Cyclic Low-cycle Fatigue Research Articles

Implications of dynamic strain aging under LCF-HCF interactions in a type 316LN stainless steel

Influence of dynamic strain aging (DSA) under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting HCF tests on specimens subjected to prior LCF cycling over a wide range of temperature from 573 to 973K. DSA was found to be pronounced at 823–873K depending on the magnitude of the stress employed under HCF cycling. DSA was seen to have contrasting implications under LCF and HCF deformation resulting in an anomalous fatigue behavior in terms of remnant HCF life under LCF-HCF interaction. LCF-HCF interaction was found to be pronounced at intermediate levels of prior LCF exposure, where the remnant HCF life is dictated by competitive damage mechanism resulting from the influence of DSA under LCF as well as HCF. Detailed fracture surface examination revealed that extensive hardening associated with DSA leads to an extended zone of faceted appearance with river markings (Stage-I crack) under HCF cycling (with or without LCF exposure). This reduces the crack growth rate, delaying the transition of crack from Stage-I to Stage-II, thereby leading to an extension of life in such cases. On the other hand, a highly striated fracture surface indicating a quick transition in crack from Stage-I to Stage-II, was observed for loading conditions with minimal or no influence of DSA, thus leading to lower life compared to the previous case.

Investigation of fracture mechanisms and substructural changes under sequential fatigue cycling involving LCF and HCF loads in a type 316LN stainless steel at 923 K

Cumulative fatigue damage under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated at 923K by conducting HCF tests on specimens subjected to prior LCF cycling at various strain amplitudes. Under such loading conditions, a critical damage marking the strongest LCF-HCF interaction was found to occur at specific pre-exposures of LCF depending inversely on the strain amplitude employed. Fracture surface examination of specimens failed in HCF with prior exposure in LCF showed formation of isolated striation pockets as the main manifestation of LCF-HCF interaction. Detailed Transmission Electron Microscopy (TEM) analysis revealed extensive formation of microtwins as prominent microstructural manifestations of LCF-HCF interaction. All these features appear within the window of effective LCF-HCF interaction and hence are a strong function of the applied strain amplitude as well as the degree of prior LCF exposure. It was found out that even though formation of major crack (critical damage) is a pre-requisite for microtwins to appear, there are several microscopic factors associated to the formation of such twins, which were discussed in detail.

Investigation of Cumulative Fatigue Damage Through Sequential Low Cycle Fatigue and High Cycle Fatigue Cycling at High Temperature for a Type 316LN Stainless Steel: Life-Prediction Techniques and Associated Mechanisms

Cumulative fatigue damage under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) cycling was investigated at 923 K (650 °C) by conducting HCF tests on specimens subjected to prior LCF cycling at various strain amplitudes. Remnant HCF lives were found to decrease drastically with increase in prior fatigue exposure as a result of strong LCF–HCF interactions. The rate of decrease in remnant lives varied as a function of the applied strain amplitude. A threshold damage in terms of prior LCF life-fraction was found, below which no significant LCF–HCF interaction takes place. Similarly, a critical damage during the LCF pre-cycling marking the highest degree of LCF–HCF interaction was identified which was found to depend on the applied strain amplitude. In view of the non-linear damage accumulation behavior, Miner’s linear damage rule proved to be highly non-conservative. Manson’s damage curve approach, suitably modified, was found to be a better alternative for predicting the remnant HCF life. The single constant (β) employed in the model, which reflects the damage accumulation of the material under two/multi-level loading conditions is derived from the regression analysis of the experimental results and validated further.

AFM Study of Microstructure Role in the Cyclic Plasticity of Martensitic Steel

The paper presents the description of FSMs (fatigue slip markings) and the evaluation of cyclic plasticity markings of a 12%Cr martensitic steel by AFM surface analyses. The microstructure of a 12%Cr martensitic steel, quenched and tempered in air, consists of prior austenite grains, packets, blocks, and laths. The low cycle fatigue (LCF) test at a total strain range Δεt=1.2% was interrupted at different life fractions for the surface relief investigation by AFM. The localization of FSMs relatively to the different microstructural interfaces of the studied steel was proposed. The principal FSMs, appeared in the first fatigue cycle, are likely to be localized at the packet and block boundaries, while the secondary one (appeared later at 44% of lifetime) are at the lath boundaries or in the laths. The height of principal and secondary FSMs increases constantly during LCF cycling.

Engineering Approach for Low-Cycle Fatigue Assessment of Porous Alloys

Most components used in gas and steam turbines are metallic parts produced by either casting or forging processes. Although process control works to eliminate defects, there can be variation in microporosity from component to component. Previously, this microporosity was only able to be detected destructively using metallography. Using computer tomography (CT), one can find voids in the range of a few tenths of a millimeter and know the location of the voids with high precision. This allows one to map the defects present in each component onto the stress and temperature fields for that component. However, there is not yet universal agreement upon a consistent method to evaluate the effect of these small porosities on a components lifetime. Having a robust analysis tool to understand the impact of microporosity would decrease development costs, decrease the time to bring a product to market, and increase the likelihood of failure-free operation. This paper presents an approach using equivalent low-cycle fatigue (LCF) material properties which avoids the need to explicitly model the morphology of the microstructure in the region of the microporosity. The homogenization methodology calculates new LCF curves depending on porosity ratios in material. This approach uses Morrows correlation factor of LCF cycles to crack initiation regarding energy amount dissipated in stable cycling (shakedown) and ultimate strain energy under monotonic loading. The paper generalizes Morrows postulate and formulates the hypothesis that energy stored and dissipated in the material under shakedown conditions corresponds directly to the number of LCF cycles to crack initiation. The paper demonstrates that the reduction of LCF life based on the porosity ratio agrees well with the experimental results. These results also show that the methodology is very sensitive to the void orientation and loading direction.

Aeromechanical Modeling of Rotating Fan Blades to Investigate High-Cycle and Low-Cycle Fatigue Interaction

Gas turbine engine components are subject to both low-cycle fatigue (LCF) and high-cycle fatigue (HCF) loads. To improve engine reliability, durability and maintenance, it is necessary to understand the interaction of LCF and HCF in these components, which can adversely affect the overall life of the engine while they are occurring simultaneously during a flight cycle. A fully coupled aeromechanical fluid–structure interaction (FSI) analysis in conjunction with a fracture mechanics analysis was numerically performed to predict the effect of representative fluctuating loads on the fatigue life of blisk fan blades. This was achieved by comparing an isolated rotor (IR) to a rotor in the presence of upstream inlet guide vanes (IGVs). A fracture mechanics analysis was used to combine the HCF loading spectrum with an LCF loading spectrum from a simplified engine flight cycle in order to determine the extent of the fatigue life reduction due to the interaction of the HCF and LCF loads occurring simultaneously. The results demonstrate the reduced fatigue life of the blades predicted by a combined loading of HCF and LCF cycles from a crack growth analysis, as compared to the effect of the individual cycles. In addition, the HCF aerodynamic forcing from the IGVs excited a higher natural frequency of vibration of the rotor blade, which was shown to have a detrimental effect on the fatigue life. The findings suggest that FSI, blade–row interaction and HCF/LCF interaction are important considerations when predicting blade life at the design stage of the engine. The lack of available experimental data to validate this problem emphasizes the utility of a numerical approach to first examine the physics of the problem and second to help establish the need for these complex experiments.

Low Cycle Fatigue Behaviour of a Turbine Blade Made of Nickel Base Wrought Alloy

Life of an aero-engine is limited by the life of the turbine blade in particular and that of hot end components in general. Design of aero-engine is always conservative in nature considering the flight safety as paramount important. Earlier engines have been assigned life in hours but the life of the component is limited by LCF cycles particularly during the start-stop cycles. In this paper LCF behaviour of a typical Russian origin nickel base wrought super alloy AP220BD used for turbine blade has been studied at room temperature (RT), 400 °C and 700 °C that corresponds to idle rating and cruise rating of a typical aero-engine. Low cycle fatigue (LCF) tests have been carried out at RT, 400°C and 700 °C at three strain amplitudes of ±0.3%, ±0.5% and ±0.8%. Hysteresis loop have been developed at each strain and temperature. It has been observed that LCF life of the nickel base wrought alloy AP220BD is not influenced significantly at strain amplitude of ±0.3% till it reaches 400° C. Reduction in LCF life with increase in strain amplitude from ±0.3% to ±0.8% is much significant compared to that of increase in temperature up to 700°c.The higher life at intermediate strain of ±0.5% may be due to DSA(dynamic strain aging) of the material. Transgranular fracture has been observed at RT & 400° C while intergranular fracture at 700° C.

Characterization of AA6061 Alloy Processed by Equal Channel Angular Pressing and Subjected to Low Cycle Fatigue

Aluminum alloy 6061 was subjected to equal channel angular pressing (ECAP) using two different processing routes BC and C, to study the evolution of the microstructure and the effect of low cycle fatigue (LCF) on the resultant microstructure. Specimens subjected to ECAP and fatigue cycling were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD patterns of the material after each pass of ECAP and after interrupted LCF tests were analyzed. Single-line approximation method of analysis was used to obtain microstructural parameters from peak broadening observed in XRD profiles. Increase in dislocation density till saturation after pass 2 and marginal changes thereafter for successive passes were observed. From the LCF tests on specimens subjected to three ECAP passes at two strain amplitudes of 0.5 and 1.0 %, cyclic stress response up to fatigue failure were obtained. The solutionized specimens exhibited continuous strain hardening at both strain amplitudes. The ECAP processed material fatigued at 0.5 % strain amplitude exhibited stable cyclic stress response, whereas the material fatigued at 1.0 % strain amplitude exhibited cyclic softening. The LCF behaviour was the same for the material processed through both BC and C routes. The TEM images of specimens and the associated selected area electron diffraction patterns indicated ultra fine grain structure after three passes of ECAP. However, some amount of grain coarsening was observed after LCF cycling.

The role of high cycle fatigue (HCF) onset in Francis runner reliability

High Cycle Fatigue (HCF) plays an important role in Francis runner reliability. This paper presents a model in which reliability is defined as the probability of not exceeding a threshold above which HCF contributes to crack propagation. In the context of combined Low Cycle Fatigue (LCF) and HCF loading, the Kitagawa diagram is used as the limit state threshold for reliability. The reliability problem is solved using First-Order Reliability Methods (FORM). A study case is proposed using in situ measured strains and operational data. All the parameters of the reliability problem are based either on observed data or on typical design specifications. From the results obtained, we observed that the uncertainty around the defect size and the HCF stress range play an important role in reliability. At the same time, we observed that expected values for the LCF stress range and the number of LCF cycles have a significant influence on life assessment, but the uncertainty around these values could be neglected in the reliability assessment.

Modeling of fatigue damage under superimposed high-cycle and low-cycle fatigue loading for a cast aluminum alloy

This study investigates the interaction between high-cycle fatigue (HCF) and low-cycle fatigue (LCF) in cast aluminum alloys. Initially, the baseline HCF and LCF lives have been determined under constant amplitude loading; LCF–HCF interaction fatigue tests were subsequently carried out through the application of periodic underloads. To account for the inherent scatter in fatigue life, probabilistic fatigue life curves were employed to establish the baseline HCF and LCF fatigue lives. The interaction fatigue damage was found to increase with the stress amplitude of the HCF cycles. Larger underloads (LCF cycles) also resulted in larger interaction damage. The impact of different numbers of HCF cycles per block (between underloads) was also studied. It was also found out that there is an additional damage caused by frequent applications of LCF cycles, which is related to the damaged Si particles in the large reversed plastic zone generated by the LCF cycles. The evolution of the crack opening stress following the application of an underload has been proposed as the major mechanism for the load interaction effect. An interaction damage model is proposed to account for these observed effects.

A novel methodology to predict the endurance domain for a material and its evolution using a generalized fracture mechanics framework

Materials in rotating machinery are typically subjected to loading conditions that combine both low cycle fatigue (LCF) and high cycle vibratory fatigue (HCF). In operating conditions, the number of allowable LCF cycles may be limited, but under no circumstances, superimposed HCF vibratory cycles can be allowed to produce fatigue damage. This paper aims at proposing an approach to account for the effect of variable amplitude LCF cycles on the endurance domain relative to HCF cycles. For this purpose, a LEFM-based approach is proposed. The material is assumed to contain flaws, and their growth is assumed to stem from crack tip plasticity. As a consequence of these “classical” assumptions, the endurance domain of the material can alternatively be viewed as an elastic domain for the crack tip region. A generalized von Mises yield criterion is hence introduced to model that elastic domain. For this purpose, rather than considering the distortional elastic energy density (von Mises), the Westergaard’s stress field is used to calculate the distortional elastic energy within a distance δ from the crack tip. A non-local yield criterion for the crack tip region is obtained. The first non-singular terms (e.g., the T-stresses) are included in the Westergaard’s stress functions to make it possible to use it also for mechanically short cracks. Then to account for the effect of LCF cycles, the elastic domain is allowed to evolve when crack tip plasticity occurs. The center of the endurance domain is defined as an internal variable that stands for the internal stress field within the crack tip region due to constrained plastic deformation. As long as the loading path remains inside the elastic domain, no plasticity, and hence no crack growth, is expected. If the yield surface is reached, plastic strain occurs and the elastic domain is displaced. In this study, elastic–plastic finite element computations are used to determine whether the crack tip region behaves elastically or plastically. For this purpose, the velocity field within the crack tip region is partitioned into elastic and plastic parts, each part being approached as the product of an intensity factor and a spatial reference field. The plastic intensity factor is then used as a global measure of the plasticity rate within the crack tip region. In this paper, the approach was applied to a forged Ti–6Al–4V titanium alloy, used in compressor blades of gas turbine engines.

Vibration Durability Assessment of Sn3.0Ag0.5Cu and Sn37Pb Solders Under Harmonic Excitation

In this paper, the vibration durability of both SAC305 and Sn37Pb interconnects are investigated with narrow-band harmonic vibration tests conducted at the first natural frequency of the test, printed wiring board, using constant-amplitude excitation. A time-domain approach, reported by Upadhyayula and Dasgupta (1998, “Guidelines for Physics-of-Failure Based Accelerated Stress Test,” Proceedings, Reliability and Maintainability Symposium, pp. 345–357), was adapted in this study for the fatigue analysis. The test board consists of daisy-chained components, to facilitate real-time failure monitoring. The response of the test specimens was characterized, and accelerated fatigue tests were conducted at different loading amplitudes to obtain a mix of low-cycle fatigue (LCF) and high-cycle fatigue data points. The SAC305 interconnects were found to have lower fatigue durability than comparable Sn37Pb interconnects, under the narrow-band harmonic excitation levels used in this study. This trend is consistent with most results from broadband vibration tests by Zhou et al. (2006, “Vibration Durability Comparison of Sn37Pb vs. SnAgCu Solders,” Proceedings of ASME International Mechanical Engineering Congress and Exposition, Chicago, IL, Paper No. 13555), Zhou and Dasgupta (2006, “Vibration Durability Investigation for SnPb and SnAgCu Solders With Accelerated Testing and Modeling,” IEEE-TC7 Conference on Accelerated Stress Testing & Reliability, San Francisco, CA), and Woodrow (2005, “JCAA/JG-PP No-Lead Solder Project: Vibration Test,” Boeing Electronics Materials and Processes Technical Report) and from repetitive mechanical shock tests by Zhang et al. (2005, “Isothermal Mechanical Durability of Three Selected Pb-Free Solders: Sn3.9Ag0.6Cu, Sn3.5Ag and Sn0.7Cu,” ASME J. Electron. Packag., 127, pp. 512–522), but counter to findings from quasistatic, LCF, and mechanical cycling studies by Cuddalorepatta and Dasgupta (2005, “Cyclic Mechanical Durability of Sn3.0Ag0.5Cu Pb-Free Solder Alloy,” Proceedings of the ASME International Mechanical Engineering Congress and Exposition, Orlando, FL, Paper No. 81171). Failure analysis revealed two competing failure modes, one in the solder and another in the copper trace under the component. Thus solder fatigue properties extracted with the help of finite element simulation of the test article should be treated as lower-bound estimates of the actual fatigue curves.

Effect of LCF on HCF crack growth of Ti-17

An improved understanding of fatigue crack growth phenomena applicable to titanium engine disks was developed through complimentary experimental and analytical investigations of Ti-17. The effect of low cycle fatigue (LCF) on the high cycle fatigue (HCF) threshold and rate of crack propagation was studied. A simplified variable-amplitude spectrum, consisting of high- R cycles, corresponding to HCF loading, and periodic R=0.1 cycles, corresponding to LCF loading, was used to demonstrate a load-interaction effect. When the ratio of HCF to LCF cycles was 100 or more the fatigue crack growth lifetimes were significantly lower than predicted using linear damage summation methods assuming no load-interaction effect. Thus, it was concluded that the LCF cycle accelerated the fatigue crack growth rate of subsequent HCF cycles, even when closure was concluded to be negligible. A phenomenological model was formulated based on hypothesized changes in the propagation resistance, K PR, and fit to the test data. The model confirmed that the periodic LCF cycles increased fatigue crack growth rates of subsequent HCF cycles.

Crack growth in IMI 829 at 550°C under combined high and low cycle fatigue

Fatigue crack growth rates have been determined for the near-alpha titanium alloy IMI 829 at an elevated temperature of 550°C using corner notched test pieces. Combined low cycle fatigue (LCF) and high cycle fatigue (HCF) tests simulate stresses induced by the start-to-stop operation, and the in-flight vibrations, of aero-engines. Growth rates from these tests were compared to those from purely low cycle fatigue in order to determine the onset of HCF crack growth. This onset was safely predicted by the fatigue threshold value determined by the load-shed method for HCF cycles having a stress ratio of 0.90. The unsafe prediction for a stress ratio of 0.82 is connected with a HCF/LCF load interaction which is apparent in the vicinity of the onset condition, particularly in tests employing the higher ratio of HCF to LCF cycles. The introduction of superimposed high cycle fatigue was reflected in the appearance of the fatigue fracture surface.

Effects of HCF loading on fretting fatigue crack nucleation

This work examines the impact of the interaction between low cycle and high cycle fatigue load spectra on the near-surface stress field associated with conditions of fretting fatigue in the context of a multiaxial fatigue life parameter. Particular focus is placed on analysis of the edge of contact region identified routinely as the site of fretting-induced fatigue damage. Effects of low-cycle fatigue (LCF)—high cycle fatigue (HCF) interaction on the extent of stick and slip are analysed in detail for contacts involving a common titanium engine alloy, Ti–6Al–4V. The effects that this LCF/HCF interaction have on the nucleation of fretting fatigue cracks are then analysed, leading to the subsequent establishment of a relationship between number of LCF cycles to failure and relative HCF frequency and amplitude.

An assessment of low cycle fatigue damage using magnetic Barkhausen emission in 9Cr–1Mo ferritic steel

A non-destructive, magnetic Barkhausen emission (MBE) technique has been used to assess various stages of low cycle fatigue (LCF) damage in 9Cr–1Mo ferritic steel. The initial decrease in the MBE peak height in the early stage of LCF cycling indicates the cyclic hardening stage, in which the formation of dislocation tangles reduces the mean free path of the domain wall movement. The increase in the MBE level again on further cycling indicates the progressive cyclic softening stage where the rearrangement of dislocation tangles into cells enhances the domain wall movement. The unaltered behaviour of MBE on continued cycling shows the saturation stage where the stabilization of dislocation substructure maintains the MBE level. Finally, a sharp increase in the MBE peak value identifies surface crack initiation and propagation, which is ascribed to the movement of additional reverse domains produced at the crack surfaces. This study establishes that the MBE technique can be used to assess the progressive degradation in the fatigue life of the ferritic steel components.

Microstructural evaluation of cumulative fatigue damage below the fatigue limit

Cumulative fatigue damage under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated at 923 K by conducting HCF tests on specimens subjected to prior LCF cycling at various strain amplitudes. Under such loading conditions, a critical damage marking the strongest LCF-HCF interaction was found to occur at specific pre-exposures of LCF depending inversely on the strain amplitude employed. Fracture surface examination of specimens failed in HCF with prior exposure in LCF showed formation of isolated striation pockets as the main manifestation of LCF-HCF interaction. Detailed Transmission Electron Microscopy (TEM) analysis revealed extensive formation of microtwins as prominent microstructural manifestations of LCF-HCF interaction. All these features appear within the window of effective LCF-HCF interaction and hence are a strong function of the applied strain amplitude as well as the degree of prior LCF exposure. It was found out that even though formation of major crack (critical damage) is a pre-requisite for microtwins to appear, there are several microscopic factors associated to the formation of such twins, which were discussed in detail.