Fatigue of commercial aluminium alloys

Fatigue crack propagation has been observed for a number of commercial aluminium alloys. Comparable data was obtained for a variety of crack and specimen geometries over a range of crack lengths for a given alloy. Where crack propagation only was of interest the initiation event has been excluded by pre-cracking the specimen using a fin of material adjacent to the crack face. By this method a controlled defect size is introduced in to the specimen. By modification of the D.C. potential drop method it has been shown possible to measure the growth of cracking from 0.12mm by this method. Crack growth from defects greater than 0.6mm have been shown to give conventional crack propagation deduced by principle of similitude. Fatigue fracture surface analysis has been conducted for cracking from both free surfaces and from blunt notches. A `quasi cleavage' feature has been identified and is shown to be prominent when the fatigue stress intensity range is below 10 MNm-3/2.

Intrinsic thresholds in polycrystalline Udimet 720

The long crack threshold behaviour of polycrystalline Udimet 720 has been investigated. Faceted crack growth is seen near threshold when the monotonic crack tip plastic zone is contained within the coarsest grain size. At very high load ratios R (=P min/P max) it is possiblefor the monotonic crack tip plastic zone to exceed the coarsest grain size throughout the entire crack growth regime and non1aceted structure insensitive crack growth is then seen down to threshold. Intrinsic threshold values were obtained for non1aceted and faceted crack growth using a constant K max, increasing K min, computer controlled load shedding technique (K is stress intensity factor). Very high R values are obtained at threshold using this technique (0.75-0.95), eliminating closure effects, so the intrinsic resistance of the material to crack propagation is reflected in these values. The intrinsic non1aceted threshold value ΔK th is lower (2.3 MN m -3/2) than the intrinsicfaceted ΔK th value (4.8 MN m -3/2). This is thought to reflect not only the effect of crack branching and deflection (in the faceted case) on the crack driving force, but also the inherent difference in resistance of the material to the two different crack propagation micromechanisms. © 1993 The Institute of Materials.

Improvement of the mechanical properties of 2124 Al/SiCp MMC plate by optimisation of the solution treatment

Static mechanical properties of 2124 Al/SiCp MMC have been measured as a function of solution temperature and time. An optimum solution treatment has been established which produces significant improvements in static mechanical properties and fatigue crack growth resistance over conventional solution treatments. Increasing the solution treatment parameters up to the optimum values improves the mechanical properties because of intermetallic dissolution, improved solute and GPB zone strengthening and increased matrix dislocation density. Increasing the solution treatment parameters beyond the optimum values results in a rapid reduction in mechanical properties due to the formation of gas porosity and surface blisters. The optimum solution treatment improves tensile properties in the transverse orientation to a greater extent than in the longitudinal orientation and this results in reduced anisotropy. © 1996 Elsevier Science Limited.

Fatigue in SiC-particulate-reinforced aluminium alloy composites

The fatigue behaviour in SiC-particulate-reinforced aluminium alloy composites has been briefly reviewed. The improved fatigue life reported in stress-controlled test results from the higher stiffness of the composites; therefore it is generally inferior to monolithic alloys at a constant strain level. The role of SiC particulate reinforcement has been examined for fatigue crack initiation, short-crack growth and long-crack growth. Crack initiation is observed to occur at matrix-SiC interface in cast composites and either at or near the matrix-SiC interface or at cracked SiC particles in powder metallurgy processed composites depending on particle size and morphology. The da/dN vs ΔK relationship in the composites is characterized by crack growth rates existing within a narrow range of ΔK and this is because of the lower fracture toughness and relatively high threshold values in composites compared with those in monolithic alloys. An enhanced Paris region slope attributed to the monotonic fracture contribution are reported and the extent of this contribution is found to depend on particle size. The effects of the aging condition on crack growth rates and particle size dependence of threshold values are also treated in this paper. © 1991.

Fatigue in metallic alloys containing non-metallic particles

This paper examines the effects of non-metallic particles on fatigue performance and, in particular, their influence on fatigue crack propagation at high ΔK (Kmax) levels. The nature and properties of a number of common non-metallic particles found in Fe- and Al- based alloys are described, and consideration is given to the consequences of mismatch of physical and chemical properties between particle and matrix. Effects of particles on fatigue in conventional alloys are illustrated and compared with the behaviour of Al/SiCp MMC. The problems associated with developing particulate reinforced MMC with adequate fatigue crack growth resistance and toughness for structural applications are discussed. © 1991.

Improved methodology to evaluate fracture properties of warm-mix asphalt using overlay test

This study developed a reliable and repeatable methodology to evaluate the fracture properties of asphalt mixtures with an overlay test (OT). In the proposed methodology, first, a two-step OT protocol was used to characterize the undamaged and damaged behaviors of asphalt mixtures. Second, a new methodology combining the mechanical analysis of viscoelastic force equilibrium in the OT specimen and finite element simulations was used to determine the undamaged properties and crack growth function of asphalt mixtures. Third, a modified Paris's law replacing the stress intensity factor by the pseudo J-integral was employed to characterize the fracture behavior of asphalt mixtures. Theoretical equations were derived to calculate the parameters A and n (defined as the fracture properties) in the modified Paris's law. The study used a detailed example to calculate A and n from the OT data. The proposed methodology was successfully applied to evaluate the impact of warm-mix asphalt (WMA) technologies on fracture properties. The results of the tested specimens showed that Evotherm WMA technology slightly improved the cracking resistance of asphalt mixtures, while foaming WMA technology provided comparable fracture properties. In addition, the study found that A decreased with the increase in n in general. A linear relationship between 2log(A) and n was established.

Keskipakoispuhaltimen siipipyörän lujuusteknisen mitoituksen kehittäminen

Tässä työssä on tutkittu prosessipuhaltimena käytettävän keskipakoispuhaltimen lujuusteknistä mitoitusta. Työn tavoitteena on ollut luoda analyyttisiä laskentakaavoja Koja Oy:n käyttöön tulevaisuuden tuotekehityksen tueksi. Tavoitteena on ollut tutkia myös siipien ja etu- ja takalevyn kiinnityshitsejä ja antaa työkaluja siipipyörän väsymismitoitukseen. Kirjallisuuskatkauksessa löytyi muutamia raportoituja tapauksia, jossa keskipakoispuhallin on vaurioitunut käytön aikana. Yhtenäisenä tekijänä kaikille tapauksille on ollut hitsausliitoksen väsyminen. Väsyttävän kuormituksen raportoiduissa tapauksissa on aiheuttanut siipipyörän värähtely. Väsyminen on alkanut siiven kiinnitysliitoksista hitsin rajaviivalta, siipipyörän ulkokehältä. Siipipyörän analysointiin on käytetty analyyttisiä laskentakaavoja ja elementtimenetelmää. Analyyttisten laskentakaavojen tuottaminen on muuten monimutkaiselle rakenteelle hyvin työlästä ja aikaa vievää. Staattisessa tarkastelussa elementtimenetelmällä siipipyörän mallintamiseen on käytetty keskipintamallia. Lisäksi laskentatiedostojen kokoa on pystytty rajaamaan syklisesti symmetrisen rakenteen ansiosta. Siipipyörän väsymistarkastelu on suoritettu tehollisen lovijännityksen menetelmällä, jossa tarkasteltavan hitsin rajaviivoille ja juureen on elementtimenetelmässä mallinnettu r=1 mm suuruinen lovi, josta jännityksen on luettu. Elementtimenetelmässä laskentaohjelmistona on käytetty Abaqus 6.14.1 -laskentaohjelmistoa. Analyyttisten laskentakaavojen kehityksessä on päästy hyvin lähelle elementtimenetelmästä saatuja vastaavia tuloksia. Analyyttisten kaavojen antamat tulokset eroavat kuitenkin sen verran virhettä, että varsinainen tarkka laskenta on syytä suorittaa ennen lopullisen konstruktion määrittämistä. Tehollisella lovijännitysmenetelmällä on saatu määritettyä rakenteen kriittisin kohta, jonka kestoikä on 36 400 sykliä. Tutkimuksen pohjalta Koja Oy on saanut hyvät edellytykset jatkaa tutkimusta lopun tuoteperheen parantamiseen.

Evaluation of Information Entropy from Acoustic Emission Waveforms as a Fatigue Damage Metric for Al7075-T6

Information entropy measured from acoustic emission (AE) waveforms is shown to be an indicator of fatigue damage in a high-strength aluminum alloy. Several tension-tension fatigue experiments were performed with dogbone samples of aluminum alloy, Al7075-T6, a commonly used material in aerospace structures. Unlike previous studies in which fatigue damage is simply measured based on visible crack growth, this work investigated fatigue damage prior to crack initiation through the use of instantaneous elastic modulus degradation. Three methods of measuring the AE information entropy, regarded as a direct measure of microstructural disorder, are proposed and compared with traditional damage-related AE features. Results show that one of the three entropy measurement methods appears to better assess damage than the traditional AE features, while the other two entropies have unique trends that can differentiate between small and large cracks.

Intergranular Corrosion and Stress Corrosion Cracking of Extruded AA6005A

A research program focused on understanding the intergranular corrosion (IGC) and stress corrosion cracking (SCC) behavior of AA6005A aluminum extrusions is presented in this dissertation. The relationship between IGC and SCC susceptibility and the mechanisms of SCC in AA6005A extrusions were studied by examining two primary hypotheses. IGC susceptibility of the elongated grain structure in AA6005A exposed to low pH saltwater was found to depend primarily on the morphology of Cu-containing precipitates adjacent to the grain boundaries in the elongated grain structure. IGC susceptibility was observed when a continuous (or semi-continuous) film of Cu-containing phase was present along the grain boundaries. When this film coarsened to form discrete Cu-rich precipitates, no IGC was observed. The morphology of the Cu-rich phase depended on post-extrusion heat treatment. The rate of IGC penetration in the elongated grain structure of AA6005A-T4 and AA6005A-T6 extrusions was found to be anisotropic with IGC propagating most rapidly along the extrusion direction, and least rapidly along the through thickness direction. A simple 3-dimensional geometric model of the elongated grain structure was accurately described the observed IGC anisotropy, therefore it was concluded that the anisotropic IGC susceptibility in the elongated grain structure was primarily due to geometric elongation of the grains. The velocity of IGC penetration along all directions in AA6005A-T6 decreased with exposure time. Characterization of the local environment within simulated corrosion paths revealed that a pH gradient existed between the tip of the IGC path and the external environment. Knowledge of the local environment within an IGC path allowed development of a simple model based on Fick's first law that considered diffusion of Al3+ away from the tip of the IGC path. The predicted IGC velocity agreed well with the observed IGC velocity, therefore it was determined that diffusion of Al3+ was the primary factor in determining the velocity of IGC penetration. The velocity of crack growth in compact tensile (CT) specimens of AA6005A-T6 extrusion exposed to 3.5% NaCl at pH = 1.5 was nearly constant over a range of applied stress intensities, exposure times, and crack lengths. The crack growth behavior of CT specimens of AA6005A-T6 extrusion exposed to a solution of 3.5% NaCl at pH = 2.0 exhibited similar behavior, but the crack velocity was ~10.5X smaller than that those exposed to a solution at pH =1.5. Analysis of the local stress state and polarization behavior at the crack tip predicted that increasing the pH of the bulk solution from 1.5 to 2.0 would decrease the corrosion current density at the crack tip by approximately 11.8X. This predicted decrease in corrosion current density was in reasonable agreement with the observed decrease in SCC velocity associated with increasing the solution pH from 1.5 to 2.0. The agreement between the predicted and observed SCC velocities suggested that the electrochemical reactions controlling SCC in AA6005A-T6 extrusions are ultimately controlled by the pH gradient that exists between the crack tip and external environment.

Damage and fracture algorithm using the screened Poisson equation and local remeshing

We propose a crack propagation algorithm which is independent of particular constitutive laws and specific element technology. It consists of a localization limiter in the form of the screened Poisson equation with local mesh refinement. This combination allows the cap- turing of strain localization with good resolution, even in the absence of a sufficiently fine initial mesh. In addition, crack paths are implicitly defined from the localized region, cir- cumventing the need for a specific direction criterion. Observed phenomena such as mul- tiple crack growth and shielding emerge naturally from the algorithm. In contrast with alternative regularization algorithms, curved cracks are correctly represented. A staggered scheme for standard equilibrium and screened equations is used. Element subdivision is based on edge split operations using a given constitutive quantity (either damage or void fraction). To assess the robustness and accuracy of this algorithm, we use both quasi-brittle benchmarks and ductile tests.

Fracture Toughening and Self-Healing of Composite Laminates by Nanofibrous Mats Interleaving

Composite laminates present important advantages compared to conventional monolithic materials, mainly because for equal stiffness and strength they have a weight up to four times lower. However, due to their ply-by-ply nature, they are susceptible to delamination, whose propagation can bring the structure to a rapid catastrophic failure. In this thesis, in order to increase the service life of composite materials, two different approaches were explored: increase the intrinsic resistance of the material or confer to them the capability of self-repair. The delamination has been hindered through interleaving the composite laminates with polymeric nanofibers, which completed the hierarchical reinforcement scale of the composite. The manufacturing process for the integration of the nanofibrous mat in the laminate was optimized, resulting in an enhancement of mode I fracture toughness up to 250%. The effect of the geometrical dimensions of the nano-reinforcement on the architecture of the micro one (UD and woven laminates) was studied on mode I and II. Moreover, different polymeric materials were employed as nanofibrous reinforcement (Nylon 66 and polyvinylidene fluoride). The nano toughening mechanism was studied by micrograph analysis of the crack path and SEM analysis of the fracture surface. The fatigue behavior to the onset of the delamination and the crack growth rate for woven laminates interleaved with Nylon 66 nanofibers was investigated. Furthermore, the impact behavior of GLARE aluminum-glass epoxy laminates, toughened with Nylon 66 nanofibers was investigated. Finally, the possibility of confer to the composite material the capability of self-repair was explored. An extrinsic self-healing-system, based on core-shell nanofibers filled with a two-component epoxy system, was developed by co-electrospinning technique. The healing potential of the nano vascular system has been proved by microscope electron observation of the healing agent release as result of the vessels rupture and the crosslinking reaction was verified by thermal analysis.

Crack mechanical failure in lithium niobate crystal under ion irradiation; novel simulation by extended finite elements

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion-generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

Crack mechanical failure in ceramic materials under ion irradiation: case of lithium niobate crystal

Swift heavy ion irradiation (ions with mass heavier than 15 and energy exceeding MeV/amu) transfer their energy mainly to the electronic system with small momentum transfer per collision. Therefore, they produce linear regions (columnar nano-tracks) around the straight ion trajectory, with marked modifications with respect to the virgin material, e.g., phase transition, amorphization, compaction, changes in physical or chemical properties. In the case of crystalline materials the most distinctive feature of swift heavy ion irradiation is the production of amorphous tracks embedded in the crystal. Lithium niobate is a relevant optical material that presents birefringence due to its anysotropic trigonal structure. The amorphous phase is certainly isotropic. In addition, its refractive index exhibits high contrast with those of the crystalline phase. This allows one to fabricate waveguides by swift ion irradiation with important technological relevance. From the mechanical point of view, the inclusion of an amorphous nano-track (with a density 15% lower than that of the crystal) leads to the generation of important stress/strain fields around the track. Eventually these fields are the origin of crack formation with fatal consequences for the integrity of the samples and the viability of the method for nano-track formation. For certain crystal cuts (X and Y), these fields are clearly anisotropic due to the crystal anisotropy. We have used finite element methods to calculate the stress/strain fields that appear around the ion- generated amorphous nano-tracks for a variety of ion energies and doses. A very remarkable feature for X cut-samples is that the maximum shear stress appears on preferential planes that form +/-45º with respect to the crystallographic planes. This leads to the generation of oriented surface cracks when the dose increases. The growth of the cracks along the anisotropic crystal has been studied by means of novel extended finite element methods, which include cracks as discontinuities. In this way we can study how the length and depth of a crack evolves as function of the ion dose. In this work we will show how the simulations compare with experiments and their application in materials modification by ion irradiation.

Impact of transition oxides on growth stresses and texture of alumina scales formed during oxidation of iron aluminides

The internal stresses and crystallographic texture in alpha-Al(2)O(3) scales grown on iron aluminides at 1100 degrees C were determined in situ using synchrotron X-ray diffraction. In the first hour of oxidation, alpha-Al(2)O(3) was formed by direct nucleation and by conversion from transition oxides (either theta-Al(2)O(3) or a mixed Fe-Al oxide). A sharp texture develops connected with the direct nucleation of alpha-Al(2)O(3), in contrast to the weaker texture observed in alpha-Al(2)O(3) originated by previous transformations, which also yielded tensile stresses in early oxidation stages. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In-situ X-ray diffraction studies during growth of Ni-Ti shape memory alloy films and their complementary ex-situ characterization

Shape Memory Alloy (SMA) Ni-Ti films have attracted much interest as functional and smart materials due to their unique properties. However, there are still important issues unresolved like formation of film texture and its control as well as substrate effects. Thus, the main challenge is not only the control of the microstructure, including stoichiometry and precipitates, but also the identification and control of the preferential orientation since it is a crucial factor in determining the shape memory behaviour. The aim of this PhD thesis is to study the optimisation of the deposition conditions of films of Ni-Ti in order to obtain the material fully crystallized at the end of the deposition, and to establish a clear relationship between the substrates and texture development. In order to achieve this objective, a two-magnetron sputter deposition chamber has been used allowing to heat and to apply a bias voltage to the substrate. It can be mounted into the six-circle diffractometer of the Rossendorf Beamline (ROBL) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France, enabling an in-situ characterization by X-ray diffraction(XRD) of the films during their growth and annealing. The in-situ studies enable us to identify the different steps of the structural evolution during deposition with a set of parameters as well as to evaluate the effect of changing parameters on the structural characteristics of the deposited film. Besides the in-situ studies, other complementary ex-situ characterization techniques such as XRD at a laboratory source, Rutherford backscattering spectroscopy(RBS), Auger electron spectroscopy (AES), cross-sectional transmission electron microscopy (X-TEM), scanning electron microscopy (SEM), and electrical resistivity (ER) measurements during temperature cycling have been used for a fine structural characterization. In this study, mainly naturally and thermally oxidized Si(100) substrates, TiN buffer layers with different thicknesses (i.e. the TiN topmost layer crystallographic orientation is thickness dependent) and MgO(100) single crystals were used as substrates. The chosen experimental procedure led to a controlled composition and preferential orientation of the films. The type of substrate plays an important role for the texture of the sputtered Ni-Ti films and according to the ER results, the distinct crystallographic orientations of the Ni-Ti films influence their phase transformation characteristics.