Static mode fatigue crack propagation and generalized stress intensity correlation for fatigue–brittle polymers

Stable fatigue crack propagation is predominantly described by the Paris power law correlation of the crack growth rate with the amplitude cyclic stress intensity. The Paris relationship works well for most ductile materials but does not capture the response for fatigue–brittle materials lacking a cyclic damage mechanism, including ceramics and many polymers. Instead, crack growth rate of fatigue–brittle materials correlates to the peak cyclic stress intensity factor, \(\hbox {K}_{\mathrm{max}}\). This work shows that \(\hbox {K}_{\mathrm{max}}\) correlation of fatigue crack growth is derived directly from static mode crack tip behavior with constant correlation coefficients, and that \(\Delta \hbox {K}\) correlations are not generally applicable for static mode crack propagation in fatigue–brittle polymers. This derivation predicts load ratio, frequency, and waveform effects, which are included in a general static mode fatigue crack propagation law. Fatigue crack propagation data of a known fatigue–brittle polymer are presented to demonstrate static mode crack propagation behavior correlation with \(\hbox {K}_{\mathrm{max}}\) with constant parameters.     

A cohesive zone model for fatigue and creep–fatigue crack growth in single crystal superalloys

A numerical analysis using cohesive zone model under cyclic loading is proposed to develop a coupled predictive approach of crack growth in single crystal. The process of material damage during fatigue crack growth is described using an irreversible cohesive zone model, which governs the separation of the crack flanks and eventually leads to the formation of free surfaces. The cohesive zone element is modeled to accumulate fatigue damage during loadings and no damage during unloadings. This paper presents the damage model and its application in the study of the crack growth for precracked specimens. The use of cohesive zone approach is validated through a convergence study. Then, a general procedure of parameters calibration is presented in pure fatigue crack growth. In the last section, an extension of the cohesive zone model is presented in the case of creep–fatigue regime at high temperature. The model showed its capability to predict with a good agreement the crack growth in the case of complex loading and complex specimen geometries.     

Low cycle fatigue and creep–fatigue interaction behaviour of 2.25Cr1MoV steel at elevated temperature

This study reports the fatigue behaviour of 2.25Cr1MoV steel under low cycle fatigue (LCF) loading and creep-fatigue interaction (CFI) loading at 355, 455 and 555 °C. Various hold durations up to 600 s were introduced in the CFI tests at the peak/valley strain under strain or stress control. In LCF tests, the steel exhibited remarkable strengthening at 455 °C, which can be ascribed to the effect of dynamic strain aging. In CFI tests, tensile holds were found more damaging than compressive holds but considerably less harmful than the combined tensile-compressive holds. A modified plastic strain energy approach based on the damage mechanisms was proposed to predict fatigue life under LCF and CFI conditions. The predictions obtained compared very favourably with the experimental results.     

Tension–tension fatigue of hybrid composite rods

The tension–tension fatigue behavior was investigated for a hybrid composite rod comprised of a unidirectional carbon fiber core and a glass fiber shell. Fatigue tests were performed at three R-ratios and four maximum applied stress levels (MAS) while recording the secant modulus at each cycle, and acoustic emission (AE) sensors were employed to monitor the activation of fatigue mechanisms. Fatigue failure occurred when the composite rod was no longer able to support the applied cyclic load. For a MAS level of 70% of the ultimate tensile stress (UTS), composite rods tested at higher R-ratios showed AE activity through a larger percentage of fatigue life, but exhibited a greater resistance to fatigue failure, whereas samples cycled at lower R-ratios displayed AE activity only near the end of fatigue life, and showed a lower resistance to fatigue failure. The hybrid composite showed modes of progressive fatigue damage at high R-ratios and low strain amplitudes in the form of longitudinal splitting of the GF shell. In contrast, failure of the CF core was catastrophic and non-progressive. The fatigue resistance and damage mechanisms of the composite rod were dependent on the MAS level and R-ratio. Fatigue cracks initiated because of fretting between the GF shell and grip surface, which led to the observed longitudinal splitting of the GF shell. Fatigue damage occurred along the GF/CF interface where non-uniform strains developed because of the clamping force of the grip on the GF surface. At an R-ratio of 0.85, a fatigue stress of 70% UTS caused catastrophic fatigue failure, while at lower stresses, composite rods did not fail and withstood cyclic loads up to 1 million cycles. The research conducted is the first to investigate the degradation in fatigue performance arising from grip/composite rod interactions and suggests that the results from the study provide new information for composite materials in industries that utilize unidirectional composites in cylindrical form.     

Fretting fatigue studies on carbon fibre/epoxy resin laminates: III—Microscopy of fretting fatigue failure mechanisms

With increasing application of carbon fibre reinforced plastic laminates (CFRP) the fretting fatigue properties become more and more important. In the case of fretting against the load-bearing 0°-fibres a reduction in fatigue life of about three orders of magnitude can be observed as a result of wear of the fretting pads on the carbon fibres. However, fretting against ±45°-plies protects the 0°-fibres and causes no reduction in fatigue life by comparison with plain fatigue.     

Three-dimensional fatigue cracking under elastic–plastic deformation

Aircraft engine structures can contain small cracks, which have developed from defects induced during material processing. Advanced structural materials such as nickel-based superalloys undergo extensive plastic deformation prior to failure, therefore, these small cracks can be subjected to localized damage with significant amount of plasticity. We have conducted a combined computational/experimental study of fatigue crack growth at room temperature and at 260°C. The experimental results have been correlated with three-dimensional finite element calculations. Material constitutive equations and a computational procedure to calculate energy release rate along the crack front are developed. It is shown that the fatigue crack growth rate is related to a power function of J_max.     

ΔK ol level effects on fatigue crack propagation

In order to determine the effects of K _ol level on fatigue life, a single peak load was applied at distinct K levels of 7.8×10.3 and 9.8×10³ p.s.i. in^1/2. Here the K _ol level was defined to be a K level at which overload was applied. Three different overload ratios of 1.5, 2.0, and 2.5 were used to determine the overload ratio effect on the recovery factor. The result showed that the recovery factor, Z, was linearly related to K as Z = qK+Z _o, where q was a function of overload ratio. The value of q decreased as the overload ratio increased in a given K _ol level and seemed to be an important factor as well as retardation cycles in determining the fatigue life. For the same overload ratio, specimens that underwent overload at a smaller K _ol level showed more improved fatigue life.Nomenclature a Crack length - a ^* Overload affected zone size - B Specimen thickness - (da/dN)_ca Crack growth rate due to constant amplitude fatigue load - (da/dN)_ol Crack growth rate after overload is applied - E Young's modulus - K Stress intensity factor - K _min Minimum stress intensity factor - K _max Maximum stress intensity factor - K _ol K level at which overload is applied - N Number of cycles - N _D Number of delayed cycles - N _f Number of cycles needed for a specimen to be completely fractured - r _p Assumed plastic zone size - S Load - _ys Yield stress - W Width - Z Recovery factor     

Low cycle fatigue resistance of Al–Li alloys

Abstract

Low cycle fatigue (LCF) resistance data from binary Al–Li, ternary Al–Li–Cu, and quaternary Al–Li–Cu–Mg alloys have been compiled and discussed. The LCF resistance is measured in terms of the variation of the number of reversals to failure 2N _fwith the plastic strain amplitude Δ? _p /2 as well as a modified average plastic strain energy per cycle (ΔW _p )_modified , obtained at different applied total strain amplitudes (Δ? _t /2). The data show the effects of microstructural features, namely dominant strengthening precipitates and the degree of recrystallisation as well as crystallographic texture. Lithium content, when in excess of 2·5 wt-%in aluminium decreases the low cycle fatigue resistance the most. The degree of aging, the degree of recrystallisation, and the degree of texture also influence the LCF resistance; among which the effect of the degree of aging is the most pronounced. The effects of lithium content in aluminium solid solution and strengthening precipitates obtainable by the change in the Li/Cu ratio are found to be marginal. Based on a modified total cyclic plastic strain energy till fracture, it is shown that most of the Al–Li alloys exhibit degradation in their LCF resistance in both hypotransition (higher fatigue lives) and hypertransition (lower fatigue lives) regions. Such degradation is attributed to the combined effects of mechanical fatigue, strain localisation through dislocation–precipitate interaction, environmental effects, and finally strain localisation through the high angle grain boundaries. In comparison with the currently used 2XXX and 7XXX series aluminium alloys, Al–Li alloys require substantial improvement before they can be considered for fatigue critical applications.     

Creep–fatigue crack development from short crack starters

Abstract

A series of isothermal strain controlled creep–fatigue tests on fully instrumented cylindrical specimens with shallow chordal crack starters has been conducted for an advanced 9%Cr turbine rotor steel at 600 and 625°C. Cyclic/hold wave shapes involving a dwell period at peak strain in tension or compression were also performed with crack development being monitored by means of electrical potential drop instrumentation. It is found that temperature, total strain range and hold period are the most influential factors on short creep–fatigue crack propagation rates and specimen life. In order to establish a reliable relationship to represent subcritical crack development for high temperature component integrity assessment, the effectiveness of candidate correlating parameters such as cyclic strain range, cyclic J integral and strain energy density factor have been evaluated. Their application to circumstances involving short crack development due to fatigue, and interacting and non-interacting creep loading are evaluated with reference to the evidence determined from post-test metallurgical examination.     

High temperature fatigue of discontinuously-reinforced metal–matrix composites

This paper reviews the current knowledge on the fatigue behavior of discontinuously-reinforced metal–matrix composites at high temperature. The effect of cyclic loading at high temperature on the micromechanims of deformation, crack nucleation, and crack propagation are dealt with. The overall performance of these composites under isothermal and thermo-mechanical fatigue loading have been examined. A brief account of the current industrial applications of discontinuously-reinforced metal–matrix composites in components subjected to fatigue at high temperature is provided     

Post-impact fatigue damage growth in fiber–metal laminates

Fiber–metal laminates (FMLs) are a family of hybrid materials currently being considered for use in airframe structural applications. Post-impact fatigue strength tests were carried out on several varieties of GLAss REinforced (GLARE) aluminum laminates. The panels were impacted in a drop weight impact tower located at the Institute for Aerospace Research of the National Research Council of Canada. Observations made by other researchers that the internal impact damage in FMLs is confined to the immediate impact site were confirmed. The impacted specimens were cycled in tension–tension fatigue until failure. Cracks developed along side the dent and also at the edges of the gauge section of the specimen. Aluminum baseline specimens had significantly lower fatigue lives than the FML specimens. The stress-state surrounding the dent is complicated and contributed to unusual fatigue crack initiation behavior in some GLARE variants.     

Environmental effect on fatigue strength of stainless steel in PWR primary water – Role of crack growth acceleration in fatigue life reduction

The influence of the pressurized water reactor (PWR) water environment on fatigue life and fatigue crack growth rate was discussed. The fatigue lives of Type 316 stainless steel in the PWR water environment were investigated using cylindrical hollow specimens. The acceleration in the crack growth due to the environment was quantified by investigating spacing of striations and crack growth tests using compact tension specimens. The growth rates obtained could be represented by the strain intensity factor. It was shown that the fatigue lives estimated by crack growth prediction agreed with those obtained by the tests. Then, it was concluded that the reduction in the fatigue life due to the PWR water environment was brought about not by enhancement of crack initiation but by the acceleration of the crack growth.     

Testing procedures for fatigue crack propagation and the ΔKeffconcept

Two different procedures are available for the experimental determination of fatigue crack propagation (FCP) velocities da/dN as a function of the loading parameters K. The first procedure is the standardized method in accordance with ASTM E 647 [1] and the second procedure is the so-called K_max-constant method. Both procedures are equivalent, meaning that under the same loading conditions (K, K_max, R) the same FCP velocity (da/dN) is measured. But, the ASTM E 647 method emphasizes the effects of closure (contact of fracture surfaces) in the low K and low K_max regime. It is shown for Al 7075–T7351, the Ni-base alloy Nicrofer 5219 Nb (annealed), the Ti-alloys Ti 6Al 4V (annealed), and Ti 6Al 6V 2Sn that K_eff is the sole driving parameter for FCP.Published in Problemy Prochnosti, No. 7, pp. 13–30, July, 1995.     

Cyclic deformation and fatigue behaviour of α-iron mono-and polycrystals

The reported studies are based on a series of cyclic deformation tests that were conducted at room temperature on decarburized high-purity -iron specimens in mono-and polycrystalline form. The experimental data cover plastic strain ranges _pl in the regime 10^–4 _pl 10^–2 and variations in cyclic plastic strain rates _pl between 10^-5 and 10^–2 s^–1. In the case of single crystals, the effect of solute carbon (30 wt. ppm) was investigated as well. The mechanical data were supplemented by detailed studies of the dislocation arrangements by transmission electron microscopy and of the surface patterns by scanning electron and optical microscopy.Detailed accounts are given of the following topics: cyclic hardening and saturation, dislocation mechanisms, shape changes due to asymmetric slip of serew dislocations, cyclic stress-strain response and fatigue crack initiation.Under conventional conditions of high _pl (10^–4 s^–1) the fatigue behaviour of -iron at room temperature reflects the low mobility of the screw dislocations which is characteristic of the lowttemperature mode of deformation of body-centred cubic (b.c.c.) metals. As a consequence the behaviour exhibits significant differences with respect to that of fatigued face-centred cubic (f.c.c.) metals such as: strongly impeded dislocation multiplication below _pl 5 × 10^–4, appreciable secondary slip at higher _pl leading to a cell structure (persistent slip bands do not form), shape changes due to asymmetric slip of screw dislocations and a relatively high effective stress level.The reduction of _pl and the presence of solute carbon atoms modify this behaviour significantly, making it more similar to that of f.c.c. metals. In all cases it was found that only the athermal component of the peak (saturation) stress but not the latter itself represents a suitable measure of the properties of the dislocation substructure.On the basis of the cyclic deformation behaviour and of observations of trans-and intergranular fatigue crack initiation it was concluded that the fatigue limit of -iron is an intrinsic property of the b.c.c. structure whose characteristics, however, are affected sensitively by interstitial impurity content and by the strain rate of the fatigue test.

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Résumé Les résultats rapportés dans le mémoire sont basés sur une série d'essais de déformation cyclique qui ont été conduits à température ambiante sur des échantillons de fer décarburés à haute pureté sous une forme mono- et polycristalline. Les données expérimentales couvrent les amplitudes de déformation plastique _pl correspondant à 10^–4 _pl 10^–2 ainsi que des variations dans la vitesse de déformation plastique comprises entre 10^-5 et 10^-2 s^-1. Dans le cas de cristaux simples, on a également étudié les effects du C en solution (30 ppm). Les données mécaniques ont été complétées par des études détaillées des arrangements des dislocations, en utilisant la microscopie électronique à transmission, ainsi que les aspects des surfaces en utilisant la microscopie optique et la microscopie électronique à balayage.On a traité dans le détail les sujets suivants: accroissement cyclique et saturation, mécanisme de dislocation, modification de forme associée à des glissements assymétriques de dislocation vis, réponse cyclique contrainte/dilatation et amorcage de la fissure de fatigue.Sous les conditions conventionnelles de haute vitesse de déformation plastique (supérieure à 10^-4 s^-1) le comportement en fatigue du fer à la température ambiante rend compte de la faible mobilité des dislocations vis, ce qui est caractéristique d'un mode de déformation à basse température des métaux cubiques centrés. En conséquence, le comportement fait état de différences significatives par rapport aux métaux cubiques faces centrés soumis à fatigue tels que: une intense multiplication des dislocations en-dessous _pl 5 × 10^–4, un glissement secondaire appréciable pour des valeurs de _pl supérieures qui conduit à une structure en cellules (c.à.d que des bandes de glissement persistantes ne se forment point), des modifications de forme dues à des glissements assymétriques de dislocation vis, ainsi qu'un niveau de contrainte effective relativement élevé.La réduction de la vitesse de déformation plastique et la presence d'atomes de C en solution modifient ce comportement de manière significative, et le rapprochent davantage de celui des métaux cubiques centrés. Dans tous les cas, on a prouvé que seule la composante athermique de la contrainte de saturation, à défaut de cette dernière, représente une mesure appropriée des propriétés des substructures de dislocation.Sur base du comportement à la déformation cyclique et des observations d'amorcage de fissure de fatigue trans-et intergranulaire, on a conclu que la limite de fatigue du fer est une propriété intrinsèque d'une structure cubique centrée dont les caractéristiques toutefois, se trouvent être sensiblement affectées par la teneur en impuretés intersticielles et par la vitesse de déformation de l'essai de fatigue.

     

Changes in bone’s micromechanical properties caused by fatigue fracture

The role of bone fatigue damage at the nanostructural level, and its effect on fatigue properties is an understudied and important subject. In this study, nanoindentation was used to probe the micro-mechanical properties of bovine tibiae subjected to fatigue loading in four-point bending. Indentation tests were conducted in the same 30 μm?×?120 μm region before fatigue loading and after loading to fracture. Using an optical microscope and scanning electron microscope, the morphology of the initial residual indentation before fatigue loading appeared the same as that after loading to fracture. The mechanical properties calculated using nanoindentation were reduced modulus and hardness, the time constant based on creep, long-term creep viscosity and the dissipated work. Differences of each parameter before loading and post fracture were examined using paired t-tests. The results show that the reduced modulus decreased significantly (p?=?9.47?×?10^–3) by 7.62–15.16% after fracture whereas the time constant of creep increased slightly (p?=?0.049) by 2.81–5.41%. There was no clear change in hardness or dissipated work with fatigue loading. Fatigue loading has the largest effect on bone’s reduced modulus compared to all other mechanical properties.

     

Combined optical–acoustic microscopy for investigating short fatigue crack propagation

Abstract

The acoustic microscope has the advantage of being able to detect microstructurally short cracks and to locate crack tips exactly. However, due to scattering of the sound waves and fringe interference effects, the surface texture relationship with the crack tip is not so clear. The optical polarising microscope on the other hand can clearly exhibit surface texture following a specimen surface treatment such as anodising of an Al–Li alloy. However anodising produces a thin layer of a brittle oxide on the surface of the specimen which will reduce the accuracy of the acoustic microscope in exactly determining the crack tip position. Whereas the acoustic microscope does not need any treatment of the specimen surface, and so does not affect the material properties, some materials such as carbon steels and anodised aluminium alloys can be very sensitive to corrosion pitting due to the lens water couplant. It follows that both of these microscopes have complementary advantages and disadvantages and therefore combining both microscopes in the same facility can permit more data to be gathered on the behaviour of very small fatigue cracks and their interactions with microstructural barriers. The present paper reports on these developments using the SIRIUS acoustic microscope facility.

MST/2048     

Multiaxial fatigue assessment of welded joints – Recommendations for design codes

In the assessment of welded joints submitted to multiaxial loading the calculations method applied, independently of the concept (nominal, structural, hot-spot or local), must consider primarily the materials ductility. While proportional loading can be assessed by von Mises, the principal stress hypothesis, the Findley method or the Gough–Pollard relationship, using any of the mentioned concepts, difficulties occur when the loading is non-proportional, i.e. the principal stress (strain) direction changes. This causes a significant fatigue life reduction for ductile steel welds, but an indifferent behaviour for semi-ductile aluminium welds. This different response to non-proportional loading can be assessed when ductility related mechanisms of fatigue failures, i.e. the mean value of plane oriented shear stresses for ductile materials and a combination of shear and normal stresses for semi-ductile materials, are properly considered.However, as these methods require a good expertise in multiaxial fatigue, for design codes used by non-fatigue experts, simpler but sound calculation methodologies are required. The evaluation of known fatigue data obtained with multiaxial constant and variable amplitude (spectrum) loading in the range N > 10⁴ cycles suggests the application of the modified interaction algorithm of Gough–Pollard. In the case of variable amplitude loading, constant normal and shear stresses are replaced by modified reference normal and shear stresses of the particular spectrum. The modification of the reference stresses is based on the consideration of the real Palmgren–Miner damage sum of D_PM = 0.5 (for spectra with constant mean loads) and the modification of the Gough–Pollard algorithm by consideration of the multiaxial damage parameter D_MA = 1.0 or 0.5, which is dependent on the material’s ductility and on whether the multiaxial loading is proportional or non-proportional. This method is already part of the IIW-recommendations for the fatigue design of welded joints and can also be applied by using hot-spot or local stresses.     

Beyond HCF – Is there a fatigue limit?

The paper gives an overview to the present state of research on fatigue strength and failure mechanisms at very high number of cycles (N>10⁷). Testing facilities are listed. A classification of materials with typical S‐N curves and influencing factors like notches, residual stresses and environment are given. Different failure mechanisms, which occur especially in the VHCF‐region like subsurface failure, are explained. There microstructural inhomogeneities and statistical conditions play an important role. Investigated materials are different metals with body‐centred cubic lattice like low‐ or high‐strength steels and quenched and tempered steels but also materials with a face‐centred cubic lattice like aluminium alloys and copper.     

Recent progress in the high-cycle fatigue behaviour of γ-TiAl alloys

The high-cycle fatigue (HCF) properties of γ-TiAl (gamma titanium aluminide) alloys are reviewed, particularly with regards to the deformation mechanisms active in the near-threshold cyclic loading regime. By examining the influence of lamellar orientation and thickness on the HCF threshold, in addition to more conventional microstructural considerations such as the grain size or the volume fraction of lamellar colonies, factors to improve the γ-TiAl microstructure for HCF are assessed. Finally, experimental methods and loading strategies are surveyed to identify techniques for improving HCF testing of γ-TiAl alloys. In this, we consider both the conservativism of differing approaches and the possibility to measure with suitable resolution the local mechanical behaviour under HCF of the lamellar γ-TiAl microstructure.

This review was submitted as part of the 2018 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Effect of temperature on high-cycle fatigue and very high cycle fatigue behaviours of a low-strength Cr–Ni–Mo–V steel welded joint

Axially push–pull fatigue tests of a low-strength Cr–Ni–Mo–V steel welded joint were conducted up to very high cycle fatigue regime at room temperature and 370 °C. The S–N curve at room temperature shows a duplex shape, while the S–N curve at 370 °C is continuously decreasing with lower fatigue strength. The welds at 370 °C undergoes dynamic strain ageing and has an enhanced load–defects interaction, leading to equal distribution of failures among different parts of the welds. The Z parameter model, with micro-defect location incorporated, having sound physical representation, is life-controlling of the welds at high temperature.