High temperature fatigue of nickel-base superalloys – A review with special emphasis on deformation modes and oxidation

Low cycle fatigue, high cycle fatigue, fatigue crack propagation and thermo-mechanical fatigue in Ni-base superalloys are reviewed in terms of fundamental deformation mechanisms, environmental effects, and interactions between environment and deformation mode. These factors are related to the chemical composition and underlying microstructure for all currently-used product forms (i.e. powder metallurgy, wrought, conventionally cast and single crystal). The basic principles that are developed are used to show how both intrinsic and extrinsic variables can be manipulated to control fatigue behaviour and as a guide for formulation of engineering life prediction models.     

Establishing criteria for root and toe cracking of load carrying cruciform joints of pressure vessel grade steel

In this investigation an attempt has been made to establish a criterion to forecast the possible crack initiation region (toe or root) in double fillet welded load carrying cruciform joints and also to know the probable failure mode. Cruciform joints were fabricated from pressure vessel grade (ASTM 517 ‘F’ grade) steel using shielded metal arc welding (SMAW) and flux cored arc welding (FCAW) processes. Fatigue crack growth experiments were carried out in a mechanical resonance vertical pulsator (SCHENCK 200 kN capacity) with a frequency of 30 Hz under constant amplitude loading (R=0). It was found that the fatigue crack initiation lives (root crack or toe crack) were relatively lower in the joints fabricated by FCAW process than the joints fabricated by SMAW process.     

Fatigue crack initiation and growth in solder alloys

In modern electronic packaging, especially surface mount technology (SMT), thermal strain is usually induced between components during processing, and in service, by a mismatch in the thermal expansion coefficients. Since solder has a low melting temperature and is softer than other components in electronic packaging, most of the cyclic stresses and strains take place in the solder. Fatigue crack initiation and fatigue crack propagation are likely to occur in the solder even when the cyclic stress is below the yield stress. It is an objective of this research to study the behaviour of fatigue crack initiation and propagation in both lead‐containing solder (63Sn‐37Pb), and lead‐free solders (Sn‐3.5Ag). The effect of alloying (Cu and Bi addition), frequency, tensile hold time and temperature on low cycle fatigue (LCF) behaviour of the solders is discussed. Mechanisms of LCF crack initiation and propagation are proposed and LCF life prediction, based on the various models, is carried out.     

Effect of frequency on very high cycle fatigue behavior of a low strength Cr–Ni–Mo–V steel welded joint

Axially fully-reversed fatigue test of a low strength Cr–Ni–Mo–V steel welded joint was conducted up to the very high cycle fatigue regime under the frequency of 110 Hz and 20 kHz. The S–N curve shows a duplex shape at low frequency while decreases continuously at high frequency. Sites of crack initiation and fracture of the welds depend on stress level and loading frequency, hence leading to changed fatigue strength. In addition, frequency effect varies among different parts of the welded joint and fatigue lifetime.     

Effect of loading type on fatigue properties of high strength bearing steel in very high cycle regime

Very high cycle fatigue tests under axial loading at frequencies of 95 Hz and 20 kHz were performed to clarify the effect of loading type on fatigue properties of a high strength bearing steel in combination with experimental result of this steel under rotating bending. As a result, this steel represents the single P-S-N (probabilistic-stress-life) curve characteristics for surface-induced fracture and interior inclusion-induced fracture, just like that under rotating bending. However, fatigue strength is lower, where the run-out stress at 10⁹ cycles is evaluated to be 588 MPa, less than that under rotating bending with about 858 MPa. Occurrence probability of larger and deeper inclusion-induced fracture is much higher than that under rotating bending. Furthermore, the formation process of fine granular area (FGA) is independent of the type and frequency of loading, which is very slow and is explained as the crack nucleation process under the special dislocation mechanism. The stress intensity factor range at the front of FGA, ΔK_FGA, is approximately regarded as the threshold value controlling the stable propagation of interior crack. For the control volume of specimen under axial loading, the estimated value of fatigue limit by FGA is similar to experimental run-out stress value at 10⁹ cycles, but that by inclusion is larger. However, the corresponding estimated results under rotating bending are all conservative.     

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.     

低周疲劳下16MnR微孔的裂纹萌生与扩展

采用宏观与微观结合的方法,在疲劳试验机上进行疲劳试验,在光学显微下观察裂纹的起裂与扩展.研究了压力容器用钢16MnR在低周疲劳下微孔(40～200μm)的裂纹萌生与扩展规律.研究表明裂纹的萌生机制:滑移带启裂和疏松带启裂,前者由剪应力起主要作用,后者由正应力起主要作用.而滑移带的局部性和裂纹开叉是低周疲劳下微裂纹的两大典型现象.微观缺陷尺寸、应力水平对疲劳寿命有显著影响,当应力水平较低时,微孔尺寸对寿命的影响明显.而当应力水平较高时(超过屈服极限),孔径对寿命的影响不敏感.在同一应力水平下,微缺陷尺寸存在临界值dt,当d＞dt时,疲劳寿命下降很多.     

Microstructural influences on the high cycle fatigue life dispersion and damage mechanism in a metastable β titanium alloy

In this work,the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al(Ti-7333)alloy is investigated.Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1.Results show that the fatigue strength is closely related to the microstructure features,especially the αp percentage.The Ti-7333 alloy with a lower αp percentage exhibits a higher scatter in fatigue data.The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lower αp percentage.Crack initiation region shows the compound αp/β facets.Faceted αp particles show crystallographic orientation and morphology dependence characteristics.Crack-initiation was accompanied by faceting process across elongated αp particles or multiple adjacent αp particles.These particles generally oriented for basal slip result in near basal facets.Fatigue crack can also initiate at elongated αp particle well oriented for prismaticslip.The β facet is in close correspondence to{110}or{112}plane with high Schmid factor.Based on the fracture observation and FIB-CS analysis,three classes of fatigue-critical microstructural configurations are deduced.A phenomenological model for the formation of αp facet in the bimodal microstructure is proposed.This work provides an insight into the fatigue damage process of the α precipitate strengthened metastable β titanium alloys.     

A model to predict S–N curves for surface and subsurface crack initiations in different environmental media

The influence of environmental media on crack propagation of a structural steel at high-cycle and very-high-cycle fatigue (VHCF) regimes was investigated with the fatigue tests in air, water and 3.5% NaCl aqueous solution. The fatigue strength in water and 3.5% NaCl solution is significantly decreased and the cracking morphology due to different driving forces is presented. A model is proposed to explain the influence of environmental media on fatigue life, which reflects the variation of fatigue life with applied stress, grain size, inclusion size and material yield stress. The model prediction is in good agreement with experimental observations.     

A fast, versatile fatigue post-processor and criteria evaluation

This paper is concerned with the development and use of a general High Cycle Fatigue (HCF) post-processor, FAST, Fatigue AnalysiS Tool. It is used to evaluate and develop HCF criteria. FAST can handle many kinds of criteria and evaluate them with fatigue test results from both standard test specimens and actual components. When a reliable criterion exists, FAST will be used for design purposes. The principles of FAST are outlined and an example is given where seven local multiaxial stress based criteria are evaluated, along with the commercial fatigue post-processor Femfat. The conclusion is that a correct treatment of the volume or stress gradient effect is more important than the choice of criterion.     

Degradation of components and structures by fatigue

This note examines briefly fatigue as a degradation mechanism and how engineers compensate for the fatigue phenomenon. Design is generally based on the stress-lifetime concept, but this concept is not very useful for lifetime assessment of operating components; thus, fatigue crack initiation and the growth of fatigue cracks become important. The numerous factors related to fatigue crack initiation and growth are briefly mentioned, and the importance of including the growth of small fatigue cracks is emphasized through the use of an illustration.     

A fractographic study exploring the relationship between the low cycle fatigue and metallurgical properties of laser metal wire deposited Ti–6Al–4V

Additive manufacturing (AM) has achieved large attention within the aerospace industry mainly because of the possibility to lower the material and the manufacturing cost. For titanium alloys several AM techniques are available today. In the present paper, the focus has been on laser metal wire-deposition of Ti–6Al–4V. Walls were built and low cycle fatigue specimens were cut out in two orientations with respect to the deposition direction. An extensive fractographic evaluation was carried out after testing and the results indicated anisotropic behaviour at low strain ranges. Defects such as pores and lack of fusion (LoF) were observed and related to the fatigue life and specimen orientation. The LoF defects are regarded to have the most detrimental influence on the fatigue life, whilst the effect of pores was not as straightforward. Noteworthy in present study is that one large LoF defect did not influence the fatigue life, which is explained by the prevalence of the LoF defect in relation to the loading direction.     

A general model to estimate life in notches and fretting fatigue

Fatigue in notched specimens and fretting fatigue are two different phenomena but they have in common the existence of a stress gradient. In these cases fatigue life estimation is usually considered as a superposition of an initiation and propagation phase. One of the main problems to estimate the fatigue life is to define the crack length where one phase finishes and the other begins. The model employed in this paper combines both phases without defining a priori the separation between them. The proposed model is applied to uniaxial and multiaxial fatigue in specimens with stress gradient: a group of fretting fatigue tests with spherical and cylindrical contact and another group of tests with notched specimens. The comparison between life estimations and experimental results allows checking the validity of the model in different conditions.