signals were measured during the whole fatigue process. The results showed that the fatigue process could be divided into four stages by observing the morphology of the fracture surface. With the increase of fatigue loading cycles, the tangential component of the magnetic field (Hx) and the normal component of the magnetic field (Hy) increased. At the notch Hx presented a “trough” shape and had a maximum value at the midpoint, while Hy at the notch rotated clockwise around the midpoint. Compared with the tangential characteristic parameters, the variation of normal characteristic parameters (i.e., maximum gradient value of Hy (Ky-max) and the variation range of Hy at the notch (∆Hyn), with the fatigue loading cycles are more similar to the variation of fatigue crack length with loading cycles), both Ky-max and ∆Hyn had a good linear relationship with fatigue crack length. Plastic deformation accumulated on both sides of the fatigue crack, and metal magnetic memory (MMM) signals measured from the specimens were able to indicate the location of the fatigue crack and the variation of the fatigue crack length. Furthermore, the distribution of magnetic signals was analyzed according to the theories of stress magnetization and magnetic flux leakage.

Many accidents are caused by fatigue in welded built-up steel structures, and so it is important to estimate the fatigue lives of such structures quantitatively for safety reasons. By assuming that fatigue cracks cannot grow without an accumulation of alternating tensile/compressional plastic strain, one of the authors identified an improved effective stress intensity factor range ΔK RPG based on the re-tensile plastic zone generating (RPG) load, which represents the driving force for fatigue cracks, and suggested that ΔK RPG should be used as the parameter to describe fatigue crack growth behavior. The “FLARP” numerical simulation code in which ΔK RPG is implemented as the fatigue crack growth parameter, was developed in order to predict fatigue crack initiation and propagation behavior. In this paper, it is demonstrated that FLARP gives accurate estimates for fatigue life by comparing the estimated fatigue crack growth curves and S–N curves with the experimental results for in-plane gusset welded joints, which are used in many welded steel structures. Moreover, the effect of induced bending moment due to the linear misalignment in the out of plane direction on the fatigue strength of in-plane gusset welded joints is investigated through numerical simulations.

The role of notch stress and surface defects on fatigue crack initiation and small-crack propagation behavior has been investigated using groove simulation specimens. The naturally initiated small-crack growth tests have been performed on a FGH4099 superalloy at 500 °C and 700 °C. The findings indicate that elevated testing temperature significantly reduced the proportion of fatigue crack initiation life, with a less pronounced effect on the proportion of life for cracks to grow to First Engineering Crack size. Competing crack initiation modes were observed in the fatigue test of groove simulation specimens. The location of maximum principal stress was dominant fatigue crack initiation sites, and for specimens with surface inclusions, the defect location can also serve as a crack initiation site. Furthermore, crack initiation modes were found to have a more pronounced effect on the small-crack growth rate. A turning point observed in the crack growth rate curves for specimens with multi-site crack initiation was attributed to crack shielding and subsequent coalescence.

Effects of unimodal secondary γ′ size and grain orientation on fatigue crack initiation and early crack propagation at room temperature have been investigated in RR1000 alloy. Fatigue tests show cracks mainly initiated at slip bands and/or pores followed by crystallographic facet formation while more diffuse strain localisation is observed around the pores by SEM-DIC analysis. Fatigue lifetime is linked to the number of crack initiations and consequent crack coalescence, which accelerates crack propagation. Slip trace analysis with EBSD mapping shows cracks are likely to propagate along the primary active slip system with a relatively high Schmid Factor. In the tests conducted here, the stochasticity of crack initiation events has more control on the overall lifetime than variations in secondary γ′ size.

Thermal fatigue tests were conducted for SB410 steel and SUS310S steel for boiler. A laboratory made thermal fatigue testing apparatus was used. This apparatus consisted principally of a heating device using oxygen and LPG gas, a temperature control device for the heated zone and a rapid cooling device for the specimen. During thermal fatigue tests the heating and cooling cycles were repeatedly loaded on the specimen placed on the specimen holder. City water was used as a cooling medium. The heating temperatures were 673K, 773k and 873K, and cooling water was sprayed on to the specimen surface through a nozzle. As it was difficult to measure the surface temperature of the specimen, small holes for a thermocouple were prepared to measure the temperature of the notch during the thermal fatigue tests. Thus the measured temperatures were used as the testing temperatures. The plate specimens with an electric discharged notch at Open image in new window Figure 1. Thermal fatigue test specimen the bottom of the mechanical U notch. The thicknesses of the specimens were 24mm and 12mm. The thermal fatigue crack initiation tests and thermal fatigue crack propagation tests were conducted up to 100 cycles for specimens with 24mm thick and 300 cycles for specimens with 12mm thick. Figure 1 shows the figure and the size of the thermal fatigue test specimen. The thermal fatigue crack length was measured at every ten cycles by use of a viewing microscope with magnification of 20 after interrupting thermal fatigue tests. The thermal fatigue cracks were examined by an optical microscope and thermal fatigue fracture surfaces were examined by a JEOL scanning electron microscope (JSM5500S). Figure2 shows crack propagation curves for SB410 steel with 12mm thick. The higher the testing temperature the smaller the number of heat cycles is. The higher the testing temperature the faster the crack propagation rate is. The same phenomenon was observed for SUS310S. The thermal fatigue crack propagation rate of SB410 steel was faster than that of SUS310S steel. Plural thermal fatigue cracks were observed on the bottom of the electric discharged notch for SB410 steel. The crack branching was observed on the specimen tested at 873K. Striation was observed on fracture surfaces of SB410 steel. Striation was predominant on fracture surfaces of SUS310S.The striation spacing per cycle ,S obtained from the measured striation spacing S versus K curve was well coincident with the da/dN K curve in the high crack propagation rate. It can be concluded that thermal fatigue crack of boiler steels propagate in association with striation. Open image in new window Figure2. Thermal fatigue crack propagation curves of SB410 steel with 12mm thick

Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy

Deformation mechanisms and strain rate sensitivity of bimodal and ultrafine-grained copper

Influence of strength and notch shape on crack initiation and propagation behavior of advanced high strength steel sheets

In the present study, torsional fatigue properties of tensile pre-strained structural carbon steels have been investigated in view-point of fatigue strength, crack initiation and propagation behavior, hardness and so on. According to the experimental results, tensile pre-strain improves the torsional fatigue strength, increases the surface hardness and affects the crack initiation and propagation characteristics.

Evolution of silicon particle damage on fatigue crack initiation and early propagation in an aluminum alloy