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     

Elevated temperature short crack fatigue behaviour in near eutectic Al–Si alloys

This paper considers two candidate automotive piston alloys and highlights the influence of microstructural features on fatigue behaviour. Fatigue initiation and subsequent short crack growth was assessed at 20, 200 and 350 °C. It is shown that both temperature and test frequency have a strong influence on the fatigue performance of the materials tested. The microstructure was quantitatively characterised in terms of the primary Si distribution. Together with post failure analysis, this allowed identification of critical microstructural features affecting both fatigue crack initiation and early growth. Large primary Si particles were found to act as preferential initiation sites by cracking or decohesion (dependent on test temperature) and are also sought out preferentially during short crack growth.     

Creep–fatigue crack growth of intermetallic compound Ni3 Al(B) at elevated temperatures

Abstract

The creep–fatigue crack growth of Ni₃ Al(B) alloy was investigated at elevated temperatures in air under four different loading waveforms. Two types of time dependent damage mechanisms have been identified: oxidation and creep effects. As compared with fatigue crack growth in air at room temperature, the effect of oxidation at the crack tip on the crack growth acceleration is significant. Creep effects, on the other hand, are dominant for tensile holding and slow–fast loading waveforms. The complicated interaction between creep–fatigue, oxidation induced embrittlement, and oxide induced crack closure determined the different fatigue crack growth behaviours for different loading waveforms at elevated temperature. The relationship between the constants C and m in the Paris formula and loading waveform were examined, and the influence of loading waveform on the crack propagation were also discussed.     

Creep–fatigue crack growth behaviour based on EBSD observations for notched specimen made of directionally solidified Ni-base superalloy

Abstract

In order to characterise the creep–fatigue crack growth behaviour of a directionally solidified Ni-base superalloy, an investigation into damage behaviour based on the electron backscattered diffraction (EBSD) method using notched specimens has been carried out. The average misorientation in the vicinity of notches increases linearly up to the initiation of cracks with the increasing creep strains under creep or creep–fatigue conditions, whereas under fatigue conditions fatigue cracks grew without an increase in misorientation. The stress holding time clearly influenced the growth behaviour of creep–fatigue cracks and the appearance of misorientation development. However, it was shown that the relationship between the average misorientation and the relative notch opening displacement was independent of test conditions such as temperatures, stresses and stress wave forms.

It is concluded that the misorientation analysis of damaged samples based on the EBSD method allows the prediction of the initiation of creep–fatigue cracks and their growth behaviours.     

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.     

Combined cycle fatigue of 7075 aluminum alloy – Fracture surface characterization and short crack propagation

Aim of this study is an interpretation of the influence of variable-amplitude (VA) cycles superimposed to low-frequency loads on fatigue life of 7075-T651 Al-alloys. Constant-amplitude (CA) 20 kHz stress/strain-life (S–N) and (ε–N)-curves with and without superimposed mean loads serve as basis. For combined fatigue loading, life-time measurements were performed. Life-time estimations based on the S–N results reveal a damaging effect of the superimposed ultrasonic vibrations in the high cycle fatigue (HCF) and the very high cycle fatigue (VHCF) regimes. The CA and VA-life time results are correlated with fractographic observations. An interpretation of fatigue lives under combined low and high-frequency VA-loading is proposed considering small/short-crack propagation and arrest mechanisms.     

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.     

Internal fatigue crack initiation in drawn Ti–6Al–4V wires

Fatigue cracks in titanium alloys are often found to initiate at faceted alpha grains. In the very high cycle fatigue regime, crack initiation tends to shift from the surface towards the interior of the material, and more initiation facets can be found on the fracture surface. In this study, fatigue tests were performed on drawn and heat-treated Ti–6Al–4V wires. Only a few samples fractured due to interior initiation. The facets at the initiation sites of these samples were not flat, but had markings on the nano-scale, and were highly inclined. A possible explanation for these aspects is the crystallographic texture of the wire, and a reflection is made on the suggested mechanisms of facet formation.

This paper is part of a Themed Issue on Euromech 570: Interface-dominated materials.     

On the fatigue crack growth–microstructure relationship in ultrafine-grained interstitial-free steel

The crack growth behavior in an ultrafine-grained (UFG) interstitial-free (IF) steel processed by equal channel angular pressing (ECAP) was investigated utilizing miniaturized compact-tension specimens with different microstructural characteristics. The current results demonstrate that both the ECAP processing route and the direction of crack growth with respect to the extrusion direction dictate the crack growth behavior in UFG IF steel. Specifically, the highest crack growth rates and the lowest threshold values were observed for the lowest grain size. Moreover, an unusual deviation from the expected direction of crack expansion was observed, where the deviation depended on the processing route and direction of crack growth. This deviation is attributed to the presence of elongated structures in the microstructure, which were mainly detectable in the UFG IF steel following a small number of pressings, and to a smaller extent in the optimized microstructures. Specifically, these elongated structures formed parallel to the material’s plastic flow during ECAP processing and moved the crack away from the expected direction of growth due to the high stress concentration zones they created along with the process-induced damages.     

The fatigue damage development in a cast Al–Si–Cu alloy

The experimental identification of fatigue damage mechanisms and evaluation of their development rate, based on changes in material respond on cycle loading, has been presented in the work. The research has been conducted on hyper-eutectic cast alloy AlSi8Cu3. The microstructure and fracture analyses were performed. The high cycle fatigue tests were conducted with frequency of 20 Hz under constant nominal stress amplitude with monitoring the strain response of material during the test. The ratcheting was found as the main mechanism of the fatigue damage. It was established that the linear fatigue accumulation law should not be used for fatigue life prediction in case of the tested cast aluminum alloy.     

Relationships between microstructure and fatigue crack propagation paths in Al–Si–Mg cast alloys

Fatigue crack propagation of long and small cracks was investigated for hypoeutectic and eutectic Al–Si–Mg cast alloys. Crack propagation behavior in the near-threshold regime and Regions II and III was related to microstructural constituents namely primary α-Al dendrites and volume fraction and morphology of eutectic Si. Long crack thresholds reflect combined closure effects of global residual stress and microstructure/roughness. The small crack threshold behavior is explained through closure independent mechanisms, specifically through the barrier effects of characteristic microstructural features specific to each alloy. In Regions II and III changes in fracture surface roughness are associated with different crack propagation mechanisms at the microstructure scale. The extent of the plastic zone ahead of the crack tip was successfully used to explain the observed changes in crack propagation mechanisms.     

Elastic–plastic material response of fatigue crack surface profiles due to overload interactions

The deformation caused by single and periodic overloads on the crack surface profile is studied using finite element fatigue crack closure simulations in a material with linear kinematic hardening. Differential surface profiles (difference of crack surface displacements before and after overloads), Δu_y, are found useful in understanding the role and the interaction between overloads. Three parameters, ΔK_OL/ΔK, ΔK and R, are found necessary to characterize deformation response of a single overload on the crack surface profile. The simulation procedure and results are discussed based on experimental and numerical studies reported in literature on overload interactions.The deformation occurred on the crack surface due to an applied single overload (hump) inhibits reversed plastic deformations by acting like a spring. Therefore, a second single overload leads to a larger deformation response even if this second overload is applied outside the overload plastic zone of the first single overload. This second deformation response is found equivalent to the response of a single overload with a higher K_min value.     

Environmental embrittlement of Al–Sn alloys and its implication for fatigue crack growth

Abstract

The environmental embrittlement of a series of Al–Sn alloys has been studied using the technique of slow strain rate testing in laboratory air, in a packing of anhydrous magnesium perchlorate, and in a salt solution (2%NaCl–0·5%Na₂CrO₄ buffered to pH 3·5). The embrittlement occurring at slow strain rates is attributed to reversible hydrogen embrittlement. Only the alloy containing a continuous network of tin phase suffers a reduction in ductility. This is attributed to an increase in hydrogen diffusivity which is demonstrated through a series of permeation tests. These results are then used to interpret fatigue crack growth tests conducted as a function of environment in the absence of closure.

MST/744     

Enhanced fatigue crack propagation resistance of Al–4.1Cu–1.1Li alloy by interrupted aging

Journal of Materials Science - The effects of the T6I6 interrupted aging on the fatigue crack propagation (FCP) behavior, mechanical properties, and microstructure of an Al-4.1Cu-1.1Li alloy were...     

Creep–fatigue interaction in two gas turbine Ni based superalloys subjected to service-like conditions

Abstract

Creep–fatigue interaction has been studied in single crystal and equiaxed Ni based superalloys, adopted for critical gas turbine component applications. Cyclic hold tests have been performed to understand the influence of creep damage and deformation on fatigue endurance, considering also the effect of the position of the hold time in the low cycle fatigue cycle. Service-like thermomechanical fatigue (TMF) benchmark tests have been carried out, involving TMF cycles based on the loading conditions at component critical locations determined by finite element (FE) simulation. Damage calculations have been performed on all the conducted tests for both materials, comparing results obtained by different methodologies (e.g. time fraction, ductility exhaustion, strain energy density). The results have been compared with actual in-service damage revealed by microstructural examination.     

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     

Multi-scale fatigue behaviour modelling of Al–Al2O3 short fibre composites

Using very heterogeneous materials in structural parts submitted to cyclic loadings, this paper presents an elasto-plastic micromechanical model. After recalling the homogenisation principle based on a mean field theory, non-linear kinematic and isotropic strain hardening is introduced into the matrix. Validation is made on an Al–3.5%Cu/SiC particle composite, and an Al–Si7Mg/Al₂O₃ fibre composite is treated as a first application. Damage is introduced into the model using a fibre failure criterion. It is based on the evolution of the volume fraction of broken fibres as a function of the maximum principal stress in the fibre family. The damage law is identified by means of in situ tensile tests performed inside the scanning electronic microscope. The number of broken fibres is determined as a function of the applied load and the number of cycles. The model predicts the fatigue behaviour, the loss of stiffness, the volume fraction of broken fibres for different volume fractions, aspect ratios, distributions of orientation and distributions of strength of the fibres. The effect of the mechanical fatigue properties of the matrix is also studied.