Search

In this work, fatigue crack growth (FCG) in the 2024-T351 aluminium alloy is studied using the plastic CTOD range, Δδp. Experimental tests were performed on 12 mm thick CT specimens in order to obtain FCG rate and in cylindrical specimens to obtain stress–strain loops. A numerical analysis replicated the experimental work in terms of material, geometry and loading conditions, but assuming pure plane strain state, in order to obtain Δδp. The material parameters were fitted using the experimental stress–strain loops. The experimental work showed an increase of FCG rate with the increase of stress ratio from R = 0.1 to R = 0.7 mm, which indicated the existence of the crack closure phenomenon. However, the analysis of the position of the first node behind the crack tip showed that there is no crack closure under plane strain state, while a maximum value of 36% was found for plane stress state. Therefore, the surfaces influence FCG rate even in 12 mm thick specimens. A nearly linear relation was found between da/dN and Δδp. The comparison with other aluminium alloys showed that there is a significant influence of material on da/dN-Δδp relation. The change from plane strain to plane stress state decreased FCG rate due to crack closure. Under plane strain state there is a minor influence of stress ratio in the range R = 0.1–0.7, also because there is no crack closure. Finally, a comparison was made between plastic CTOD and cumulative plastic strain at the crack tip. Well defined relations were found, showing that both parameters can be used to quantify crack tip deformation. The authors would like to acknowledge the financial support from Project PTDC/CTM-CTM/29101/2017 – POCI-01-0145-FEDER-029101 funded by FEDER funds through COMPETE2020 - Programa Operacional Competitividade e Internacionalizaç˜ao (POCI) and by national funds (PIDDAC) through FCT/MCTES. This research is also sponsored by FEDER funds through the program COMPETE – Programa Operacional Factores de Competitividade – and by national funds through FCT – Fundaçao ˜ para a Ciˆencia e a Tecnologia –, under the project UIDB/00285/2020, UIDB/00481/2020 and UIDP/00481/2020. Financial support from the Junta de Andalucia through grant reference UMA18-FEDERJA-250 of Programa Operativo FEDER and Subprograma Estatal de Movilidad, del Plan Estatal de I+D+i through grant reference CAS18/00251 is also greatly acknowledged. The authors would also like to acknowledge the funding for open access charge: Universidad de Málaga/CBUA.

Accurate knowledge of the plastic zone of fatigue cracks is a very direct and effective way to quantify the damage of components subjected to cyclic loads. In this work, we propose an ultra-fine experimental characterisation of the plastic zone based on Vickers micro-indentations. The methodology is applied to different compact tension (CT) specimens made of aluminium alloy 2024-T351 subjected to increasing stress intensity factors. The experimental work and sensitivity analysis showed that polishing the surface to #3 μm surface finish and applying a 25 g-force load for 15 s produced the best results in terms of resolution and quality of the data. The methodology allowed the size and shape of both the cyclic and the monotonic plastic zones to be visualised through 2D contour maps. Comparison with Westergaard’s analytical model indicates that the methodology, in general, overestimates the plastic zone. Comparison with S355 low carbon steel suggests that the methodology works best for alloys exhibiting a high strain hardening ratio.

A study on the accuracy of the approximation of the displacement field around of crack tip in a sample made from bridge steel (S355) is main objective of contribution. Linear elastic fracture mechanics (LEFM) theory in framework of multi-parameter formulation, i.e. postulated by Williams is used to determine of coefficients of Williams power series terms. Over deterministic method was used to calculate the terms based on the least squares regression technique, applied on data from numerical simulation and experiment on S355 steel grades. Comparison between the stress fields (by principal stress s1 and von Mises stress sHMH) obtain from experimental measurement DIC, Hybrid method and obtain from reconstruction by using various number of Williams power terms are quantified in order to get key information around the crack tip region on bridge steel specimens.

The numerical analysis of the plasticity induced crack closure requires the development of the plastic wake. Transient behaviour is observed when the crack starts to grow. The plastic wake length has an influence on the crack closure results and a great impact on the computational cost. Previous works have analysed the influence of this parameter considering bi-dimensional specimens in either plane strain or plane stress conditions. Lately, some three-dimensional models have appeared in order to analyse the crack closure phenomenon. The main scope of this study is to quantify and comprehend the minimum length required to stabilise the crack opening and closure values considering a three-dimensional model. On this purpose, a CT aluminium specimen has been modelled three-dimensionally considering a straight crack front and several calculations have been made in order to evaluate the influence of the simulated plastic wake. The numerical analysis is made in terms of crack closure and opening values.

The fatigue crack retardation mechanisms operating after an overload event are investigated for a bainitic steel using high spatial resolution energy dispersive synchrotron X-ray diffraction. The elastic crack-tip strain fields are mapped at mid-thickness of compact tension samples at R-ratios of 0.1 and 0.4. The same overload stress intensity factor (KOL = 60 MPa m1/2) is applied in each case with the cracks then propagating under the same applied stress intensity range, ΔKapp = 27 MPa m1/2. The competing retardation mechanisms are directly quantified and separated, with the associated fatigue crack growth (FCG) rates then being predicted according to a 2-parameter Walker-type assessment and validated against those measured. The stress intensity factor associated with the overload residual stress field is calculated using a weight function approach. For R=0.1, shielding from residual stress controls retardation when crack growth through the overload plastic zone, rp OL, is small (specifically p OL). For more extensive crack growth, discontinuous crack closure controls the retardation behaviour, with significant load transfer across opposing crack faces being observed at minimum load (for R=0.1). These crack face tractions are associated with the plastic asperity created during overload. The traction forces holding the crack faces open at minimum load are, for the first time, used to directly quantify the associated stress intensity factor, Kmin tract as a function of crack growth. While no crack shielding is expected, nor observed, for R=0.4, the variation in FCG rate after overload is explained by changes in effective R-ratio.

In this work we present a novel methodology to analyse the extent of crack tip plasticity inside engineering materials. The plastic zone developed ahead of a fatigue crack in a bainitic steel compact tension specimen is studied. The bulk of the steel is probed with synchrotron X-ray diffraction technique to measure the elastic strain field deep inside the material. The procedure involves combining the experimental synchrotron data with numerical data to obtain the equivalent strain map. This map is then converted into a stress field where the boundary of the plastic zone can be directly extracted. The procedure is tested on a real fatigue crack and the results are compared to plane stress and plane strain models. The promising results will enable key progress on understanding the mechanisms that generate the damage at the tip of fatigue cracks in engineering components.

In this work, we present a detailed numerical analysis of the evolution of the plastic zone through the thickness of compact tension aluminium alloy specimens. An ultra-fine non-linear finite element model was used to characterise with great detail the size and shape of the plastic zone for different thicknesses. The numerical plastic zone was compared in size and shape with the plane strain and plane stress Dugdale plastic zones. In general, the maximum plastic zone was found to be in the interior but closer to the surface than to the mid plane. We also propose an equivalent bi-dimensional plastic zone for converting the complex 3D distribution into a 2D plastic zone. Finally, the numerical results were validated using experimental displacement data measured optically from the surface of the specimen.

International audience; Controlled dielectric breakdown is becoming the main solid-state nanopore fabrication technique worldwide. This paper presents the construction of a system for the fabrication of a single solid-state nanopore in a membrane using this technique. We include a basic introduction to nanopore theory. We also detail the building process of the low-cost equipment for nanopore fabrication, so that this paper can be used as a guide for building a similar system. Finally, we include the results of the creation of two nanopores and their characterization.

The fatigue overload behaviours of coarse grain (~30 μm), ultrafine grain (360 nm) and nanocrystalline (~30 nm) Ni are compared under constant amplitude loading at R = 0.1 after a 100% overload. Synchrotron X-ray diffraction is applied to map the elastic crack-tip strain fields at the mid-thickness of a compact tension specimen of nanocrystalline Ni at various stages through the loading cycles, from which the variation in stress before, during and after overload is estimated. Digital image correlation is used to measure the crack length and the displacement fields at the specimen surfaces for both grain sizes, from which the fatigue crack growth (FCG) rate, crack opening displacement and stress intensity factor range are determined. The FCG for coarse grain Ni is most significantly retarded whereas the nanocrystalline Ni is least affected by the overload due to an increased yield stress and a more planar crack surface morphology. As a result, FCG retardation by plasticity, surface roughness and residual stress-induced crack closure are reduced. Keywords: Nanocrystalline, Retardation mechanisms, Energy dispersive X-ray diffraction, Crack path, J-integral

In this work the biaxial behavior of 316 stainless steel is studied under the lens of critical plane approach. A series of ten experiments were developed on dog bone shape hollow cylindrical specimens made of type 316 stainless steel. Five different loading conditions were assessed, with (i) only axial stress, (ii) only hoop stress, (iii) proportional combination of axial and hoop stresses, (iv) non-proportional combination of axial and hoop stresses with square shape and (v) non-proportional combination of axial and hoop stresses with L-shape. The fatigue analysis is performed following four different critical plane theories, namely Wang-Brown, Fatemi-Socie, Liu I and Liu II. The efficiency of all four theories is studied in terms of the accuracy of their life predictions.

A mathematical processing technique and an algorithm for the numerical correction of experimental displacement fields on the specimen surface in the vicinity of a crack tip with consideration of the displacement of a body as a rigid whole under its loading and the actual position of a crack tip were proposed. The initial experimental displacement fields in the vicinity of a fatigue crack tip of a compact specimen were determined by digital image correlation. This approach was successfully used for a compact specimen and the possibility of its expansion to obtain the distributions of the stress intensity factors and the nonsingular T stresses along the spatial crack front was shown.

The crack propagation is influenced by what happens at both the surface and the interior of the component. Traditional experimental tools allow the surface behaviour to be characterised accurately but the information obtained from the bulk is much more limited. In this work, a study of both bulk and surface behaviour is presented. The material behaviour is studied by powerful 3D ultrafine finite element analysis in terms of the crack tip plasticity for a range of different conditions. The analysis is performed on a wedged opening loaded specimen made of an Aluminium alloy. The results are then validated with full-field digital image correlation data. The validation is used to enhance the model used in our analyses.

Structural components are often subjected to complex multiaxial loading conditions. The study of fatigue cracks under such conditions is not easy from an experimental point of view and most works tend to focus more on the simpler but less realistic case of uni-axial loading. Consequently, there are many uncertainties related to the load sequence effect that is now well known and is not normally incorporated into the growth models. The current work presents a new methodology for evaluating overload effect in biaxial fatigue cracks. The methodology includes evaluation of mixed-mode (ΔK I and ΔK II ) stress intensity factor and the Crack Opening Displacement for samples with and without overload cycle under biaxial loading. The methodology is tested under two different load levels and a range of crack lengths. All crack-tip information is obtained with a hybrid optical-analytical methodology. It combines experimental full-field digital image correlation data and Williams' elastic model describing the crack-tip field.

A study on the approximation of the stress field in the vicinity of crack tip in a compact tension specimen made from Al 2024-T351 is presented. Crack tip stress tensor components are expressed using the linear elastic fracture mechanics (LEFM) theory in this work, more precisely via its multi-parameter formulation, i.e. by Williams power series (WPS). Determination of coefficients of terms of this series is performed using a least squares-based regression technique known as over deterministic method (ODM) for which displacements data obtained experimentally via optical measurements are taken as inputs. The stress fields reconstructed based on the displacement data obtained experimentally by means of optical measurements are verified by means of the stress field approximations derived for the normalized CT specimen via hybrid elements.

In this work, the multiaxial fatigue behaviour of 316 and 304 stainless steel was studied. The study was based on the critical plane approach which is based on observations that cracks tend to nucleate and grow in specific planes. Three different critical plane models were employed to this end, namely Fatemi&ndash, Socie (FS), Smith&ndash, Watson&ndash, Topper (SWT) and the newly proposed Sandip&ndash, Kallmeyer&ndash, Smith (SKS) model. The study allowed equi-biaxial stress state, mean strain and non&ndash, proportional hardening effects to be taken into consideration. Experimental tests including different combinations of tension, torsion and inner pressure were performed and were useful to identify the predominant failure mode for the two materials. The results also showed that the SKS damage parameter returned more conservative results than FS with lower scatter level in both materials, with prediction values between FS and SWT.

Rare earth doped phosphate glasses are attractive materials for the engineering of photonic devices, due to their easy processing, good thermal stability, excellent optical properties and high rare-earth ions solubility [1]. Besides, phosphate glasses with a P 2 O 5 content of 50 mol% have been shown to be suitable for fiber drawing. It is well known that if the rare-earth ions are located in crystalline phase of desired nature and structure, the spectroscopic properties of the glasses can be enhanced [2]. Therefore, efforts have been focused on the development of new glass-ceramics (GCS) obtained from the heat treatment of glasses, as these engineered materials possess some of the glass properties (large flexibility of composition and geometry) but also some advantages of the RE-doped single crystals (high absorption and emission as well as long lifetimes).

This work investigates the fatigue behaviour under multiaxial loading conditions of 316 stainless steel commonly used in offshore and marine structures. Cylindrical hollow specimens were tested under uniaxial tensile stress, hoop stress and different combinations of tensile and hoop stresses. Both proportional and non-proportional loading were studied in the experiments. Prediction of the fatigue life was performed with Wang-Brown, Fatemi-Socie and Liu critical plane models. A detailed analysis and discussion of the performance of the different models is presented with an emphasis on the versatility for different situations.

In this work a novel approach to estimate in situ the stress intensity factor (SIF) through the thickness of metal specimens is presented. It is based on a hybrid methodology that combines powerful synchrotron X-ray diffraction data with an elastic analytical model describing the strain field around the crack tip. A sensitivity analysis is conducted to understand the largest sources of error and their impact on the estimated SIF values. The accuracy in locating the crack tip position was found to affect the quality of the SIF estimation. Accordingly a procedure is developed to help locate the crack tip position. The methodology is tested on ultra-fine grained aluminium alloy 5091 and on a bainitic steel. It is recommended that an area of interest having a size at least 3 times larger than the plane strain plastic zone is mapped, providing good SIF estimations (within 8%) for all cases studied.

In this work Suman-Kallmeyer critical plane model is assessed and compared to few other critical plane models. The analysis is based on both in-phase proportional loading and out-of-phase non-proportional loading applied to a low carbon steel. A wide range of loading scenarios were studied by combining axial and torsional loads, producing fatigue lives in the range of 103 to 106 cycles. The study allowed low and high cycle fatigue regimes to be assessed. In addition, the hardening effect was also investigated. Finally, a comparison with well established critical plane models is also performed.

International audience

The effect of the incorporation of Er2O3-doped particles on the structural and luminescence properties of phosphate glasses was investigated. A series of different Er2O3-doped TiO2, ZnO, and ZrO2 microparticles was synthesized using soft chemistry and then added into various phosphate glasses after the melting at a lower temperature than the melting temperature. The compositional, morphological, and structural analyses of the particles-containing glasses were performed using elemental mapping by field emission-scanning electron microscopy (FE-SEM) with energy dispersive x-ray spectrometry (EDS) and x-ray diffraction (XRD). Additionally, the luminescence spectra and the lifetime values were measured to study the influence of the particles incorporation on the spectroscopic properties of the glasses. From the spectroscopic properties of the glasses with the composition 50P2O5-40SrO-10Na2O, a large amount of the Er2O3-doped particles is thought to dissolve during the glass melting. Conversely, the particles were found to survive in glasses with a composition 90NaPO3-(10 &minus, x)Na2O-xNaF (with x = 0 and 10 mol %) due to their lower processing temperature, thus clearly showing that the direct doping method is a promising technique for the development of new active glasses.

A generalised approach for determining the stress intensity factor in the bulk of non-transparent materials is presented. The new approach combines experimental elastic strain data, measured with powerful synchrotron X-ray diffraction, with an elastic model based on Williams’ series development. The stress intensity factor is calculated using a multi-point over-deterministic method where the number of experimental data points is higher than the number of unknowns describing the elastic field surrounding the crack-tip. The tool is tested on X-ray strain measurements collected on a bainitic steel. In contrast to surface techniques the approach provides insights into the crack tip mechanics deep within the sample and makes full use of the plastic zone data. Satisfactory results are obtained with 1 term in the series development and a smaller region of interest. By increasing the number of terms, it is possible to improve the accuracy in the SIF predictions but this requires a larger region of interest.

Two complementary experimental techniques have been used to study the evolution of crack-tip strain fields in a thin (plane stress dominated) compact tension sample following a single overload (OL) event. The total strain has been characterised at the surface by digital image correlation (DIC), while the elastic strain field in the bulk (interior) behaviour has been characterised by means of synchrotron X-ray diffraction (XRD). Surface and bulk information allowed us to visualise the evolution of the strain fields before the OL event, during the OL event, just after it and at various stages after it. Unlike previous work, complete 2D maps of elastic strains around the crack-tip were acquired at 60 µm spatial resolution by XRD. The strain data were used to estimate the effective crack driving force at the surface and at the mid plane. The DIC shows less crack opening displacement after overload and the XRD a lower crack-tip peak stress after OL until the crack has grown past the compressive crack-tip residual stress after which the behaviour returned to that for the baseline fatigue response. While the compressive residual stress introduced by the OL offsets the crack-tip stress field as it grows through the overload plastic zone, the changes in crack-tip stress over each cycle are the same before and at all stages after OL.

This work presents a new methodology for evaluating crack initiation under biaxial conditions. The methodology consists of evaluating a number of crack parameters automatically with digital processing of high-magnification images of the crack. Five different strain conditions were evaluated on a low carbon ferritic–pearlitic steel specimen with tubular shape. A hole of 150 μm diameter was drilled to enforce the crack to initiate at a particular spot. Different combinations of axial and torsional strains were analysed during the initiation stage of the crack. Fatigue crack propagation curves clearly showed oscillations due to microstructure. It was also observed that these oscillations decreased as the torsional component of the strain was increased. Driving force was also evaluated in crack opening and sliding direction through crack opening displacement and crack sliding displacement. The results demonstrate that crack opening displacement is sensitive to microstructural barriers and crack sliding displacement is sensitive to crack deviations caused by the microstructure.

This work aimed to study the fatigue behaviour of S355 low carbon steel under uniaxial and biaxial loading conditions and to measure the overload effects under both types of loads. The analysis is performed with a local strain approach and with damage tolerance approach. Local strain based investigation was based on Fatemi-Socie critical plane model. This allows the life reduction caused by growing the crack from a stress raiser to be quantified. The fatigue life reduction caused by applying simultaneously torsional and tension loads was also evaluated with Fatemi-Socie model. In addition, the crack opening displacement (COD) and the effective stress intensity factor (SIF) were evaluated experimentally via full-field technique of digital image correlation (DIC). Both COD and SIF have been useful to understand the transient effects caused by a 40% overload under the two types of load. In addition, COD measurements allowed the offset compliance method for evaluating the opening load to be applied on a cylindrical specimen subjected to tension load. SIF estimation was also useful to quantify the mode mixity (I + II) developed during the experiment under the two types of load.

The parametric study of the effect of material properties on fatigue crack growth (FCG) rarely has been addressed in literature. The consideration of plastic CTOD as crack driving force opened the opportunity to predict FCG rate numerically and therefore to develop parametric studies focused on the effect of loading, geometrical and material parameters. The objective here is to study the effect of kinematic saturation stress, XSat, and kinematic saturation rate, CX, on FCG using this numerical approach. The increase of the kinematic parameters reduced the plastic CTOD and therefore the FCG rate. The variation is non-linear and the rate of variation of δp decreases with the increase of XSat and CX. The effect of the kinematic saturation stress, XSat, is more relevant than the effect of kinematic saturation rate, CX. On the other hand, a small effect of kinematic hardening parameters on crack closure was found. Finally, the increase of the number of load cycles between crack increments produced a great reduction of crack closure but no effect of plastic CTOD on models without contact of crack flanks.

Titania particles doped with various concentrations of Erbium were synthesized by the sol-gel method followed by different heat treatments. The shape and the grain growth of the particles were noticeably affected by the concentration of Erbium and the heat treatment conditions. An infrared emission at 1530 nm, as well as green and red up-conversion emissions at 550 and 670 nm, were observed under excitation at 976 nm from all of the synthesized particles. The emission spectra and lifetime values appeared to be strongly influenced by the presence of the different crystalline phases. This work presents important guidelines for the synthesis of functional Er3+ -doped titania particles with controlled and tailored spectroscopic properties for photonic applications. publishedVersion

The crack initiation and propagation behaviour of St52-3N steel is investigated under biaxial conditions. Short crack behaviour is studied and pearlite bands are found to modify the crack growth rate. The behaviour under proportional and non-proportional (90°) loading is studied with different critical plane models; Wang–Brown, Fatemi–Socie, Smith–Watson–Topper, Liu I and Liu II. Smith–Watson–Topper and Liu I are found to best predict the crack orientation under proportional loading and Fatemi–Socie and Liu II under non-proportional loading. The best predictions in terms of fatigue life are obtained by Fatemi–Socie, both under proportional and non-proportional loads.

This work presents a novel methodology for characterising fatigue cracks under biaxial conditions on a low carbon steel. It allows both short crack and early propagation stages to be studied in tubular specimens. Short crack growth is studied with a long-distance microscope acquiring images of the bare metal surface. Results showed oscillations in crack growth rate due to microstructure. Early propagation stage is studied with high magnification Digital Image Correlation (DIC) technique for measuring displacement and strain crack-tip fields. By applying micro-speckle pattern on the metal surface it is possible to achieve high magnification for DIC technique. Ultra-fine black and white speckles were created by electro-spray technique. The validity of this novel technique is demonstrated by direct comparison with extensometer measurements, under combined tension–compression and torsion conditions. It was also possible to estimate satisfactorily the mixed-mode stress intensity factor.

In this work we present the results of in situ synchrotron X-ray diffraction measurements of fatigue crack-tip strain fields following a 100% overload (OL) under plane strain conditions. The study is made on a bainitic steel with a high toughness and fine microstructure. This allowed a very high (60 mu m) spatial resolution to be achieved so that fine-scale changes occurring around the crack-tip were captured along the crack plane at the mid-thickness of the specimen. We have followed the crack as it grew through the plastic/residually stressed zone associated with the OL crack location. We observed two effects; one when the enhanced plastic zone is ahead of the crack and one after it has been passed. Regarding the former it was found that the compressive stress at the crack-tip initially falls sharply, presumably due to the increased plastic stretch caused by the OL. This is associated with a concomitant fall in peak tensile stress at K-max, the elastic excursion between K-min and K-max remaining essentially unchanged from before OL. Subsequently discontinuous closure as seen previously for plane stress caused by crack face contact at the OL location limits the elastic strain range experienced by the crack tip and thereby retards crack growth. (C) 2014 Elsevier Ltd. All rights reserved.

This work uses the strip yield model implemented in NASGRO software to estimate fatigue life under random loading. Simulated results were compared with experimental data previously obtained by the authors using different random loading processes in Al2024-T351. Test data under constant amplitude loading from different authors have been considered in order to characterize the material behaviour and fit the model parameters. The two different strip yield model implemented in NASGRO software were considered. The ratio of simulated to experimental fatigue lives was between 0.71 and 1.52 considering all options and between 0.87 and 1.12 with the best option. Ministerio de Economía y Competitividad DPI2012-33382 Junta de Andalucía TEP-3244

In this invited proceeding, we report our latest results on the development of Er3+ doped phosphate glass-ceramics. Those new glass-ceramics were processed using different techniques, such as direct doping of particles into the glass melt and glass-ceramics methods. First, we explain the challenges to balance the survival and dispersion of Er3+ doped particles when preparing phosphate glass-ceramics using the direct doping of particles. Then, we discuss the impact of the glass crystallization on the luminescence properties of Er3+ ions.

Most mechanical components are subjected to the complex fatigue loading conditions, where both amplitude and direction of loading cycles change over the time. The estimation of damage caused by these complex loading scenarios are often done by simplified uniaxial fatigue theories, which ultimately leads to higher factor of safety during the final design considerations. Critical plane-based fatigue theories have been considered more accurate for computing the fatigue damage for multiaxial loading conditions in comparison to energy-based and equivalent stress-based theories. Two recently developed fatigue theories have been evaluated in this work for the available test data. Test data includes significant amount of biaxial load paths.

Er-doped phosphate glass ceramics were fabricated by melt-quenching technique followed by a heat treatment. The effect of the crystallization on the structural and luminescence properties of phosphate glasses containing Al2O3, TiO2, and ZnO was investigated. The morphological and structural properties of the glass ceramics were characterized by Field Emission-Scanning Electron Microscopy (FE-SEM), X-ray Diffraction (XRD), and micro-Raman spectroscopy. Additionally, the luminescence spectra and the lifetime values were measured in order to study the influence of the crystallization on the spectroscopic properties of the glasses. The volume ratio between the crystal and the glassy phases increased along with the duration of the heat treatment. The crystallization of the glass ceramics was confirmed by the presence of sharp peaks in the XRD patterns and different crystal phases were identified depending on the glass composition. Sr(PO3)2 crystals were found to precipitate in all the investigated glasses. As evidenced by the spectroscopic properties, the site of the Er3+ ions was not strongly affected by the heat treatment except for the fully crystallized glass ceramic which does not contain Al2O3, TiO2, and ZnO. An increase of the lifetime was also observed after the heat treatment of this glass. Therefore, we suspect that the Er3+ ions are incorporated in the precipitated crystals only in this glass ceramic. publishedVersion

Er-doped phosphate glasses were fabricated by melt-quenching technique. The changes in their thermal, structural and luminescence properties with the addition of Al2O3, TiO2 or ZnO were studied. Physical and thermal properties were investigated through density measurement and differential thermal analysis. Structural characterization was performed using the Raman and Infrared spectroscopy. In order to study the influence of the composition on the luminescence properties of the glasses, the refractive index, the luminescence spectra and the lifetime values were measured. The results show that with the addition of Al2O3 and TiO2 the phosphate network becomes more connected increasing the glass transition temperature, whereas the addition of ZnO does not show significant changes in the optical, thermal and structural properties but it leads to a larger emission cross-section at 1540 nm as compared to the other glasses. As the site of the Er3+ is not strongly affected by the change in the glass composition, we think that the emission properties of the glasses depend on the glass structure connectivity, which has an impact on the Er3+ ions solubility., 32 pages, 9 figures, 1 table

In this paper a methodology for evaluating crack initiation under biaxial conditions is presented. The methodology consists of evaluating the crack length automatically with digital processing of highmagnification images of the crack. The methodology was applied to study five different strain conditions on a low carbon ferritic-pearlitic steel specimen with tubular shape. A hole of 150 μm diameter was drilled to enforce the crack to initiate at a particular spot. Different combinations of axial and torsional strains were analysed during the initiation stage of the crack. The setup employed allowed detection of the crack to within 6 μm from the edge of the hole on average and monitoring of the crack during early stages. Fatigue crack propagation curves clearly showed oscillations due to microstructure. It was also observed that these oscillations decreased as the torsional component of the strain was increased. Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Junta de Andalucía through Proyectos de Excelencia grant reference TEP-3244 and of Spanish Ministerio de Economía y Competitividad through grant reference DPI2012-33382

This work presents a set of experiments devoted to studying the crack initiation stage under different combined tension-compression and torsion loads. Two different load levels were applied, producing very different fatigue lives. Lower strains generated lives approximately 10 times longer than higher strains. Results allowed retardation and acceleration effects due to microstructure to be clearly visualised.

The aim of this work is to investigate the effect of the loading rate on the crack growth behavior in carbon fiber reinforced polymer composites, under mode I monotonic tensile loading. For such objective, inter-laminar fracture toughness tests were performed on double cantilever beam (DCB) specimens, according to the ASTM-D5528 standard. The initial crack tip opening displacement (CTOD) was obtained by employing the digital image correlation (DIC) technique. Then, the strain energy release rate and the R-curve were obtained from the force-displacement curve, based on the modified beam theory (MBT), the compliance calibration method (CC) and the modified compliance calibration method (MCC). No significant variation was observed between results of these methods. Obtained results also indicated no considerable changes in the maximum force, by increasing the loading rate. The relation between the initial CTOD and the crack length was nonlinear, due to fiber bridging during testing. In addition, increasing the loading rate caused extended steady state fiber bridging zones.

The displacement field of compact tension (CT) specimens have been mapped by digital image correlation (DIC) local to growing fatigue cracks to study overload effects for plane stress and plane strain. We have extracted crack opening displacement (ΔCOD) and stress intensity (K) determined by a Muskhelishvili fit to the crack tip displacement field to infer the closure load. In both cases a classical knee was observed upon unloading consistent with closure which disappeared during the accelerated growth following OL, before increasing during retardation. In both cases following OL the crack growth rate is perturbed for a distance similar to the plastic zone.