Papers by Keyword: Fatigue Crack Propagation

Abstract: This work investigates the influence of initial residual stresses after additive manufacturing, specifically directed energy deposition, in 5xxx aluminum alloys on the fatigue crack propagation behavior. For this purpose, initial plane stress states (compressive as well as tensile) are introduced along the crack path on a C(T)50 specimen via eigenstrains, mimicking possible residual stress states after both directed energy deposition and possible post-processing. The evolution of the stress intensity factor difference is determined and used to calculate the crack propagation rate via Walker’s equation. The stress state of the vicinity of the crack tip dictates the crack behavior: Compressive stresses perpendicular to the crack path exhibit crack closure, resulting in slower propagation rates. Finally, the influence of a more local distribution of the residual stresses on the fatigue crack propagation is investigated, highlighting the importance of the position of compressive stresses relative to the crack tip for effective crack growth retardation.

Abstract: This study presents a detailed analysis of the catastrophic failure of a Pelton turbine bucket, revealing a complex mechanism involving multiple interacting factors. Through a root cause analysis (RCA), the primary crack was identified to have originated in a high-stress concentration zone, exacerbated by pre-existing discontinuities. The turbine runner had accumulated approximately 90,000 service hours, suggesting a low-stress, high-cycle fatigue as the initial damage mechanism. However, the rapid crack propagation was driven by an abrupt shift in the fatigue regime, transitioning to high-stress, low-cycle fatigue induced by severe impact loads during counter-jet entry. This phenomenon led to the fracture of the bucket segment. This work emphasizes the importance of considering the synergistic interaction of accumulated fatigue, pre-existing discontinuities, and changes in the loading regime in the design and maintenance of Pelton turbines, to prevent premature failures and ensure the structural integrity of these critical components.

Abstract: In present work, a high Zn-containing Al-Zn-Mg-Cu alloy with different grain sizes was fabricated by extrusion and related precipitation characteristics and mechanical property were investigated after uniform heat treatments. The results showed that precipitation characteristics for the three alloys were almost the same. Matrix precipitates were GPII zone and η' phase and possessed small size and dense distribution while grain boundary precipitates exhibited discontinuous distribution. The rank of strength and fracture toughness for the three alloys are SG>MG>LG. Tearing ridges had been found on all the fracture surface while only LG alloy possess obvious dimple characteristics. The a-N curve showed that crack length list is MG>LG >SG under a same cycle number. The da/dN-ΔK curve also proved that fatigue crack propagation (FCP) rate of MG alloy is slightly larger than that of LG alloy, both were apparently larger than that of SG alloy. The width of fatigue striations on FCP fracture surface also backed it. Besides, obvious transgranular cracking characteristics and apparent secondary cracks were found on the FCP fracture surface.

Abstract: The deformation parameters of aluminum alloys during thermal deformation have a significant impact on the alloy's properties. The industrial free forging of the Al-Zn-Mg-Cu alloy was carried out at deformation rates of 10 mm/s and 20 mm/s, respectively, at a deformation temperature of 430°C and a deformation degree of 60% in this study. The microstructure was determined using EBSD, and the mechanical characteristics were examined. According to EBSD observations, the recrystallization fraction of the alloy is nearly identical under both deformation rates; however, the average grain size of the alloy with a deformation rate of 10 mm/s is 10.8 μm larger than that of the alloy with a deformation rate of 20 mm/s. As the deformation rate increased from 10 mm/s to 20 mm/s, the alloy's yield strength and fracture toughness increased. The resistance to fatigue crack propagation, on the other hand, displayed the reverse pattern. That is, the alloy with a 20 mm/s deformation rate had a higher FCP rate than the alloy with a 10 mm/s deformation rate. In summary, the influence of deformation rate on the microstructure and mechanical properties of a high alloy Al-Zn-Mg-Cu alloy was investigated.

Abstract: Fatigue crack propagation is closely associated to the chemical composition of Al-Zn-Mg-Cu alloys. In this work, two Al-Zn-Mg-Cu alloys with a variation of zinc content was investigated and multiple aging treatments were exerted on them and a same regime was selected for further fatigue analysis by tensile property tests. The corresponding precipitation characteristics and fracture surface were observed. The results showed that the alloy with lower zinc content (LZ alloy) possessed an inferior strength value compared with the alloy with higher zinc content (HZ alloy) under the same three stage aging treatments while the elongation had no obvious difference. In contrast, the LZ alloy had a higher fatigue crack propagation rate than the HZ alloy. The observation of fracture surface also proved it. The precipitation observation demonstrated that both had GP zones and η' phase, which possessed the majority. Quantitative analysis of precipitates exhibited that the HZ alloy had a larger proportion of large size precipitates than the LZ alloy. The mechanism of the interaction between dislocation and precipitate was employed to elaborate the difference.

Abstract: Specimens from Ni-Cr-Fe wrought superalloy INCONEL 718 were used for fatigue push-pull test at elevated temperature 700 °C. Fatigue loading was with the coefficient of cycle asymmetry R = -1. Temperature of fatigue test was chosen from two reasons; one is that limit operating temperature for this type of alloy is 650 – 700 °C due to precipitation of stable but incoherent orthorhombic Ni₃Nb delta phase; the second reason for this temperature is fact that metastable body-centred tetragonal Ni₃Nb gamma double prime phase starts to transform to delta phase and from that reason there is an expectation for mechanical properties decreasing due to increased volume of delta phase. For evaluation of fatigued specimens were Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) observation used. Also methods of quantitative metallography – coherent testing grids were used for evaluation of delta phase volume. Some references describe that higher volume of delta phase, mostly situated at grain boundaries, act as initiation sites for cracks and therefore decreasing mechanical properties. Employed SEM and TEM analysis confirm the increased volume of delta phase in specimens after fatigue loading but a negative influence on dynamic properties, such fatigue lifetime, for this alloy was not confirmed.

Abstract: The fatigue crack propagation of Al-Zn-Mg-Cu alloys could be influenced by the content of main alloying element. In the present work, two Al-Zn-Mg-Cu alloys with a same Zn/Mg ratio were treated by two stage over-aging aging treatment and typical T7651 states were extracted via mechanical properties. Fatigue crack propagation of the two alloys were tested and the related precipitation characteristics and fracture morphology were observed. The results showed that the alloy with higher Zn, Mg contents possessed a better fatigue crack propagation resistance compared with the alloy with lower Zn, Mg contents. The corresponding fracture morphology also showed the difference of fatigue striation, which provided an additional support. The precipitation observation demonstrated that the both alloys possessed GPII zone, η' phase and η phase while the alloy with higher Zn, Mg contents had a larger average precipitate size and a larger proportion of large size precipitates compared with the alloy with lower Zn, Mg contents. Cut and bypass mechanisms of dislocation-precipitate interactions were used to explain the difference of fatigue crack propagation between the two alloys.

Abstract: The usual approaches for Fatigue Crack Growth (FCG) assessment are based on semi-empirical models that consider the stress intensity factor range of fracture mechanics, ΔK, as the governing driving force for crack propagation. However, FCG data incorporating compressive loads have brought discussions and controversies. The compressive loads are not taken into account in the usual life prediction codes and the negative portion of the loading cycle is neglected. In the present work, constant amplitude fatigue crack growth tests with load ratios-1 and 0 are performed, for two diferent stess levels, in AA 6005 alloy centre-notched middle tension M(T) specimens and the obtained results are discussed. It is shown that the crack closure concept, that usually work well for positive ratios, is not enough to satisfactorily explain the negative load ratio effects. The results also showed how the negative part of the load affected the propagation rate of the crack, mainly for larger cracks and smaller loads. Finally, it is shown that a previously proposed expression for the stress distribution ahead of the crack can shed some light on this question.

Abstract: FGH96 superalloy has been used in aircraft engine turbine discs due to the superior damage tolerance. This work investigated the characteristics of microscopic inclusions and their influence on fatigue crack propagation in hot isostatic pressed and heat treated commercial FGH96 superalloy. Low cycle fatigue crack growth tests were conducted. Several kinds of microscopic inclusions were identified by optical microscope (OM), scanning electric microscope (SEM) and energy dispersive spectrometer (EDS). Microscopic inclusions and secondary cracks were analyzed near the main cracks. Combined with the finite element method (FEM) results, it can be proposed that the microscopic inclusions have no critical influence on fatigue crack propagation in the specimens.

Abstract: In the fracture mechanics, the failure of mode-III crack belongs to the anti-plane shear (also called out-plane shear or longitudinal shear). Fatigue crack growth (FCG) behavior under mode-III reversed load (load ratio R=-1) was studied by using 30Cr2Ni4MoV rotor steel with types of circumferential notches. Under the remote cyclic torsional load, the precrack process was applied to specimens with notch in the round bar, and the ideal sharp-crack was obtained. Then, mode-III FCG test under the remote cyclic torsional load and the axial constant small load was carried out until to fatigue failure. The axial constant small load is aimed at reducing the cracks interaction in the mode-III FCG experiment. Due to experimental data of 30Cr2Ni4MoV rotor steel, the mode-III FCG rate in the range of KIII from 12MPa·m1/2 to 40 MPa·m1/2 and the threshold value were got. Furthermore, a prediction model for mode-III FCG is proposed with considering the small scale yield of plane stress crack tip zone and the plastic strain energy density failure criteria. The predicted mode-III FCG rate is found to agree well with the experimental curves of 30Cr2Ni4MoV rotor steel.