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.     

3D Investigation of Damage During Strain-Controlled Thermomechanical Fatigue of Cast Al–Si Alloys

The strain-controlled thermomechanical fatigue behavior is investigated for three cast near-eutectic Al–Si alloys with different Ni, Cu, and Mg contents. Synchrotron tomography and neutron diffraction experiments are used to correlate 3D microstructural features with damage initiation and evolution. The results show that the alloy with lower Cu, Ni, and Mg concentrations has up to 45% higher thermomechanical fatigue resistance for cooling/heating rates of 5 and 15 K s⁻¹. In addition, this alloy also exhibits damage formation at later stages during thermomechanical fatigue and slower damage accumulation compared to other alloys. This difference in behavior is a consequence of its higher ductility, which is a result of the lower volume fraction and global interconnectivity of the 3D hybrid networks formed by Si and intermetallics and the absence of large primary Si clusters which act as preferred crack initiation sites during the early stages of thermomechanical fatigue.     

Statistical characterization of the geometric properties of particles in 7075‐T6 aluminium alloy

Corrosion in aluminium alloys is initiated and sustained at constituent particles within the metal matrix via a localized galvanic process. These particles are also known to play a critical role in fatigue crack initiation and growth. Consequently, statistical characterization of particle geometrical features is critical when modelling corrosion and fatigue. A key statistic is particle size distribution, which was extensively modelled here via imaging of unstressed and fatigued 7075‐T6 aluminium alloys. Fatigued samples were obtained from the outer wing panels of teardown specimens from retired military aircraft. The purpose of this effort was therefore to analyse extensive sets of particle geometry data obtained via microscopy using advanced multimodal statistical modelling and to appropriately characterize the properties of constituent particles and fatigue cracks found in these specimens. The resulting distribution functions for the underlying modes are assumed to be three‐parameter Weibull distribution functions.     

The effects of loading waveform and microstructure on the fatigue response of titanium aero-engine compressor disk alloys

Microstructural variations produced from manufacturing processes and their influence on fatigue crack growth in titanium disks were investigated. Charpy‐tests on titanium disk material were performed and materials with fracture energy values in the range of 3.8–19.1 J/cm² were selected for tests under cyclic loads. Results of Charpy‐tests were compared with fractographic features related to fatigue crack growth in Ti?6Al?3Mo?0.4Si and Ti?6Al?3Mo?2Cr alloys with a two‐phase (α + β) lamellar structure under various cyclic waveforms using specimens made from compressor disks. The material sensitivity to cyclic load waveform can be seen for in‐service disks using a criteria based on fracture energy values determined in Charpy‐tests. A difference in fatigue crack growth periods of 2.5 times was discovered for specimens made from the disk with a filament type microstructure and the mainly globular two‐phase structure of the Ti?6Al?3Mo?0.4Si alloy. The shorter crack growth period correlated with the mainly facetted pattern formation with local zones of fatigue striations when fatigue crack growth is along the planes of the filaments. Fatigue striations are the major fracture surface relief when crack growth occurs in the perpendicular direction to the plane of the filaments. A quantitative fractographic method for estimating the crack growth period for in‐service failed disks was performed for the case of crack development along planes of such microstructural filaments created during the manufacturing process. Specimen tests involving a hold‐time in the cyclic loads are recommended for in‐service accepted titanium disks using a criteria based on the fracture energy value. Selection of disks based on these criteria can indicate a material sensitivity to cyclic load waveforms.     

Analysis of fatigue crack initiation and S-N response of model cast aluminium piston alloys

Fatigue crack initiation and S-N fatigue behaviour of hipped model Al7Si-Sr and Al0.7Si piston alloys have been investigated after overaging at 260 °C for 100 h to provide a practical simulation of in-service conditions. The results show that hipping did not affect the S-N behaviour of Al7Si-Sr. This is attributed to the lack of significant change in porosity distribution in this alloy because of its low porosity levels even in the unhipped state. However, hipping profoundly improved the fatigue performance of alloy Al0.7Si due to the significant reduction in porosity. In this investigation, it was observed that porosity was rendered impotent as a fatigue crack initiator in both hipped alloys. Instead, fatigue cracks were observed to originate mainly from intermetallic particles (particularly the Al₉FeNi phase) in both alloys and sometimes from oxide particles in Al0.7Si alloy. Fatigue cracking was also frequently observed at intermetallic clusters in hipped Al0.7Si. The observed scatter in fatigue life is discussed in terms of the size of fatigue crack initiating particles and the overall particle size distribution which follows a power law distribution function.     

Fatigue crack propagation in Ti3 Al based alloys

Abstract

Effects of microstructure, stress ratio, and environment on the fatigue crack growth resistance of Ti–23Al–9Nb–2Mo–1Zr–1·2Si and Ti–23Al–11Nb–0·9Si (at.-%) Ti₃ Al based alloys have been studied at room and elevated temperatures. Only modest effects of microstructure on fatigue crack growth resistance have been obtained at room temperature, and these tend to reduce further at the elevated temperatures of 600 and 700°C both in air and in vacuum. At room temperature the fatigue crack growth resistance of Ti₃ Al based alloys is controlled primarily by the thickness of the retained βphase rather than by its volume fraction and the microstructure with a larger average thickness of retained β laths shows improved fatigue crack growth resistance. However, in some microstructures, the spatial distribution of the β phase can also be deduced to be important. A marked difference on crack growth resistance is obtained for stress ratios of 0·1 and 0·5 both at room temperature and at a temperature of 600°C. The mechanisms of fatigue crack growth in air and vacuum are discussed.     

Crack initiation and propagation characteristics of a dual-phase Mg–Li alloy under high-cycle and very-high-cycle fatigue regimes

Ultralight Mg–Li alloys are promising aerospace materials as they are the lightest structural alloys at present; however, their fatigue behaviors remain to be explored. This work focuses on the fatigue strength and crack initiation behavior of an extruded dual-phase Mg–Li alloy (LZ91) under high-cycle and very-high-cycle fatigue regimes. The fatigue limit of LZ91 alloy at 10⁹ cycles was determined to be 78 MPa, and the fatigue ratio is 0.46. Microstructural characterization demonstrates that fatigue cracks tend to initiate at β-Li phase-enriched regions. The α-Mg phase presents a < > fiber texture with a basal plane that has low deformation in the extrusion direction and acts as an enhanced phase in relation to the β-Li phase. Deformation discrepancies cause localized cyclic plasticity at the Li phase that leads to fatigue crack initiation.     

The influence of defect and temperature on the fatigue behaviours of Al‐Si‐Cu‐Mg‐Ni alloy

High‐cycle fatigue (HCF) properties of two Al‐Si‐Cu‐Mg‐Ni alloys with different defect sizes named as alloys A (smaller ones) and B (bigger ones) were investigated at 350°C and 425°C, respectively. The results indicate that fatigue strengths of both alloys decrease as the temperature increases. Fatigue cracks originated from pores and oxide films at both temperatures. They propagated preferentially through cracked matrix at 350°C and debonded interface and grain boundary at 425°C. Alloy A exhibits higher fatigue life and fatigue strength than alloy B at 350°C due to its smaller pore sizes. However, it has slightly worse fatigue properties than alloy B at 425°C because the fatigue crack initiation is controlled by oxide film at this temperature and is not affected by its size. This indicates that there is a transition of predominant initiation site from pores to oxide films when the temperature increases. The fatigue strength estimated through defect size is consistent with the experimental results at 350°C, while unsuitable at 425°C.     

A crystal plasticity based methodology for fatigue crack initiation life prediction in polycrystalline copper

Fatigue failure is the dominant mechanism that governs the failure of components and structures in many engineering applications. In conventional engineering applications due to the design specifications, a significant proportion of the fatigue life is spent in the crack initiation phase. In spite of the large number of works addressing fatigue life modelling, the problem of modelling crack initiation life still remains a major challenge in the scientific and engineering community. In the present work, we present a methodology for estimating fatigue crack initiation life using macroscale loading conditions and the microstructural phenomenon causing crack initiation. Microstructure sensitive modelling is used for predicting potential crack initiation life by employing randomly generated representative microstructures. The microstructural parameters contributing to crack initiation life are identified and accounted for by computing lattice level energy dissipation during fatigue crack initiation. This model is coupled with experimental results to improve the predictive capabilities and identification of potentially damaging weak points in the microstructures. The estimated values for crack initiation life were found to be in good agreement with the experimentally observed values of initiation life. The results have shown that this kind of approach could be successfully used to predict crack initiation life in polycrystalline materials. This work successfully provides an approach for estimating crack initiation life based upon numerical computations accounting for the microstructural phenomenon.     

Ausbreitungsverhalten mikrostrukturell kurzer Ermüdungsrisse – experimentelle Charakterisierung und mechanismenorientierte Simulation

Propagation behaviour of microstructural short fatigue cracks – experimental characterization and mechanism‐based simulation This paper presents results of an interdisciplinary research project, which was undertaken by the authors during the past seven years on the subject of experimental characterization and modelling of microstructurally short fatigue crack growth. Fatigue testing was carried out on the austenitic‐ferritic duplex stainless steel X2 CrNiMoN 22–5‐3. It was shown clearly that microstructural features like grain size, phase distribution or yield stress of the grain containing the crack tip control an advancing short crack. On the basis of these findings, a mechanism‐oriented model was established, which is able to simulate the growth of microstructurally short fatigue cracks in a physically reasonable way. Microstructural parameters like grain size, grain distribution, grain orientation etc. are taken into account by assigning measured values to a modelled microstructure, in which the crack growth simulation takes place. The comparison of experimentally observed and calculated values shows excellent agreement.     

Fatigue crack propagation behaviour of Al–Zn–Ce alloys

This paper presents the investigation on fatigue crack growth behaviour of Al–Zn and Al–Zn–Ce alloys. Fatigue tests were carried out on as‐cast and heat‐treated CT specimens according to ASTM E647 testing standard. The test results showed that the addition of rare earth element (cerium) and heat treatments (T6 and T5) had very strong influence on fatigue strength. This enhancement was due to metallurgical changes in the alloy system. Cerium eliminates the porosities and refines microstructures of the alloy, showing the improved fatigue crack growth behaviour. In addition, the fatigue fractured specimens were examined using a scanning electron microscope to clarify the fracture initiation points.     

Low‐cycle fatigue behavior of a newly developed cast aluminum alloy for automotive applications

The drive for increasing fuel efficiency and decreasing anthropogenic greenhouse effect via lightweighting leads to the development of several new Al alloys. The effect of Mn and Fe addition on the microstructure of Al‐Mg‐Si alloy in as‐cast condition was investigated. The mechanical properties including strain‐controlled low‐cycle fatigue characteristics were evaluated. The microstructure of the as‐cast alloy consisted of globular primary α‐Al phase and characteristic Mg₂Si‐containing eutectic structure, along with Al₈(Fe,Mn)₂Si particles randomly distributed in the matrix. Relative to several commercial alloys including A319 cast alloy, the present alloy exhibited superior tensile properties without trade‐off in elongation and improved fatigue life due to the unique microstructure with fine grains and random textures. The as‐cast alloy possessed yield stress, ultimate tensile strength, and elongation of about 185 MPa, 304 MPa, and 6.3%, respectively. The stress‐strain hysteresis loops were symmetrical and approximately followed Masing behavior. The fatigue life of the as‐cast alloy was attained to be higher than that of several commercial cast and wrought Al alloys. Cyclic hardening occurred at higher strain amplitudes from 0.3% to 0.8%, while cyclic stabilization sustained at lower strain amplitudes of ≤0.2%. Examination of fractured surfaces revealed that fatigue crack initiated from the specimen surface/near‐surface, and crack propagation occurred mainly in the formation of fatigue striations.     

Characteristics of Faigue Crack Initiation in Ti-5Al-4Sn-2Zr-1Mo-0.7Nd-0.25Si Alloy

The characteristics of fatigue crack initiation in Ti-5AI-4Sn-2Zr1Mo-O.7Nd-O.25Si alloy wereStudied. Two modes Of fatigue crack initiation were found. The Nd-rich phase particles displaybetter resistance to fatigue crack initiation than the matrix at lower stress.     

Microstructure‐sensitive fatigue modelling of medical‐grade fine wire

This work presents a modelling methodology to assess the sensitivity to microstructure in high‐cycle fatigue performance of fine wires made from MP35N alloy (35Ni‐35Co‐20Cr‐10Mo in wt%) used as conductors in cardiac leads. The model consists of a microstructure generator that creates a mesh of a statistically representative microstructure, a finite element analysis using a crystal plasticity constitutive model to determine the local response behaviour of the microstructure, and a postprocesser using fatigue indicating parameters to assess the likelihood of fatigue crack initiation. The fatigue crack initiation potency for selected microstructure attributes, boundary and interface conditions, and loading profiles is determined by computing the Fatemi‐Socie fatigue indicating parameter over a physically relevant volume of scale. Case studies are used to investigate (1) the influence of nonmetallic inclusion proximity to the wire surface on fatigue potency and (2) the transition life demarcating lives primarily controlled by fatigue crack initiation versus microcrack fatigue growth.     

Enhancement of mechanical properties of Al–Mg–Si alloys by means of manganese dispersoids

Abstract

The effects of Mn dispersoids on the enhancement of mechanical properties in Al–Mg–Si(–Mn) alloys have been studied to develop a new high Mn alloy which does not need an aging heat treatment after a shaping process (i.e. extrusion process). By adding Mn to Al–Mg–Si alloys, sphere- or rod shaped Mn dispersoids of a size ranging from 0·05 to 0·5 μm are formed by the use of proper heat treatments. The as extruded alloys containing 1·0 wt-%Mn are measured to have higher tensile properties with good ductility, as compared with those of the commercial Al alloy 6N01 (Al–0·69Mg–0·79Si–0·48Cu–0·27Zn–0·37Mn–0·3Cr– 0·11Ti, wt-%). These phenomena are obtained from the dispersion hardening effect and homogeneous deformation by Mn dispersoid particles acting as obstacles to dislocation movement. Comparing the fatigue crack growth behaviour between the high Mn alloys and the commercial 6N01 alloy in the as forged condition, high Mn alloys are shown to have higher fatigue crack growth resistance and show a more tortuous crack path. This result can be explained by the increasing energy absorption through crack deflections and tortuous crack paths by the Mn dispersoids.     

Experimental and numerical study on crack initiation under fretting fatigue loading

Press-fitted railway axles and wheels are subjected to fretting fatigue loading with a potential hazard of crack initiation in press fits. Typically, the resistance against crack initiation and propagation in press fits is investigated in full-scale tests, which procedure is both costly and time consuming. In this context, combined experimental and numerical approaches are of increasing practical importance, as these may reduce the experimental effort and, moreover, provide a basis for the transferability of experimental results to different axle geometries and materials. This study aims at evaluating stress–strain conditions under which fretting fatigue crack initiation is likely to occur. Experiments on small-scale specimens under varying fretting fatigue load parameters and their finite-element modelling to characterize the resulting stress–strain fields are performed. Subsequently, different multiaxial fatigue parameters are applied to predict crack initiation under fretting fatigue conditions.     

Nanograin layer formation at crack initiation region for very‐high‐cycle fatigue of a Ti–6Al–4V alloy

The microstructural features and the fatigue propensities of interior crack initiation region for very‐high‐cycle fatigue (VHCF) of a Ti–6Al–4V alloy were investigated in this paper. Fatigue tests under different stress ratios of R = ?1, ?0.5, ?0.1, 0.1 and 0.5 were conducted by ultrasonic axial cycling. The observations by SEM showed that the crack initiation of VHCF presents a fish‐eye (FiE) morphology containing a rough area (RA), and the FiE and RA are regarded as the characteristic regions for crack initiation of VHCF. Further examinations by TEM revealed that a layer of nanograins exists in the RA for the case of R = ?1, while nanograins do not appear in the FiE outside RA for the case of R = ?1, and in the RA for the case of R = 0.5, which is explained by the Numerous Cyclic Pressing model. In addition, the estimations of the fatigue propensities for interior crack initiation stage of VHCF indicated that the fatigue life consumed by RA takes a dominant part of the total fatigue life and the related crack propagation rate is rather slow.     

Fatigue behaviour prediction of steel reinforcement bars using an adapted Navarro and De Los Rios model

Fatigue cracks tend to initiate on the rebar surface and therefore, the surface conditions may control their fatigue behaviour. This study investigates the influence of surface microstructure and roughness dispersion on the scatter and fatigue life of hot rolled (HR)–cold worked (CW) and quenched and self-tempered (QST) rebars. The stochastic nature of the fatigue life is mainly affected by the scatter of short cracks in the crack initiation phase. A model adapted from Navarro and De Los Rios (N–R) was developed to predict the crack initiation, including short crack growth, and long crack propagation phases. The crack initiation phase includes the dispersion inherent to the grain size, grain orientation ratio and multiple phases i.e., ferrite–pearlite and martensite as well as the roughness dispersion determined on the rebar surface and the influence of the rib geometry. The stress concentration factor due to the rib geometry was considered as a constant parameter. In the long crack propagation phase, all microstructural features are considered as constants. The model results were compared to experimental data from the literature.     

FATIGUE CRACK INITIATION IN ALUMINIUM ALLOYS UNDER PROGRAMMED BLOCK LOADING

Abstract— Tests for fatigue crack initiation were carried out on two different aluminium alloys. Results and analysis of initiation under constant amplitude loading are presented; elastic and elastoplastic analyses are applied. Initiation under programmed block loading is investigated and damage accumulation is discussed. Tests were performed on two notch root radii:5 and 0.5mm. The electric potential method was used to detect fatigue crack initiation. Three point bending tests on smooth specimens were carried out to follow the evolution of damage during the crack initiation phase.     

Crystal plasticity finite‐element modelling of cyclic deformation and crack initiation in a nickel‐based single‐crystal superalloy under low‐cycle fatigue

Nickel‐based single‐crystal superalloys are predominantly used for turbine blades in aircraft engines and land‐based gas turbines. Understanding and predicting the fatigue failure of Ni‐based single‐crystal superalloys are critical to ensure the safety of these components during operation. In this paper, low‐cycle fatigue experiments were carried out to investigate cyclic deformation of a nickel‐based single‐crystal superalloy MD2, recently developed by GE Power, with different crystallographic orientations. Specialty in situ scanning electron microscope (SEM) tests were also conducted to study the slip‐controlled initiation of short cracks under low‐cycle fatigue. In particular, the stress–strain response for both [001] and [111] orientations was used to calibrate a crystal plasticity model, which allowed us to simulate the activation of crystallographic slip systems and predict the initiation of short fatigue crack. Using the accumulated shear strain as a criterion, the simulations confirmed that the slip system with the maximum accumulated shear strain appeared to control the crack initiation. The location and direction of slip traces and short cracks, captured by the crystal plasticity finite‐element simulations, agreed with the in situ SEM observations. The modelling tool will be valuable for assessing the structural integrity of critical gas turbine blades.