Effects of chromic acid anodizing on fatigue behavior of 7050 T7451 aluminum alloy

The effect of chromic acid anodizing (CAA) surface treatment on 7050 T7451 aluminum alloy was presented in this study in terms of fatigue behavior. CAA is a treatment against corrosion by producing aluminum oxide layer (Al2O3) at the surface. However, fatigue performance of 7050 T7451 is affected by the coating. In this study, eight different CAA processes were examined with regard to etching stage of pre-treatments by using an alkaline etchant and/or acid etchants with various immersion times. Optical microscopic examinations were applied in order to determine pitting characteristics for the selection of CAA process parameters before fatigue tests. A CAA process was selected among eight processes in terms of pitting characteristics in order to apply fatigue specimens. Four fatigue test groups were determined to investigate bare condition of 7050 T7451 and sub-stages of the CAA particularly. Constant amplitude axial fatigue tests were conducted on specimens at 91 Hz at stress ratio (R) -1 until run-out criteria, which was 106 cycles. Fatigue life reduction was determined due to pretreatments of CAA. Fracture surfaces of the specimens were examined by scanning electron microscope (SEM) to investigate morphology and crack initiation sites.

Influence of Mean Strain on Fatigue Life of Stainless Steel in PWR Water Environment

Influence of application of the mean strain on the fatigue life was investigated for Type 316 stainless steel in the simulated pressurized water rector (PWR) primary water environment. Low-cycle fatigue tests were conducted for a constant mean strain by controlling the strain range to be 1.2%. The applied strain rates were 0.4, 0.004, or 0.001%/s. The applied mean strain was 15% in nominal strain. In addition, cold worked specimens were also used for the tests without applying the mean strain. The cold working simulated the application of mean strain without increase in surface roughness due to application of plastic deformation. By inducing the cold working at low temperature, the influence of martensitic phase on the fatigue life was also examined. The PWR water environment reduced the fatigue life and the degree of the fatigue life reduction was consistent with the prediction model of the code issued by the Japan Society of Mechanical Engineers (JSME) and NUREG/CR-6909. Increases in the maximum peak stress and stress range caused by cold working did not cause any apparent change on the fatigue life. It was revealed that the 10.5 wt% martensitic phase and the increase in the surface roughness caused by application of 15% mean strain did not bring about further fatigue life reduction. The current JSME and NUREG/CR-6909 models were applicable for predicting the fatigue in the PWR water environment even when the mean strain or cold working was applied.

Investigations Into Fatigue of OPB Loaded Offshore Mooring Chains

The Chain FEARS (Finite Element Analysis of Residual Strength) JIP developed a correlated FEA method for determining fatigue life of Tension-Tension (T-T) loaded offshore mooring chain. The developed first principles method incorporated the non-linear effects of proof loading, accounted for mean chain tension, accounted for material grade, employed a multi-axial fatigue method to account for complex stress fields within the chain, and was based on a parent material S-N curve. It was anticipated that this high fidelity approach could be applied more universally to successfully address a broader spectrum of emergent factors experienced in-field which have caused chain fatigue life reduction and adversely impacted mooring system integrity. These emergent factors include an array of chain degradation modes including; uniform, pitting and mega pitting corrosion; preferential butt weld corrosion; abrasive, contact and interlink wear; and load duties other than Tension-Tension (TT) such as Out-Of-Plane Bending (OPB), In-Plane-Bending (IPB) and Bending-Tension (BT). The objective of the investigations conducted by AMOG Consulting following completion of the Chain FEARS JIP research was to explore the extent to which the developed fatigue method could be applied more universally to address these emergent factors as they pertained to residual fatigue life. Application of the method identified a number of trends in fatigue performance for links subject to hawse pipe and fairlead interaction, and demonstrated good correlation against available guidance on fairlead performance and against OPB fatigue test data. The investigation confirms that the first principles FEA fatigue method can be applied more universally to successfully address a broader spectrum of emergent factors experienced in-field relating to fatigue life reduction.

Influence of Material Ductility on Fatigue Life under Multiaxial Proportional and Non-Proportional Normal and Shear Stresses

Current experiences show that a non-proportional loading of ductile materials such as wrought steels, wrought aluminium or magnesium alloys, not welded or welded, causes a significant fatigue life reduction under an out-of-phase shear strain or shear stress superimposed on a normal strain or normal stress compared with proportional in-phase loading. However, when ductility, here characterised by tensile elongation, is reduced by a heat treatment or by another manufacturing technology such as casting or sintering, the afore-mentioned life reduction is compensated or even inversed, i. e. longer fatigue life results compared with proportional loading. Some actual results, determined with additive manufactured titanium, suggest that microstructural features such as manufacturing-dependent internal defects like microporosities should be considered in addition to the ductility level. This complex life behaviour under non-proportional loading cannot always be estimated. Therefore, in experimental proofs of multiaxial loaded parts, especially safety-critical components or structures, with real or service-like signals, emphasis must be placed on retaining non-proportionalities between loads and stresses/strains, respectively.

Frequency Effect of Torsion on Rotating Bending Fatigue Behavior of Gas Tungsten Arc (GTA) Welded AISI 1018 and AISI 4140 Welded Joints

Fatigue failures of welded structure are subjected to occur due to multiaxial fatigue load and torsion. In the current research work, the frequency effect of torsion on rotating bending fatigue load is analyzed on AISI 1018 steel and AISI 4140 steel. To perform rotating bending torsional fatigue test of welded and un-welded specimens, a unit was designed and manufactured. Gas Tungsten Arc (GTA) welding was carried out on round bar of AISI 1018 steel and AISI 4140 steel welded using ER70-S2 filler metal for welded specimen. Later, the influence of torsional frequency on rotating bending with torsion is analyzed on both base metal and welded structures. The frequency of torsion was applied on the specimens were - 500 cycle, 1000 cycle, 1500 cycle, 2000 cycle and 2500 cycle. From the analysis, fatigue life of AISI 1018 and AISI 4140 base metal specimens (rotating bending and torsion) were not affected when torsion was applied at different frequencies. However, fatigue behavior of welded AISI 1018 and AISI 4140 specimens were highly affected by the frequency of torsion. For torsion applied at every 500 cycles, 83.8% reduction of fatigue life was observed for AISI 1018 welded specimen with respect to AISI 1018 base metal specimens. In addition, torsion applied for every 1000, 1500, 2000, and 2500 cycles; 81.9%, 80%, 77.1%, and 72.4% reduction on fatigue life were observed for AISI 1018 welded specimen compare to AISI 1018 base metal specimens, respectively. Welded AISI 4140 specimens experienced less change in fatigue life compare to welded AISI 1018 specimens. For torsion applied at every 500 cycles, 73.9% fatigue life reduction was observed for welded AISI 4140 specimens compare to AISI 4140 base metal specimens. For torsion applied at every 1000, 1500, 2000, and 2500 cycles; 65.2%, 60.8%, 50%, and 43.5% fatigue life reduction occurred on welded AISI 1018 specimen. Moreover, hardness measurements for welded specimens of AISI 4140 and AISI 1018 were performed longitudinally. For welded AISI 1018 specimen, 14.8% and 9.7% hardness decrease was observed longitudinally compare to AISI 1018 base metal specimen at heat-affected zone (HAZ) and from heat-affected zone through weld zone, respectively. For welded AISI 4140 specimen, 26.3% reduction of hardness value was observed compare to AISI 4140 base metal through heat-affected zone for longitudinal analysis. Moreover, fracture surface analysis was performed on the welded and non-welded specimens to understand the fracture behavior.

Research on the Fatigue Performance of TC6 Compressor Blade under the CCF Effect

This paper studied the influence of high and low combined fatigue (CCF) on compressor blade fatigue performance. We investigated the coupling between low cycle fatigue (LCF) loading from centrifugal force with high cycle fatigue (HCF) loading from vibration and determined the blade disc vibration frequency using static analysis at maximum rotational speed. We designed and constructed a combined fatigue test rig, and CCF tests were performed on a TC6 compressor blade to analyze fatigue life characteristics. Results showed that CCF could significantly shorten blade life compared with pure LCF and that larger HCF caused more significant fatigue life reduction. Fatigue source characteristics and CCF fracture appearance were observed and analyzed using a scanning electron microscope (SEM).

Research on the Application of Fen for Environmentally Assisted Fatigue Evaluation of the Austenitic SS Pipe Under Combined Transient Loads

Accumulative test data indicate that the effects of the light water reactor (LWR) environment could cause the fatigue resistance of primary pressure boundary components materials to be significantly reduced. Environmentally assisted fatigue (EAF) is the abbreviation of the environmentally assisted fatigue. In 2007, Nuclear Regulatory Commission (NRC) issued RG. 1.207. It was updated in 2014. And, it requires that the effects of LWR environment on the fatigue life reduction of metal components should be considered for new design plants. And it suggests to use environmental correction factor, Fen, to account for EAF. NRC regulation (NUREG), NUREG/CR-6909 (NRC, 2013, “Effect of LWR Coolant Environments on the Fatigue Life of Reactor Materials,” U.S. Nuclear Regulatory Commission, Argonne, IL, Standard no. NUREG/CR-6909), presents the detail Fen calculation formula. Fen is a function of temperature, strain rate, dissolved oxygen level in water, and sulfur content of the steel. Accordingly, Fen calculation will present a comparatively conservative result. Depending on the experience of the primary pressure boundary piping transient operation, Fen varies during each transient. More uncertainty and confusion are raised during the application of the Fen method. The research work in this paper includes: first, the typical character of piping thermal transient is derived based on the existing experience. Second, small specimen EAF tests are conducted depending on the above derived combined loading characters. Then effort is taken to improve the application of the Fen method for the combined multitransient loading conditions. And the results are compared with those of the lowest instantaneous Fen method and equalization of the weighted Fen method. Finally, a designed test plan is presented.

Influence of Mean Strain on Fatigue Life of Stainless Steel in PWR Water Environment

Change in the fatigue life due to application of the mean strain was investigated for Type 316 stainless steel in simulated pressurized water rector (PWR) primary water environment. The tests were conducted by controlling the strain range to 1.2% for different strain rates of 0.4, 0.004, or 0.001%/s. The applied mean strain was 15% in nominal strain. In addition, cold worked specimens were also subjected to the tests without applying the mean strain. The tests using the cold worked specimens were regarded as the tests with the mean strain without increase in surface roughness due to application of plastic deformation. By inducing the cold working at low temperature, the effect of martensitic phase on the fatigue life was also examined. It was shown that the fatigue life of the stainless steel was reduced in the PWR water environment and the degree of the fatigue life reduction was consistent with the prediction model prescribed in the code issued by the Japan Society of Mechanical Engineers (JSME) and NUREG/CR-6909. Increases in peak stress and stress range due to cold working did not cause any apparent influence on the fatigue life. It was also shown that the 10.5 wt% martensitic phase induced by the low temperature cold working and the increase in the surface roughness caused by application of 15% mean strain did not bring about further fatigue life reduction. It was concluded that the effects of the mean strain, cold working, and martensitic phase were minor on the fatigue life in the PWR water environment. The current JSME and NUREG/CR-6909 models were applicable for predicting the reduction in fatigue due to the PWR water environment even if the mean strain or cold working was applied.