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.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Basic issues in the mechanics of high cycle metal fatigue

Mechanics issues related to the formation and growth of cracks ranging from subgrain dimension to up to the order of one mm are considered under high cycle fatigue (HCF) conditions for metallic materials. Further efforts to improve the accuracy of life estimation in the HCF regime must consider various factors that are not presently addressed by traditional linear elastic fracture mechanics (LEFM) approaches, nor by conventional HCF design tools such as the S-N curve, modified Goodman diagram and fatigue limit. A fundamental consideration is that a threshold level for ΔK for small/short cracks may be considerably lower than that for long cracks, leading to non-conservative life predictions using the traditional LEFM approach. Extension of damage tolerance concepts to lower length scales and small cracks relies critically on deeper understanding of (a) small crack behavior including interactions with microstructure, (b) heterogeneity and anisotropy of cyclic slip processes associated with the orientation distribution of grains, and (c) development of reliable small crack monitoring techniques. The basic technology is not yet sufficiently advanced in any of these areas to implement damage tolerant design for HCF. The lack of consistency of existing crack initiation and fracture mechanics approaches for HCF leads to significant reservations concerning application of existing technology to damage tolerant design of aircraft gas turbine engines, for example. The intent of this paper is to focus on various aspects of the propagation of small cracks which merit further research to enhance the accuracy of HCF life prediction. Predominant concern will rest with polycrystalline metals, and most of the issues pertain to wide classes of alloys.     

A MICROCRACK GROWTH LAW FOR MULTIAXIAL FATIGUE

Within the past decade, critical plane approaches have gained increasing support based on correlation of experimentally observed fatigue lives and microcrack orientations under predominately low cycle fatigue (LCF) conditions for various stress states. In this paper, we further develop an engineering model for microcrack propagation consistent with critical plane concepts for correlation of both LCF and high cycle fatigue (HCF) behavior, including multiple regimes of small crack growth. The critical plane microcrack propagation approach of McDowell and Berard serves as a starting point to incorporate multiple regimes of crack nucleation, shear growth under the influence of microstructural barriers, and transition to linear crack length-dependent growth related to elastic-plastic fracture mechanics (EPFM) concepts. Microcrack iso-length data from uniaxial and torsional fatigue tests of 1045 steel and IN 718 are examined and correlated by introducing a transition crack length which governs the shift from nonlinear to linear crack length dependence of da/dN. This transition is related to the shift from strong microstructural influence to weak influence on the propagation of microcracks. Simple forms are introduced for both the transition crack length and the crack length-dependence of crack growth rate within the microcrack propagation framework (introduced previously by McDowell and Berard) and are employed to fit the 1045 steel and IN 718 microcrack iso-length data, assuming preexisting sub-grain size cracks. The nonlinear evolution of crack length with normalized cycles is then predicted over a range of stress amplitudes in uniaxial and torsional fatigue. The microcrack growth law is shown to have potential to correlate microcrack propagation behavior as well as damage accumulation for HCF-LCF loading sequences and sequences of applied stress states.     

An assessment of the role of near-threshold crack growth in high-cycle-fatigue life prediction of aerospace titanium alloys under turbine engine spectra

Increasingly accurate life prediction models are required to utilize the full capability of current and future advanced materials in gas turbine engines. Of particular recent interest are predictions of the lifetimes of engine airfoil materials that experience significant intervals of high-frequency, high-cycle fatigue (HCF). Conventional life management practices for HCF in the turbine engine industry have been based principally on a total-life approach. There is a growing need to develop damage tolerance methods capable of predicting the evolution and growth of HCF damage in the presence of foreign object damage (FOD), low cycle fatigue (LCF), and surface fretting fatigue. To help identify key aspects of the HCF life prediction problem for turbine engine components, a review is pressented of the extensive results of an Air Force research contract with Pratt & Whitney on the high strength titanium alloy Ti-8Al-1Mo-1V. Data from this representative turbine-airfoil material are used to examine the applicability of linear elastic fracture mechanics methods for prediction of service lifetimes under load spectra that include high cycle fatigue. The roles of fatigue crack initiation and growth are examined for materials that are nominally-defect-free, as well for materials that have experienced significant prior structural damage. An assessment is presented of the potential utility of the conventional threshold stress intensity factor range, ΔK _th, defined by testing specimens containing large cracks. Although the general utility of a large-crack-ΔK _th approach is questionable due to the potentially rapid growth of small fatigue cracks, the low allowable stresses involved in turbine engine high cycle fatigue appear to limit and simplify the small-crack problem. An examination is also presented of the potential effects of high-cycle fatigue and low-cycle fatigue (HCF/LCF) interactions.     

Experimental characterization of short crack nucleation and growth during cycling in lean duplex stainless steels

Considering that many applications of Lean Duplex Stainless Steels (LDSSs) involve cyclic loading, the aim of this paper is to study short crack initiation and growth during low (LCF) and high cycle fatigue (HCF) in AL 2003 (UNS S32003). Electron Backscattered Diffraction (EBSD) analysis of plastically active grains allows to determinate the slip systems and their associated Schmid factor (SF). Additionally, the dislocation structure developed during cycling is observed by transmission electron microscopy (TEM). Whereas in HCF cracks nucleate at grains boundaries, during LCF cracks nucleate along intrusion/extrusions in ferritic grains and as they reach austenitic grains grow along active slip systems or by double slip system. Moreover, phase boundaries and grain boundaries act as effective barrier against crack propagation.     

Influence of foreign object damage on fatigue crack growth of gas turbine aerofoils under complex loading conditions

Foreign object damage (FOD) has been identified as one of the main life limiting factors for aeroengine blades, with the leading edge of aerofoils particularly susceptible. In this work, a generic edge ‘aerofoil’ geometry was utilized in a study of early fatigue crack growth behaviour due to FOD under low cycle fatigue (LCF), high cycle fatigue (HCF) and combined LCF and HCF loading conditions. Residual stresses due to FOD were analyzed using the finite element method. The longitudinal residual stress component along the crack path was introduced as a nodal temperature distribution, and used in the correction of the stress intensity factor range. The crack growth was monitored using a nanodirect current potential drop (DCPD) system and crack growth rates were correlated with the corrected stress intensity factor considering the residual stresses. The results were discussed with regard to the role of residual stresses in the characterization of fatigue crack growth. Small crack growth behaviour in FODed specimens was revealed only after the residual stresses were taken into account in the calculation of the stress intensity factor, a feature common to LCF, HCF and combined LCF + HCF loading conditions.     

Prediction of Fatigue Lives of MAR‐M247 LC Based on the Crack Closure Concept

Low cycle fatigue (LCF), high cycle fatigue (HCF), and combined LCF and HCF tests are carried out on MAR‐M247 LC at 650 °C in air environment. Under combined LCF and HCF loading, block striations form on the fracture surface which are used to complete an effective crack growth curve by using the linear summation model. Crack growth lives starting from equivalent initial flaw sizes are calculated by the crack closure code FASTRAN and compared with experimental fatigue lives. Under HCF loading, predicted and experimental fatigue lives agree well for lifetimes above 10⁵ cycles. Lower lifetimes are overestimated indicating that the linear summation model is not valid for MAR‐M247 LC in this loading range. Interactions between the non‐crystallographic HCF crack growth and striated crack growth that is caused by the LCF loading are probably responsible for this behavior.     

On the life-controlling microstructural fatigue mechanisms in ductile metals and alloys in the gigacycle regime

The high-cycle fatigue (HCF) behaviour of ductile metals and alloys, and the life-controlling microstructural fatigue mechanisms known from HCF are reviewed critically with respect to their possible role in the gigacycle or ultra-high-cycle fatigue (UHCF) regime. Arguments are presented to support the hypothesis that, at the very low amplitudes of the UHCF regime, fatigue crack initiation, resulting from cyclic strain localization, and slow early Stage I fatigue crack propagation are the life-controlling mechanisms and that these processes can essentially be described in terms of the microstructurally irreversible portion of the cumulative cyclic plastic strain. Emphasis is placed on the important role of the so-called slip irreversibility which decreases as the amplitude becomes lower and lower. Finally, the Manson–Coffin law is reformulated for very low amplitudes in terms of microstructurally relevant parameters, and a fatigue life diagram is developed, based on these preceding microstructural considerations. Important features of this diagram are: (i) the plastic strain fatigue limit in the HCF regime which is related to the threshold for cyclic strain localization in persistent slip bands; and (ii) the transition from this plastic strain fatigue limit to a threshold of negligible slip irreversibility at still lower amplitudes in the UHCF regime.     

Effect of predamage from low cycle fatigue on high cycle fatigue strength of Ti-6Al-4V

Effects of prior low cycle fatigue (LCF) cycling on the subsequent high cycle fatigue (HCF) limit stress corresponding to a life of 10⁷ cycles are investigated for Ti-6Al-4V at room temperature. Tests are conducted at 420 Hz on an electrodynamic shaker-based system at several different LCF maximum loads and under subsequent HCF at R=0.1, 0.5 and 0.8 using a step loading procedure. Under these load combinations, which include the possibility of overload or underload effects if cracks form, there is no statistically significant effect of the prior LCF on the subsequent HCF limit stress. While LCF loading at a high stress of 900 MPa is seen to result in strain ratcheting, no distinct features on the fracture surface and different mechanisms of crack propagation from those obtained at lower maximum loads were observed. LCF loading up to 50% of expected life did not produce any indications of crack formation from either the stress limit data or the fracture surfaces.     

Low and high‐cycle fatigue properties of an ultrahigh‐strength TRIP bainitic steel

The scope of this study is to characterize the mechanical properties of a novel Transformation‐Induced Plasticity bainitic steel grade TBC700Y980T. For this purpose, tensile tests are carried out with loading direction 0, 45 and 90° with respect to the L rolling direction. Yield stress is found to be higher than 700 MPa, ultimate tensile strength larger than 1050 MPa and total elongation higher than 15%. Low‐cycle fatigue (LCF) tests are carried out under fully reverse axial strain exploring fatigue lives comprised between 10² and 10⁵ fatigue cycles. The data are used to determine the parameters of the Coffin–Manson as well as the cyclic stress–strain curve. No significant stress‐induced austenite transformation is detected. The high‐cycle fatigue (HCF) behaviour is investigated through load controlled axial tests exploring fatigue tests up to 5 × 10⁶ fatigue cycles at two loading ratios, namely R = ?1 and R = 0. At fatigue lives longer than 2 × 10⁵ cycles, the strain life curve determined from LCF tests tends to greatly underestimate the HCF resistance of the material. Apparently, the HCF behaviour of this material cannot be extrapolated from LCF tests, as different damage, cyclic hardening mechanisms and microstructural conditions are involved. In particular, in the HCF regime, the predominant damage mechanism is nucleation of fatigue cracks in the vicinity of oxide inclusions, whereby mean value and scatter in fatigue limit are directly correlated to the dimension of these inclusions.     

Fretting fatigue crack analysis in Ti–6Al–4V

A study was conducted to verify the efficacy of a fracture mechanics methodology to model the crack growth behavior of fretting fatigue-nucleated cracks obtained under test conditions similar to those found in turbine engine blade attachments. Experiments were performed to produce cracked samples, and fretting fatigue crack propagation lives were calculated for each sample. Cracks were generated at 10⁶ cycles (10%-of-life) under applied stress conditions previously identified as the fretting fatigue limit conditions for a 10⁷ cycle fatigue life. Resulting cracks, ranging in size from 30 to 1200 μm, were identified and measured using scanning electron microscopy. Uniaxial fatigue limit stresses were determined experimentally for the fretting fatigue-cracked samples, using a step loading technique, for R=0.5 at 300 Hz. Fracture surfaces were inspected to characterize the fretting fatigue crack front indicated by heat tinting. The shape and size of the crack front were then used in calculating ΔKth values for each crack. The resulting uniaxial fatigue limit and ΔKth values compared favorably with the baseline fatigue strength (660 MPa) for this material and the ΔKth value (2.9 MPa√m) for naturally initiated cracks tested at R=0.5 on a Kitagawa diagram.Crack propagation lives were calculated using stress results of FEM analysis of the contact conditions and a weight function method for determination of ΔK. Resulting lives were compared with the nine million-cycle propagation life that would have been expected in the experiments, if the contact conditions had not been removed. Scatter in the experimental results for fatigue limit stresses and fatigue lives had to be considered as part of an explanation why the fatigue life calculations were unable to match the experiments that were modeled. Analytical life prediction results for the case where propagation life is observed to be very short experimentally were most accurate when using a coefficient of friction, μ=1.0, rather than for the calculations using μ=0.3     

Short fatigue crack propagation during low-cycle,high cycle and very-high-cycle fatigue of duplex steel – An unified approach

The present paper reviews experimental results on the fatigue damage of austenitic–ferritic duplex steel under various load levels ranging from LCF to VHCF, placing the focus towards the relationship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. A combination between fatigue testing of electropolished specimens and analytical electron microscopy (SEM/EBSD, TEM) revealed that under LCF loading conditions almost all the ferrite and the austenite grains showed plasticity, while under HCF and VHCF loading conditions, slip band formation was limited to the softer austenite grains and a low plastic activity is observed in the ferrite. Once being formed, the bands generate high stress concentrations, where they impinge the α–γ phase boundaries, eventually, leading to the crack initiation. This is discussed by applying a numerical simulation approach based on the finite-element (FEM) and the boundary-element (BEM) method.     

Manifestations of dynamic strain aging under low and high cycle fatigue in a type 316LN stainless steel

Influence of Dynamic strain aging (DSA) under low cycle fatigue (LCF) and high cycle fatigue (HCF) loading was investigated by conducting LCF and HCF tests on specimens over a wide range of temperature from 573 to 973 K. DSA was found to be highly pronounced in the temperature range of 823–873 K. DSA was seen to have contrasting implications under LCF and HCF deformation. The cyclic hardening owing to DSA caused an increase in the cyclic stress response under LCF, leading to decrease in cyclic life. On the other hand, the DSA-induced strengthening suppressed the crack initiation phase under HCF where the applied stress remains fixed, leading to an increase in the cyclic life.     

Fatigue crack nucleation and growth in filled natural rubber

Rubber components subjected to fluctuating loads often fail due to nucleation and the growth of defects or cracks. The prevention of such failures depends upon an understanding of the mechanics underlying the failure process. This investigation explores the nucleation and growth of cracks in filled natural rubber. Both fatigue macro‐crack nucleation as well as fatigue crack growth experiments were conducted using simple tension and planar tension specimens, respectively. Crack nucleation as well as crack growth life prediction analysis approaches were used to correlate the experimental data. Several aspects of the fatigue process, such as failure mode and the effects of R ratio (minimum strain) on fatigue life, are also discussed. It is shown that a small positive R ratio can have a significant beneficial effect on fatigue life and crack growth rate, particularly at low strain range.     

High- and low-cycle fatigue influence of silicon,copper, strontium and iron on hypo-eutectic Al–Si–Cu and Al–Si–Mg cast alloys used in cylinder heads

In this publication, ambient condition fatigue investigations with different types of Al–Si–Cu and Al–Si–Mg cast alloys in rotating-bending high-cycle fatigue (HCF) and push–pull low-cycle fatigue (LCF) regimes have been performed with varying Si, Cu, Fe and Sr contents. The cast alloys investigated here are common used in cylinder heads for automotive application. Because the cylinder head is one of the most fatigued parts in combustion chamber engines, the microstructural knowledge of the damage process provides a tool of construction and its material selection. The investigations were also supported with an in-situ microstructural crack observation in high plasticity rotating-bending regimes. The specimens were directly processed out of serial produced T79 heat-treated cylinder heads to provide the equal microstructure for testing as under operational conditions.The observations clearly identified the effects of the individual alloying elements both under low- and high-cycle fatigue. The crack propagation speed and the crack paths were majorly influenced by the eutectic silicon. Additional, the precipitation hardening due to copper affected significantly the fatigue endurance, too. In high plasticities the silicon’s influence got almost lost and only the matrix strength was crucial. Thus, increased fatigue strength in high loaded LCF regimes was observed for alloys with less copper content, thus higher ductility. By contrast, improved HCF and low loaded LCF endurance was only achieved when the matrix strength was increased by copper’s precipitation hardening. Crack branching and deflections strongly influenced the microstructural damage of the ductile AlSi7Mg(Sr) and hence, gained its fatigue strength. Iron phases could not identified as harmful inclusions, since the phases were similar in size of other hard phase elements like the other primary intermetallic phases like ${\text{Al}}_2\text{Cu}$ and β-$\text{Si}$ phases under notch stress aspects, by the well defined solidification process in the test section. Because the crack nucleation mainly occurred on Si particles, strontium as a refinement agent influenced the early crack onset and accordingly the fatigue in total. Thus, the AlSi6Cu4(Sr) had increased lifetimes compared to AlSi6Cu4 both in HCF and LCF. Further, the presented results provide a modification of the Manson–Coffin approach to describe the relationship between plastic strain and lifetime, valid for all proposed alloys with only one set of parameters. Thus, it was possible to perform the fatigue calculation with a reduced range of scatter.     

Analyses of the fatigue crack propagation process and stress ratio effects using the two parameter method

In situ SEM observations (Zhang JZ. A shear band decohesion model for small fatigue crack growth in an ultra-fine grain aluminium alloy. Eng Fract Mech 2000;65:665–81; Zhang JZ, Meng ZX. Direct high resolution in-site SEM observations of very small fatigue crack growth in the ultra fine grain aluminium alloy IN 9052. Script Mater 2004;50:825–28; Halliday MD, Poole P, Bowen P. New perspective on slip band decohesion as unifying fracture event during fatigue crack growth in both small and long cracks. Mater Sci Technol 1999;15:382–90) have revealed that fatigue crack propagation in aluminium alloys is caused by the shear band decohesion around the crack tip. The formation and cracking of the shear band is mainly caused by the plasticity generated in the loading part of a load cycle. This shear band decohesion process has been observed to occur in a continuous way over the time period during the loading part of a cycle. Based on this observation, in this study, a new parameter has been introduced to describe fatigue crack propagation rate. This new parameter, da/dS, defines the fatigue crack propagation rate with the change of the applied stress at any moment of a stress cycle. The relationship between this new parameter and the conventional da/dN parameter which describes fatigue crack propagation rate per stress cycle is given.Using this new parameter, it is proven that two loading parameters are necessary in order to accurately describe fatigue crack propagation rate per stress cycle, da/dN. An analysis is performed and a general fatigue crack propagation model is developed. This model has the ability to describe the four general type of fatigue crack propagation behaviours summarised by Vasudevan and Sadananda (Vasudevan AK, Sadananda K. Fatigue crack growth in advanced materials. In: Fatigue 96, Proceedings of the sixth international conference on fatigue and fatigue threshold, vol. 1. Oxford: Pergamon Press; 1996. p. 473–8).     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

Microstructure-based fatigue modeling of cast A356-T6 alloy

High cycle fatigue (HCF) life in cast Al-Mg-Si alloys is particularly sensitive to the combination of microstructural inclusions and stress concentrations. Inclusions can range from large-scale shrinkage porosity with a tortuous surface profile to entrapped oxides introduced during the pour. When shrinkage porosity is controlled, the relevant microstructural initiation sites are often the larger Si particles within eutectic regions. In this paper, a HCF model is introduced which recognizes multiple inclusion severity scales for crack formation. The model addresses the role of constrained microplasticity around debonded particles or shrinkage pores in forming and growing microstructurally small fatigue cracks and is based on the cyclic crack tip displacement rather than linear elastic fracture mechanics stress intensity factor. Conditions for transitioning to long crack fatigue crack growth behavior are introduced. The model is applied to a cast A356-T6 Al alloy over a range of inclusion severities.     

SMALL CRACK GROWTH IN THE ALUMINIUM–LITHIUM ALLOYS 8090 AND 8091

The relationship between microstructure and the fatigue behaviour of small cracks has been examined for the aluminium–lithium alloys 8090 and 8091 after peak ageing at 170°C. Duplex ageing and pre-stretching were used to vary the distribution of S'precipitates and thus the distribution of slip. No effect of S'distribution an small crack growth was observed in either alloy. This is thought to be due to a combination of the lack of closure and lower overall slip reversibility in small cracks. Small cracks in 8091 were found to grow slower than in 8090 due to differences in grain shape rather than texture. Small cracks in both alloys were observed to grow much faster than long cracks for equivalent ΔKs. This difference was reduced when small crack data were compared with long crack data generated at R= 0.7 due to the reduced closure. The use of ΔJ made long and small crack growth rates still more comparable.