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Effect of notch on fatigue performance of marine shaft made of 34CrNi3Mo alloy steel under torsional loading
... 34CrNi3MoVA steel is a kind of alloy steel with versatility and broad application prospects in the manufacturing of crankshafts, engine rotors, and turbine impellers [7,8], which are often subjected to fatigue loads with various types of waveforms. So, this study intended to perform fatigue loading on the 34CrNi3MoVA steel by using loading waveforms of cosine wave (CW), triangular wave (TW), sawtooth wave (SW), and reverse sawtooth wave (RSW). ...
Mechanical components often experience fatigue loading from various waveform conditions during their operational lifespan. However, the underlying mechanisms through which variations in loading waveform affect the fatigue life of components remain unclear. Thus, this study conducted tension–compression fatigue experiments on 34CrNi3MoVA steel specimens under the same stress amplitude with different waveforms (cosine, triangular, sawtooth, and reverse sawtooth) to investigate the effects of loading waveform variations on the cyclic strain hardening behaviors, the fatigue fracture failure, and the fatigue life. The results indicated that specimens under different waveforms all exhibited cyclic strain hardening. The fatigue cyclic hardening level progressively increased in the order of cosine, triangular, and sawtooth waveforms, resulting in a continuous increase in cyclic saturation strain amplitude. The analysis of fatigue fractures demonstrated a consistent increase in both the initiation and propagation zone areas in the order of cosine, triangular, and sawtooth waveforms, and the boundary between the propagation and final fracture zones gradually shifted from a straight to a curved shape. The influence mechanisms of cyclic loading waveforms on the fatigue life of specimens were analyzed based on the energy dissipation, leading to the development of a universal fatigue life prediction model applicable to different waveform conditions, the model was then verified with the reverse sawtooth wave specimens and resulted in a prediction error less than 15%. The study is expected to serve as a significant guide for predicting and evaluating the fatigue life of mechanical components under various fatigue loading conditions.
... 34CrNi3Mo is a kind of high-strength alloy structural steel with good comprehensive mechanical properties and workability [1]. It has been widely used in the production of engine rotors and marine diesel engine crankshafts, where fatigue resistance is critical [2]. It is well known that the fatigue crack growth threshold (ΔK th ) is an important indicator of the fatigue performance of materials. ...
In this study, the short fatigue crack growth behavior of 34CrNi3Mo high-strength steel was investigated through
in-situ fatigue testing under scanning electron microscopy. An innovative method was proposed to determine the
fatigue crack growth threshold by a changing load strategy, which shows high consistency with the standard test
for compact tensile specimens. The difference in misorientation along distinct directions at the crack tip has been
identified as a factor controlling crack deflection regardless of slip system types.
There is a need to study the evolutionary laws of the risks in the navigation environments of complex marine areas. This can promote shipping safety using an early-warning system. The present study determines shipping flows and meteorological conditions in a marine area on the basis of meteorological and automatic identification system (AIS) data. It also determines the uncertainty evolution law of the navigation environment’s influencing factors. Moreover, a navigation risk evolution system for ships in complex marine areas was developed. A case study was carried out in a coastal area of China on the basis of the determined evolutionary laws. Evolution in the navigational environment risk within the case study area was analyzed. The results showed that the hydrometeorology wind factor has the greatest impact on the risk of ship collisions. This work was not only able to show advances in navigational collision environmental evolution laws but also provides a theoretical reference for the evaluation and early warning of risks in shipping environments.
The present study evaluates the fracture surface response of fatigued 34CrNiMo6 steel bars with transverse blind holes subjected to bending with torsion loading. The analysis of the geometric product specification was performed by means of height parameters Sx, functional volume pa-rameters Vx, and fractal dimension Df. Surface topography measurements were carried out using an optical profilometer with focus variation technology. The experimental results show that the doubling the bending to torsion moment ratio B/T from B/T = 1 to B/T = 2, maintaining the same normal stress amplitude, greatly reduces both Sa, Vv as well as the fractal dimension Df of the an-alyzed specimen fractures by 32.1%, 29.8%, and 16.0%, respectively. However, as expected, a two-fold increase in the B/T ratio, maintaining the same normal stress amplitude, resulted in a larger number of cycles to fatigue crack initiation, Ni, which can be explained by the lower shear stress level. These experiments prove that parameters Sx, Vx, Df are smaller for larger Ni values, which is an important finding. In addition, it was found a high consistency of surface topography measurements for the two sides of the broken specimens. The proposed methodology is both relia-ble and applicable for other engineering applications involving different geometries and loading conditions.
Since hot-dip galvanizing causes a heat effect on cold-worked steel substrate and produces a coating layer comprised of distinct phases with varying mechanical properties, the fatigue mechanism of hot-dip galvanized steel is very complex and hard to clarify. In this study, AISI 1020 steel that has been normalized to minimize susceptibility to the heat effect was used to clarify the effect of the galvanizing layer on the tensile and fatigue properties. The galvanizing layer causes a reduction in the yield point, tensile strength, and fatigue strength. The reduction in the fatigue strength was more significant in the high cycle fatigue at R = 0.5 and 0.01 and in the low cycle fatigue at R = 0.5. The galvanizing layer seems to have very little effect on the fatigue strength at R = −1.0 in the low and high cycle fatigue. Since the fatigue strengths at R = 0.01 and −1.0 in the low cycle fatigue were strongly related to the tensile strength of the substrate, the cracking of galvanized steel was different than that of non-galvanized steel. The fatigue strength of galvanized steel at R = 0.5 dropped remarkably in the low cycle fatigue in comparison to the non-galvanized steel, and many cracks clearly occurred in the galvanizing layer. The galvanizing layer reduced the fatigue strength only under tension-tension loading. We believe that the findings in this study will be useful in the fatigue design of hot-dip galvanized steel.
Torsional fatigue tests in the high and very high cycle regime were performed with martensitic stainless steel 17-4PH. Tests were conducted with a servo-hydraulic machine and an ultrasonic equipment at cyclic frequencies of 15–35 Hz and 19 kHz, respectively. Fatigue lifetimes were significantly longer for smooth specimens tested at 19 kHz. Fatigue limits were not affected by testing frequency. This behaviour was explained by the presence of δ ferrite grains, in which crack initiation at conventional frequency is accelerated, but their size is too small to significantly decrease the fatigue limit. In the presence of detrimental defects – i.e. small drilled holes that were introduced into test specimens –, no frequency effect could be observed. The geometry of non-propagating shear cracks emanating from δ ferrite grains and in the martensitic matrix was measured by stepwise polishing of a run-out specimen. Fracture-mechanics considerations suggest that the threshold condition for the propagation of Mode-I cracks determines the fatigue limit under torsional loading.
The paper contains the results of fatigue tests of smooth and notched specimens made of 10HNAP (S355J2G1W) subjected to proportional cyclic loading with use of mean values stress. The results obtained for specimens under bending, torsion and one combination of bending with torsion for four mean values have been compared. The experimental data have been collected in the tables and shown in the figures with use of various σa(τa)-N fatigue characteristics for which parameters of the regression equations have been determined. The influence of average values on the allowable stress amplitudes and amplitude of moments at the level close to the fatigue limit depending on the angle α determining loading combination and the average stress is also shown. The greatest effect of the notch on fatigue life compared to smooth specimens is observed at symmetrical loads. At unsymmetrical loads with non-zero mean stress, this effect clearly weakens or disappears.
The demand for high-strength components for commercial vehicles has recently increased. Conventional gas nitrocarburizing has been used to increase strength and productivity of the crankshaft. A potential-controlled nitriding process was recently developed to control the crystal structure of the nitride compound layer. It has been found that this treatment improves the bending fatigue strength compared with conventional treatment, and has the potential to cope with the increase in crankshaft strength. However, the effect of torsional fatigue strength has not been studied. Therefore, in this study, the influence of the crystal structure of the nitride compound layer on torsional fatigue strength was investigated. Two kinds of test specimens with different crystal structures of the compound layer were prepared using gas nitriding treatment with controlled nitriding potential for an alloy steel bar (JIS-SCM435). Torsional fatigue tests were carried out using these test specimens. Although the compound layer of these test specimens had different crystal structures, the hardness distribution and residual stress distribution on the diffusion layer were almost the same. The relationship between stress amplitude and number of cycles to failure (S-N curve) showed that the torsional fatigue limits of the specimens were almost the same. This indicates that the crystal structure of the nitride compound layer did not affect the torsional fatigue limits, because the origin of the torsional fatigue failure is inside the specimen.
The torsional fatigue behavior of a high-strength spring steel, SWOSC-V, was investigated. Of particular interest were the influences of small scratches and superimposed static shear stress in the very high cycle fatigue regime. To achieve the research objectives, an ultrasonic torsional fatigue testing machine was developed. S-N curves showed the fatigue limit for the tested material condition (i.e. with no residual stresses at specimen surface). This study revealed that a notable influence of the stress ratio, R, on the torsional fatigue behavior was not explicitly recognized. Concerning the influence of small scratches, it was found that the square-root-area parameter model can predict the lower bound of the torsional fatigue limit, irrespective of the value of R.
This study provides a theoretical and experimental investigation of the effect of static shear stresses on the high-cycle fatigue behaviour of a 34CrNiMo6 high-strength steel under quenched and tempered conditions. Torsion S–N curves under different mean shear stresses were obtained. Experimental results show that an increase in mean shear stress yields a gradual reduction in shear-stress amplitude that the material can withstand without failure. The results for this steel agree well with the Smith's hypothesis for ductile steels, which states that the effect of the torsional mean stresses on the torsional fatigue limit is negligible as long as the maximum shear stress is within the torsional yield strength. Taking into account the results collected from the literature and the experimental results on torsional fatigue of 34CrNiMo6 steel, an extension of the theory of Crossland is proposed to include the mean-shear-stress effect. Its application to the torsional fatigue case with mean shear stresses can be interpreted in terms of a balance of the energy of distortion. Macro-analyses of the specimen fracture appearance were conducted to obtain the fracture characteristics for different mean-shear-stress values under torsion fatigue loading.
The applicability of traditional S-N curve in fatigue failure life cycle of component was not strong. The high cycle fatigue tests of HRB400 high strength steel bars connected by flash butt welding were carried out based on single point method, and a series of small sample data were got. Accordingly, double-logarithmic double-dogleg S-N curve was studied and proposed which was suitable for the fatigue failure life cycle of component. The improved Chauvenet criterion was proposed to select the test data of the first segment of dogleg line, and then the equation for the first segment was obtained according to a linear regression principle. Based on the gradient obtained by three-parameter exponential function curve and the fatigue strength determined by staircase method of the second segment, the equation for the second segment was worked out. Double-logarithmic double-dogleg S-N curve suitable for the fatigue failure life cycle of component was obtained by the simultaneous equations of two segments. On this basis, the calculation method considering dependability and reliability was further studied and proposed for double-logarithmic double-dogleg S-N curve suitable for the fatigue failure life cycle of component. © 2017, Editorial Department of China Railway Science. All right reserved.
This paper presents the results of torsion fatigue of widely used bearing steels (through hardening with bainite, martensite heat treatments, and case hardened). An MTS torsion fatigue test rig (TFTR) was modified with custom mechanical grips and used to evaluate torsional fatigue life and failure mechanism of bearing steel specimen. Tests were conducted on the TFTR to determine the ultimate strength in shear (Sus) and stress cycle (S-N) results. Evaluation of the fatigue specimens in the high cycle regime indicates shear driven crack initiation followed by normal stress driven propagation, resulting in a helical crack pattern. A 3D finite element model was then developed to investigate fatigue damage in torsion specimen and replicate the observed fatigue failure mechanism for crack initiation and propagation. In the numerical model, continuum damage mechanics (CDM) were employed in a randomly generated 3D Voronoi tessellated mesh of the specimen to provide unstructured, nonplanar, interelement, and inter/transgranular paths for fatigue damage accumulation and crack evolution as observed in micrographs of specimen. Additionally, a new damage evolution procedure was implemented to capture the change in fatigue failure mechanism from shear to normal stress assisted crack growth. The progression of fatigue failure and the stress-life results obtained from the fatigue damage model are in good agreement with the experimental results. The fatigue damage model was also used to assess the influence of topological microstructure randomness accompanied by material inhomogeneity and defects on fatigue life dispersion.
Since the number of fatigue crack propagation data is related to cost and time and the crack propagation data are generally finite, adopting median rank or average rank as the probability value of a test datum will result in significant error. On the basis of analyzing disadvantages of traditional methods to identify distribution functions, which begin with overall fitting effect, this paper proposed a method by applying numerical simulation and fuzzy linear regression. It can be used to determine the distribution function of crack propagation data. Through fuzzy linear regression, the result that crack propagation size under certain load cycles are in logarithmic normal distribution can be acquired. In the same way, the number of load cycles is also in logarithmic normal distribution when the crack propagates to a certain size.
In this study, different fracture surfaces caused by fatigue failure were generated from 18Ni300 steel produced by selective laser melting (SLM). Hollow round bars with a transverse hole were tested under bending-torsion to investigate the crack initiation mechanisms and fatigue life. Next, the post-failure fracture surfaces were examined by optical profilometer and scanning electron microscope. The focus is placed on the relationship between the fatigue features (e.g., bending-torsion ratio, fatigue crack initiation angles, and fatigue life) and the fracture surface topography parameters (e.g., height parameter Sx, volume parameters Vx, maximal pit and valley angles). The analysis was carried out using the entire fracture surface of the tested specimens. It was found that the decrease of the shear stress level significantly reduces the value of the fracture surface parameters. A fatigue life prediction model based on both the surface topography values and the applied load was proposed. Fatigue life predictions for different loading ratios agreed well with the experimental results and were slightly better than those of other existing models. The proposed model can be helpful for post-mortem analysis of engineering components sub-jected to multiaxial fatigue.
Present work is aimed at performing fatigue tests on notched C-Mn steel tubes. Tests were conducted on different sizes of notches under remote axial strain-controlled conditions. Individual effects of peak equivalent strain amplitude and strain gradient are brought out on experimental fatigue life. The localized measured strains are compared with corresponding outcome of FE analyses.
Fatigue life for notched specimens was predicted based on peak equivalent strain amplitude/existing critical plane model considering stress/strain information at peak/characteristic distance locations. The characteristic distance location with critical plane model and peak equivalent strain amplitude results in improved fatigue life predictions. https://authors.elsevier.com/c/1gQB7WK0QXzRc
A bulk-ship was not working normally when anchoring to wait for entering port. Inspection in site found that the intermediate shaft fractured. The intermediate shaft had accumulatively serviced for 43,000 h before fractured. Fractographic investigation indicated that the multiple origins fatigue fracture was the dominant failure mechanism of the shaft. The cracking propagated on two fracture planes at approximately ± 45° with respect to the shaft axis. The material of failed shaft was found to be a low carbon steel of type ASTM 1020, rather than the specified the medium carbon steel of type ASTM 1035. The average values of yield strength and ultimate strength of shaft material were respectively 16% and 20% lower than the lower limit of technical specification due to low carbon content, as a result of the decreased fatigue strength of shaft material. The fracture occurred at the fillet root of flange connected to the propeller-shaft, where high stress concentration was caused by geometry design. Deep circumferential machining grooves were present on the fillet root of flange for additive stress concentration. The inhomogeneities acted as potential sites for fatigue crack initiation. The low fatigue strength of shaft material promoted initiation and propagation of fatigue cracks in the weaker region under the reversed torsion loading. The static strength analysis of intermediate shaft was performed to assess the design implications. The measured average ultimate strength of failed intermediate shaft was 12% lower than the lower limit in Rule for Classification of Sea-going Steel Ships issued by China Classification Society. The safety factor of the failed intermediate shaft is lower than the permitted one.
Objectives
- We aimed to investigate how pre-bending affects the mechanical properties, specifically fatigue, of stainless-steel plates.
Methods
- 3.5mm LCP 10-hole plates were pre-bent in 1, 2 and 3 locations to the same overall degree and fatigue testing performed. Finite Element Analysis (FEA) was performed in Strand7 (version 2.4.6) to better understand the failure point of the plates in four-point bending.
Results
- Six different plate pre-bending conditions were tested for resistance to fatigue failure. Increasing the number of pre-bends improved the fatigue resistance with two pre-bends having a mean 509,304 cycles to failure and three pre-bends 491,378 cycles to failure. The region of highest stress and the point of fatigue failure were at the plate's minimum cross-sectional area, which was predicted by the FEA and confirmed with mechanical testing. For plates pre-bent in two locations, the fatigue failure always occurred in the screw hole not in between the positions of the two pre-bends. Non-linear FEA simulation confirmed that work hardening occurs around pre-bend locations, conferring increased fatigue resistance to the holes next to, or between, pre-bend locations.
Conclusions
We found that contrary to orthopaedic folklore, pre-bending of plates is not detrimental to fatigue resistance of the stainless-steel plates we tested. Pre-bending plates in a single plane increased the fatigue properties of the 10-hole stainless-steel plate tested.
The torsional fatigue response of as-built direct metal laser sintered (DMLS) stainless steel (SS) GP1 of reduced specimen size and horizontal build orientation subjected to cyclic loading at a highly plastic shear strain has been investigated. The average shear modulus, G, was found to be 55.36 GPa, with shear stress history revealing cyclic hardening to stabilization and softening to fracture. Findings suggest that stainless steel GP1 of reduced specimen size experiences fatigue crack initiation at voids near the surface, resulting in a star-spline brittle fracture response, and premature torsional fatigue failure within the low cycle fatigue regime. A noninteraction deformation constitutive model through finite element simulations has been used to successfully simulate the deformation response of the gage section under torsional fatigue conditions.
Fatigue crack initiation behaviour of notched 34CrNiMo6 high-strength steel bars with transverse blind holes and lateral U-shaped notches subjected to proportional bending-torsion loading is studied. Crack initiation at the geometric discontinuities is examined in situ using a high-magnification digital camera. Crack initiation mechanisms and failures modes are investigated after the fatigue testing program via scanning electron microscopy. Fatigue crack initiation lifetime is predicted using an averaged Total Strain Energy Density approach in conjunction with the Theory of Critical Distances. Crack initiation sites, crack surface angles and surface crack paths at the geometric discontinuities are successfully determined a priori from the maximum value of the first principal stress, the first principal direction, and the first principal stress field at the notch surface, respectively. Overall, fatigue lifetime predictions are very well correlated with the experimental observations.
This study discusses a multiaxial low cycle fatigue (LCF) life of notched specimen under proportional and non-proportional loading conditions at room temperature. Multiaxial LCF tests controlled by strain were carried out using a smooth and a circumferentially notched round-bar specimens of type 316L stainless steel (316LSS). Four kinds of notched specimens of which elastic stress concentration factors Kt are 1.5, 2.5, 4.2 and 6.0 were employed. The strain paths employed were proportional loading and non-proportional loading. The former is push-pull loading and reversed torsion loading and the latter is circular loading achieved by the strain path of which axial and shear strains are loaded by 90° sinusoidal out-of-phase. Total axial strain and total shear strain ranges were the same ranges based on von Mises. An evaluation of the fatigue life for the notched specimen is discussed based on the experimental results and also by employing an inelastic finite element analysis (FEA). The fatigue life is decreased with increase in Kt at each strain path and is underestimated by a maximum local strain range evaluated by FEA result. A proposed strain range defined by a local mean strain value near the notch calculated from FEA by taking into account the non-proportional effect is an appropriate parameter for life evaluation of 316LSS notched specimen under non-proportional loading.
In order to evaluate the mean S-N curves and fatigue limits of materials, an evaluation method based on small data samples was proposed. Limited data of stress vs. life was obtained from 14 specimens, using measurement methods proposed by the Japanese Society of Mechanical Engineers (JSME). The new method proposed by the authors uses the linear theory of cumulative damage and assumes that the distribution of fatigue strength is irrelevant to fatigue life, and approximately identical at different numbers of cycles. The data at the breakoff point from the S-N curves were transformed into one set of stress. The optimal value of Nc, at the turning point of the inclined line and the horizontal line was decided according to the converted stress. The mean S-N curve and the distribution of fatigue limits could then be obtained. The results of fatigue limit evaluation using the method proposed in this paper had higher confidence levels than those based on the JSME method. The proposed method for small sample data sets improved evaluations of fatigue limits.
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S-N Curve of Fatigue Life Cycle of Component Based on Small Sample Data
- Xingwang